We propose an efficient code-assignment scheme utilizing multiple rake combiners to avoid the code-blocking problem in third-generation (3G) and beyond wideband code-division multiple-access (WCDMA) systems based upon orthogonal variable spreading factor (OVSF) channelization codes. The multiple rake combiners are required to be equipped with each base station (BS) and user equipment (UE). The major benefit of the proposed assignment scheme is in handling nonquantized data rates, making internal code fragmentation approximately zero. The task of code assignment to handle quantized data rates is divided into three steps. In the first step, the number of terms satisfying the capacity equation is found. The code combinations for all the terms in step 1 are found in step 2, which can be further subdivided into terms producing unique combinations and multiple combinations of terms. The procedure for finding the optimal code combination according to the number of rake combiners available at the UE and BS is given in step 3. Simulation results show the performance improvement in terms of reduction in blocking probability compared with existing single-code assignment schemes.

Multiple Rake Combiners and Performance Improvement in 3G and Beyond WCDMA Systems

This paper investigates computationally efficient suboptimal dynamic code assignment (DCA) schemes with call admission control (CAC) for orthogonal variable spreading factor code-division multiple-access systems. We examine two different approaches. The first approach reduces the complexity of the DCA scheme by partitioning the total resource (either capacity or service class) into several mutually exclusive subsets and assigns each subset of resource to a group of users in proportion to the corresponding traffic load. The second approach reduces the complexity of the optimal CAC scheme with the Markov decision process over an infinite time horizon by a suboptimal CAC policy, which is designed by observing the behavior of system dynamics over only two consecutive stages upon the arrival of a call. It is demonstrated by numerical evaluation that the proposed schemes achieve an average data throughput close to that of the optimal DCA-CAC performance while demanding a much lower computational complexity in their design and implementation.

/pdf/computationally-efficient-dynamic-code-assignment-schemes-34gvq86e7e.pdf

Computationally Efficient Dynamic Code Assignment Schemes With Call Admission Control (DCA-CAC) for OVSF-CDMA Systems

The tradeoff between the throughput and the blocking probability of various code assignment schemes for OVSF-CDMA (orthogonal variable spreading factor CDMA) systems is investigated, and the optimal fixed and dynamic code assignment schemes that maximize the average data throughput under a blocking probability constraint are proposed. The optimal schemes are obtained using constrained optimization through the Markov decision process and linear programming. The performance of each scheme is numerically evaluated. Numerical results demonstrate that the proposed new schemes can achieve a satisfactory balance between the average throughput and the blocking probability.

/pdf/optimal-code-assignment-and-call-admission-control-for-ovsf-3jzuo6xy9k.pdf

Optimal code assignment and call admission control for OVSF-CDMA systems constrained by blocking probabilities

Orthogonal variable spreading factor-based code division multiple access systems allocate vacant codes when new call(s) arrive. This study proposes a same rate assignment scheme which allocates a new call to most favourable code in the region crowded by same rate calls. This does not add to blocked codes because of previous calls but utilise the unused scattered capacity created by previous calls. A vacant code whose parent code is handling maximum number of ongoing calls of same rate is the most favourable code for new call assignment under that sub tree. This reduces higher rate blocking because of scattering of lower rate calls due to the fact that same rate calls will be assigned in the same portion of code tree. If a tie occurs for two parent codes it is resolved using pattern recognition of codes. In the proposed scheme, the most favourable search starts from higher layers that reduce number of code searches before assignment significantly. The proposed single code and multi code schemes are compared with other schemes based on code blocking probability, throughput and number of code searches required before assignment. Simulation results indicate that the proposed scheme outperforms other schemes for various traffic distributions.

Same rate and pattern recognition search of orthogonal variable spreading factor code tree for wideband code division multiple access networks

In this paper, call admission and code allocation schemes are proposed to provide service differentiation in the forward link of wideband code-division multiple-access (WCDMA) systems. In particular, this paper proposes multiple leaf code reservation (MLCR) schemes, where different numbers of orthogonal variable spreading factor (OVSF) leaf codes (i.e., codes of the lowest layer of the OVSF code tree) are reserved to differentiate users with different bandwidth requirements. Leaf codes are only reserved for as long as the call admission process lasts. Once the decision of whether a new request is admitted or not has been made, a Code Dereservation procedure is carried out to increase flexibility in the code assignment phase. The performance of these MLCR strategies with/without code reassignments is then evaluated. Analysis shows that MLCR schemes are also useful in improving fair access among different traffic classes. In addition, perfect fair access among requests with different data rates can be achieved when code reassignments are jointly employed with the proposed OVSF-code reservation schemes.

Call admission and code allocation strategies for WCDMA systems with multirate traffic

Enhanced Fixed and Dynamic Code Assignment Policies for OVSF-CDMA Systems.

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Enhanced Fixed and Dynamic Code Assignment Policies for OVSF-CDMA Systems.