Video Transmission for Third Generation Wireless Communication Systems

Journal of Research of the National Institute of Standards and Technology, March-April, 2001 by H. Gharavi, S. M. Alamouti

2.5 The Spreading and Scrambling Codes

There are two major types of codes specified for WCDMA channels: orthogonal spreading codes and non-orthogonal long and short scrambling codes. The orthogonal spreading codes are used for channelization and the short and long scrambling codes are used for reducing inter-cell and intra-cell interference. In this paper, we do not discuss the detailed specifications of these codes. Instead, we will give you an overview of the different families of codes and their functions.

2.5.1 Orthogonal Variable Spreading Factor (OVSF) Codes

These spreading codes, also known as channelization codes, are used to ensure orthogonality between the channels with different spreading factors and rates and are hence called Orthogonal Variable Spreading Factor (OVSF) codes [4]. On the downlink, the orthogonal spreading codes are used to separate the transmissions of the various traffic and control channels within a given cell. On the uplink, the use of spreading codes is not coordinated by the system. The base station and the mobile station agree only on the number of orthogonal channels and hence the number of codes, but the spreading codes used by the mobile stations is not negotiated. Therefore, two mobile stations in a given cell may use the same spreading code on the uplink. In such cases, the resulting interference is suppressed by the uplink scrambling codes which are unique to every mobile station in a given cell.

The OVSF codes are generated using the code tree shown in Fig. 5. The construction of these codes is very similar to Hadamard codes. Two branches emanate from each code in the code tree. The top emanating branch is simply the code from the mother branch repeated twice and the bottom branch is that same code followed by its negation. Not all the OVSF codes are mutually orthogonal. A given code can be used in a cell if and only if there are no other codes used on the path from that given code, to the root of the tree, or any code belonging to the sub-tree generated from that specific code. For instance, if [C.sub.4,1] is used, then [C.sub.2,1] and [C.sub.1,1] may not be used in the same cell as they are in the path to the root. Also, [C.sub.8,1] and [C.sub.8,2] and all other codes derived from [C.sub.4,1] cannot be used in the same cell. Depending on the required data rate, the length of the spreading code applied on a given DPCCH/DPDCH channel may vary from 4 to 512 on the downlink and 4 to 256 on the uplink ( see Table 1).

2.5.2 The Non-Orthogonal Scrambling Codes

There are three classes of scrambling codes used in WCDMA; 1) downlink long scrambling codes, 2) uplink short scrambling codes, 3) uplink long scrambling codes. In order to reduce the interference from the channels in adjacent cells in the downlink, every base station uses a long scrambling code. The downlink long scrambling codes are from the well-known family of Gold codes [5] built by the product of two maximum linear PN (Pseudo Noise) sequences. These codes have good cross-correlation properties and are widely used in commercial spread spectrum systems. The long code used on the forward DCDPCH/DCCPCH is 38 400 chips of a [2.sup.15] - l Gold Code.