Elsevier

Computer Networks

Volume 50, Issue 7, 15 May 2006, Pages 1003-1021
Computer Networks

A capacity acquisition protocol for channel reservation in CDMA networks

https://doi.org/10.1016/j.comnet.2005.05.032Get rights and content

Abstract

In this paper, a capacity acquisition protocol is proposed for channel reservation in CDMA networks. Under this protocol, a cell is virtually divided into three regions (i.e., inner region, forced handoff region, and active handoff region). A new call in the active handoff region works in soft handoff mode upon its admission, while a new call in the inner and forced handoff regions works in single mode. However, in the forced handoff region, calls working in single mode can be forced into soft handoff mode, when extra capacity is needed by soft handoff calls. As a result, no explicit channel reservation is required before a call enters soft handoff. By adjusting the size of the forced handoff region, the capacity acquisition can adapt to traffic load and guarantee a desired call dropping probability. To evaluate the capacity acquisition protocol, an analytical model is derived and is also validated through computer simulations. Numeric results illustrate that the capacity acquisition protocol significantly reduces the call dropping probability.

Introduction

Dropping an on-going call is more disturbing than blocking a new call. To resolve this problem, channel reservation for handoff calls can be used in a call admission control (CAC) algorithm to prioritize handoff calls over new arrival calls. However, channel reservation in CDMA networks is nontrivial because of the special features of soft handoff. It is well known that soft handoff of CDMA networks reduces interference and increases the interference-sensitive capacity [1]. This feature must be taken into account by the channel reservation scheme in a CAC algorithm. In addition, two other important features found in this paper need to be considered. One is that soft handoff and the need of channel reservation occur at different times. The other is that a call working in single mode releases a certain amount of capacity when it is forced into soft handoff mode. Based on these features, a novel channel reservation scheme called capacity acquisition protocol is proposed in this paper for soft handoff calls in CDMA networks. This protocol focuses on uplink operation of a CDMA network.

In the capacity acquisition protocol, three regions, i.e., inner region, forced handoff region, and active handoff region, are introduced for each cell. When an admitted call moves into or a new call arrives at the active handoff region, it works in soft handoff mode. In the forced handoff region, an admitted new call works in single mode and thus only communicates with one base station. Since a call in soft handoff mode consumes less capacity than it does in single mode, some capacity is implicitly reserved by a new admitted call in the forced handoff region. When more capacity is needed by soft handoff calls, it can be acquired from new admitted calls in the forced handoff region by forcing them into soft handoff mode. Thus, before a call enters soft handoff, no explicit channel reservation is required to reserve capacity.

The size of forced handoff region in the capacity acquisition protocol can be adjusted according to the traffic load in the network. As long as traffic load is lower than an upper bound, a target call dropping probability is guaranteed by the load-adaptive protocol.

To date, many algorithms have been proposed for call admission in CDMA networks [2], [3], [4], [5], [6], [7]. Neither the signal-to-interference ratio (SIR)-based CAC algorithm in [2] nor the interference level-based CAC algorithm in [3] considers interference reduction by soft handoff. The interference reduction brought by soft handoff is not considered either in [4], although a call in the soft handoff region can access two base stations. Compared to the schemes in [2], [3], [4], the CAC analytical model proposed in [5] achieves better performance, because it takes into account the capacity increase factor introduced by soft handoff. According to this model, the larger is the size of soft handoff region in a cell, the smaller is the call blocking probability of a CDMA network. No differentiation is performed between soft handoff and new arrival calls in [5]. Algorithms reserving fixed channels [4] for soft handoff calls waste resources. In [6], a “look around” CAC algorithm is proposed to reduce dropped calls. Soft guard channels are exclusively used for handoff calls, which also results in low resource utilization. In [7], an adaptive channel reservation scheme is proposed for soft handoff calls in a CDMA network. When a user with an on-going call moves into a reservation region, it starts a channel reservation procedure, so channels are reserved individually for each handoff call. Thus, each handoff call does not have fixed reservation of capacity and the utilization is improved. However, this method still wastes a large amount of capacity, because the reserved capacity for a soft handoff call are held useless during the period from the approval of reservation request to the initiation of a soft handoff call. In addition, when traffic load (i.e., new call arrival rate) is high, the resource utilization of the adaptive reservation scheme is not guaranteed to be more efficient than that of a fixed reservation scheme, because many users need to have reserved channels. In other words, this scheme is not actually adaptive to traffic load.

The rest of this paper is organized as follows. The capacity acquisition protocol is proposed in Section 2, and is analyzed in Section 3. The analytical model is justified by simulations in Section 4 where analytical results are used to compare the new scheme with other channel reservation schemes for CDMA networks. In Section 5, a load-adaptive capacity acquisition protocol and its performance are presented. Practical issues of the capacity acquisition protocol is discussed in Section 6. The paper is concluded in Section 7.

Section snippets

Capacity acquisition based on soft handoff

The capacity acquisition protocol is motivated by the features of soft handoff.

Assumptions

In a CDMA network, due to the factors such as irregular cell boundaries, traffic characteristics, and the movement of mobile terminals [5], it is complicated to find an accurate model of soft handoff. To simplify the analysis, assumptions are given as follows:

  • Residual times of a call in all regions are generally distributed [8].

  • The new call arrival rate is a Poisson process with rate λn.

  • The new calls are uniformly distributed in the cells.

  • The call holding time Tc is exponentially distributed

Performance of the capacity acquisition protocol

In this section the analytical model is first verified through simulations. Based on the validated analytical model, the capacity acquisition protocol is then compared with other channel reservation schemes.

A load-adaptive capacity acquisition protocol and its performance

Given a fixed forced handoff region in the capacity acquisition protocol, if it is small, the call dropping probability will be very large when the new call arrival rate is high. On the other hand, if the size of the forced handoff region is large, many calls will unnecessarily work in soft handoff mode when traffic load is low, which wastes system capacity because soft handoff calls decrease the available channels on the forward link of a base station [9]. In order to keep the call dropping

Practicality of the capacity acquisition protocol

There are a few practical issues concerning the implementation of the capacity acquisition protocol.

  • SINR measurement. SINR is used in the capacity acquisition protocol to determine if enough capacity has been acquired. SINR measurement is usually available as one of the building blocks for power control in CDMA networks. The accuracy of SINR measurement depends on the performance of the estimation algorithm. Tradeoff between accuracy and measurement periods must be considered for SINR

Conclusions

An implicit channel reservation was proposed for soft handoff calls in CDMA networks. The effectiveness of this scheme was based on the features of soft handoff. An analytical model was also derived for the new channel reservation scheme. It was validated through computer simulations. The proposed capacity acquisition protocol was shown to outperform other existing channel reservation schemes for CDMA networks. It was also extended into a traffic-load adaptive channel reservation scheme. How to

Xudong Wang received his B.E. and Ph.D. degrees from Shanghai Jiao Tong University, Shanghai, China, in 1992 and 1997, respectively. From 1998 to 2003, he was with the Broadband and Wireless Networking (BWN) Lab at Georgia Institute of Technology. He also received the Ph.D. degree from Georgia Institute of Technology in 2003. Currently, he is a senior researcher with Kiyon, Inc., where he leads a research and development team working on MAC, routing, and transport protocols for wireless mesh

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Xudong Wang received his B.E. and Ph.D. degrees from Shanghai Jiao Tong University, Shanghai, China, in 1992 and 1997, respectively. From 1998 to 2003, he was with the Broadband and Wireless Networking (BWN) Lab at Georgia Institute of Technology. He also received the Ph.D. degree from Georgia Institute of Technology in 2003. Currently, he is a senior researcher with Kiyon, Inc., where he leads a research and development team working on MAC, routing, and transport protocols for wireless mesh networks. His research interests also include software radios, cross-layer design, and communication protocols for cellular, mobile ad hoc, sensor, and ultra-wideband networks.

He is a guest editor for the special issue on wireless mesh networks in IEEE Wireless Communications Magazines. He has been a technical committee member of many international conferences, and a technical reviewer for numerous international journals and conferences. He has two patents pending in wireless mesh networks. He is a member of IEEE, ACM, and ACM SIGMOBILE.

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