Effective Bandwidth in CDMA Cellular Networks with Soft Handoff

The notion of effective bandwidth measures the amount of resource required by a user under statistical mult iplexing with bursty traffic. This notion has been applied to Code Division Multiple Access (CDMA) systems in [1] to compute the resource requirement by considering the stochastic variat ions of the traffic and multi-cell interference. In this paper, we compute the effective bandwidth values under a more realistic multi cell environment by considering lognormal shadowing and So ft Handoff. The computed effective bandwidth can be used to perform admission control in CDMA cellular systems. I. I NTRODUCTION In a Code Division Multiple Access (CDMA) system, if the received Signal power to Interference noise density Rat io (SIR) at the Base Station (BS) for a Mobile Station (MS) falls below a certain threshold, then Quality of Service (Qo S) requirement for the mobile station may not be met leading to erroneous packet reception. This condition is called outag e in CDMA. Various admission control policies have been proposed in the literature [2], [3], [4], [5] that determine the number of users to be admitted for a desired probability of outage. In [2], an analytical expression for probability of outage taking multiple cell interference into considera tion has been derived. In [3], the authors have derived outage probability with SIR based admission control. In [5], the authors have given an expression for blocking probability under SIR based admission control. More recently, in [4], a comprehensive performance analysis of voice and data CDMA systems has been considered. This analysis has evaluated th e outage probability for voice traffic and the average system throughput and mean delay for data traffic. The analysis has also considered multiple cell scenario. Due to soft capacity of CDMA link layer, admission control for a given probability of outage achieves a kind of statisti cal multiplexing gain (albeit at the link layer) similar to that observed in packet switched networks where the admission control is performed at the network layer for QoS measure lik e packet loss and delay. In packet switched networks, the traf fic is usually bursty in nature. If the resources are allocated b ased on the peak rate, then it may lead to inefficient resource util ization and no statistical multiplexing gain may be achieved. T o address this problem, a new metric called effective bandwid th for each user is defined such that when the sum of the effective bandwidth values of all users remains less than or equal to the total system resource such as the system bandwidth, then the QoS guarantees are also satisfied. The value of effective bandwidth for a particular user may lie between the peak rate and the average rate of the user and depends upon the QoS requirement as well as the statistical characterization of the user’s traffic. In packet switched network context, effecti ve bandwidth has been computed for various traffic models and QoS performance criteria [6], [7], [8], [9], [10], [11]. Eff ective bandwidth leads not only to a simple admission control polic y but also effectively increases the capacity of the system. In [1], the notion of effective bandwidth has been extended to CDMA networks for admission control. This notion not only exploits the randomness in the outer cell interference but also the stochastic variation of the traffic sources. The aut hors have computed an admissible region bounded by a finite number of hyperplanes. While the authors have considered th e variations in the traffic rate and the interference in a multi ple cell scenario, they have not considered the effect of shadow ing and soft handoff. In this paper, we extend the work of [1] to compute the effective bandwidth in a multicell scenario wit h soft handoff and lognormal shadowing. We assume that each user is in soft connection with two of the nearest base statio ns and power controlled by the one which requires it to transmit with minimum power. The computed effective bandwidth leads o a simple admission control policy in a practical CDMA system with soft handoff and shadowing. The rest of the paper is organized as follows. Section II describes the system model and how effective bandwidth arises in CDMA systems. In Section III, we describe how effective bandwidth can be computed taking both shadowing and source variations into account. The paper discusses the results obtained in Section IV and conclude in Section V. II. SYSTEM MODEL AND EFFECTIVE BANDWIDTHS IN CDMA Consider a CDMA cellular system as shown in Fig. 1a with voice calls only. The total bandwidth is assumed to be W Hz. The users are assumed to be uniformly distributed in space and also in perfect power control with their own BS. The users’ signals are assumed to undergo lognormal shadowing. We do not consider multipath fading effects. Thi s s a reasonable assumption if the holding times of the voice calls are sufficiently long to average out the multipath fadi ng.