Code reservation schemes at the forward link in WCDMA

Abstract

We examine resource reservation schemes for the management of orthogonal variable spreading factor (OVSF) codes at the forward link of 3G mobile communications systems employing WCDMA. Like in every multi-service network, calls with different rate requirements will perceive very dissimilar system performance at the forward link in 3G systems if no measures are taken and the channelization code tree is treated as a common pool of resources. Assuming that the traffic level for each class is known in advance, we introduce complete sharing (CS), complete partitioning (CP) and hybrid partitioning (HP) policies to manage the code tree. At the resource reservation level, we develop an efficient method to partition the available codes based on the offered traffic load of each class of calls and the size of the tree. The resulting partition is optimal in the sense that the maximum blocking probability of the different rate classes is minimized. At the call level, we use a real-time scheme to assign free codes to incoming requests, and evaluate its performance in terms of blocking probability per traffic class and utilization of codes in conjunction with the partitioning method used. It turns out that code blocking, which is encountered on this type of systems, further deteriorates the unfairness conditions at the forward link. Our simulation results show that fair access to codes by different rate calls is assured more by CP and less by HP schemes, at the expense of slightly lower code utilization at medium to high loads, compared to the CS scheme. Also, hybrid schemes absorb small traffic deviations more efficiently than CP, which is generally optimized for certain traffic mixes.

Introduction

High data rate and variable data rate services ranging from 144 to 384 kbps for wide and up to 2 Mbps for low coverage and mobility are anticipated in 3G mobile communication systems. In WCDMA, the separation of simultaneous data transmissions from a user or base station, as well as the separation of the various data transmissions of a single user is done in a two-step process, spreading and scrambling. During spreading, each individual data stream is spread to the chip rate by the application of a spreading code or else a channelization code, and after that, the spread signal is scrambled by the application of a scrambling code operating at the chip rate. In the inverse link, each mobile station has the whole set of channelization codes available to separate its own transmissions. On the other hand, in the forward link, the whole set of channelization codes are used to separate data transmissions from all users of one cell.

WCDMA supports multiple rate transmission using single orthogonal channelization codes with variable spreading factors (SFs), the so-called orthogonal variable spreading factor (OVSF) codes. Single OVSF code operation requires less hardware complexity at the user equipment, compared to multi-code operation, and can provide only data rates, which are a power of two times the lowest basic rate. Obviously, OVSF codes at the forward link are valuable and scarce resources, and this is the reason why the OVSF code assignment problem has been receiving great attention recently.

The problem is similar to that faced by designers of multi-rate networks, who need to compromise the requirement of supporting various multi-rate services with two mutually conflicting goals: (a) provide a reasonable level of performance without letting the demand from some type of service excluding the other types, and at the same time (b) keep the utilization of resources as high as possible. This situation is studied extensively in the area of ATM networks, fixed or wireless (see for example Ref. [1] and the references therein). Alternative methodologies in the area of ATM networks can be found in Ref. [2].

To cope with this problem, priority based schemes for different service classes are presented in Ref. [3], while a new protocol that dynamically assigns OVSF codes on a timeslot basis to provide for data rate guarantee for bursty traffic is presented in Ref. [4]. According to Ref. [5], a target position is defined for each traffic class and codes close to the target position are dynamically allocated to calls of the same class. The tuning of target positions is not considered, however, and this problem is left open. Another work in this area [6], where three different rate classes (voice, video and data) are considered, proposes a leftmost code assignment procedure for voice terminals, combined with reassignments at the release of a connection to make room for video traffic requests. In Ref. [7], the situation of code blocking is introduced, where an incoming call may be blocked even if there is spare capacity to handle the new call. Code blocking may appear only on incoming calls with rate greater than the lower code rate available and is due to the scattering of lower layer free codes along the tree. This phenomenon further deteriorates the performance perceived by higher rate calls.

In this paper, we introduce resource partitioning schemes to provide for fair access of different rate calls to the available limited set of OVSF codes at the forward link of WCDMA systems. The division of codes is made according to some prior knowledge of network operators about the traffic level of different classes. The procedure is optimal in the sense that the maximum blocking probability of the different rate calls is minimized and efficient in the sense that computation of the size of different partitions is relatively fast. The latter is due to the special structure and properties of the channelization code tree.

At the two extremes, we have complete sharing (CS) strategy, in which the whole code tree is considered as a common pool of resources, and complete partitioning (CP) strategy, where each traffic rate class is allocated a separate non-overlapping partition of the code tree for use. Between the two extremes, hybrid partitioning (HP) strategies are developed and the performance of resource reservation strategies are studied in terms of blocking per class and code utilization. Simulation results show that: (a) fair access to codes is assured more by CP and less by HPs, especially at medium to high loads, at the expense of slightly lower code utilization compared to CS; (b) when reassignments are used, the same behavior is shifted towards higher loads; and (c) HPs are more immune against traffic variations than CP, which is usually optimized for certain traffic mixes.

The paper is organized as follows. The system model is presented in Section 2. The long term resource reservation policies, as well as the method to compute the size of each partition are presented in Section 3. The real-time assignment operation is developed in Section 4. Finally, Section 5 presents and discusses the simulation results, and the paper is concluded in Section 6.