Third-generation Code Division Multiple Access Research Articles

Quality-Based OVSF Code Assignment and Reassignment Strategies for WCDMA Systems

In third-generation code division multiple access (3G CDMA) mobile cellular systems, orthogonal variable spreading factor (OVSF) codes are applied in downlink to maintain the orthogonality between different channels. Since the number of available codes is limited, efficiently utilizing OVSF codes becomes an important issue. The code assignment and reassignment problem focuses on the subject of minimizing the number of code relocations. Previous works were based on some assumptions, including fixed service data rates and a code-limited system capacity. In this paper, we investigate this issue from a different perspective, by applying variable service data rates and an interference-limited system capacity. We have proposed three OVSF code assignment and reassignment strategies for this scenario, including sparse-first, sparse-first/rightmost, and modified sparse-first/rightmost, with a view to achieving the best service quality. The proposed strategies are well evaluated and compared with other strategies, based on the number of code reassignments, service quality, and throughput, via simulation. It is found that our proposed strategies outperform others under the scenario with variable service data rates and an interference-limited system capacity.

On the Performance of Open-Loop Transmit Diversity Techniques for IS-2000 Systems: A Comparative Study

Third-generation code-division multiple access (CDMA) cellular systems incorporate a downlink transmission technique called transmit diversity (TD). This paper provides a comprehensive investigation of the performance and practical implementation issues of open-loop transmit diversity schemes for the IS-2000 third-generation cellular CDMA standard. Discussed in detail are orthogonal transmit diversity (OTD) and space-time spreading (STS) diversity schemes. STS is a TD technique that is motivated by space-time coding principles originally described for narrowband systems. OTD is a TD technique that obtains diversity not at the symbol level, but in the decoding process, and has performance that is in general lower bounded by STS, which obtains diversity combining prior to decoding. Thus, STS always outperforms OTD, with the improvement particularly significant in the presence of weak convolutional codes. Probability of error analysis is performed for STS under the assumptions of imperfect channel estimates, correlation between antennas, and unequal pilot power allocations. Extensions to STS are provided for the multicarrier version of the standard and four transmit antennas. Simulation studies are performed to detail the performance of both open-loop TD schemes with convolutional coding and closed-loop power control consistent with the 3GPP2/IS-2000 standard. Many of these results were generated in the course of the IS-2000 standardization procedure. Performance is studied in radio environments which experience flat Rayleigh fading, frequency selective Rayleigh fading, spatially selective fading, as well as Ricean fading with various K factors. Some additional results are presented for cases where mobile receivers have two receive antennas. Implementation issues are also considered including the impact of antenna delay differences on performance, transmitter and receiver architectures, and computational complexity.

Locally optimal soft handoff algorithms

The design of soft handoff algorithms for cellular radio systems is considered. The design problem is posed as a tradeoff between three metrics: the rate of handoffs, the mean size of the active set, and the link quality. It is argued that the algorithm that optimizes the tradeoff among these metrics is impractical. Hence, a locally optimal (LO) handoff algorithm is derived as a practical approximation to the optimal handoff algorithm. The LO algorithm is shown to yield a significantly better tradeoff than the static threshold handoff algorithm used in second-generation code-division multiple-access (CDMA) systems. It is also shown that the dynamic threshold algorithm, which is an ad hoc algorithm proposed for third-generation CDMA systems, achieves nearly the same performance as the LO algorithm. Thus, an analytical justification is developed for the dynamic threshold algorithm. Further, the handoff algorithm design is separated into independent design problems on the forward and reverse links. The forward link LO algorithm is shown to be computationally intensive but is also shown to be closely approximated by the simpler reverse link LO algorithm.

File transmission over wireless fast fading downlink

The third-generation code division multiple access (3G CDMA) mobile communication networks are designed to transmit voice and data over wireless channels. Fundamental problems need to be solved for such mobile communication networks to operate efficiently. One of the problems is finding optimal scheduling algorithms for file transmission over the fading downlink channel from a base station to mobile users. Here, the problem is solved in the case of transmission of a given number of files. Optimal algorithms that minimize the delay and the transmission time are found. Also, specific nonoptimal algorithms are considered and compared numerically with the optimal ones. We consider a slotted downlink with channels to the mobile users that are dependent in the same slot but independent for different slots. Additionally, some generalizations are presented.