Page: WCDMA Channels
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Forward WCDMA Channels

All Forward link WCDMA channels are transmitted by a Base Station to Mobile station. A list of all Forward W CDMA channels, their numbers and functions are shown in following below table.

S.No. Forward (Down link) Channel Number Function Pattern / Rate Processing Blocks
1 Pilot Channel 1 Helps in Clock Recovery at MS All zero pattern Modulo - 2 operation
2 Sync Channel 1 Frame synchronization at MS 16 Kbps Convolution encoder, Block interleaver, Symbol Repetition, Modulo - 2 operation
3 PagingChannel 7 To page the MS and send instructions to it. 16 Kbps Convolution encoder, Block interleaver, Symbol Repetition, Modulo - 2 operation
4 Forward Traffic Channel (Addition of signaling and power control bits ) Several To carry voice and Data Traffic 16, 32, 64 Kbps Convolution encoder, Block interleaver, Symbol Repetition, Modulo - 2 operation MUX

The pilot channel is directly modulo - 2 added to different orthogonal codes of different chip rates depending upon the bandwidth, while in other channels, the signals are processed (Convolutionally encoded, block interleaved and symbol repetition) before modulo - 2 addition with the appropriate orthogonal code. The processing of Forward WCDMA Pilot, Sync and Paging channels is illustrated in Figure 15, 16 and 17 respectively.

Figure 15: Forward WCDMA Pilot Channel Processing

Figure 16: Forward Paging Channel Processing

Figure 17: Forward WCDMA Sync Channel Processing

Forward Traffic Channel processing: It is different from control channels. The system allows multiplexing the control information (signaling bits 2, 4 Kbps) into the traffic channel stream (16, 32, 64 Kbps) to send and receive data between the MS and BS. The traffic channel bits and signaling channel bits are processed by separate convolution encoder (rate one half code), block interleaver and symbol repetition stage. The power control bits at 2 Kbps rates are processed through a symbol repetition stage. The traffic channel data, the signaling channel data and power control bits are then multiplexed. The resultant signal carries voice or data, power control bits and signalling channel data. The resultant multiplexed signal is then further scrambled by decimated long code with modulo - 2 operation. The use of 1 out of 64 bits is called decimation. The scrambler is constructed using every $64^{th}$ bit from a long code generator. The scrambler prevents long sequences of 0s or 1s from appearing in the data stream. The long code mask chosen for each channel establishes the voice or data privacy for that channel. Finally the resultant signal is modulo - 2 added to the Walsh (or Hadamard) code for the channel being used. The complete processing blocks are illustrated in Figure 18.

Figure 18: Forward WCDMA Traffic Channel Processing

Figure 19: Forward WCDMA Channel Modulator

Forward CDMA Channel Modulator:

-The I and Q signals from each channel (pilot, sync, paging and traffic) are modulo - 2 added to an I and Q pseudo random noise sequence (also called as PN code).

• The I and Q spread signals are the base band filtered.

• Similar such filtered signals from all channels are inputted to a linear filter with gain control. Different channels are allocated different power levels depending upon the quality of received signal with gain control block.

• The I & Q base band signals are then modulated by the I & Q carrier signals, combined together and amplified sent to the antenna. The resultant signal from WCDMA modulator is a Quadrature phase shift signal.

Reverse WCDMA Channels:

All reverse WCDMA channels are transmitted from the Mobile station to Base Station. Different frequency bands are used on this reverse path. The reverse WCDMA channel consists of access channel and reverse traffic channel. Their functions, data rate and blocks used for processing are illustrated in below table.

S.No. Reverse (Up link) Channel Number Function Rate Processing Blocks
1 Access Channel 1 - Access the BS
- To respond to the page
- To initiate the call
- Processing of message between MS & BS
16 Kbps Convolution encoder, Block interleaver, Symbol Repetition, Modulo - 2 operation
2 Traffic Channel 1 To send the voice and traffic bits 16, 32, 64 Kbps (encoded speech) Data up to 64 Kbps Convolution encoder, Block interleaver, Symbol Repetition, Modulo - 2 operation
3 Signaling bits (They are multiplexed with traffic channel bits) 1 To send the signaling information 4, 2 Kbps Convolution encoder, Block interleaver, Symbol Repetition, Modulo - 2 operation

Reverse Access channel processing is illustrated in Figure 20. The channel operates at 16 Kbps. The information bits are passed through various processing blocks. And final orthogonal spread signal is sent to the modulator.

Figure 20: Reverse Access Channel Processing

Reverse Traffic Channel processing is illustrated in Figure 21 and 22. It carries the mobile stations’s encoded speech at 16, 32 or 64 Kbps or data up to 64 Kbps. The reverse traffic channel bits and signaling information bits are processed separately through various blocks (convolution encoder, block interleaver, symbol repetition and modulo - 2 sum with orthogonal code). The final spread traffic channel bits and signaling bits are sent to modulator.

Reverse WCDMA Modulator: The modulation process on reverse link is different from forward link as illustrated in Figure 23.

• The Inphase (I) and quadrature (Q) signals are separated after convolution encoding.
• The I channel adds the pilot channel and traffic channel data in linear adder. The pilot channel is added at a reduced power level (-10 dB below the traffic channel).
• The Q channel linearly adds the other traffic channel and signaling information.
• Both I and Q channels are then modulo - 2 summed with the same pilot PN sequence, band pass filtered and sent to the modulator (carrier multiplication).
• The resultant modulation is quadrature phase shift keying, because no phase delay is used on Q channel.

Figure 21: Reverse Traffic Channel Processing

Figure 22: Reverse Signaling (Reverse traffic Channel information ) Processing

Figure 23: Reverse CDMA Channel Modulator

Modulation Parameters

S.No. Modulation Block Function Specifications
1 Convolution Encoding Encoding of traffic, control and signaling bits Rate: One and half code
2 Bit Repetition The bits are repeated if any channel data rate is less than 64 Kbps, to make a constant 64 Kbps Nominal data rate: 64 Kbps
3 Block Interleaving To reduce the consecutive errors due to deep fading, no two adjacent bits are retransmitted near to each other. And errors are randomized. Block interleaver span: 5,10 and 20 ms
Rate: 64 kbps
4 Orthogonal Codes The data and signaling information are modulo 2 added to an orthogonal code on both forward and reverse link to perform spreading. 1 of 256, 1 of 512 Walsh codes for 5 Hz and 10 MHz, 1 of 768 Hadamard code at 15 MHz BW, Long code on reverse link
5 Direct PN Spreading The data stream is modulated with a PN sequence to establish the data and voice privacy on the channel. Data rate: 4.096, 8.196 or 12.288 Mcps,
Code: $x^{32}+x^{22}+x^{2}+x+1$
6 Base band filtering After PN modulation, the signal is filtered by base band filter -
7 Power Control (Closed loop) The BS sends power control message to MS about once every millisecond increase or decrease the power level depending upon received signal strength Maximum EIRP for mobile phone:
Class 1: +23 dBm
Class 2: +13 dBm,
Class 3: +3dBm

Comparison of WCDMA and IS-95

The main differences between WCDMA and IS95 are illustrated in below table.

S.No. Technical Parameter WCDMA IS-95 Remarks
1 Channel Bandwidth 5 MHz 1.25 MHz Greater Frequency Diversity
2 Chip rate 3.84 Mcps 1.2288 Mcps Increase in chip rate by the factor of 3, provides greater multipath resolution
3 Spredaing factor Up to 512 Fixed 64 -
4 Data rates Up to 2 Mbps Up to 9.6 Kbps Variable data rates can be achieved due to range of spreading factors & OVSF codes
5 Frame size 10 ms 20 ms -