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Code Division Multiple Access (CDMA) phần 8

nhu cầu khai thác các lợi ích của quang phổ lây lan. Mạng này có thể được dựa trên trình tự trực tiếp hoặc nhảy tần. Trong các mạng như vậy, xem xét chính là sự phân công của dạng sóng lan rộng, và chúng tôi đã thảo luận về phương pháp chính được giao mã. | SPREAD SPECTRUM PACKET RADIO NETWORKS 125 FIGURE4.7 System throughput for FHMA system with no side information N 128 perfect codes . wish to exploit the benefits of spread spectrum. Such networks can be based on either direct sequence or frequency hopping. In such networks a primary consideration is the assignment of spreading waveforms and we discussed the primary methods of code assignment. Due to the potential for severe near-far situations in ad hoc networks frequency-hopped approaches are typically more appropriate. Thus we focused on the network throughput for FHMA systems. 127 CHAPTER 5 Multiuser Detection As discussed in previous chapters the conventional matched filter receiver treats multiple access interference MAI which is inherent in CDMA as if it were additive noise because after despreading theMAI tends toward a Gaussian distribution see Chapter2 for details . However we have also seen that this MAI is the limiting factor in the capacity of CDMA systems. As a result capacities for single-cell CDMA systems can be substantially lower than those for orthogonal multiple access techniques such as TDMA or FDMA. In addition if one of the received signals is significantly stronger than the others the stronger signal will substantially degrade the performance of the weaker signal in a conventional receiver due to the near-far problem. Thus CDMA performance can be greatly enhanced by receivers designed to compensate for MAI. Multiuser receivers sometimes referred to as multiuser detection is one class of receivers that use the structure of MAI to improve link performance 55 . This chapter presents an overview of multiuser receivers and their usefulness for CDMA particularly at the base station. As detailed in Chapter 3 cellular or personal communication system PCS design consists of two distinct problems the design of the forward link from the base station to the mobile and the design of the reverse link from the mobile to the base station . The forward link .