This paper presents a mathematical technique for determining the optimum packet size in multi-rate multi-code Code Division Multiple Access (CDMA) network. This scheme optimizes the channel capacity and tries to meet the multi-rate transmission demands of all the users by varying the size of packets. Throughput is defined as the expected value of the total number of successful transmitted packets/slot. The Analysis shows that as we increase the size of packets keeping processing gain, error detection and correction capability (t) of the network fixed, throughput drops with increase in the number of transmitted packets/slot. It is found that channel capacity for packet length L = 64, 128, 256, 512 and 1,024 in terms of throughput are 120, 72, 50, 38, 30 for t = 10, respectively, and are 420, 140, 79, 50 and 38 for t = 20, respectively. It is concluded that to optimize the channel capacity, we have to transmit possible large size packets as per the flow control of the network.

The current second generation (2G) mobile systems such as GSM and IS-95 are primarily designed to support voice service. Due to large market demand multimedia communication including voice, data and video will be one of the most important applications in third generation (3G) mobile systems (Wang et al., 2003). Future wireless networks are expected to provide Quality of Service (Qos) support for multimedia traffic including voice, videos and data. Because these traffic streams have different requirements of information rate and performance (Tao and Zhisheny, 2002), future generation systems will have to accommodate a wide variety of data rates such as low data rate or high data rate. There are different multiple access techniques which have been proposed for these bit rate systems, among them Time Division Multiple Access (TDMA) and CDMA seem to be the main contenders (Hedayat et al., 1999). CDMA for mobile and personal communication has attracted much attention in recent years because of its numerous advantages like a zero channel access time, a gradual degradation in throughput as the number of users increases and immunity to interference and interception over other systems in wireless communication (Gordon, 1998). In Europe a 3G standard for mobile communication, Universal Mobile Telecommunications System (UMTS) has been proposed based on a Wideband CDMA (W-CDMA) air interface. It is believed that W-CDMA will play a dominant role in future mobile communication like commercial services in the Third Generation Partnership Project (3GPP) and commercial W-CDMA services on a global scale (Sow-Hiroyuki et al., 2001). CDMA has proven successful for large scale cellular voice system, but there is some skepticism about whether CDMA will be well suited to non-voice traffic (Taeyoon et al., 2005). This has motivated research on MC-CDMA which allow variable data rates by allocating multiple codes and hence varying degrees of capacity to different users. Meanwhile MC-CDMA has emerged as a powerful alternative to conventional DS-CDMA in mobile wireless communication and has been shown to have superior performance to single carrier CDMA in multipath fading. Multi-Code-CDMA (MC-CDMA) is one of the techniques that has been proposed to support multi-rate operation in a packetized CDMA network (Subramanian and Wong, 1999). It provides easy integration of data stream having variable rates into a common unified architecture. In a MC-CDMA system if a user needs n times the basic source rate, k spreading codes are assigned to it. The user converts its packet streams into k basic rate parallel packet streams, which are spread using k different codes, respectively. Intuitively with MC-CDMA, the different data rates requirements of the users can easily catered by assigning an enough number of codes to each user. Multi-code Direct Sequence-CDMA (DS-CDMA) schemes offer simple way to provide different data rates to different user in system.

Telecommunications Journal, Multi-Rate Multi-Code Cdma, Broadband Packet Wireless Access, 2G Mobile Systems, Multimedia Communication, Third Generation Mobile Systems, Universal Mobile Telecommunications System, Time Division Multiple Access, MC-CDMA Network, Error Detection, Multi-rate Transmission Network.