AN2254 Freescale Semiconductor / Motorola, AN2254 Datasheet - Page 3

• Manufacturer Part Number: AN2254
• Description: Scrambling Code Generation for WCDMA
• Manufacturer: Freescale Semiconductor / Motorola

to spreading, is the scrambling operation. Because scrambling is used on top of spreading as shown Figure 1, it
does not change the bandwidth of the signal, but only makes the signals from different users separable from each
other [3].
With I-Q/code multiplexing, also called dual-channel quaternary phase shift keying (QPSK) modulation, the power
levels of the dedicated physical data channel (DPDCH) and the dedicated physical control channel (DPCCH)
typically differ. This is especially true as data rates increase and can lead in extreme cases to binary phase shift
keying (BPSK) type transmission when the branches are independently transmitted. This situation is avoided by
using a scrambling operation after the spreading with channelization codes. The transmission of two parallel
channels, DPDCH and DPCCH, leads to multicode transmission, which increases the peak-to-average power ratio
[3]. The spreading modulation solution shown in Figure 2 keeps the transmitter power amplifier efficiency the
same as for normal balanced QPSK transmission in general.
2.1 Generating Long Complex Scrambling Codes
All uplink physical channels are subjected to scrambling with a complex-valued scrambling code. In WCDMA
uplink transmissions, the scrambling code can either be short or long. There are 2
and these codes are assigned by higher layers. The long codes are essentially Gold codes. Large sets of Gold codes
have low cross-correlation properties so that as many users as possible can use the channel with minimum mutual
interference. According to 3GPP specifications, Gold codes are generated with a system of 25-stage linear
feedback shift registers, as shown in Figure 3 [4].
Freescale Semiconductor
DPDCH
DPCCH
Scrambling Code Generation for WCDMA on the StarCore™ SC140/SC1400 Cores, Rev. 1
Data
C
C
D
Figure 2. I-Q/Code Multiplexing With Complex Scrambling.
C
Figure 1. Relation Between Spreading and Scrambling
Symbol Rate
Channelization
Code
Q
I
Chip Rate
Multiplex
IQ
Scrambling
Code
I+jQ
24
long uplink scrambling codes,
Scrambling Codes for WCDMA
Chip Rate
C
SCRAMB
Modulation
To QPSK
3