OPTIMIZED SCHEDULING OF SCANNING FOR MS INITIATED HO IN MOBILE WIMAX
A Thesis Presented to The Academic Faculty By:
Nafiz Imiaz Bin Hamid (042424) Adnan Mahmud (042443)
In Partial Fulfillment of the Requirements for the Degree B.Sc. Engineering in Electrical and Electronic Engineering
Islamic University of Technology November 2008
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ABSTRACT Mobile WiMAX is one of the most promising technologies for broadband wireless communication based on the IEEE 802.16e-2005 standard. Mobile WiMAX bearing the IEEE standard 802.16e-2005 brings wireless broadband to a new dimension due to the support of nomadism.IEEE 802.16e standard for 4G Cellular Technology-Mobile WiMAX is proposed to support mobility. As a result, handover has become one of the most important QoS factors.
Because of lack of clarification in the conventional algorithm for handover process, there exists wastage of channel resource along with undesired delay. Again, it causes significant degradation in the overall system performance. Existing standard for handover includes scanning as one of the most vital processes. But scanning requests without proper response and delayed initiation of scanning only deteriorates the scenario. This threatening handover delay is a huge obstacle for the optimum performance of mobile WiMAX. In our project, various problematic scenarios of handover procedure have been shown with the log file analyzer tool along with “SlickEdit” using data from a WiMAX trial network and from that an optimized handoff scheme is proposed .associated with suitable pre-scanning algorithm, fast dedicated ranging and pre-registration process. It reflects the modification of the scheduling process of scanning to reduce redundant scanning requests and overall handover operation delay.
It should be admitted that working with WiMAX is a very challenging job. It obviously requires firm background knowledge in telecommunication; most of which had to be initiated in the beginning of our project work. Again, though we basically intended to do the major part of our work in MAC layer; because of gaining consolidated background knowledge we had to go through the PHY level. Lots of obstacles came through our path;one of the most vital ones was the unavailability of WiMAX trial network in Bangladesh. Still we didn’t lose heart and stuck to our plan of reaching the destination.
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ACKNOWLEDGEMENTS The research undertaken would not have been possible without the guidance and assistance that we have been able to enjoy.
Firstly, we would like to thank our thesis supervisor Assistant Professor Mohammad Tawhid Kawser, for providing me the right balance of guidance and independence in our research.. We are immensely indebted to him for his advice both in technical and non-technical matters. He has always been willing to take the time to help us and offer advice. The level of assistance that we got from him actually worked a great extent in our coming up to this level. The vital inspirations coming from him to us time to time consolidated our determination and eagerness towards the successful completion of the project. His perpetual help did more than enough to give a constant pace to our work without the existence of a co-supervisor.
Again valuable advice from many lecturers of our department signifying the value of a good project work accelerated our working speed and dedication in the project. We would like to extend many thanks to them along with our classmates for creating a healthy research environment, which was very conducive for us. It should be mentioned that during the initial level of our work we got various suggestions of improvement from Engg.Mohammad Zahed Ali –Manager/section chief of Network product division of ZTE Corporation along with Engg. Hasnat Jamil Dipu– Product and Solution manager of CDMA-WiMAX Dept. of Huawei Technologies Bangladesh Ltd.We express our heartiest gratitude towards their raising such selfless helping hands.
Our special thanks go to our parents for being with us all the time in our hardship and pains and being such great well-wishers. But the greatest help came undoubtedly from the Almighty Creator of the universe. We are very much grateful to Him for remaining with us and providing us necessary patience to go on.
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TABLE OF CONTENTS Page ACKNOWLEDGEMENTS LIST OF TABLES LIST OF FIGURES LIST OF SYMBOLS AND ABBREVIATIONS CHAPTER 1. Introduction 1.1. Salient Features 1.2. Reference Model 1.3. IEEE 802.16e-2005 Technology
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1 2 3 5
2. Physical Layer (PHY) Description 2.1. TDD Frame Structure 2.2. OFDMA Basics 2.3. OFDMA Symbol Structure and Sub-Channelization 2.3.1. Subcarrier permutation and Diversity permutation 2.3.2. Contiguous permutation 2.4. Scalable OFDMA 2.5. Other Advanced PHY Layer Features
6 6 8 10 11 12 13 14
3. MAC Layer Description 3.1. Quality of Service (QoS) Support 3.2. MAC Scheduling Service 3.3. MAC Addresses and MAC Frames 3.3.1. MAC Addresses and Other Addresses 3.3.2 MAC Frames 3.3.3 MAC Header Format 3.4. Key Management Messages 3.5. Hybrid Automatic Repeat Request (HARQ) Mechanism
16 16 18 20 20 20 21 22 24
4. Mobility, Handover And Power-Save Modes 4.1. Mobility Management 4.1.1. Power Management 4.1.1.1. Sleep mode 4.1.1.2. Idle mode 4.2. Handover considerations
26 26 26 26 26 27
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4.3. Network Topology Acquisition 4.3.1. Network Topology Advertisement 4.3.2. MS Scanning of Neighbor BSs 4.3.3 Association Procedure 4.4 The Handover Process 4.4.1 Cell Reselection 4.4.2 Handover Decision and Initiation 4.4.3 Synchronization to a Target BS Downlink 4.4.4 Ranging and Network Re-entry 4.4.5 Termination of MS Context 4.4.6 Handover Cancellation 4.5. MDHO and FBSS
28 28 29 30 31 32 32 33 33 35 35 36
5. An Optimized Method For Scheduling Process Of Scanning 5.1. Previous work 5.2. Shortcoming in Scheduling Process for Scanning in IEEE 802.16e 5.3. Proposed Scheme 5.4. Performance Analysis
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6. Conclusion
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REFERENCES APPENDIX –I (Definitions)
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47 49
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LIST OF TABLES Table 2.1. OFDMA Scalability Parameters Table 2.2. Supported Code and Modulations Table 2.3. Mobile WiMAX PHY Data Rates with PUSC Sub-Channel Table 3.1. Mobile WiMAX Applications and Quality of Service Table 3.3.1. Key Management Messages Table 3.3.2. Key Management Messages (Continued) Table 5.4: SS Power info RSSI all data using XCAP-X Table 5.4.1. Scanning related logging message using XCAP-X
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LIST OF FIGURES Figure 1.1: Mobile WiMAX System Profile Figure 1.2: IEEE Standard 802.16 protocol layering showing SAPs Figure 2.1.WiMAX OFDMA Frame Structure Figure 2.1.1. Basic Architecture of an OFDM system Figure 2.1.2. Insertion of Cyclic Prefix Figure 2.1.3. WiMAX OFDMA Frame Structure Figure 2.2.1. Basic Architecture of an OFDM system Figure 2.2.2. Insertion of Cyclic Prefix Figure 2.3. OFDMA Sub-Carrier Structure Figure 2.3.1. DL Frequency Diverse Sub-Channel Figure 2.3.2. Tile Structure for Uplink PUSC Figure 2.3.3 DL PUSC, FUSC cluster, bin &AMC zone concept Figure 3.1. Mobile WiMAX QoS Support Figure 3.2. General format of a MAC frame or MAC PDU Figure 3.4. Incremental Redundancy (IR) HARQ Figure 4.3. Network Topology Advertisement Figure 4.3.2. Scanning of Neighbor BS Figure 4.4. Handover process stages Figure 4.4.1. Summary of network re-entry steps Figure 4.4.2. MS Initiated Hard HO as seen by MS Figure 4.5. Handover algorithm for MDHO/FBSS Figure 5.3. Proposed method for scheduling process of scanning Figure 5.4. SS power info RSSI and CINR all data Figure 5.4.1. Logging message elaboration using XCAP-X Figure 5.4.2. User defined cell measurement
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LIST OF SYMBOLS AND ABBREVIATION 3GPP
3G Partnership Project
3GPP2
3G Partnership Project 2
AAS
Adaptive Antenna System also Advanced Antenna System
ACK
Acknowledge
AES
Advanced Encryption Standard
AG
Absolute Grant
AMC
Adaptive Modulation and Coding
A-MIMO Adaptive Multiple Input Multiple Output (Antenna) AMS
Adaptive MIMO Switching
ARQ
Automatic Repeat reQuest
ASN
Access Service Network
ASP
Application Service Provider
BE
Best Effort
BRAN
Broadband Radio Access Network
CC
Chase Combining (also Convolutional Code)
CCI
Co-Channel Interference
CCM
Counter with Cipher-block chaining Message authentication code
CDF
Cumulative Distribution Function
CDMA
Code Division Multiple Access
CINR
Carrier to Interference + Noise Ratio
CMAC
Cipher-based Message Authentication Code
CP
Cyclic Prefix
CQI
Channel Quality Indicator
CSN
Connectivity Service Network
CSTD
Cyclic Shift Transmit Diversity
CTC
Convolutional Turbo Code
DL
Downlink
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DOCSIS Data Over Cable Service Interface Specification DSL
Digital Subscriber Line
DVB
Digital Video Broadcast
EAP
Extensible Authentication Protocol
EESM
Exponential Effective SIR Mapping
EIRP
Effective Isotropic Radiated Power
ErtPS
Extended Real-Time Polling Service
ETSI
European Telecommunications Standa
FBSS
Fast Base Station Switching
FCH
Frame Control Header
FDD
Frequency Division Duplex
FFT
Fast Fourier Transform
FUSC
Fully Used Sub-Carrier
HARQ
Hybrid Automatic Repeat Request
HHO
Hard Hand-Off
HiperMAN
High Performance Metropolitan Area Netw
HMAC
Hash Message Authentication Code
HO
Hand-Off or Hand Over
HTTP
Hyper Text Transfer Protocol
IE
Information Element
IETF
Internet Engineering Task Force
IFFT
Inverse Fast Fourier Transform
IR
Incremental Redundancy
ISI
Inter-Symbol Interference
LDPC
Low-Density-Parity-Check
LOS
Line of Sight
MAC
Media Access Control
MAI
Multiple Access Interference
MAN
Metropolitan Area Network
MAP
Media Access Protocol
MBS
Multicast and Broadcast Service
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MDHO
Macro Diversity Hand Over
MIMO
Multiple Input Multiple Output
MMS
Multimedia Message Service
MPLS
Multi-Protocol Label Switching
MS
Mobile Station
MSO
Multi-Services Operator
NACK
Not Acknowledge
NAP
Network Access Provider
NLOS
Non Line-of-Sight
NRM
Network Reference Model
nrtPS
Non-Real-Time Polling Service
NSP
Network Service Provider
OFDM
Orthogonal Frequency Division Multiplex
OFDMA Orthogonal Frequency Division Multiple Acces PER
Packet Error Rate
PF
Proportional Fair (Scheduler)
PKM
Public Key Management
PUSC
Partially Used Sub-Carrier
QAM
Quadrature Amplitude Modulation
QoS
Quality of Service
QPSK
Quadrature Phase Shift Keying
RG
Relative Grant
RR
Round Robin (Scheduler)
RRI
Reverse Rate Indicator
RTG
Receive/transmit Transition Gap
rtPS
Real-Time Polling Service
RUIM
Removable User Identity Module
SDMA
Space (or Spatial) Division (or Diversity) Multiple Access
SF
Spreading Factor
SFN
Single Frequency Network
SGSN
Serving GPRS Support Node
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SHO
Soft Hand-Off
SIM
Subscriber Identify Module
SIMO
Single Input Multiple Output
SINR
Signal to Interference + Noise Ratio
SLA
Service Level Agreement
SM
Spatial Multiplexing
SMS
Short Message Service
SNIR
Signal to Noise + Interference Ratio
SNR
Signal to Noise Ratio
S-OFDMA
Scalable Orthogonal Frequency Division Multiple Access
SS
Subscriber Station
STC
Space Time Coding
TDD
Time Division Duplex
TEK
Traffic Encryption Key
TTG
Transmit/receive Transition Gap
TTI
Transmission Time Interval
TU
Typical Urban (as in channel model)
UE
User Equipment
UGS
Unsolicited Grant Service
UL
Uplink
UMTS
Universal Mobile Telephone System
USIM
Universal Subscriber Identify Module
VoIP
Voice over Internet Protocol
VPN
Virtual Private Network
VSF
Variable Spreading Factor
VSM
Vertical Spatial Multiplexing
WiFi
Wireless Fidelity
WAP
Wireless Application Protocol
WiBro
Wireless Broadband (Service)
WiMAX Worldwide Interoperability for Microwave Access
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CHAPTER 1 INTRODUCTION
WiMAX, the Worldwide Interoperability for Microwave Access, is a 4G telecommunication technology that provides the wireless transmission of data in a variety of ways, ranging from point-to-point links to full mobile cellular-type access. The technology provides broadband speed without the requirement of cables. The technology is based on the IEEE 802.16 standard (also called WirelessMAN). The name "WiMAX" was created by the WiMAX Forum, which was formed in June 2001 to promote conformity and interoperability of the standard. The forum describes WiMAX as "a standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL (and also to High Speed Packet Access).
The WiMAX technology, based on the IEEE 802.16-2004 Air Interface Standard rapidly proved itself as a technology playing a key role in fixed broadband wireless metropolitan area networks. An amendment to 802.16-2004, IEEE 802.16e-2005 (formerly known as IEEE 802.16e), addressing mobility, was concluded in December2005. This implemented a number of enhancements to 802.16-2004, including better support for Quality of Service and the use of Scalable OFDMA, and is sometimes called “Mobile WiMAX”, after the WIMAX forum for interoperability. Mobile WiMAX is a broadband wireless solution that enables convergence of mobile and fixed broadband networks through a common wide area broadband radio access technology and flexible network architecture. The Mobile WiMAX Air Interface adopts Orthogonal Frequency Division Multiple Access (OFDMA) for improved multi-path performance in non line-ofsight environments. Scalable OFDMA (SOFDMA) is introduced in the IEEE 802.16e amendment to support scalable channel bandwidths from 1.25 to 20 MHz [3] [5].
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Figure 1.1: Mobile WiMAX System Profile
1.1. Salient Features Mobile WiMAX systems offer scalability in both radio access technology and network architecture, thus providing a great deal of flexibility in network deployment options and service offerings. Some of the salient features supported by Mobile WiMAX are:
High Data Rates: The inclusion of MIMO antenna techniques along with flexible sub-channelization schemes, Advanced Coding and Modulation all enable the Mobile WiMAX technology to support peak DL data rates up to 63 Mbps per sector and peak UL data rates up to 28 Mbps per sector in a 10 MHz channel.
Quality of Service (QoS): The fundamental premise of the IEEE 802.16 MAC architecture is QoS. It defines Service Flows which can map to DiffServ code points or MPLS flow labels that enable end-to-end IP based QoS. Additionally, sub-channelization and MAP-based signaling schemes provide a flexible mechanism for optimal scheduling of space, frequency and time resources over the air interface on a frame-by-frame basis.
Scalability: Despite an increasingly globalized economy, spectrum resources for wireless broadband worldwide are still quite disparate in its allocations. Mobile WiMAX technology therefore, is designed to be able to scale to work in different channelizations from 1.25 to 20 MHz to comply with varied worldwide requirements as efforts proceed to achieve spectrum harmonization in the longer term. This also allows diverse economies to realize the multi-faceted benefits of the Mobile WiMAX technology for their specific geographic needs such as
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providing affordable internet access in rural settings versus enhancing the capacity of mobile broadband access in metro and suburban areas.
Security: The features provided for Mobile WiMAX security aspects are best in class with EAP-based authentication, AES-CCM-based authenticated encryption, and CMAC and HMAC based control message protection schemes. Support for a diverse set of user credentials exists including; SIM/USIM cards, Smart Cards, Digital Certificates, and Username/Password schemes based on the relevant EAP methods for the credential type.
Mobility: Mobile WiMAX supports optimized handover schemes with latencies less than 50 milliseconds to ensure real-time applications such as VoIP perform without service degradation. Flexible key management schemes assure that security is maintained during handover.
1.2. Reference Model The Figure 1.2. illustrates the reference model and the scope of the standard IEEE 802.16 [1].
The MAC comprises three sublayers.
The Service-Specific convergence Sublayer (CS) provides any transformation or mapping of external network data, received through the service access point (SAP), into MAC SDUs received by the MAC Common part sublayer (CPS) through the MAC SAP.
This includes classifying external network network data units (SDUs) and associating them to the proper MAC service flow identifier (SFID) and connection identifier (CID).
It may also include such functions as payload header suppression (PHS). Multiple CS specifications are provided for interfacing with various protocols.
The internal format of the CS payload is unique to the CS, and the MAC CPS is not required to understand the format of or parse any information from the CS payload.
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Figure 1.2: IEEE Standard 802.16 protocol layering showing SAPs The MAC CPS provides the core MAC functioanlity of the system access, bandwidth allocation, connection establishment, and connection maintenance. It receives data from various CSs, through the MAC SAP, classified to particular MAC connections. Quality of Service (QoS) is applied to the transmission and scheduling data over PHY. The MAC also contains a separate security sublayer providing authentication, secure key exchange, and encryption. Data, PHY control, and statistics are transferred between the MAC CPS and the PHY via the PHY SAP (which is implementation specific). The PHY definition includes multiple specifications, each appropriate to particular frequency range and application.
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1.3. IEEE 802.16e-2005 Technology The 802.16 standard essentially standardizes 2 aspects of the air interface - the physical layer (PHY) and the Media Access Control layer (MAC). This section provides an overview of the technology employed in these 2 layers in the current version of the 802.16 specification (which is strictly 802.16-2004 as amended by 802.16e-2005, but which will be referred to as 802.16e for brevity) [3]. PHY 802.16e uses Scalable OFDMA to carry data, supporting channel bandwidths of between 1.25 MHz and 20 MHz, with up to 2048 sub-carriers. It supports adaptive modulation and coding, so that in conditions of good signal, a highly efficient 64 QAM coding scheme is used, whereas where the signal is poorer, a more robust BPSK coding mechanism is used. In intermediate conditions, 16 QAM and QPSK can also be employed. Other PHY features include support for Multiple-in Multiple-out (MIMO) antennas in order to provide good NLOS (Non-line-of-sight) characteristics (or higher bandwidth) and Hybrid automatic repeat request (HARQ) for good error correction performance. MAC The 802.16 MAC describes a number of Convergence Sublayers which describe how wireline technologies such as Ethernet, ATM and IP are encapsulated on the air interface, and how data is classified, etc. It also describes how secure communications are delivered, by using secure key exchange during authentication, and encryption using AES or DES (as the encryption mechanism) during data transfer. Further features of the MAC layer include power saving mechanisms (using Sleep Mode and Idle Mode) and handover mechanisms.A key feature of 802.16 is that it is a connection oriented technology. The subscriber station (SS) cannot transmit data until it has been allocated a channel by the Base Station (BS). This allows 802.16e to provide strong support for Quality of Service (QoS).
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CHAPTER 2 PHYSICAL LAYER (PHY) DESCRIPTION 2.1. TDD Frame Structure The 802.16e PHY supports TDD and Full and Half-Duplex FDD operation; however the initial release of Mobile WiMAX certification profiles only include TDD. With ongoing releases, FDD profiles will be considered by the WiMAX Forum to address specific market opportunities where local spectrum regulatory requirements either prohibit TDD or are more suitable for FDD deployments. To counter interference issues, TDD does require system-wide synchronization; nevertheless, TDD is the preferred duplexing mode for the following reasons: •
TDD enables adjustment of the downlink/uplink ratio to efficiently support asymmetric downlink/uplink traffic, while with FDD, downlink and uplink always have fixed and generally, equal DL and UL bandwidths.
•
TDD assures channel reciprocity for better support of link adaptation, MIMO and other closed loop advanced antenna technologies.
•
Unlike FDD, which requires a pair of channels, TDD only requires a single channel for both downlink and uplink providing greater flexibility for adaptation to varied global spectrum allocations.
•
Transceiver designs for TDD implementations are less complex and therefore less expensive.
Figure 2.1: Illustration of different FDD mode operation
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Figure 2.1.1 TDD Frame Uplink and Downlink
Figure 2.1.2. General format of a TDD frame (OFDM PHY)
Figure 2.1.3 illustrates the OFDM frame structure for a Time Division Duplex (TDD) Implementation [4] [5]. Each frame is divided into DL and UL sub-frames separated by Transmit/Receive and Receive/Transmit Transition Gaps (TTG and RTG, respectively) to prevent DL and UL transmission collisions. In a frame, the following control information is used to ensure optimal system operation:
Preamble: The preamble, used for synchronization, is the first OFDM symbol of the frame.
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Frame Control Header (FCH): The FCH follows the preamble. It provides the frame configuration information such as MAP message length and coding scheme and usable sub-channels.
DL-MAP and UL-MAP: The DL-MAP and UL-MAP provide sub-channel allocation and other control information for the DL and UL sub-frames respectively.
UL Ranging: The UL ranging sub-channel is allocated for mobile stations (MS) to perform closed-loop time, frequency, and power adjustment as well as bandwidth requests.
UL CQICH: The UL CQICH channel is allocated for the MS to feedback channel-state information.
UL ACK: The UL ACK is allocated for the MS to feedback DL HARQ acknowledge.
Figure 2.1.3. WiMAX OFDMA Frame Structure
2.2. OFDMA Basics The Physical layer of Mobile WiMAX is based on OFDMA technology. Orthogonal Frequency Division Multiplexing (OFDM) is a multiplexing technique that subdivides the bandwidth into multiple frequency sub-carriers as shown in Figure 2.2.1 In an OFDM system, the input data stream is divided into several parallel sub-streams of reduced data rate and each sub-stream is modulated and transmitted on a separate orthogonal sub-carrier. The increased symbol duration improves the robustness of OFDM 8
to delay spread. Furthermore, the introduction of the cyclic prefix (CP) can completely eliminate Inter-Symbol Interference (ISI) as long as the CP duration is longer than the channel delay spread. The CP is typically a repetition of the last samples of data portion of the block that is appended to the beginning of the data payload as shown in Figure2.2.2 The CP prevents inter-block interference and makes the channel appear circular and permits low-complexity frequency domain equalization. A perceived drawback of CP is that it introduces overhead, which effectively reduces bandwidth efficiency. While the CP does reduce bandwidth efficiency somewhat, the impact of the CP is similar to the “roll-off factor” in raised-cosine filtered single-carrier systems. Since OFDM has a very sharp, almost “brick-wall” spectrum, a large fraction of the allocated channel bandwidth can be utilized for data transmission, which helps to moderate the loss in efficiency due to
the
cyclic
prefix.
Figure 2.2.1. Basic Architecture of an OFDM system
OFDM exploits the frequency diversity of the multipath channel by coding and interleaving the information across the sub-carriers prior to transmissions. OFDM modulation can be realized with efficient Inverse Fast Fourier Transform (IFFT), which enables a large number of sub-carriers (up to 2048) with low complexity. In an OFDM system, resources are available in the time domain by means of OFDM symbols and in the frequency domain by means of sub-carriers. The time and frequency resources can be organized into sub-channels for allocation to individual users. Orthogonal Frequency
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Division Multiple Access (OFDMA) is a multiple-access/multiplexing scheme that provides multiplexing operation of data streams from multiple users onto the downlink sub-channels and uplink multiple access by means of uplink sub-channels [5].
Figure 2.2.2. Insertion of Cyclic Prefix
2.3 OFDMA Symbol Structure and Sub-Channelization The OFDMA symbol structure consists of 3 types of sub-carriers as shown in Figure 2.3: •
Data sub-carriers for data transmission
•
Pilot sub-carriers for estimation and synchronization purposes
•
Null sub-carriers for no transmission; used for guard bands and DC carriers
Active (data and pilot) sub-carriers are grouped into subsets of sub-carriers called subchannels. The WiMAX OFDMA PHY supports sub-channelization in both DL and UL. The minimum frequency-time resource unit of sub-channelization is one slot, which is equal to 48 data tones (sub-carriers) [5].
Figure 2.3. OFDMA Sub-Carrier Structure
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2.3.1. Subcarrier permutation and Diversity permutation There are two types of sub-carrier permutations for sub-channelization; diversity and contiguous. The diversity permutation draws sub-carriers pseudo-randomly to form a sub-channel.
It provides frequency diversity and inter-cell interference averaging.
The diversity permutations include DL FUSC (Fully Used Sub-Carrier), DL PUSC (Partially Used Sub-Carrier) and UL PUSC and additional optional permutations.
With DL PUSC,for each pair of OFDM symbols, the available or usable subcarriers are grouped into clusters containing 14 contiguous sub-carriers per symbol period, with pilot and data allocations in each cluster in the even and odd symbols as shown in Figure 2.3.1
Figure 2.3.1. DL Frequency Diverse Sub-Channel A re-arranging scheme is used to form groups of clusters such that each group is made up of clusters that are distributed throughout the sub-carrier space. 9 A sub-channel in a group contains two (2) clusters and is made up of 48 data subcarriers and eight (8) pilot sub-carriers. 9 The data sub-carriers in each group are further permutated to generate subchannels within the group. Therefore, only the pilot positions in the cluster are shown
in
Figure 2.3.1.
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9 The data sub-carriers in the cluster are distributed to multiple subchannels.Analogous to the cluster structure for DL, a tile structure is defined for the UL
PUSC
whose format is shown in Figure 2.3.2
Figure 2.3.2.Tile Structure for Uplink PUSC 9 The available sub-carrier space is split into tiles and six (6) tiles, chosen from across the entire spectrum by means of a re-arranging/permutation scheme, are grouped together to form a slot. The slot comprises 48 data sub-carriers and 24 pilot sub-carriers in 3 OFDM symbols.
2.3.2. Contiguous permutation The contiguous permutation groups a block of contiguous sub-carriers to form a sub channel. The contiguous permutations include DL AMC and UL AMC, and have the same structure. ¾ A bin consists of 9 contiguous sub-carriers in a symbol, with 8 assigned for data and one assigned for a pilot. ¾ A slot in AMC is defined as a collection of bins of the type (N x M = 6), where N is the number of contiguous bins and M is the number of contiguous symbols. Thus the allowed combinations are [(6 bins, 1 symbol), (3 bins, 2 symbols), (2 bins, 3 symbols), (1 bin, 6 symbols)]. AMC permutation enables multi-user diversity by choosing the sub-channel with the best frequency response. In general, diversity sub-carrier permutations perform well in mobile applications while contiguous sub-carrier permutations are well suited for fixed, portable, or low mobility
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environments. These options enable the system designer to trade-off mobility for throughput [5].
Figure 2.3.3 DL PUSC, FUSC cluster, bin &AMC zone concept
2.4. Scalable OFDMA The IEEE 802.16e-2005 Wireless MAN OFDMA mode is based on the concept of scalable OFDMA (S-OFDMA). S-OFDMA supports a wide range of bandwidths to flexibly address the need for various spectrum allocation and usage model requirements. The scalability is supported by adjusting the FFT size while fixing the sub-carrier frequency spacing at 10.94 kHz. Since the resource unit sub-carrier bandwidth and symbol duration is fixed, the impact to higher layers is minimal when scaling the bandwidth. The S-OFDMA parameters are listed in Table 1. The system bandwidths for two of the initial planned profiles being developed by the WiMAX Forum Technical Working Group for Release-1 are 5 and 10 MHz (highlighted in the table) [5].
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Table 2.1. OFDMA Scalability Parameters
2.5. Other Advanced PHY Layer Features Adaptive modulation and coding (AMC), Hybrid Automatic Repeat Request (HARQ) and Fast Channel Feedback (CQICH) were introduced with Mobile WiMAX to enhance coverage and capacity for WiMAX in mobile applications. Support for QPSK, 16QAM and 64QAM are mandatory in the DL with Mobile WiMAX. In the UL, 64QAM is optional. Both Convolutional Code (CC) and Convolutional Turbo Code (CTC) with variable code rate and repetition coding are supported. Block Turbo Code and Low Density Parity Check Code (LDPC) are supported as optional features. Table 2.2 summarizes the coding and modulation schemes supported in the Mobile WiMAX profile.
Table 2.2. Supported Code and Modulations
The combinations of various modulations and code rates provide a fine resolution of data rates as shown in Table 3 which shows the data rates for 5 and 10 MHz channels with PUSC sub-channels. The frame duration is 5 milliseconds. Each frame has 48 OFDM
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symbols, with 44 OFDM symbols available for data transmission. The highlighted values indicate data rates for optional 64QAM in the UL [5].
Table 2.3: Mobile WiMAX PHY Data Rates with PUSC Sub-Channel
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CHAPTER 3 MAC LAYER DESCRIPTION The 802.16 standard was developed from the outset for the delivery of broadband services including voice, data, and video. The MAC layer is based on the time-proven DOCSIS standard and can support bursty data traffic with high peak rate demand while simultaneously supporting streaming video and latency-sensitive voice traffic over the same channel. The resource allocated to one terminal by the MAC scheduler can vary from a single time slot to the entire frame, thus providing a very large dynamic range of throughput to a specific user terminal at any given time. Furthermore, since the resource allocation information is conveyed in the MAP messages at the beginning of each frame,the scheduler can effectively change the resource allocation on a frame-by-frame basis to adapt to the bursty nature of the traffic.
3.1. Quality of Service (QoS) Support WiMAX was developed from the outset to meet the stringent requirements for the delivery of broadband services. The WiMAX QoS is specified for each service flow. The connection-oriented QoS therefore, can provide accurate control over the air interface. With fast air link, asymmetric downlink/uplink capability, fine resource granularity and a flexible resource allocation mechanism, Mobile WiMAX can meet QoS requirements for a wide range of data services and applications. Since the air interface is usually the bottleneck, the connection-oriented QoS can effectively enable the end-to-end QoS control. The service flow parameters can be dynamically managed through MAC messages to accommodate the dynamic service demand. Service flows provide the same control mechanism in both the DL and UL to improve QoS in both directions. Furthermore, since the sub-channels are orthogonal, there is no intra-cell interference in either DL or UL. Therefore, the DL and UL link quality and QoS can be easily controlled by the base station scheduler. The high system throughput also allows efficient multiplexing and low data latency. Therefore, with fast air link, high system throughput, symmetric downlink/uplink capacity, fine resource granularity and flexible resource
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allocation Mobile WiMAX can support a wide range of data services and applications with varied QoS requirements as summarized in Table 3.1
Table 3.1. Mobile WiMAX Applications and Quality of Service In the Mobile WiMAX MAC layer, QoS is provided via service flows as illustrated in Figure 8. This is a unidirectional flow of packets that is provided with a particular set of QoS parameters. Before providing a certain type of data service, the base station and user-terminal first establish a unidirectional logical link between the peer MACs called a connection. The outbound MAC then associates packets traversing the MAC interface into a service flow to be delivered over the connection. The QoS parameters associated with the service flow define the transmission ordering and scheduling on the air interface. The connection-oriented QoS therefore, can provide accurate control over the air interface. Since the air interface is usually the bottleneck, the connection-oriented QoS can effectively enable the end-to-end QoS control. The service flow parameters can be dynamically managed through MAC messages to accommodate the dynamic service demand. The service flow based QoS mechanism applies to both DL and UL to provide 17
improved QoS in both directions. Mobile WiMAX supports a wide range of data services and applications with varied QoS requirements [5].
Figure 3.1. Mobile WiMAX QoS Support
3.2. MAC Scheduling Service The Mobile WiMAX MAC scheduling service is designed to efficiently deliver broadband data services including voice, data, and video over time varying broadband wireless channel. The MAC scheduling service has the following properties that enable the broadband data service [5]: Fast Data Scheduler: The MAC scheduler must efficiently allocate available resources in response to bursty data traffic and time-varying channel conditions. The scheduler is located at each base station to enable rapid response to traffic requirements and channel conditions. The data packets are associated to service flows with well defined QoS parameters in the MAC layer so that the scheduler can correctly determine the packet transmission ordering over the air interface. The CQICH channel provides fast channel information feedback to enable the scheduler to choose the appropriate coding and modulation for each allocation. The adaptive modulation/coding combined with HARQ provide robust transmission over the time- varying channel.
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Scheduling for both DL and UL: The scheduling service is provided for bothDL and UL traffic. In order for the MAC scheduler to make an efficient resource allocation and provide the desired QoS in the UL, the UL must feedback accurate and timely information as to the traffic conditions and QoS requirements. Multiple uplink bandwidth request mechanisms, such as bandwidth request through ranging channel, piggyback request and polling are designed to support UL bandwidth requests. The UL service flow defines the feedback mechanism for each uplink connection to ensure predictable UL scheduler behavior. Furthermore, with orthogonal UL sub-channels, there is no intra-cell interference. UL scheduling can allocate resource more efficiently and better enforce QoS. Dynamic Resource Allocation: The MAC supports frequency-time resource allocation in both DL and UL on a per-frame basis. The resource allocation is delivered in MAP messages at the beginning of each frame. Therefore, the resource allocation can be changed frame-by-frame in response to traffic and channel conditions. Additionally, the amount of resource in each allocation can range from one slot to the entire frame. The fast and fine granular resource allocation allows superior QoS for data traffic. QoS Oriented: The MAC scheduler handles data transport on a connection-byconnection basis. Each connection is associated with a single data service with a set of QoS parameters that quantify the aspects of its behavior. With the ability to dynamically allocate resources in both DL and UL, the scheduler can provide superior QoS for both DL and UL traffic. Particularly with uplink scheduling – the uplink resource is more efficiently allocated, performance is more predictable, and QoS is better enforced. Frequency Selective Scheduling: The scheduler can operate on different types of sub-channels. For frequency-diverse sub-channels such as PUSC permutation, where sub-carriers in the sub-channels are pseudo-randomly distributed across the bandwidth, sub-channels are of similar quality. Frequency-diversity scheduling can support a QoS with fine granularity and flexible time-frequency resource scheduling. With contiguous permutation such as AMC permutation, the subchannels may experience different attenuation. The frequency-selective
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scheduling can allocate mobile users to their corresponding strongest subchannels. The frequency-selective scheduling can enhance system capacity with a moderate increase in CQI overhead in the UL.
3.3. MAC Addresses and MAC Frames 3.3.1. MAC Addresses and Other Addresses Each SS has a 48-bit universal MAC address, as defined in the standard . This type of address is often known as the IEEE 802 MAC address. It uniquely defines the SS for all possible vendors and equipment types. It is used during the initial ranging process to establish the appropriate connections for an SS. It is also used as part of the authentication process by which the BS and SS each verify the identity of the other. This is also the case in Mesh mode where each node has a unique IEEE 802 MAC address. A 802.16 BS has a 48-bit Base Station ID (BSID). This is different from the MAC address of the BS. It includes a 24-bit operator indicator. The BSID can then be used for operator identification. It is used, for example, in the Downlink Channel Descriptor (DCD) MAC management message. In the Mesh mode, another address in addition to the MAC address is used. When authorized to access, a candidate SS node receives a 16-bit Node Identifier (Node ID) upon a request to an SS identified as the Mesh BS. The Node ID is the basis of node identification in the Mesh mode.
3.3.2 MAC Frames A MAC PDU is known as a MAC frame. It has the general format shown in Figure 3.2. Each MAC frame starts with a fixed-length MAC header. This header may be followed by the payload of the MAC PDU (MPDU). A MPDU may contain a CRC (Cyclic Redundancy Check). If present, the MPDU payload contains one or more of the following:
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zero or more subheaders included in the payload;
zero or more MAC SDUs;
fragment(s) of a MAC SDU.
Figure 3.2: General format of a MAC frame or MAC PDU The payload information may vary in length. Hence, a MAC frame length is a variable number of bytes. This format allows the MAC to tunnel various higher-layer traffic types without knowledge of the formats or bit patterns of those messages.
3.3.3 MAC Header Format Two MAC header formats are defined in the standard: 9 The Generic MAC Header (GMH): This is the header of MAC frames containing either MAC management messages or CS data. The CS data may be user data or other higher layer management data. The generic MAC header frame is the only one used in the downlink. 9 The MAC header without payload where two types are defined: Type I and Type II. For MAC frames with this type of header format, the MAC header is not followed by any MPDU payload and CRC. This frame name has been introduced by the 16e amendment. Previously, in 802.16-2004, the bandwidth request header was defined to request additional bandwidths . With 16e, the bandwidth request header becomes a specific case of MAC header formats without payload. In the uplink, the single-bit Header Type (HT) field, at the beginning of the MAC header, makes the distinction between the generic MAC header and the MAC header without payload formats: zero for the GMH and one for a header without payload [4].
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3.4. Key Management Messages Management messages are broadcast and sent on 3 CIDs in each direction: Basic, Primary and Secondary •
Uplink Channel Descriptor
•
DL-MAP
•
Downlink Channel Descriptor
•
DSA-REQ
•
UL-MAP
•
DSA-RSP
The basic connection is used by the BS MAC and SS MAC to exchange short, time-urgent MAC management messages. This connection has a Basic CID The primary management connection is used by the BS MAC and SS MAC to exchange longer, more delay-tolerant MAC management messages. This connection has a Primary Management CID The secondary management connection is used by the BS and SS to transfer delay tolerant, standards-based messages. These standards are the Dynamic Host Configuration Protocol (DHCP), Trivial File Transfer Protocol (TFTP), Simple Network Management Protocol (SNMP), etc. The secondary management messages are carried in IP datagram, as for IP CS PDU formats). Hence, secondary management messages are not MAC management messages. Use of the secondary management connection is required only for managed SSs [2] [5].
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Table 3.3.1. Key Management Messages
Table 3.3.2. Key Management Messages (Continued)
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3.5. Hybrid Automatic Repeat Request (HARQ) Mechanism The Hybrid ARQ (HARQ) mechanism uses an error control code in addition to the retransmission scheme to ensure a more reliable transmission of data packets (relative to ARQ). The main difference between an ARQ scheme and an HARQ scheme is that in HARQ, subsequent retransmissions are combined with the previous erroneously received transmissions in order to improve reliability. HARQ parameters are specified and negotiated during the initialization procedure. A burst cannot have a mixture of HARQ and non-HARQ traffic. The HARQ scheme is an optional part of the 802.16 standard MAC. HARQ may only be supported by the OFDMA physical interface [4] [8]. For the downlink HARQ, a fast ACK/NACK exchange is needed. Uplink slots ACK (ULACK) in the OFDMA frame allow this fast feedback .Two main variants of HARQ are supported: ¾ Incremental Redundancy (IR) for CTC and CC. The PHY layer encodes the HARQ packet generating several versions of encoded subpackets. Each subpacket is uniquely identified by a SubPacket IDentifier (SPID). Four subpackets can be generated for a packet to be encoded. For each retransmission the coded block (the SPID) is different from the previously transmitted coded block. ¾ Chase Combining (CC) for all coding schemes. The retransmission is identical to the initial transmitted block. The PHY layer encodes the HARQ packet generating only one version of the encoded packet. An SS may support IR and an SS may support either CC or IR.
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Figure 3.4.Incremental Redundancy (IR) HARQ
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CHAPTER 4 MOBILITY, HANDOVER AND POWER-SAVE MODES 4.1. Mobility Management Battery life and handoff are two critical issues for mobile applications. Mobile WiMAX supports Sleep Mode and Idle Mode to enable power-efficient MS operation. Mobile WiMAX also supports seamless handoff to enable the MS to switch from one base station to another at vehicular speeds without interrupting the connection [4].
4.1.1. Power Management Mobile WiMAX supports two modes for power efficient operation-Sleep Mode and Idle Mode.
4.1.1.1. Sleep mode •
Sleep Mode is a state in which the MS conducts pre-negotiated periods of absence from the Serving Base Station air interface.
•
These periods are characterized by the unavailability of the MS, as observed from the Serving Base Station, to DL or UL traffic.
•
Sleep Mode is intended to minimize MS power usage and minimize the usage of the Serving Base Station air interface resources.
•
The Sleep Mode also provides flexibility for the MS to scan other base stations to collect information to assist handoff during the Sleep Mode.
4.1.1.2. Idle mode •
Idle Mode provides a mechanism for the MS to become periodically available for DL broadcast traffic messaging without registration at a specific base station as the MS traverses an air link environment populated by multiple base stations.
•
Idle Mode benefits the MS by removing the requirement for handoff and other normal operations and benefits the network and base station by eliminating air interface and network handoff traffic from essentially inactive MSs while still
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providing a simple and timely method (paging) for alerting the MS about pending DL traffic [2] [5] [4].
4.2. Handover considerations One of the major goals of the 802.16e amendment is to introduce mobility in WiMAX. Consequently, mobile WiMAX profiles are based on 802.16e. Mobility is based on handover. Handover operation (sometimes also known as ‘handoff’) is the fact that a mobile user goes from one cell to another without interruption of the ongoing session (whether a phone call. data session or other). The handover can be due to mobile subscriber moves, to radio channel condition changes or to cell capacity considerations. Handover is a mandatory feature of a cellular network. In this chapter the handover (HO) is described as defined in 802.16e [2] [4]. There are three handoff methods supported within the 802.16e standard – Hard Handoff (HHO), Fast Base Station Switching (FBSS) and Macro Diversity Handover (MDHO). Of these, the HHO is mandatory while FBSS and MDHO are two optional modes. •
Hard handover, also known as break-before-make. The subscriber mobile station (MS) stops its radio link with the first BS before establishing its radio link with the new BS. This is a rather simple handover.
•
Soft handover, also known as make-before-break. The MS establishes its radio link with a new BS before stopping its radio link with the first BS. The MS may have two or more links with two or more BSs, which gives the soft handover state. The soft handover is evidently faster than the hard handover.It is also known as Macro Diversity Handover (MDHO).
•
Fast BS Switching (FBSS). This is a state where the MS may rapidly switch from one BS to another. The switch is fast because the MS makes it without realizing the complete network entry procedure with regard to the new BS.
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4.3. Network Topology Acquisition 4.3.1. Network Topology Advertisement A BS broadcasts information about the network topology using the MOB_NBR-ADV (Neighbour Advertisement) MAC management message. This message provides channel information about neighbouring BSs normally provided by each BS's own DCD/UCD message transmissions. The MOB_NBR-ADV does not contain all the information of neighbouring BSs, UCD and DCD. The standard indicates that a BS may obtain that information over the backbone and that availability of this information facilitates MS synchronisation with neighbouring BS by removing the need to monitor transmission from the neighbouring (handover target) BS for DCD/UCD broadcasts. The BSs will keep mapping tables of neighbour BS MAC addresses and neighbour BS indexes transmitted through the MOB_NBR-ADV message, for each configuration change count, which has the same function as for the DCD message. BSs supporting mobile functionality must be capable of transmitting a MOB_NBR-ADV MAC management message at a periodic interval to identify the network and define the characteristics of the neighbour BS to a potential MS seeking initial network entry or handover. The standard indicates that the maximum value of this period is 30 seconds [4] [14].
Figure 4.3. Network Topology Advertisement
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4.3.2. MS Scanning of Neighbor BSs Beside the BS’s advertisement, the MS can actively scan for neighboring BSs and estimate their suitability as target BS. Figure 4.3.2 shows the MSC during the MS scanning interval. A MS initiates the scanning process by transmitting the Scanning Interval Allocation Request (MOB_SCN-REQ). The message contains the estimated scan duration and, for scanning multiple times, the interleaving interval and the number of iterations. Additionally, the MS indicates the intended scanning of one or several neighboring BSs. Like this, the BS can negotiate over the backbone a unicast ranging opportunity (instead of contention-based ranging) for the intended neighboring BSs. The uni-cast opportunity will be granted to the MS at a specific rendezvous time [14].
Figure 4.3.2. Scanning of Neighbor BS
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4.3.3 Association Procedure Association is an optional initial ranging procedure occurring during the scanning interval with respect to one of the neighbor BSs . The function of Association is to enable the MS to acquire and record ranging parameters and service availability information for the purpose of proper selection of a handover target BS and/or expediting a potential future handover to a target BS. Recorded ranging parameters of an Associated BS may be further used for setting initial ranging values in future ranging events during a handover. Upon completion of a successful MS initial ranging of a BS, if the RNG-RSP message (sent by the BS) contains a service level prediction parameter set to 2, the MS may mark the BS as Associated in its MS local Association table of identities, recording elements of the RNG-RSP to the MS local Association table and setting an appropriate ageing timer [2] [4] [14]. There are three levels of Association as follows:
Association Level 0: Scan/Association without coordination. The serving BS and the MS negotiate the Association duration and intervals (via MOB_SCN-REQ). The serving BS allocates periodic intervals where the MS may range neighbouring BSs. The target BS has no knowledge of the MS. The MS uses the target BS contention-based ranging allocations.
Association Level 1: Association with coordination. Unilaterally or upon request of the MS (through the MOB_SCN-REQ message), the serving BS provides Association parameters to the MS and coordinates Association between the MS and neighbouring BSs. The target BS reserves a CDMA initial ranging code and an initial ranging slot (transmission opportunity) in a specified dedicated ranging region (rendezvous time). The neighbouring BS may assign the same code or transmission opportunity to more than one MS, but not both. There is no potential for collision of transmissions from different MSs.
Association Level 2: network assisted association reporting. The MS may request to perform Association with network assisted Association reporting by sending the MOB_SCN-REQ message, including a list of neighbouring BSs, to the 30
serving BS with scanning type = 0b011. The serving BS may also request this type of Association unilaterally by sending the MOB_SCN-RSP message with the proper indication. The serving BS will then coordinate the Association procedure with the requested neighbouring BSs in a fashion similar to Association Level 1. With Level 2, the MS is only required to transmit the CDMA ranging code to the neighbour BSs. The MS does not wait for RNG-RSP from the neighbour BSs. Instead, the RNG-RSP information on PHY offsets is sent by each neighbour BS to the serving BS over the backbone. The serving BS may aggregate all ranging information into a single MOB_ASC_REPORT, MOB_ASC-REP, Association result report, and message.
4.4 The Handover Process The IEEE 802.16 standard states that the handover decision algorithm is beyond its scope. The WiMAX Forum documents do not select a handover algorithm either. Only the framework is defined. The MS, using its current information on the neighbour BS or after a request to obtain such information, evaluates its interest in a potential handover with a target BS. Once the handover decision is taken by either the serving BS or the MS, a notification is sent over the MOB_BSHO-REQ (BS Handover Request) or the MOB_MSHO-REQ (MS Handover Request) MAC management messages, depending on the handover decision maker: the BS or MS. The handover process steps are described in the following [4]. The handover process is made of five stages which are summarized in Figure 4.4.
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Figure 4.4. Handover process stages
4.4.1 Cell Reselection Cell reselection refers to the process of an MS scanning and/or association with one or more BS in order to determine their suitability, along with other performance considerations, as a handover target. The MS may use neighbour BS information acquired from a decoded MOB_NBR-ADV message or may make a request to schedule scanning intervals or sleep intervals to scan, and possibly range, the neighbour BS for the purpose of evaluating the MS interest in the handover to a potential target BS.
4.4.2 Handover Decision and Initiation A handover begins with a decision for an MS to make a handover from a serving BS to a target BS. The decision may originate either at the MS or the serving BS. The handover
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decision results in a notification of MS intent to make a handover thruugh the MOB_MSHO-REQ (MS HO REQ) message (handover decision by the MS) or the MOB_BSHO-REQ (BS HO REQ) message (handover decision by the BS). The BS may transmit a MOB_BSHO-REQ message when it wants to initiate a handover. This request may be recommended or mandatory. In the case where it is mandatory, at least one recommended BS must be present in the MOB_BSHO-REQ message. If mandatory, the MS responds with the MOB_HO-IND message, indicating commitment to the handover unless the MS is unable to make the handover to any of the recommended BSs in the MOB_ BSHO-REQ message, in which case the MS may respond with the MOB_HO-IND message with proper parameters indicating HO reject. An MS receiving the MOB_BSHO-REQ messsage may scan recommended neighbour BSs in this message. In the case of an MS initiated handover, the BS transmits an MOB_BSHO-RSP message upon reception of the MOB_MSHO-REQ message.
4.4.3 Synchronization to a Target BS Downlink Synchronization to a target BS downlink must be done. If the MS had previously received a MOB_NBR-ADV (MAC management) message including a target BSID, physical frequency, DCD and UCD, this process may be shortened. If the target BS had previously received handover notification from a serving BS over the backbone, then the target BS may allocate a non-contention-based initial ranging opportunity.
4.4.4 Ranging and Network Re-entry The MS and the target BS must conduct handover ranging. Network re-entry proceeds from the initial ranging step in the Network Entry process: negotiate basic capabilities, PKM authentication phase, TEK establishment phase, registration (the BS may send an unsolicited REG-RSP message with updated capabilities information or skip the REGRSP message when there is no TLV information to be updated) and the other following Network Entry optional steps (lP connectivity, etc.).
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Network re-entry may be shortened by target BS possession of MS information obtained from the serving BS over the backbone network. Depending on the amount of that information, the target BS may decide to skip one or several of the Network Entry steps (Figure 4.4.1). Handover ranging can then be a simplified version of initial ranging. To notify an MS seeking handover of possible omission of re-entry process management messages during the current handover attempt (due to the availability of MS service and operational context information obtained over the backbone network), the target BS must place, in the RNG-RSP message, an HO Process Optimization TLV indicating which reentry management messages may be omitted. The MS completes the processing of all indicated messages before entering Normal Operation with the target BS.
Figure 4.4.1. Summary of network re-entry steps
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Regardless of having received MS information from a serving BS, the target BS may request MS information from the backbone network.
4.4.5 Termination of MS Context This is the final step of a handover. Termination of the MS context is defined as the serving BS termination of the context of all connections belonging to the MS and the discarding of the context associated with them, i.e. information in queues, ARQ state machine, counters. timers, header suppression information. etc. This is accomplished by sending the MOB__HO-IND message with the HO_IND_type value indicating a serving BS release.
4.4.6 Handover Cancellation An MS may cancel HO at any time prior to expiration of the Resource_Retain_Time interval after transmission of the MOB_HO-IND message. Resource_Retain_Time is one of the parameters exchanged during the registration procedure (part of Network Entry). The standard indicates that Resource_Retain_Time is a multiple of 100 milliseconds and those 200 milliseconds is recommended as default [2] [4] [5].
Figure 4.4.2 MS Initiated Hard HO as seen by MS
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4.5. MDHO and FBSS In addition to the HO procedures previously discussed, there are two optional HO modes, MDHO and FBSS. The MDHO or FBSS capability can be enabled or disabled in the REG-REQ/RSP message exchange. With MDHO or FBSS enabled, the MS shall perform the MDHO & FBSS HO decision stages.
A MDHO begins with a decision for an MS to transmit to and receive from multiple BSs at the same time. A MDHO can start with either MOB_MSHO-REQ or MOB_BSHO-REQ messages. Again a FBSS handover begins with a decision for an MS to receive/transmit data from/to the Anchor BS that may change within the Diversity Set. A FBSS handover can be triggered by either MOB_MSHO-REQ or MOB_BSHO-REQ messages.
For diversity Set Selection/Update an MS may scan the neighbor BS and select BSs that are suitable to be included in the diversity set. The MS reports the selected BSs and the diversity set update procedure is performed by the BS and the MS. Again for anchor BS Selection/Update an MS is required to continuously monitor the signal strength of the BSs that are included in the diversity set. The MS selects one BS from its current Diversity Set to be the Anchor BS and reports the selected Anchor BS on CQICH or MOB_MSHO-REQ message.
Figure 4.5. Handover algorithm for MDHO/FBSS
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The MS entering into MDHO region scans the preamble CINR of neighbor BSs as shown in the Figure 4.5. If the preamble CINR margin between the Anchor BS and a neighbor BS is smaller than H_add, the MS transmits MOB_MSHO-REQ message to request to add the neighbor BS to the Active Set list. Again, if the margin between target BS and a serving BS exceeds the H_delete level then by sending MOB_MSHO-REQ, MS asks for dropping the serving BS from the diversity set. The decision of the type of HO, i.e. normal HO or MDHO/FBSS, is performed by the BS after receiving the MOB_MSHOREQ message from the MS. One more criterion that is needed by the BS to make HO decision is the difference in frame arrival time between the Anchor BS and that of the neighbor BS. The arrival time difference needs to be smaller than CP (Cyclic Prefix) in order to effectively support MDHO and FBSS [2] [5].
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CHAPTER 5 An Optimized Method For Scheduling Process Of Scanning 5.1. Previous work Handover of IEEE 802.16e broadband wireless network had been studied in several literatures. Up to now various fast handover schemes had been proposed. An enhanced fast handover algorithm was proposed to reduce the waste of the wireless channel resources and handover delay in [10] [11]. Target BS estimation using mean CINR and arrival time differences had been proposed for reducing unnecessary neighbor BS scanning and association process in [10]. Single neighbor BS scanning, fast ranging and pre-registration had been proposed in [11]. Again, a scheme for the reduction of data transmission delay and packet loss probability for real-time downlink service had been proposed in [7]. A delay timer to reduce the initiation of unnecessary handover had been proposed in [9]. In that proposal the timer deals with unnecessary handover caused by inaccurate signal level assessment.
5.2. Shortcoming in Scheduling Process for Scanning in IEEE 802.16e The overall handover process in IEEE 802.16e MAC layer can be divided into two phases: Network topology acquisition phase i.e. pre-handover phase and the real handover execution phase. The 1st one deals with the neighbor advertisement message by serving BS and scanning by MS to select suitable target BS.
The current standard gives the availability of the channel information of the neighboring BSs in the serving BS MOB-NBR-ADV message which removes the need for MS to monitor transmission from the neighboring BS for DCD/UCD broadcast. Again, periodic scanning is also allowed by MS for the reduction of scanning request and response message along with the provision of T44 timer for retransmitting scanning request [2].
According to the present specification, when the Trigger Action in the DCD message is encoded as 0x3, the MS shall send the MOB_SCN-REQ message to the BS to begin the
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neighbor BS scanning process when the trigger condition is met. MS sends MOB_SCNREQ message repeatedly if serving BS does not suggest any neighbor BS for scanning. It may arise that MS keeps trying with MOB_SCN-REQ message repeatedly, but the serving BS also keeps sending MOB_SCN-RSP suggesting no neighbor BSs for scanning. Such repeated exchange of MOB_SCN-REQ and MOB_SCN-RSP messages is obviously a waste of wireless resource. Again, throughput also gets reduced as scanning and data traffic can’t take place simultaneously [2]. So, it makes certain degradation in system performance.
The current standard also does not explicitly suggest the beginning of scanning when the signal quality is getting too poor but still serving BS denies the request for scanning.
5.3. Proposed Scheme The previous discussion shows that a major waste of resource may be caused by redundancy in attempts to begin scanning and association process of neighbor BSs when Serving BS rejects scanning requests. This can also cause delay in essential scanning and thus, in actual HO. As a result, target BS estimation via CINR measurement prior to scanning request can be greatly helpful. Such measurement already exists in the proposition [10] [11]. An optional implementation of a Trigger Value and Trigger Averaging Duration in DCD from serving BS is also already in use which suggests MS to request for scanning when the CINR of the serving BS stays below this trigger value for Trigger Averaging Duration. We propose few additional steps along with the aforementioned CINR measurement and use of trigger value in DCD in order to avoid redundant attempts to begin scanning as well as expedite beginning of necessary scanning under certain condition. Thus, the main purpose of the proposal is to reduce wireless channel resource waste and latency in HO decision.
Figure 5.3.gives a brief overview of our proposed scheme for scanning with MS initiated Hard HO. We propose introduction of a second Trigger Value (Lower Trigger Value) and Trigger Averaging Duration in DCD in addition to the aforementioned existing Trigger Value (Higher Trigger Value) and Trigger Averaging Duration. If the CINR of serving
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BS stays below Higher Trigger Value for Trigger Averaging Duration, MS will measure CINR of all BSs in MOB_NBR-ADV which satisfies QoS requirements. On the basis of CINR values, MS selects up to three top neighbor BSs and sends MOB_SCN-REQ for scanning of the selected neighbor BSs. If serving BS sends MOB_SCN-RSP suggesting certain neighbor BSs for scanning with or without association, MS will scan Neighbor BSs as per suggestion. Optionally, MS may also scan few Neighbor BSs which were not suggested by serving BS. In this case, MS will select neighbor BSs only from what were requested in MOB_SCN-REQ and scan them without association.
However, if serving BS sends MOB_SCN-RSP suggesting no neighbor BSs for scanning, MS will send MOB_SCN-REQ again. If serving BS suggests no neighbor BSs for scanning over MOB_SCN-RSP, MS will send MOB_SCN-REQ for the third time. Now, if serving BS sends MOB_SCN-RSP suggesting no neighbor BSs for scanning for the third time, MS will pause sending MOB_SCN-REQ for certain period in order to reduce wireless channel resource waste even if the CINR of serving BS is then found to be below Higher Trigger Value. After waiting for certain period, MS may resume sending MOB_SCN-REQ if the CINR of serving BS is still below Higher Trigger Value. The length of this period can be set by the manufacturer of MS.
As long as the CINR of serving BS is found to be in between Higher Trigger Value and Lower Trigger Value, MS will repeat sending MOB_SCN-REQ three times and then pausing if it gets no recommended BS from serving BS scanning response message. The manufacturer of MS may gradually increase the period for pause with time.
Now, if CINR of serving BS goes below Lower Trigger Level and stays as such for its associated Trigger Averaging Duration, MS will immediately resume sending MOB_SCN-REQ and check MOB_SCN-RSP from serving BS. If serving BS suggests certain neighbor BSs for scanning with or without association in MOB_SCN-RSP, MS will do exactly what is described above for the case, CINR of serving BS falls below Higher Trigger Level. However, if serving BS suggests no neighbor BSs for scanning over MOB_SCN-RSP, MS will send request two more times like before. If serving BS
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sends response suggesting no neighbor BSs for the third time; in this case, MS will spontaneously trigger ‘scanning without association’ without waiting for any further events. Here, MS will select only the top three neighbor BSs based on their CINR for scanning.
Figure 5.3. Proposed method for scheduling process of scanning
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5.4. Performance Analysis When MS sends MOB_SCN-REQ and serving BS responds over MOB_SCN-RSP, it may happen that serving BS suggests no neighbor BSs for scanning for various reasons. Our proposal mostly deals with this particular situation.
In our proposal, in both the cases, when CINR of serving BS is in between Higher Trigger Value and Lower Trigger Value and when it is lower than Lower Trigger Value; MS sends MOB_SCN-REQ as many as three times if serving BS keeps suggesting no neighbor BSs for scanning over MOB_SCN-RSP. These three attempts allow the provision that serving BS may like to check on the continuity of the changed CINR level of serving BS before suggesting neighbor BSs for scanning. On the other hand, consecutive three MOB_SCN-RSP messages suggesting no neighbor BSs can be enough to indicate that serving BS is disinclined to suggest any neighbor BSs for scanning. Therefore, a pause in sending MOB_SCN-REQ is suggested in this case. This pause can save wastage of wireless channel resource from exchange of MOB_SCN-REQ and MOB_SCN-RSP messages. However, after certain period, MS resumes sending MOB_SCN-REQ if the CINR of serving BS is still below Higher Trigger Value. MS performs measurement only if CINR of serving BS falls below Higher Trigger Value. Also, MS performs scanning only when either CINR of serving BS is in between Higher Trigger Value and Lower Trigger Value and serving BS suggests neighbor BSs for scanning or CINR of serving BS goes below Lower Trigger Value. Thus, measurement or scanning will not take place if they are not enough justified. This can save unnecessary time allocation and power consumption of MS.
When MS triggers scanning but BS suggests no neighbor BS for scanning; MS selects top three neighbor BSs based on measured CINR value and scans them. So, unnecessary scanning of neighbor BSs with improper CINR value is eliminated here. When the CINR of serving BS becomes too poor, MS may soon go out of service and so an attempt for handover is very much expected then. However, if serving BS does not realize this dire need of MS, it might still suggest no neighbor BSs for scanning.
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Table 5.4: SS Power info RSSI all data using XCAP-X
Figure 5.4. SS power info RSSI and CINR all data
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Table 5.4.1. Scanning related logging message using XCAP-X
Figure 5.4.1: Logging message elaboration using XCAP-X
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Figure 5.4.2. User defined cell measurement Our proposal introduces Lower Trigger Level which allows MS to begin scanning in this situation. MS, after getting denied three times by serving BS, will spontaneously begin scanning. This can actually expedite the decision for handover and also help avoid radio link failure. So, from these analyses, it can be undoubtedly said that our proposed scheme of scheduling process speeds up necessary scanning initiation and brings a satisfactory change reducing wireless resource waste.
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CHAPTER 6 CONCLUSION The advent of 4G wireless cellular technology-mobile WiMAX based on IEEE 802.16e standard was basically destined to meet higher data rates with mobility along with enhanced Quality of Service (QoS). But due to the HO delay & wireless resource waste there occurs significant level of service level degradation. Handover is obviously a vital factor in IEEE 802.16e broadband wireless access network. In our project work an optimized HHO scheme is proposed with a modified scan scheduling in mobile WiMAX network. Frequent exchange of scanning related messages results in wastage of wireless channel resource. So, a method was proposed here with two trigger levels in DCD in terms of CINR value to reduce unnecessary scanning and quicken the initiation of justified scanning. Starting with MAC layer handover process in mobile WiMAX along with the related major MAC management messages; this gives a probable problematic scenario of the scheduling process of scanning followed by our proposed scheme for solution and performance analysis verifying the efficacy of the stated proposal. In fine, the proposed scheme enhances the overall performance radically and solves the obstacles mentioned.
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REFERENCES [1]IEEE 802.16-2004, IEEE Standard for Local and Metropolitan Area Networks, Air Interface for Fixed Broadband Wireless Access Systems, October 2004.
[2]IEEE 802.16e, IEEE Standard for Local and Metropolitan Area Networks, Air Interface for Fixed Broadband Wireless Access Systems, Amendment 2: Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1, February 2006 (Approved: 7 December 2005). [3]Wikipedia, the free encyclopedia, http://www.wikipedia.org. [4]WiMAX: Technology for Broadband Wireless Access- by Loutfi Nuaymi, John Wiley & Sons 2007 [5]Mobile WiMAX – Part I:A Technical Overview and Performance Evaluation (WiMAX Forum) –August,2006 [6]WiMAX-A
Wireless
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Rao,G.
Radhamani [7] Choi S., “Fast Handover Scheme for Real-time Downlink Services in IEEE 802.16e
BWA Systems”, In: Proceeding of IEEE 61st Vehicular Technology
Conference 2005. Vol. 3 2028~2032.
[8 ]Mobile WiMAX – Part II: A Comparative Analysis (WiMAX Forum) - May 2006
[9] Becvar Z., Zelenka J., “Implementation of Handover Delay Timer into WiMAX.” 6th Conference on Telecommunication. Peniche, Portugal, 2007.
[10] Lee D.H., Kyamekya K., Umondi J.P., “Fast Handover Algorithm for IEEE 802.16e Broadband Wireless Access System”, ISWPC 2006.
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[11] Wang L., Liu F., Ji Y., “Performance Analysis of Fast Handover Schemes in IEEE 802.16e Broadband Wireless Networks” , Asia Pacific Advanced Network, Network Research Workshop, August 2007, China.
[12] Yun J. Kavehrad M., “PHY/MAC Cross-Layer Issues in Mobile WiMAX”. Bechtel Telecommunications Technical Journal, Bechtel Corporation, January 2006, Volume 4, No. 1, PP. 45-56
[13] Chow J., Garcia G., “Macro-and Micro-mobility Handoffs in Mobile IP Based MBWA Networks”. IEEE Communications Society, Globecom 2004
[14] Hoymann C., Grauer M., “WiMAX Mobility Support.”
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APPENDIX-I DEFINITIONS Adaptive Antenna System (AAS): A system adaptively exploiting more than one antenna to improve the coverage and the system capacity. Adaptive Modulation: A systems ability to communicate with another system using multiple burst profiles and a system's ability to subsequently communicate with multiple systems using different burst profiles. Automatic Repeat Request (ARQ) block: A distinct unit of data that is carried on an ARQ-enabled connection. Such a unit is assigned a sequence number, and is managed as a distinct entity by the ARQ state machines. Block size is a parameter negotiated during connection establishment. Base Station (BS): A generalized equipment set providing connectivity, management, and control of the subscriber station (SS). Basic Connection: Connection that is established during Subscriber Station (SS) initial ranging and used to transport delay-intolerant medium access control (MAC) management messages. Broadband: Having instantaneous bandwidths greater than around 1 MHz and supporting data rates greater than about 1.5 Mb/s. Broadband Wireless Access (BWA): Wireless access in which the connection(s) capabilities are broadband. Burst Profile: Set of parameters that describe the uplink or downlink transmission properties associated with an interval usage code. Each profile contains parameters such as modulation type, forward error correction (FEC) type, preamble length, guard times, etc. Channel Identifier (ChID): An identifier used to distinguish between multiple uplink channels, all of which are associated with the same downlink channel.
Connection: A unidirectional mapping between base station (BS) and subscriber station (SS) medium access control (MAC) peers for the purpose 49
of transporting service flow's traffic. Connections are identified by a connection identifier (CID). Connection Identifier (CID): A 16 bit value that identifies a connection to equivalent peers in the MAC of the base station (BS) and subscriber station (SS). It maps to a service flow identifier (SFID), which define the Quality of Service (QoS) parameters of the service flow associated with the connection. Security associations (SAs) also exist between keying material and CIDs. DC Sub-carrier: In an orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) signal, the subcarrier whose frequency would be equal to the RF center frequency of the station. Downlink (DL): The direction from the base station (BS) to the subscriber station (SS). Downlink Channel Descriptor (DCD): a MAC message that describes the PHY characteristics of a downlink channel. Downlink Interval Usage Code (DIUC): An Interval Usage code specific to downlink. Downlink Map (DL-MAP): A MAC message that defines burst start times for both time division multiplex and time division multiple access (TDMA) by a subscriber station (SS) on the downlink. Frame: A structured data sequence of fixed duration used by some PHY specifications. A frame may contain both an uplink sub-frame and a downlink sub-frame. Frequency Division Duplex (FDD): A duplex scheme in which uplink and downlink transmission use different frequencies but are typically simuultaneous. Node: A term associated with a mesh network station. A node, due to the nature of mesh, may behave as a BS, SS, or both, and will generate and forward data to other nodes.
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Packing: The act of combining multiple service data units (SDUs) from a higher layer into a single medium access control data unit (PDU). Payload header suppression (PHS): The process of suppressing the repetitive portion of payload headers at the sender and restoring the headers at the receiver. Physical Slot (PS): A unit of time, dependent on the PHY specification, for allocating bandwidth. Point to Point (PtP): A mode of operation whereby two link exists between two network entities. Privacy Key Management (PKM) Protocol: A client/server model between the base station (BS) and subscriber station (SS) that is used to secure distribution of keying material. Protocol Data Unit (PDU): The data unit exchanged between peer entities of the same protocol layer. On the downward direction, it is the data unit generated for the next lower layer. On the upward direction, it is the data unit received from the previous lower layer. Receive/ Transmit Transition Gap (RTG): A gap between the uplink burst and the subsequent downlink burst in a time division duplex (TDD) transceiver. This gap allows gap for the base station (BS) to switch from receive to transmit mode and SSs to switch from transmit to receive mode. During this gap, the BS and SS are not transmitting modulated data but simply allowing the BS transmitter carrier to ramp up, the transmit/receive (Tx/Rx) antenna switch to accurate, and the SS receiver sections to activate. Not applicable for FDD systems.
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