Digital Standard 3GPP W-CDMA (FDD)
SMIQ
1125.5555.03
E-9
2.230
2.14.3.4 Effects on Crest Factor
The crest factor of the signal is mainly influenced by the following parameters:
•
Link direction
By modifying the Q component of the scrambling code in the uplink, it is possible to avoid every
second zero crossing. In contrast to the downlink, the crest factor is reduced by approx. 1.8 dB.
(Standard case DPCCH + 1DPDCH, the effect decreases with increasing number of channels.)
•
Timing offset (see 2.14.3.3.5)
In the case of more than 10 channels, an increment of 1 (i.e. the first channel is assigned 0, the
second 1, etc.) causes a low crest factor. With a small number of channels, an increment of 3 or 5
permits to obtain even better values.
•
Sequence length if timing offset is active
Longer sequences cause higher peak values and hence higher crest factors since the probability of
even higher peak values increases.
•
Number of channels
More channels cause higher crest factors.
•
Selection of channelization codes
Neighbouring channelization codes usually cause higher crest factors than numbers distributed over
the full range of available values.
•
Example:
16 channels with channelization codes 0 to 15: 16.7 dB
16 channels with channelization codes 0, 4, 8, to 60: 12,45 dB
•
Value of clipping level (see section 2.14.2.10)
2.14.3.5 Orthogonality of Channels
To enable a 3GPP W-CDMA demodulator to restore the information of the individual channels from the
sum signal, only certain combinations of
•
scrambling code sequences,
•
channel power and
•
symbol rate and channelization codes
are allowed.
2.14.3.5.1 Ideal
Scenario
In an ideal scenario the measured channel power (with code domain power analyzers) is equal to the
transmitted power (setting value in the channel configuration table), Even with great differences between
the channel powers the information of the channels can be restored 100%. For this purpose however
orthogonality of the channels is required. Orthogonality is given in all cases described in section 2.14.3.3
Orthogonality is given for instance if DPCH channels are used only and all of them have the same
symbol rate but different channelization codes. In addition, identical scrambling code values have to be
set for all channels, i.e. all channels have to belong to a base station.
In this case the bit streams SC
i
and SC
q
' are identical for all channels. Since the channelization code
sequences of two channels are orthogonal, the chip sequences Si and Sq of two channels are also
orthogonal.
2.14.3.5.2 Real
Scenario
A real scenario usually consist of signals from different base/mobile stations that are operated at
different scrambling codes and have different delays. In this case there is no orthogonality, the channel
powers overlap and the bit error rates increase with decreasing channel power (therefore power control
is extremely important for all CDMA systems).
