8, we can see that the spreading factor can vary from SF 4 to SF 512, depending
on the data rate that is being transmitted.
Let us now turn our attention to the spreading operations performed. The uplink spreading functionality
is shown in Fig. 1.22. Here, the separate dedicated data and control channels are channelized
(and spread) by an Orthogonal Variable Spreading Factor (OVSF) sequence. We have labeled the
OVSF codes as Walsh codes, due to their similarity. The resulting sequence is then made to be complex
valued, and then, QPSK spreading is applied prior to entering the pulse-shaping filter. Note that,
in order to acknowledge the terminology used by 3GPP, this QPSK spreading operation is also called
scrambling within the standard. On one hand, this is clearly understandable since the OVSF multiplication
did indeed spread the information frequency bandwidth. The second PN multiplication is acting
more like a scrambling function than a frequency-spreading function.
The QPSK spreading (or scrambling) is actually called hybrid PSK (HPSK). This was introduced
to the UE transmitter since this technique will dramatically reduce the number of phase trajectories
that come dangerously close to the origin.
WIRELESS TOPICS 19
One Radio Frame, Tf = 10 msec
TPC
NTPC Bits
Slot #0 Slot #1 Slot #i Slot #14
TSlot = 2560 Chips, 10 .
Pages:
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60