4.3 Downlink

In most existing CDMA systems, the same detection schemes are used in both uplink and downlink. Since downlink CDMA signals possess some intrinsic structures that are not available in uplink. we will discuss in the remainder of this chapter high-performance downlink signal reception by exploiting structural information.

In CDMA downlink, all users' signals are transmitted synchronously. By setting the relative delays to zero in (1.2), we obtain the expression for downlink CDMA signal as:

The time varying spreading waveform w _i,kt is given in (1.5) of Chapter 1.

In downlink modulation, individual data sequences are first spread by orthogonal spreading codes (e.g., the Walsh codes). The spread sequences are then combined and overlaid with a long pseudorandom mask code. The lth chip within the kth symbol can thus be decomposed as:

where c _i(l) is the short and periodic-spreading code for the ith user; and o _k( l) ? { 1} is the overlaid mask code that is common for all users. Note that the compound-spreading codes are still mutually orthogonal for each symbol period if { c _i( l)} _i=1 ^P satisfy: Because of this inherent orthogonality, downlink signal reception is usually performed in two steps: (1) channel equalization that compensates the "common" multipath channel effect and restores the orthogonality. and (2) despreading that extracts the desired signal. The issue to be studied here is the design of channel equalizers.

For general purposes,...