Adaptive OFDM System for Multi-Rate Multi-User Services

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{0, 1, 2, 3, 4, 5 , 6}. This set consists of no bit, BPSK, QPSK, 8PSK, 16QAM, 32QAM, and 64QAM. Users per OFDM symbol are reallocated from allocated bit size and IFFTEFT size for each service. lFFT/FFT size consists of set R=(O, 1, 2,4, 8, 16, 32, 64, 128, 256, 512). We assume that we assign &={I}, single camer for telephone service,
camers for indoor high-rate data transmission. When subcamers are allocated to each user, we use a method of grouping adjacent subcamers allocated to each user [9]. In this paper, we focus on an algorithm that maximizes the total throughput by assigning different bit and rate for each user. Also, another goal is to minimize the average computational complexity. We show the performance of our algorithm in a multi-user OFDM in frequency selective fading channels with various different delay time and attenuation. We have analyzed this system from a throughput and computational complexity point of view. The organization of this paper is as follows. Section I1 provides the system model of multi-rate adaptive OFDM. In section 111, a capacity analysis, bit allocation, multi-rate algorithm, and user allocation method are provided. Computer simulation results are presented in section IV to show the BER performance and throughput according to RMS delay spread with exponential delay profile. Finally, we conclude in section V. SYSTEM MODEL The structure of an adaptive OFDM system for multi-rate multi-user system is shown in Fig. 1. The channel state information for all users at the base station is already known. The base station transmitter is controlled by adaptive bit allocation and rate decision algorithm according to the channel state information and the transmit power. This system has a variable user number and each user has different data rate rk bit per OFDM symbol. In the transmitter, each subcamer is adaptively modulated by adaptive bit allocation. After adaptively modulated, the different number of subcamers is reallocated according to the required bit rate for each service. The serial date from the variable k users are fed into the subcamer and adaptive modulator use a corresponding modulation scheme according to the number of bit assigned to a subcamer. We define bk,nas the number of bits of kth user assigned to the nth subcamer. The adaptive modulator allows bk,nto assign value ik
Dept. of Electrical and Computer Engineering, SUNY at Stony Brook. Stony Brook, NY, USA parksy@
Sangjin Hong
Dept. of Electrical and Computer Engineering, SUNY at Stony Brook. Stony Brook, NY,USA snjhong@
0-7803-7442-8/02/$17.00 02002 IEEE
Adaptive OFDM System for Multi-Rate Multi-User Services in Wireless Communications
Jaekyoung Moon
Dept. of Electrical and Computer Engineering, SUNY at Stony Brook. Stony Brook, NY, USA jkmoon@
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Keywords Adaptive OFDM modulation, Multi-rate Multi-user Services.
INTRODUCTION When OFDM is used for a multi-user application to provide a highly flexible and efficient communication system, configuration of the multi-user OFDM system relies on required applications and a computation complexity [9]. Therefore, we focus on a variable rate allocation method combined with an adaptive modulation, which support a multi-rate multi-user OFDM system for several services with a simple hardware complexity. Instantaneous channel state information for each subcamer is available in a base station transmitter. Therefore, the base station knows the channel conditions for all users and makes a use of the same channel conditions [2][3][5][7][8]. In this paper, we consider a problem of multi-rate support in an adaptive OFDM system. We assume that we assign a single camer for telephone service, 2 8 camers for lowrate data, 16 64 camers for high-rate data, and 128 5 12
Abstract This paper investigates performance and throughput of an adaptive bit allocation and variable rate algorithm depending on channel state information (CSI) and a transmit power in a multi-user OFDM transmission. The purpose of the presented multi-rate OFDM system using adaptive modulation is to increase bandwidth eflciency by controlling constellation size and to adapt proper service for each user according to channel conditions and the transmit power while maintaining transmission ' quality of service s (QoS) at the receiver. A fixed-rate adaptive OFDM system only exploits instantaneous channel state information by using several modulation levels, but it is inappropriate to a multi-rate multi-user services. On the contrary, multi-rate adaptive OFDM system is very eficient to multi-user system requiring various services, because this system exploits not only instantaneous channel state information but also the transmit power. Computer simulation results show that the capacity of the adaptive multi-rate OFDM system can be adaptively changed for various services. And performance and throughput analysis of this system in multipath fading channel with different RMS delay spread is also presented.