Multi-stage Neural Networks for Channel Assignment in Cellular Radio Networks Hyuk-Soon Lee, Dae-Won Lee, and Jaewook Lee Department of Industrial Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk 790-784, Korea. {fnledo,woosuhan,jaewookl}@postech.ac.kr

Abstract. In this paper, we consider the channel assignment problem in cellular mobile communication systems, which assigns a channel to every requested call with the minimum span of channels subject to interference constraints. In this paper, a multi-stage heuristic algorithm using neural networks for the channel assignment problem is proposed. The proposed algorithm is devised to find first a good initial feasible assignment, then a locally optimal assignment, and finally a overall best quality assignment. The proposed algorithm has been applied to the Philadelphia benchmark instances. Experimental results show that the proposed method is competitive with the other existing algorithms.

1

Introduction

In the last few years, demands for the cellular mobile service are growing rapidly. Since the available electromagnetic frequency spectrum is limited, the efficient use of frequency spectrum is regarded as of major importance. In a cellular mobile network, the service area of the system is divided into a large number of cells. When a user requests a call for this system, a channel is assigned there to provide the communication service. However, due to the nature of radio transmissions, calls generated in a cell may cause interference with calls generated in the other cells. The objective of the channel assignment problem (CAP) is to assign a channel to every customer’s call while satisfying the constraints imposed to avoid the radio interference among the channels assigned to the same cell or in relatively adjacent cells. The mathematical model for the CAP considered in this paper is as follows: minimize maxi,k fik subject to |fik − fjl | ≥ cij ∀i, k, j, l(i = j, k = l) fik is nonnegative integer

(1)

where N is the number of cells in the system, mi , 1 ≤ i ≤ N is the number of channels required in cell i, cij , 1 ≤ i, j ≤ N is the channel separation constraint between a call in cell i and a call in cell j, and fik , 1 ≤ i ≤ N, 1 ≤ k ≤ mi is the frequency assigned to the kth call in the ith cell. F. Yin, J. Wang, and C. Guo (Eds.): ISNN 2004, LNCS 3174, pp. 287–292, 2004. c Springer-Verlag Berlin Heidelberg 2004 

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In this paper, we propose a novel multi-stage algorithm to find a conflict-free frequency assignment with the minimum number of total frequencies. In the first stage, a good initial assignment is found by using a so-called frequency insertion strategy (FIS). In the second stage, this initial assignment is adapted to a locally optimal assignment using an adaptive local search. In the final stage, a parallel neural-network algorithm is employed to find a better optimal assignment than the assignment obtained from the second stage. This paper is organized as follows. In Section 2, a novel multi-stage algorithm is proposed. In Section 3, some experimental results and comparison for the benchmark problems are discussed. Section 4 contains a conclusion.

2 2.1

The Proposed Method Initial Assignment Stage

In the first stage, a good initial feasible assignment is found using a frequency insertion strategy (FIS) [4]. The FIS initially permits constraint-violating assignment pretending that we do not have enough frequencies and then inserts necessary frequencies to resolve the violation by sliding the relevant calls, increasing the number of frequencies required. The basic procedure of FIS is as follows: Step 1. Assign the frequency to a call that results in least increment of maximum frequency. Step 2. If the call to which the frequency assigned causes constraint violation, this violation may be avoided by using frequency rearrangement called frequency iteration. In step 2, to avoid constraint violations, we calculate conflicts from frequency 1 to maximum frequency and assign the calls to the minimum-conflict channel as widening bandwidth following conflict size. A simple example of FIS is given for illustration in Fig. 1. 2.2

Local Search Stage

The basic idea behind using local search for CAP is as follows; Given a current ordered list of calls xp , we construct a new neighbor xp by selecting two calls and swapping their positions in the call ordering list. The neighborhood N (x) is searched in this way for a new configuration xp+1 for which f (xp+1 ) < f (xp ). During the local search stage, frequency exhaustive strategy (FES), which assigns each call to the least possible frequency [2], is used instead of FIS. When it converges to the local minimum assignment or fails to find such a solution before limit is reached, the local-search algorithm terminates. One distinguished feature of the proposed local search distinguished from existing ones such as CAP3 [3] is that it obtains new call ordering from the result of FIS and uses it as an initial call list of a local search in the Stage

Multi-stage Neural Networks for Channel Assignment

289

Fig. 1. A simple example of frequency assignment using FIS

II. Existing local search algorithms for CAP mostly use a node-degree ordering heuristics [2] 2] to generate an initial list of calls, whereas the proposed method obtains a new call list according to the order of calls assigned to ascending frequency order obtained by FIS of Stage I. In the case of example in Fig. 1, an initial call list for local search is not [4 4 4 1 2 3],but [4 1 3 4 2 4]. 2.3

Neural Network Assignment Stage

In the final stage, neural-network assignment is employed to find a better solution using the result of assignment in the Stage II. After releasing calls in some cells, the calls are assigned to channels once again by using neural-network. To find the assignment that contains no violate constraint within the M channels, we use a hystereses McCulloch-Pitts neuron model where the output of the ith neuron Vi as follows [6]:  if Ui > U T P (upper trip point)  1 0 if Ui < LT P (lower trip point) Vi (t + ∆t) = (2)  Vi (t) if LT P ≤ Ui ≤ U T P where Ui is the input of ith neuron. To solve N -cell-M -channel CAP, a total N × M binary neuron is used. The motion equation of the ijth neuron in the N -cell-M -channel problem is given by : dUij dt

m j+(cii −1) N j+(cip −1) = −A( q=1 Viq −di )−B( q=j−(c Viq + p=1 q=j−(c Vpq ) ii −1) ip −1) q=j,1≤q≤M

p=i cip >0

1≤q≤M

(3)

where A and B are coefficients, Vij is output of neuron#ij that represents the assignment of channel #j to cell #i, and di is the demand of cell #i. The first

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H.-S. Lee, D.-W. Lee, and J. Lee

Fig. 2. The 21-cell cellular networks of the Philadelphia problem

term has nonzero output, if a total of di frequencies are not required for cell #i. The second term has nonzero output if the assignment of frequency violates the constraints. All the input values Uij are updated while all the outputs Vij are fixed. Then all the outputs Vij are updated while all the input values Uij are fixed. The energy function E corresponding to Eq.(3) is defined as follows: E=

m n m A  B  ( Viq − di ) + ( 2 q=1 2 i=1 j=1

j+(cii −1)



q=j−(cii −1) q=j,1≤q≤M

Viq +

N 

j+(cip −1)



p=1 q=j−(cip −1) p=i 1≤q≤M cip >0

Vpq )Vij

(4)

The energy function E is zero when no constraints are violated. And the goal of neural network model described by Eq.(3) is to find zero point of the energy function E. 2.4

Algorithm

Algorithm 1 (Multi-stage algorithm using neural-network algorithm for CAP) % Initial assignment stage using FIS 1) Set lower bound of maximum frequency, max iterate imax 2) Assign frequency using FIS algorithm. And X is a new call ordering list obtained from the result of FIS, and fmax is max frequency. % Local search stage 3) Select a call aik in maximum frequency cell, and a call ajl randomly (j = i). 4) Make new call ordering list X  by swapping aik and ajl from X.  5) Assign frequency to call ordering list X  using FES. fmax is max frequency.   6) If fmax < fmax , then X ← X  , fmax ← fmax . Go to 3) until iteration number is equal to the max iterate imax ,  or fmax = lower bound

Multi-stage Neural Networks for Channel Assignment

291

Table 1. Demands of Philadelphia instances # 1 2 3 4 5 6 7 8 9

Reuse Demand vector √ √ distance (2√ 3,√ 3, 1, 1, 1, 0) (8,25,8,8,8,15,18,52,77,28,13,15,31,15,36,57,28,8,10,13,8) ( √7, √3, 1, 1, 1, 0) (8,25,8,8,8,15,18,52,77,28,13,15,31,15,36,57,28,8,10,13,8) (2√ 3,√ 3, 1, 1, 1, 0) (5,5,5,8,12,25,30,25,30,40,40,45,20,30,25,15,15,30,20,20,25) ( √7, √3, 1, 1, 1, 0) (5,5,5,8,12,25,30,25,30,40,40,45,20,30,25,15,15,30,20,20,25) (2√ 3,√ 3, 1, 1, 1, 0) (20,20,20,20,20,20,20,20,20,20,20,20,20,20,20,20,20,20,20,20,20) ( √7, √3, 1, 1, 1, 0) (20,20,20,20,20,20,20,20,20,20,20,20,20,20,20,20,20,20,20,20,20) (2 √3, 3, 1, 1, 1, 0) (16,50,16,16,16,30,36,104,154,56,26,30,62,30,72,114,56,16,20,26,16) (2√ 3,√2, 1, 1, 1, 0) (8,25,8,8,8,15,18,52,77,28,13,15,31,15,36,57,28,8,10,13,8) (2 3, 3, 1, 1, 1, 0) (32,100,32,32,32,60,72,208,308,112,52,60,124,60,144,228,112, 32,40,52,32)

Table 2. Minimum bandwidth of algorithms tested on benchmark problem LB CRF CRR CCF CCR DRF DRR DCF DCR RSD CAP3 Proposed 1 427 543 464 460 476 543 521 475 504 485 463 427 2 427 543 468 451 501 543 466 447 495 486 463 427 3 258 360 345 296 296 346 296 304 297 299 302 284 4 258 347 285 285 273 346 270 280 269 296 271 261 5 240 302 299 275 261 295 298 280 251 296 264 249 6 179∼189 289 223 206 197 295 220 220 197 226 232 209 7 856∼857 1088 928 922 939 1088 1046 952 1003 1010 945 856 8 525∼528 646 547 582 588 646 544 586 577 626 584 544 9 1714∼1725 2178 1856 1846 1889 2178 2096 1905 2016 1956 1893 1714 Note: The boldface represent the values to reach lower bounds and underlines represent the best solutions among the 12 algorithms.

% Neural-network assignment stage 7) Release calls in some cells which include max frequency cell. 8) Assign neural-network assignment within the fmax . If the assignment is success, then go to 9). If the assignment fails, then stop. 9) Decrease fmax by 1 and go to 7)

3

Simulation Results

In this section, we conduct several experiments for the purpose of illustrating the results of our paper. The proposed algorithm described in the previous section is tested on Philadelphia benchmark problem [1] as shown in Fig. 2. Table 1 represents the demand vectors and reuse distances of all instances. dj in the reuse distance (d0 , d1 , ..., dk ) means that at least dj unit distance is needed to avoid interference supposing that the difference between all pairs of assigned frequencies is more than j. And the ith element of demand vector shows the demand of cell i.

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Table 2 compares the quality of the solutions obtained from Sivarajan’s 8 algorithms [2], RSD [5], CAP3 [3], and our proposed method. As shown in Table 2, our method finds optimum solutions (i.e. lower bounds) for four out of nine cases and yields the best solutions among the 11 algorithms in all the cases except only one cases. We released 7 cells in order according to the closeness to the maximum frequency.

4

Conclusion

In this paper, a novel multi-stage algorithm for CAP has been proposed. Firstly, the proposed method is performed by means of FIS and local search for a good initial point. And then after releasing the calls in cells which include maximum frequency cell, channels are assigned using neural network algorithm. The performance is verified through Philadelphia benchmark problems. An application of the method to more large-scale benchmark problems remains to be investigated.

Acknowledgement. This work was supported by by Com2 Mac-KOSEF.

References 1. Anderson, L.G: A Simulation Study of Some Dynamic Channel Assignment Algorithms in a High Capacity Mobile Telecommunications System. IEEE Transactions on Communications, 21 (1973) 1294-1301 2. Sivarajan, K.S., McEliece, R.J., Ketchum, J.W.: Channel Assignment in Cellular Radio. in Proc. 39th IEEE Vehicular Tech. Conf. May1-3 (1989) 846-850 3. Wang, W., Rushfold, C.K.: An Adaptive Local-Search Algorithm for the ChannelAssignment Problem. IEEE Trans. Veh. Tech., Vol. 45, No.3 (1996) 4. Shin, W.-Y.: Frequency Insertion Strategy for Channel Assignment Problem. M.S. Dissertation, Dept.I.E. POSTECH (2003) 5. Battiti, R., Bertossi, A., Cavallaro, D.: A Randomized Saturation Degree Heuristic for Channel Assignment in Cellular Radio Network. IEEE Trans. Veh. Tech., Vol. 50, No. 2 (2001) 6. Funabiki, N., Okutani, N.: A Three-Stage Heuristic Combined Neural-Network Algorithm for Channel Assignment in Cellular Mobile Systems. IEEE Trans. Veh. Tech., Vol. 49, No. 2 (2000)

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H.-S. Lee, D.-W. Lee, and J. Lee. In this paper, we propose a novel multi-stage algorithm to find a conflict-free frequency assignment with the minimum number of total frequencies. In the first stage, a good initial assignment is found by using a so-called frequency insertion strategy (FIS). In the second stage, this initial ...

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