Non-blocking OVSF codes and enhancing network capacity for 3G wireless and beyond systems

Abstract

Orthogonal variable spreading factor (OVSF) codes are employed as channelization codes in wideband CDMA. Any two OVSF codes are orthogonal if and only if one of them is not a parent code of the other. Therefore, when an OVSF code is assigned, it blocks all of its ancestor and descendant codes from assignment because they are not orthogonal to each other. Unfortunately, this code-blocking problem of OVSF codes can cause a substantial spectral efficiency loss of up to 25%. This paper presents non-blocking OVSF (NOVSF) codes to increase substantially the utilization of channelization codes without having the overhead of code reassignments. In addition, an encoding algorithm is presented to increase network capacity and support higher data rates when NOVSF codes are employed.

Introduction

The third generation (3G) wireless standards UMTS/IMT-2000 use the wideband CDMA (WCDMA) to support high data rate and variable bit rate services with different quality of service (QoS) requirements. In WCDMA, all users share the same carrier under the direct sequence CDMA (DS-CDMA) principle [1]. In the 3GPP specifications [2], orthogonal variable spreading factor (OVSF) codes [3] are used as channelization codes for data spreading on both downlink and uplink. OVSF codes also determine the data rates allocated to calls. Because OVSF codes require a single RAKE combiner at the receiver, they are preferable to multiples of orthogonal constant spreading factor (SF) codes which need multiple RAKE combiners at the receiver.

When a particular code is used in OVSF, its descendant and ancestor codes cannot be used simultaneously because their encoded sequences become indistinguishable. Therefore, the OVSF code tree has a limited number of available codes. Because one OVSF code tree, along with one scrambling code, is used for transmissions from a single source that may be a base station or mobile station, the same OVSF code tree is used for the downlink transmissions and therefore the base station must carefully assign the OVSF codes to the downlink transmissions. The asynchronous uplink transmissions do not suffer from this limitation since each mobile station as a single source uses a unique scrambling code with the spreading codes of its OVSF code tree, where scrambling code makes signals from different mobile stations separable from each other. But, if the uplink is synchronous, the OVSF code limitations of the downlink are also valid for the uplink. The use of OVSF codes in downlink and synchronous uplink guarantees that there is no intra-cell interference in a flat fading channel. Since the maximum number of OVSF codes is hard-limited, the efficient assignment of OVSF codes has a significant impact on resource utilization.

Any two OVSF codes are orthogonal if and only if one of them is not a parent code of the other. Therefore, when an OVSF code is assigned, it blocks all of its ancestor and descendant codes from assignment because they are not orthogonal. This results in a major drawback of OVSF codes, called blocking property [4]: a new call cannot be supported because there is no available free code with the requested SF, even if the network has excess capacity to support it. To alleviate the effects of the blocking property of OVSF codes, various schemes such as code reassignment schemes [4], [5], [6], [7], time sharing of channels, statistical multiplexing of bursty data traffic [8] are proposed in the literature.

This paper presents three types of non-blocking OVSF (NOVSF) codes, the preliminary version of which appeared in Ref. [9]. NOVSF codes are non-blocking in the sense that no code assignment blocks the assignment of any other code. All NOVSF codes are orthogonal to each other and, therefore, can be assigned simultaneously as far as orthogonality is concerned. Three different techniques are discussed to obtain the proposed three types of NOVSF codes. The first technique proposes eight OVSF codes with SF 8 that are shared in time. The second technique is involved with the rearrangement of OVSF code trees as follows. Initially, there are X orthogonal codes with the SF of X, where X is either four or eight. Each of these X orthogonal codes first generates Y orthogonal codes with the SF Y and then the generated Y codes are placed on a distinct layer of NOVSF code tree. If the SF ranges from 4 to 32, this type of code tree may be a very desirable for broadband fixed wireless networks, where highest SF is not expected to exceed 32. The third technique introduces a very structured way of generating NOVSF codes starting with SF of 4, if there is no upper bound for SF.

3G systems including WCDMA are designed to be service-independent in order to accommodate a flexible introduction of new services. The capacity and QoS requirements increase as more support is needed to support end-user services such as multimedia and high-speed packet data services. However, the total aggregate data rate in downlink of 3G systems is still few Mbps. Therefore, this paper proposes an encoding technique that first determines the patterns in the input data. If some patterns are repeated much more frequently than the others, then each of the most frequently used patterns is mapped to more than one time slot in the first type of NOVSF codes, so that a chip sequence can represent more than 1 bit. When this mapping is done properly, the network capacity can be increased.

The remainder of this paper is organized as follows. In Section 2, the tree-structured generation and blocking properties of OVSF codes are described. Section 3 presents the proposed NOVSF codes. In Section 4, an encoding technique for binary patterns in input data is introduced to enhance network capacity when the first type of NOVSF codes is used. Concluding remarks are made in Section 5.