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spreading and scrambling in the uplink of wcdma
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         Spreading and Scrambling  in the Uplink  of WCDMA

            The most common PN code families are Walsh-Hadamard codes, m-sequences, Gold codes and Kasami codes. Walsh-Hadamard codes are orthogonal on zero code delay whereas the m-sequence, Gold codes and Kasami codes are nonorthogonal with varying cross-correlation properties. Walsh-Hadamard codes and Gold codes are used in uplink communication of WCDMA. In this section, we will emphasize Walsh-Hadamard codes and Gold-codes.

The concept of spreading process is introduced . In WCDMA, a PN sequence is used to scramble the signal in addition to the spreading signal. The purpose of the scrambling signal is to separate terminals or base station from each other. Scrambling is used on top of spreading. It does not change the signal bandwidth but only makes the signals from different source separable from each other. With the scrambling, it would not matter if the actual spreading were done with identical code for several transmitters. The relationship between spreading and scrambling is shown in Fig 3.3 [9].


In the uplink communication of WCDMA, Walsh-Hadamard codes are used to spread data, and Gold codes are used to scramble data, so called scrambling code.

There are 2 types of uplink dedicated physical channels, the uplink Dedicated Physical Data Channel  (uplink DPDCH) and the uplink Dedicated Physical Control Channel (uplink DPCCH). The uplink DPDCH is used to carry dedicated data generated at Layer 2 and above. There may be zero, one, or several uplink DPDCHs on each Layer 1 connections. The uplink DPCCH is used to carry control information generated at Layer 1. The Layer 1 control information consists of known pilots bits to support channel estimation for coherent detection, transmit power-control (TPC) commands, and an optional transport-format indicator. There is one and only one DPCCH  on each Layer 1  connection.

In the uplink The data modulation of both the DPDCH and the DPCCH is Binary Phase Shift Keying (BPSK). The modulated DPCCH is mapped to the Q-channel, while the first DPDCH is mapped to the I-channel. Subsequently added DPDCHs are mapped alternatively to the I or the Q-channel. Spreading Modulation is applied after data modulation and before pulse shaping. The spreading

 

modulation used in the uplink is dual channel QPSK. Spreading modulation consists of two different operations. The first one is spreading where each data symbol is spread to a number of chips given by the spreading factor. This increases the bandwidth of the signal. The second operation is scrambling where a complex valued scrambling code is applied to spread signal. Fig. shows the spreading and modulation for an uplink user . The uplink user has a DPDCH and a DPCCH.

The bipolar data symbols on I and Q branches are independently multiplied by different channelization codes. The channelization codes are known as Orthogonal Variable Spreading Factor (OVSF) codes or Walsh-Hadamard codes.  

The resultant signal is multiplied by a complex scrambling code. The complex scrambling code is a unique signature of the mobile station. Next, the scrambled signal is pulse shaped. Square-Root Raised Cosine filters with roll-off factor of 0.22 are employed for pulse shaping. The pulse shaped signal is subsequently upconverted as shown-