B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

11. 8 Modern Practical DSSS CDMA Systems 643
Figure 11. 16
Decision feedback
MUD
receiver based
on successive
interference
cancellation
(assuming that
all M users are
ranked in the
order of
descending
gains).
(1)
®
c, (t) t
L@
c2(I) t
and
Step 2. Detect the £th (strongest) user symbol via
e = 1
Step 3. Cancel the first (strongest) user interference from the received signals
(i) (i) r;:;-p R (k)
'(C)
Ye+1 = Ye - gcy re i,£ s k i = £+ I, ... , M
Step 4. Let £ = e + 1 and repeat step 2 until £ = M.
A decision feedback MUD receiver requires very little computation, since the interference
cancellation step requires only O(M 2 ) complexity. It is a very sensible and low-complexity
receiver. Given correct symbol detection, strong interference cancellation from received weak
signals completely eliminates the near-far problem. The key drawback or weakness of the
decision feedback receiver lies in the effect of error propagation. Error propagation takes place
when, in step 2, a user symbol si£) is detected incorrectly. As a result, this erroneous symbol
used in the interference cancellation of step 3 may in fact strengthen the MAI. This leads to
the probability of more decision errors of the subsequent user symbol, siHl), which in turn
can cause more decision errors. Analysis on the effect of error propagation can be found in
Refs. 14 and 15.
11.8 MODERN PRACTICAL DSSS CDMA SYSTEMS
Since the 1990s, many important commercial applications have emerged for spread spectrum,
including cellular telephones, personal communications, and position location. Here we discuss
several popular applications of CDMA technology to illustrate the benefits of spread spectrum.
11 .8.1 CDMA in Cellular Phone Networks
Cellular Networks
The cellular network divides a service area into smaller geographical cells (Fig. 11.17). Each
cell has a base station tower to connect with mobile users it serves. All base stations are wired