An Optimal Admission Control Policy for CDMA Multiple Antenna Systems with QoS Constraints Wei Sheng and Steven D. Blostein Department of Electrical and Computer Engineering Queen’s University, Kingston, Ontario, Canada Emails: wsheng@ee.queensu.ca, Steven.Blostein@queensu.ca Abstract— An optimal admission control (AC) policy is pro- posed for a CDMA multiple antenna system, which can maxi- mize the system throughput while simultaneously guaranteeing all quality-of-service (QoS) requirements in both physical and network layers. While previous research can only ensure the QoS requirements for appropriately chosen automatic-retransmission- request (ARQ) parameters, the proposed AC policy aims to ensure the QoS requirements for arbitrary ARQ parameters, which saves system resources. Numerical examples show that the proposed AC policy can achieve a significant performance gain in terms of network layer performance as well as system throughput. I. INTRODUCTION In current third generation (3G) systems such as high speed uplink packet access (HSUPA), a threshold-based admission control (AC) policy is employed, which admits a user request if the load reported is below the AC threshold. Although the AC decision can be improved upon by taking advantage of resource allocation information [1], and it is simple to implement, this threshold-based AC policy cannot guarantee long-term QoS requirements in the network layer [2], such as blocking probability. Recently there are several admission control (AC) policies proposed in the literature, which can guarantee QoS constraints in both physical and network layers [2]- [3]. To employ the powerful error-control capability, a suboptimal packet admission control (PAC) problem is studied in [4], which includes the impact of an automatic-retransmission-request (ARQ) scheme. To the best of our knowledge, [4] is the first paper in the literature which addresses the cross-layer AC design by including ARQ schemes. However, the solution in [4] depends on an approximate power control feasibility condition (PCFC) and as a result is only a suboptimal solution. Furthermore, in [4], ARQ parameters, such as the number of allowed retransmissions and target packet-error-rate (PER), should be chosen appropriately to meet the QoS requirements, which adds to system complexity. To overcome this problem, an optimal AC policy for arbitrary ARQ parameters is desired. In our companion paper [7], the optimal AC problem is studied for multiple antenna systems, in which an outage probability constraint as well as all the other QoS constraints are incorporated into the semi-Markov decision process. For simplicity, however, error-control schemes are ignored in [7]. In this paper, by considering the impact of ARQ schemes, we further investigate optimal and suboptimal AC policies with QoS constraints. The proposed AC policies can guarantee the QoS for arbitrary ARQ parameters and maximize the overall system throughput. A truncated ARQ scheme is employed in this paper, which retransmits an error packet until correctly received or a maximum number of retransmissions is reached. We remark that the proposed work in this paper differs from the existing algorithms in [4] in the following aspects: a) a separate reduced-outage-probability (ROP) algorithm is neces- sary in [4], while the proposed AC policies in this paper do not require any ROP algorithm, saving system resources; b) In [4], ARQ is employed to reduce outage, while in our proposed AC policy, with a guaranteed outage probability constraint, ARQ is employed to increase user capacity, which results in improved network layer performance; c) In [4], the AC policy is derived to optimize network layer performance only, while in this paper we aim to optimize average throughput which represents the overall system performance across different layers. The proposed AC policy can be derived offline and then stored in a lookup table. Whenever an arrival or departure occurs, an optimal action can be obtained by table lookup, resulting in low enough complexity for admission control at the packet level. Similar to call/connection level admis- sion control, in a packet-switched system a packet admis- sion control policy decides if an incoming packet can be accepted or blocked in order to meet quality-of-service (QoS) requirements. In a packet-switched network, blocking a packet instead of blocking the whole user connection can be more spectrally efficient. In this paper, we consider the packet level AC problem. The rest of this paper is organized as follows. The signal model and problem formulation are presented in Section II. Section III investigates the physical layer performance and provides an analytical expression for outage probability. Optimal CAC policies are proposed in Section IV. Numerical results are presented in Section V. II. SIGNAL MODEL A. Signal model in the physical layer A single-cell uplink CDMA beamforming system is consid- ered which has M antennas at the BS. A spatial matched filter receiver is employed, and the system can support J classes of packets. Different classes of packets are characterized by different QoS requirements. Let K denote the total number of packets in the system. The received signal-to-interference ratio (SIR) for a desired 1525-3511/08/$25.00 ©2008 IEEE This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the WCNC 2008 proceedings. 1826 Authorized licensed use limited to: Queens University. Downloaded on May 2, 2009 at 19:47 from IEEE Xplore. Restrictions apply.