SOFT HANDOFF AND CAPACITY OF CDMA COMMUNICATION NETWORKS

Abstract

An enormous growth has been experienced by wireless communication over the last decade and the world of communication is becoming more mobile than ever. Mobile cellular systems have evolved from the first generation analog systems such as Advanced Mobile Phone System (AMPS) to the second generation digital systems such as Global System for Mobile (GSM) and IS-95 systems. Meanwhile, there has been a parallel and rapid growth in Internet-based services and mobile communication which had paved a path for third generation mobile systems. The third generation (3G) systems can not only provide the traditional voice services as an extension of the wired telephone systems but also support mobile multimedia and mobile Internet-based services. 3G systems provide advanced and flexible Quality of Service (QoS) support and efficient utilization of available radio resources by means of packet and circuit switching. It has a goal of providing universal coverage and to enable users with services any time any where. The wireless users access or share to a resource using multiple access techniques. There are three multiple access techniques namely, Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA). FDMA is a technique whereby spectrum is divided up into frequencies and then assigned to users. The number of users is restricted by the number of available frequency channels. TDMA improves spectrum capacity by splitting each frequency into time slots. This allows each user to access the entire radio frequency channel for the short period of a call. Other users share the same frequency channel at different time slots. The number of users is restricted by the number of time slots in a frame. The CDMA is based on spread spectrum technology. It increases spectrum capacity by allowing all users to occupy all channels at the same time. Transmissions are spread over the whole radio band and each voice or data call is assigned a unique code to differentiate from the other calls carried over the same spectrum. The number of users can be increased as large as the number of available orthogonal codes. The capacity of a communication network is the maximum number of the subscribers that can be accommodated for a given quality of service parameters. The capacity of CDMA systems is limited by interference. With CDMA based systems like Wideband Code Division Multiple Access (WCDMA), one of the most adopted 3G air interface, capacity of the system becomes more flexible since all users share the same spectrum allocation and use codes to identify themselves from others. Every cell can use the whole bandwidth and the capacity of the whole system is limited by the total interference that occurs from other users or other base stations and the background noise. Handoff, an important feature of CDMA systems, guarantees the continuity of the services when a mobile station (MS) moves across the cellular boundaries. First and second generation of mobile networks employed hard handoff (HHO) while the third generation soft handoff (SHO). With hard handoff, a definite decision is made on whether to handoff or not. Handoff is initiated and executed on a positive decision but user does not attempt to have simultaneous traffic channel communication with the two or more base stations. With soft handoff, a conditional decision is made on whether to handoff or not. A hard decision will eventually be made to communicate with only one. This decision is usually based on pilot signal strength from two or more base stations which are involved. This normally happens when it is clear that the signal from one base station (BS) is considerably stronger than those from the others. Users would have simultaneous traffic channel communication with all candidate base stations in the interim period. Soft handoff has its own advantages and disadvantages. It gives a smoother transmission, less ping-pong effects and macro-diversity gain to the system. However, there is extra resource consumption in employing soft handoff. Thus, optimization becomes crucial for better performance of SHO. Until now, a number of algorithms based on maximizing the macro-diversity gain and minimizing the handoff failure rate have been proposed. For the last one decade there has been extensive work over SHO in the uplink direction of CDMA based systems but the same is not with SHO in downlink direction. Further, in today's arena, downlink is now more complicated due to user demand of Internet based services over mobile terminal. In this thesis, downlink soft handoff effects in WCDMA cellular mobile networks have been investigated and a new approach of power control in downlink direction is presented. Capacity of CDMA and WCDMA systems has also been analyzed. A fuzzy logic based system for soft handoff in CDMA based systems is also presented. The objective of the present research work is to study and analyze the soft handoff effects in downlink direction of WCDMA cellular mobile networks and to modify the possibilities of enhancements for the existing model, to apply fuzzy approach to handoff in CDMA systems and to evaluate the capacity of CDMA and WCDMA systems based on different parameters. The objectives are HI • To study and analyze soft handoff effects of the WCDMA cellular mobile network in downlink direction • To evaluate power control effects during soft handoff • To develop a modified power control strategy in downlink direction of WCDMA systems during soft handoff for maximizing the downlink capacity • To evaluate the performance of soft handoff provided by the macro diversity due to two base stations simultaneously involved in communication • Performance analysis of the soft handoff algorithm using fuzzy technique To full fill the above objectives, the analysis is carried out as follows: 1. To analyze the soft handoff effects over downlink direction of WCDMA networks In the present study effects of soft handoff in downlink direction of WCDMA cellular networks are analyzed. Power control is one of the most crucial aspect of all mobile stations as all mobile stations operate over battery, life of which is limited, further safety reasons are also associated with power control. Power control aspect along with interference is discussed and analyzed. Power allocation is evaluated on two fronts, one when soft handoff is not considered and one with soft handoff. Further, power allocation with soft handoff is evaluated in presence and absence ofshadowing component. 2. To evaluate the performance of the soft handoff provided by the macro diversity The Soft handoff capability provided by macro diversity is estimated in which a mobile is usually linked via multiple (typically two) signal paths to both the current base station and the target base stations. The signal to noise ratio is very weak in the soft handoff region. The signal to noise ratio (SNR) is improved by combining the signal from the different base stations. We have analyzed the effect of SNR on the system IV capacity. The capacity at the boundary without-macro diversity and with macro-diversity is calculated with different voice activity factors. The effect of voice activity factor and interference factor on the system capacity is also evaluated. 3. An Optimized method of power control during soft handoff A new approach of power control in downlink direction of WCDMA system during soft handoff is given. Both shadowing and non-shadowing cases are discussed and compared. CDMA systems are interference limited systems, i.e. performance of CDMA system depends mainly on interference reduction and this new approach shows a significant reduction in interference. The downlink capacity is increased to a good extent and hence improves the system performance in general. 4. Performance analysis of the soft handoff algorithm using fuzzy technique Fuzzy handoff algorithm for wireless communication proposed by George Edwards et al. has been applied to calculate the received signal strength (RSS) in macro-cellular system. The parameter TUP and TDown, which dynamically vary, depending on the traffic load in each cell are used in the soft handoff algorithm. The effect of number of base station (noBs), number of remaining channels of serving base stations (CHrm) and bit energy to noise plus interference ratio (Eb/Io) have been evaluated for call blocking probability of on going calls and outage probability with traffic load. The fuzzy logic technique is applied for the three parameters (Eb/Io, noBs, CHm,) and the performance of the soft handoff algorithm has been analyzed.