Software Radio in Mobile Communication Systems: Issues and Challenges Santosh Shah, S L Maskara & V Sinha The LNM Institute of Information Technology, Jaipur (Rajasthan)-303012 Tel: 0141-5189212, Fax: 0141-5189214 [email protected], [email protected], [email protected] Abstract ― Application of analog techniques in the early evolution of mobile communication has today been replaced by digital technology. Mobile communication is special in two major ways. The first is a Base Station (BS) and the second is the Mobile Station (MS) i.e. handheld mobile terminal. Both employ software coding, channel coding, interleaving, cipher techniques, modulation, and multiple access. Digital signal processing (DSP) is required to carry out all these functions. Digital hardware takes care many of these functions but today digital signal processing is also used for large number of such functions. In addition to the basic communication functions for the proper operation of mobile communication it is necessary to employ resource and mobility management. Discussions remain confined to GSM only. Index Terms ― Software radio, Base station, Mobile station, Reconfigurable, GSM, Challenges.

I. INTRODUCTION Mobile communication has grown exponentially ever since its emergence and is still growing phenomenally. Infact in the entire history of telecommunications the rate of growth of mobile communication has been unprecedented. Use of sophisticated technology for widespread application of information transfer has been the most important factor for the success of mobile communication. The third generation (3G) mobile communication uses CDMA instead of TDMA. In CDMA the DSP plays a greater role than in TDMA. Today a mobile communication system uses many different frequency bands. For the convenience of the users it is important that a single terminal, which can be programmed depending on the service available in a given region, functions for all the multiple accessing techniques and associated technologies. The DSP can provide such a desired flexibility. Considering the various signal processing functions and the multiband and multimode operations required in mobile communications, the

software approach is more attractive than the hardware. This concept has given birth to the nomenclature of software defined radio. Advances in the analog to digital conversion and processor technologies have made it possible to go for the software radios, in where majority of the communication functions of a radio link are performed by easily reconfigurable and possibly down-loadable software. There are many issues and challenges in the implementation of base stations and mobile stations based on the software radio approach. The layout of a generic GSM network consisting of several functional components and interfaces is schematically shown in fig. 1. The GSM network can be divided into three major parts. The Mobile Station (hand-held terminal) is carried by the subscriber. The Base Station provides the radio link with the Mobile Station. Mobile service Switching Centre (MSC) is the main part of the Network Subsystem. It performs switching of the calls between the mobile users, and between fixed land line network and mobile user. This also handles the mobility management operations. Um interface is also known as radio link or air interface and via Um MS and BS subsystem communicates. BS subsystem communicates with the MSC via another interface called A. We have given stress in this paper on MS and BS. Mobile Station: Mobile Station (MS) or mobile equipment represents the terminal equipment used by mobile subscriber and supported by the GSM wireless system. The MS consists of the handheld terminal and a smart card called the Subscriber Identity Module (SIM). This module may be removed from the handheld terminal. A subscriber with an appropriate SIM can use the network services by using different terminals, which can interact with the related networks. The mobile equipment is uniquely identified by International Mobile Equipment Identity (IMEI). Similarly the subscriber having SIM card can be identified by International Mobile Subscriber Identity (IMSI).

Base Station: Base Station has two parts Base Transmitter Station (BTS) and the Base Station Controller (BSC). These two communicate across the Abis interface. A BTS per cell in a particular region handles the radio-link protocols with the MS. BTS are designed based on terrain to be served and the expected traffic for the cell. The BSC manages the radio resources for one or more BTS(s). This also handles frequency hopping, handover, radio and channel setup. Infact BSC provides the connectivity between MS and MSC. In this paper we have included the general challenges of BS and MS in the section II, software implementation of BS in the section III, software implementation of MS in the section IV, conclusion, and finally references. II. GENERAL CHALLENGES General challenges for BS: Base Station handles the radio-link protocols with the mobile station. Frequency of radio link may vary in accordance with the various air interface standard application (i.e. TDMA, FDMA, CDMA etc). So reconfigurability is a challenge to both the BS and MS. There is also a big challenge that which station should be reconfigurable in accordance with another. General challenges for MS: The challenge areas of mobile nodes are protocols, multimode personalities, wideband modes, co-site interference, and radio interference by itself as shown in table 1 [1]. The protocol defines the mobility of the network, means switching from one channel to another. There

are four to six channels operating simultaneously from high frequency (HF-2MHz) to ultra high frequency (UHF-3GHz), this can be possible by the Global Mobility (GloMo) program, which is an area of research in Defense Area of Research Program in America (DARPA). The mobile node should be multimode personality, so that it can operate about 30 different air interface standards. For example Joint Tactical Radio Systems (JTRS) and SPEAKeasy technologies have multimode personalities. Mobile node could be able to hopes thousand times per second over about 250 MHz. this is possible only with the fast settling tunable Local Oscillator (LO), an ideal software radio could approach the wideband modes. Cosite interference caused by the high power troposcatter system, which is nonlinearly intermodulated in a metal structure. MEMS narrowband filters may be the solution for this type of interference. In the time domain duplex (TDD) radio generates the interference by itself, for example some times may be needed to both listen and transmit in the same ADC band simultaneously. This interference may be reduced by active cancellation. III. SOFTWARE IMPLEMENTATION OF BS Ideal implementation of a BS in software would mean implementation of all subsystems, both for uplink and downlink, in software; i.e. except for the antenna and r.f., all subsystems should be amenable for implementation in software. We describe only one or two modules. The sequence of operation of a GSM sender ands receiver [2] is depicted in fig. 2.,

Uplink modules basically contain the GSM speech coder maps speech into digital blocks. For example, coder used in GSM phase 1 compresses the speech signal to 13 kb/s rate using the Rectangular Pulse Excited Linear Predictable coding with Long Term Prediction (RPE-LTP) technique as per GSM 06-10 specification. And in the GSM phase 2 half rate (HR) Enhance Full Rate (EFR), scheme achieves a rate of 5.6 kb/s. EFR provides same quality and better performance than RPE-LTP. These algorithms produce a speech block of 260 bits every 20 ms. Channel coding / decoding: Channel coding [3] adds redundancy to the blocks. Again, in GSM, the block of 260 bits is divided into three classes (Ia, Ib, and II). CRC protection is used to protect the class Ia bits. Then class Ib bits are added to this result. Then the complete class I sequence is convolutional encoded with the rate r = 1/2 and constraint length k = 5 code thus obtain 378 bits are multiplexed with the 78 unprotected class II bits giving complete coded speech frame of 456 bits. Reverse operation is

done at the decoder. Interleaver / De-interleaver: 456 bits of one block from the channel coding are then split into eight groups of 57 bits. Each group of 57 bits is then carried in different eight bursts. Interleaving [4] means is to decorrelate the relative positions of the coded bits within the code words. Interleaving algorithms avoid the risk of loosing consecutive data bits. Group of 57 bits is carried by a different burst and in a different TDMA frame. Burst contains 148 bits including two successive 57 bits of 456 bits speech code. These two speech blocks are even and odd blocks in a burst (i.e. block A and block B), block A takes the even positions inside the burst and bits of block B, the odd positions. De–interleaving takes the reverse operation to the interleaving, from the burst of 148 bits, extracts the 114 bits of speech code and managed into a 456 bits buffer to get the audio blocks. Ciphering / Deciphering: Ciphering is used to change the data patterns. The ciphering method

does not depend on the type of data to be transmitted, but is only applied to normal bursts. This is achieved by performing an XOR operation between a pseudorandom bit sequences and the 114 bits of each bursts. The deciphering is exactly the same operation like ciphering. Modulation / demodulation: GSM system uses the Gaussian Modulation Shift Keying (GMSK) with the modulation index h = ½, filter Bandwidth Times bit period (BT) equal to 0.3 and a modulation rate of 270.8 kbps. A NRZ signal is integrated and fed to Gaussian filter. The output of filter is then split into in phase and quadrature components and then finally modulated by shift keying. GMSK differs from MSK, for it uses pre-modulation Gaussian filter. The timedomain impulse response of the filter is described by the following equation. and B is half-power bandwidth. IV. SOFTWARE IMPLEMENTATION OF MS The evolution of software radio architecture from digital radio to ideal software radio may be explained in three phases. The first possible handset evolution implements the bitstream processing (FEC, MUX / DEMUX, framing etc.), source coding & deciding, and control functionality in a software on DSP / microcontroller / programmable logic. This is known as digital radio. In the second phase of evolution basenband processing modulation / demodulation comes in to the software processing. With this the radio would be able to adopt the new modulation / demodulation techniques under either self-adaptive or downloaded control. The major change in the evolution of overall architecture comes in the third phase [5] of handset evolution which is shown in fig. 3, which implements the intermediate frequency (IF)

module in software. This phase of evolution will give the new direction to multimode, multiband, and multipersonalities architecture of the radio. This will allow adopting to multiple radio air interface standards by software reconfigurability. IF processing in digital is still a challenge area at higher frequencies. At high frequency high speed ADCs as well as DSPs with low power consumption and high MIPS or GIPS are required [6]. These are the two present technological challenges. V. CONCLUSION The software radio technology makes it possible to stay with a single handheld terminal anywhere in the globe. A single handheld terminal or any type of network designed with this technology can adopt any network (air interface standard) by reconfigurability. REFERENCES [1] Joseph Mitola III, “Technical Challenges in the Globalization of Software Radio”IEEE Communication Magazine, pp. 84-89, February 1999. [2] Thierry Turletti, “Complexity of a Software GSM Base Station”. IEEE communication Magazine, February 1999. [3] Kambiz C Zangi, “Software Radio Issues in Cellular Base Stations”. IEEE Communication Magazine, Vol. 17 No. 4 April 1999. [4] Thierry Turletti, “Towards the Software Realization of a GSM Base Station”. IEEE Communication Magazine, Vol. 17 No. 4 April 1999. [5] Walter H. W. Tuttlebee, “Software-Defined Radio: Facets of a Developing Technology” IEEE Communication Magazine, pp. 38-44, April 1999. [6]. Alan Gatherer, “DSP-Based Architectures for Mobile communications: Past, Present and Future”, IEEE Communication Magazine, pp. 84-90, January 2000.