BER Simulation for WCDMA System in Multipath Fading Channel

The goal for the third generation (3G) of mobile communications system is to seamlessly integrate a wide variety of communication services. One of the most promising approaches to 3G is to combine a Wideband Code Division Multiple Access (WCDMA) air interface with the fixed network of Global System for Mobile communications (GSM). In this paper we investigate the bit error rate (BER) of a WCDMA system at both uplink and downlink for different channel conditions at a multiuser environment. Simple rake diversity combining is employed at the receiver. Performance improvement due to error correction coding scheme is also shown. 1 WCDMA: Air Interface for 3G Third generation cellular systems are being designed to support wideband services like high speed Internet access, video and high quality image transmission with the same quality as the fixed networks. Research efforts have been underway for more than a decade to introduce multimedia capabilities into mobile communications. Different standard agencies and governing bodies are trying to integrate a wide variety of proposals for third generation cellular systems. One of the most promising approaches to 3G is to combine a Wideband CDMA (WCDMA) air interface with the fixed network of GSM. Several proposal supporting WCDMA were submitted to the International Telecommunication Union (ITU) and its International Mobile Telecommunications for the year 2000 (IMT2000) initiative for 3G. All these schemes try to take advantage of the WCDMA radio techniques without ignoring the numerous advantages of the already existing GSM networks. The standard that has emerged is based on ETSI's Universal Mobile Telecommunication System (UMTS) and is commonly known as UMTS Terrestrial Radio Access (UTRA) [1]. The access scheme for UTRA is Direct Sequence Code Division Multiple Access (DSCDMA). The information is spread over a band of approximately 5 MHz. This wide bandwidth has given rise to the name Wideband CDMA or WCDMA. There are two different modes namely: Frequency Division Duplex (FDD) Time Division Duplex (TDD). Since different regions have different frequency allocation schemes, the capability to operate in either FDD or TDD mode allows for efficient utilization of the available spectrum. We investigated a WCDMA system operating in the FDD mode. So all the system description here holds for the FDD mode only. 2 WCDMA Physical Layer: This section provides a layer 1 (also termed as physical layer) description of the radio access network of a WCDMA system operating in the FDD mode. 2.1 Physical Channel Structure WCDMA defines two dedicated physical channels in both links: Dedicated Physical Data Channel (DPDCH) to carry dedicated data generated at layer 2 and above and Dedicated Physical Control Channel (DPCCH) to carry layer 1 control information. Each connection is allocated one DPCCH and zero, one or several DPDCHs. In addition, there are common physical channels defined as: Primary and secondary Common Control Physical Channels (CCPCH) Synchronization Channels (SCH) for cell search, Physical Random Access Channel (PRACH). 2.2 Spreading and Modulation In the uplink the data modulation of both the DPDCH and the DPCCH is Binary Phase Shift Keying (BPSK). The spreading modulation used in the uplink is dual channel QPSK. Figure 1 shows the spreading and modulation for an uplink user. The uplink user has a DPDCH and a DPCCH. The channelization codes used for spreading are known as Orthogonal Variable Spreading Factor (OVSF) codes. The spreading factor for the control channel is always set at the highest value, which is 256. This improves the noise immunity at the control channel by taking advantage of the highest possible processing gain. The complex scrambling code is a unique signature of the mobile station (MS). j Channelization Code(CD) Scrambling Code(CSC) Data (DPDCH)