Variable Spreading Factor-orthogonal Frequency And Code Division Multiplexing Research Articles

Adaptive Load Balancing MDP-Based Approach of Two-Dimensional Spreading for VSF-OFCDM in 4G Next-Generation Cellular Communications

The variable spreading factor-orthogonal frequency and code division multiplexing (VSF-OFCDM) system has been proposed as the forward link interface for achieving a high data rate in fourth-generation (4G) mobile communications. VSF-OFCDM allocates the orthogonal channelization codes of an orthogonal variable spreading factor (OVSF) code tree in 2-D spreading in the time and frequency domains but suffers from two disadvantages, namely, moderate utilization and low transmission quality. Moderate utilization is caused by code blocking, and the low transmission quality is due to the multicode interference from high channel loading in the time domain of the 2-D spreading. A tradeoff thus exists between code blocking and multicode interference. A lower code blocking means a higher interference. Achieving high utilization while also providing high transmission quality is a critical issue that should be addressed in 4G VSF-OFCDM. Therefore, this paper proposes a 2-D spreading approach that is based on an adaptive load balancing with Markov decision process (which is denoted by ALM). The ALM approach consists of three phases. First, for balancing the channel load, the adaptive 2-D spreading phase is proposed to select the 2-D spreading combinations, which have low channel load and high frequency diversity, as the candidates. Second, a dynamic recombination of the 2-D spreading is presented to decrease the channel load while supporting the transmission quality from high frequency diversity if the channel load of a time-domain code exceeds the defined channel load threshold. Third, the cost-based Markov decision process (MDP) code-selection approach is adopted to minimize the code blocking. The MDP models the state of the OVSF code tree as an MDP and then defines the costs for all the candidate channelization codes. The MDP phase selects the least-cost channelization code as the optimal solution if several 2-D spreading combinations can satisfy the channel load limitation. Additionally, the time complexity of the ALM is analyzed. The numerical results indicate that the proposed approach outperforms the other approaches in transmission quality ratio, fractional reward loss, and total number of reassignments.

Performance Analysis of Orthogonal Frequency and Code Hopping Multiplexing

Beyond-3G wireless communication systems should support packet traffic with both low/medium rate and high rate services. Scheduling-based resource allocation schemes may cause a problem in accommodating low/medium rate services due to a rate mismatch and a large signaling overhead. Dedicated resource allocation/multiplexing schemes are also inadequate for accommodating packet-type traffic that is characterized by low channel activity and high burstiness. Therefore, we propose a new orthogonal frequency and code hopping multiplexing (OFCHM) scheme for downlink multiplexing to provide efficient support of packet traffic with low/medium data rates in a spread OFDM environment. The performance of the proposed scheme is evaluated in terms of the statistical multiplexing gain, the block error rate, and the overall capacity by using both an analytical approach and simulation. The proposed scheme yields a larger overall downlink capacity than a dedicated resource allocation/multiplexing scheme. A comparison study with the variable spreading factor-orthogonal frequency and code division multiplexing (VSF-OFCDM) system through system-level simulations is also performed. The comparison results show that the OFCHM system yields better performance than the VSF-OFCDM system in packet delay, packet delay variance, and fairness, and has a system throughput advantage with low/medium rate services.

Performance comparison of optimum and MMSE receivers with imperfect channel estimation for VSF-OFCDM systems

Performances of optimum and minimum mean square error (MMSE) receivers for variable spreading factor orthogonal frequency and code division multiplexing (VSF-OFCDM) systems are compared in this paper. In VSF-OFCDM systems, the existence of multicode interference (MCI) in the frequency domain due to frequency-selective fading channels dramatically degrades the system performance. A quasi-analytic bit error rate performance is presented in the presence of imperfect channel estimations. Numerical results show that with linear computation complexity the MMSE receiver can improve system performance significantly by suppressing the MCI, although it cannot perform as well as the optimum receiver. Thus, with a small number of code channels, an optimum receiver can be employed to achieve better performance, whereas the MMSE receiver is suitable for a system with a large number of code channels due to simple complexity. In addition, the MMSE receiver is more robust than the optimum receiver to the different configurations of system parameters, e.g., spreading factors. Moreover, it is found that pilot channel power should be carefully assigned by making tradeoffs between the channel estimation quality and the received SNR for each code data channel.

Field Experiments on Throughput Performance above 100 Mbps in Forward Link for VSF-OFCDM Broadband Wireless Access

This paper presents throughput performance along with power profiles in the time and frequency domains over 100 Mbps based on field experiments using the implemented Variable Spreading Factor-Orthogonal Frequency and Code Division Multiplexing (VSF-OFCDM) transceiver with a 100-MHz bandwidth in a real multipath fading channel. We conducted field experiments in which a base station (BS) employs a 120-degree sectored beam'antenna with the antenna height of 50 m and a van equipped with a mobile station (MS) is driven at the average speed of 30 km/h along measurement courses that are approximately 800 to 1000 m away from the BS, where most of the locations along the courses are under non-line-of-sight conditions. Field experimental results show that, by applying 16QAM data modulation and Turbo coding with the coding rate of R = 1/2 to a shared data channel together with two-branch antenna diversity reception, throughput over 100 and 200 Mbps is achieved when the average received signal-to-interference plus noise power ratio (SINR) is approximately 6.0 and 14.0 dB, respectively in a broadband channel bandwidth where a large number of paths such as more than 20 are observed. Furthermore, the location probability for achieving throughput over 100 and 200 Mbps becomes approximately 90 and 20% in these measurement courses, which experience a large number of paths, when the transmission power of the BS is 10 W with a 120-degree sectored beam transmission.

PROSPECTS FOR MOBILE COMMUNICATION SYSTEMS AND KEY TECHNOLOGIES CAPABLE OF SUPPORTING EXPANDING MOBILE MULTIMEDIA SERVICES

The first commercial service of the 3rd generation (3G) mobile communication system, IMT-2000 (International Mobile Telecommunications), was launched in October 2001 in Japan. This is the first 3G service employing Wideband Code Division Multiple Access (W-CDMA) as its air interface between mobile terminals and base stations. This new system, 3G, is expected to accelerate the deployment of future mobile multimedia services, which substantially got started with the "i-mode" service in February 1999 in Japan. Research activities into future mobile communication systems capable of supporting a vastly expanded market for mobile multimedia services are underway worldwide. This paper describes the vision, service trends, and technical challenges of such future systems. Broadband packet wireless access, Variable Spreading Factor Orthogonal Frequency and Code Division Multiplexing (VSF-OFCDM), are promising candidates for realizing future mobile communication systems that provide higher transmission rates and capacity than 3G systems.