tailieunhanh - McGraw Hill - 2002 - W-CDMA and cdma2000 for 3G Mobile Networks_4

Tham khảo tài liệu 'mcgraw hill - 2002 - w-cdma and cdma2000 for 3g mobile networks_4', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 98 Chapter 3 The maximum amount of multipath delay that can be exploited in a rake receiver is usually limited and is determined by the power delay profile. As an example for a city like New York it lies in the range of ms. Thus in UMTS W-CDMA where the chip rate is Mc s the delay is about 1-10 chips. Although multipath diversity is a property of all CDMA systems it is only W-CDMA that provides multipath diversity for small cells that is the micro and pico cells . To see this consider IS-95 where the carrier bandwidth is MHz. In this case because the chip rate is Mc s and because the delay must be at least one chip long to achieve multipath diversity the difference in path lengths must be at least 244 meters. On the other hand for W-CDMA with 5 MHz bandwidth the chip rate is Mc s and so this path difference is reduced to 81 meters. The multipath diversity employed in a rake receiver leads to an improvement in performance. For example the value of Eb N0 required to ensure a bit error rate of 10 3 on a fading channel is about 10 dB assuming BPSK modulation a 4-branch rake receiver and equal gain combining. The required Eb N0 for the same bit error rate is 14 dB with two branches and about 24 dB with one branch that is without any multipath diversity 21 . The maximal ratio combining has the best performance. If most of the signal energy is contained in only one branch a conventional receiver will perform better than a rake receiver that uses equal gain combining 33 because in this case branches with very little signal power will only add to the noise. Multiuser Detection Consider the uplink transmissions in UMTS. Here the user data on various physical channels such as dedicated physical data channels dedicated physical control channels and so on is first spread with a channelization code and then scrambled with a user-specific PN code. Because channelization codes are mutually orthogonal and thus more resistant to multiuser interference