A comparative study of the effects of microdiversity and macrodiversity on CDMA forward-link capacity

Lin Dai, Shi-dong Zhou, Yan Yao
2002 IEEE International Conference on Communications. Conference Proceedings. ICC 2002 (Cat. No.02CH37333)  
Abstrucr-Both microdiversity and macrodiversity can effectively overcome the harmful effect of fading. Most of previous work focused on their benefits to the reverse link in CDMA systems. However, their effects on the forward link are less well understood. In this paper, we present an analytical model on forward-link capacity in an H-port macrodiversity system with power control. Maximum Ratio Transmission (MRT) is considered instead of switched selection. We also make a comparison between the
more » ... erformance of microdiversity and macrodiversity on the forward link. The simulation results show that, both types of diversity can bring benefits to the forward-link capacity. However, with macrodiversity higher capacity can be obtained at the cost of complexity. I INTRODUCTION Diversity is an effective means to counteract the harmful effect of channel fading. Macroscopic diversity (macrodiversity) is one kind of diversity to overcome large-scale fading effects [14]. In CDMA systems, combining signals from widely separated base station (BS) antennas allows exploitation of macrodiversity gains. Microscopic diversity (microdiversity) is another type of diversity that can be obtained usually by placing an antenna array at each BS [2]. Since all signals received at the mobile from the same BS propagate through the same path, microdiversity can effectively counteract small-scale multipath fading effects. The effect of microdiversity on CDMA system performance has been thoroughly studied. Numerous papers show that with antenna arrays both reverse-link capacity' and forward-link capacity can be increased greatly [2]-[5]. In contrast, the effect of macrodiversity is not well understood. Most of previous studies focus on the benefits of selection-based macrodiversity (soft handotl) to the ' In this paper, the term "capacity" refers to the number of users that can be supported at the desired quality-of-service requirement. This should be distinguished from the information theoretic-capacity of a channel. reverse link [6]-[lo], while the forward-link analysis is less considered. As we know, since multimedia capabilities are ongoing to be introduced into mobile communications, highly asymmetrical services must be provided by the forward link in CDMA systems. Therefore researches on improving CDMA forward-link capacity are becoming more and more necessary. Several authors investigate the performance of macrodiversity on the forward link [ 1 I]-[ 131. However, [ 1 l] and [ 121 only give the simulation model, and in [ 131 no power allocation scheme is adopted and only the case of two BS's macrodiversity is studied. In this paper, we present an analytical model on forward-link capacity with macrodiversity in CDMA systems. We assume that there are H BS's that form a macrodiversity group. Different transmit powers are allocated at those BS's in order to maximize the forward-link capacity. Additionally, a comparison is made between the performance of microdiversity and macrodiversity on the forward link. It actually reflects the effect of antenna topology on capacity and can act as a guideline for cell planning. In our analysis, fast fading is assumed not to affect the average power level. Therefore, only the path loss and log-normal shadowing fading are considered. The forward-link capacity with microdiversity and macrodiversity are analyzed in Section II and Section III, respectively. Numerical results are given in Section IV. Finally, SectionV contains our concluding remarks. II CDMA FORWARD-LINK CAPACITY WITH MICRODIVERSITY Consider a CDMA forward-link system with coherent demodulation, which is achieved by sending a CDMA pilot with all the traffic channels. The conventional matched filter receiver is adopted at the mobile. We assume that each BS uses a multi-element antenna array to transmit signals to mobiles. Let H be the number of antenna elements (H>l). It is well known that with an antenna array, the BS must also beamform on the forward link in order to effectively 0-7803-7400-2/02/$17.00 © 2002 IEEE 1893
doi:10.1109/icc.2002.997177 dblp:conf/icc/DaiZY02 fatcat:puexpyz3gbgcvfrq4r4alkulta