10 Downlink spreading example.
X 2P . cos[2pfct]
C2(t)
b2(t)
X
C1(t)
b1(t)
+
User #1
User #2
X
S(t)
X
Z1(t)
X LPF
V1(t)
X1(t)
X
Z2(t)
X LPF
V2(t)
X2(t)
BS
Mobile Receiver for User #1
Mobile Receiver for User #2
I1(t) = b1(t ??“t1) ?† ?†
I2(t) = b2(t ??“t2) ?† ?†
C1(t ??“ t1) ?†
C2(t ??“ t2) ?†
2P . cos[2pfct ??“ f1]
2P . cos[2pfct ??“ f2]
PG . Tc
dt
0
??«
PG
. Tc
dt
0
??«
FIGURE 7.11 MS example receiver operations for the downlink spreading scenario.
If we consider User 1 only, we can write the following equations, assuming ideal carrier recovery
(coherent detection). Although the above block diagrams imply carrier recovery should occur prior to
despreading, in practice, it is not necessary to perform spectral down conversion with both phase and
frequency coherence. By this, we mean a ???sloppy??? spectral down-conversion operation can occur
where the residual frequency offsets will be compensated by the automatic frequency control (AFC)
algorithm. Similarly, the residual phase/amplitude offsets will be compensated by the channel estimation
(CE) algorithm. For this discussion, let us work with the following equation for User 1:
(7.14)
After the multiplication of the locally generated PN sequence for User 1, we have an expression for
the first part of despreading operation, which is given below:
(7.
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