Code Division Multiplexing System Research Articles

Joint Transmitter–Receiver Frequency-Domain Equalization in Generalized Multicarrier Code-Division Multiplexing Systems

Joint transmitter-receiver optimization in generalized multicarrier code-division multiplexing (GMC-CDM) systems is investigated in this paper. The optimization consists of a one-tap post-frequency-domain equalizer (post-FDE) and a one-tap pre-FDE. While the one-tap post-FDE is optimized under the criterion of minimum mean square error (MMSE), the one-tap pre-FDE is achieved through three stages of optimization, which are operated at different levels and motivated to achieve, possibly, different objectives, including maximum throughput and maximum reliability. Specifically, in our three-stage pre-FDE, the first-stage pre-FDE is operated at the symbol level, concerning only the symbols within a group. The second-stage pre-FDE is carried out at the group level for harmonization among the groups. Finally, the third-stage pre-FDE handles group partition. In this paper, the error and throughput performance of the GMC-CDM systems is investigated when assuming communications over frequency-selective Rayleigh fading channels. It can be shown that the reliability or throughput of the GMC-CDM systems can be significantly improved by employment of the proposed pre- and post-FDE schemes. Furthermore, the pre- and post-FDE algorithms obtained can be implemented with high flexibility, which facilitates a GMC-CDM system to achieve a good tradeoff between its throughput and reliability.

Iterative method of joint demodulation and filtering of communications channel parameters in digital information transmission systems

The iterative method, which can be applied to estimation of the unknown vector of discrete parameters in combination with filtering of the unknown vector of continuous signal parameters, is proposed. It is demonstrated that this method makes it possible to construct highly efficient algorithms for joint demodulation and filtering of communications channel parameters in code-division multiple-access and orthogonal frequency-division multiplexing systems.

Performance of MC-CDM Systems With MMSEC Over Rayleigh Fading Channels

In this paper, we investigate the performance of multicarrier code-division multiplexing (MC-CDM) systems with minimum mean square error combining (MMSEC) over a Rayleigh fading channel. An approximated expression on the symbol error rate (SER) for MMSEC is derived. The approximated expression provides an accurate approximation of the true SER, particularly for systems with higher modulation and a small spreading factor. Through the derived error performances, we also calculate the achievable diversity order. MC-CDM systems with MMSEC achieve the same asymptotical diversity order of 1, regardless of the spreading factor. Within practical signal-to-noise ratio (SNR) ranges, the effective diversity order is further increased as a larger spreading factor is used. The derived analytical expressions are verified by numerical and simulation results.

Coded overlapped code division multiplexing system with Turbo product structure

Coded overlapped code division multiplexing system with Turbo product structure (TPC-OVCDM) is introduced, and trellis coded modulation (TCM) code is employed as error correcting code for uncoded overlapped code division multiplexing (OVCDM) system. In such a scheme, row code and column code are TCM and OVCDM spreading code, respectively. Data bits are only encoded directly by TCM and transformed into a matrix. Each column of this matrix is then permuted by symbol interleaver before being encoded by OVCDM spreading code. During iterative decoding process in the receiver, two constituent decoders use symbol by symbol BCJR algorithm in the log domain. The order of decoding two sub-codes is determined by the encoding order. The proportion of TCM coding and OVCDM coding affects system performance. For fixed coding structure and symbol interleaver, the performance of TPC-OVCDM systems of different proportions of additive white Gaussian noise (AWGN) channel have been simulated. The results show that TPC-OVCDM system of reasonable proportion can achieve significant coding gain, compared with uncoded OVCDM system under the condition of same spectral efficiency at bit error rate (BER) level of 10 −5

Hybrid detectors for wireless orthogonal frequency division multiplexing with code division multiplexing systems based on reliability information

Orthogonal frequency division multiplexing with code division multiplexing (OFDM-CDM) is attractive for the next generation high-speed wireless systems due to the fact that the performance of OFDM-CDM systems can be considerably improved by employing a joint detection scheme such as the maximum likelihood (ML) detector. However, the complexity of the ML detector increases rapidly as the number of orthogonal spreading codes and/or the number of bits per modulation symbol increase. In this study, the authors introduce a unified detection model and propose two hybrid detectors, which combine zero forcing (ZF) with successive interference cancellation (SIC) and sphere detection (SD) algorithms, respectively. After obtaining the initial solution from the front-end ZF receiver, the proposed back-end algorithms are adopted to extend the potential solution list and search for the final result. The objective is to utilise the combination of a simplified linear equaliser and a comprehensive detection scheme to achieve enhanced performance and offer alternatives to the more complex and channel-estimation-sensitive minimum mean squared error (MMSE) scheme. The results show that the proposed hybrid detectors are able to achieve superior performance compared to the MMSE scheme and provides a significant performance improvement compared to the conventional OFDM counterpart.

Hybrid WDMA/OCDM system with the capability of encoding multiple wavelength channels by employing one encoder and one corresponding optical code

A hybrid wavelength division multiple access (WDMA)/optical code division multiplexing (OCDM) system is proposed, where the optical code is not the same as the address of every optical network unit (ONU); rather, the code is a virtual fiber of hybrid passive optical network (PON). To our knowledge, this is the first report analyzing a single encoder/decoder with a single corresponding optical code being exploited to encode/decode multiple wavelength signals simultaneously. This system enables OCDM to become transparent to ONU so that the existing wavelength division multiplexing (WDM) PON can be upgraded. Thus, redesigning the optical line terminal and ONU can be easily accomplished, and greatly decreasing the number of encoder/decoder becomes possible. In experiment, we only employ two encoder/decoder pairs to combine two WDM-PONs in one fiber. Simulation results confirm the feasibility of the proposed system.

Performance Analysis of Spectral Amplitude Encoding OCDM Systems Over a Linear Dispersive Optical Channel

We propose a new model of pulse propagation over a linear dispersive optical channel and use this model to comprehensively analyze the performance of spectral amplitude encoding optical code-division multiplexing (SAE/OCDM) systems under the impact of group velocity dispersion (GVD). We quantitatively discuss the impact of GVD on various aspects of SAE/OCDM system performance, including the number of supportable users and the maximum transmission length and power penalty. We also investigate the system performance with several types of optical fibers, such as single-mode fiber (SMF), dispersion-shifted fiber (DSF), and nonzero DSF. In addition, we discuss techniques to compensate for GVD so that the performance of SAE/OCDM systems can be improved.

Variable power and rate allocation using simple CQI for multiuser OFDMA-CDM systems

This paper presents variable power and rate allocation (PRA) for a multiuser orthogonal frequency division multiple access code division multiplexing (OFDMA-CDM) system which exploits both orthogonality among users and symbol-level diversity. To predict the link performance, we analyze the postprocessing SNR as the transmit power (TP) varies when the minimum mean square error combiner (MMSEC) is employed. It is shown from analysis that the MMSEC SNR is nonlinear in TP, but is bounded by the harmonic and the arithmetic means of the subcarrier SNRs. From the observation, we propose two variable PRA methods using a simple channel quality indication and a required TP estimation method. Simulation results show that the proposed methods not only guarantee the target performance but also increase the throughput at the same or slightly increased feedback overhead compared with the fixed TP allocation case.

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 improvement of hybrid subcarrier multiplexing optical spectrum code division multiplexing system using spectral direct decoding detection technique

This paper proposes a hybrid subcarrier multiplexing/optical spectrum code division multiplexing (SCM/OSCDM) system for the purpose of combining the advantages of both techniques. Optical spectrum code division multiple-access (OSCDMA) is one of the multiplexing techniques that is becoming popular because of the flexibility in the allocation of channels, ability to operate asynchronously, enhanced privacy and increased capacity in bursty nature networks. On the other hand, subcarrier multiplexing (SCM) technique is able to enhance the channel data rate of OSCDMA systems. In this paper, a newly developed detection technique for the OSCDM called spectral direct decoding (SDD) detection technique is compared mathematically with the AND subtraction detection technique. The system utilizes a new unified code construction named KS (Khazani–Syed) code. The results characterizing the bit-error-rate (BER) show that SDD offers a significant improved performance at BER of 10 −9.

A Low Complexity Architecture for OFCDM Downlink Transmitter Using Joint Time-Frequency Spreading and IFFT

In this letter, a low complexity transmitter is proposed for the downlinks of orthogonal frequency code division multiplexing (OFCDM) systems. The principle is based on a joint time-frequency spreading and inverse fast Fourier transform (TFS-IFFT), which combines the frequency spreading with partial stages of IFFT, so as to simplify the real-time processing. Compared with the conventional one, the proposed OFCDM transmitter is of lower real-time computational complexity, especially for those with large spreading factor or low modulation level. Furthermore, the proposed TFS-IFFT can also be applied to other frequency spreading systems, such as MC-CDMA, for complexity reduction.

Performance analysis of MMSE detectors for high speed VSF-OFCDM

This letter investigates the effect of Doppler frequency shift on the performance of MMSE receivers for high speed variable spreading factor (VSF) orthogonal frequency and code division multiplexing (OFCDM) systems in broadband wireless communications. In VSF-OFCDM systems, the existence of multicode interference (MCI) resulting from multiple code channels occupying the same bandwidth degrades the performance. A full MMSE receiver is analytically studied in this letter, exploiting correlation properties among transmitted data on different subcarriers to combat the MCI. Furthermore, considering the fact that the fast time-varying fading destroys the orthogonality of the time domain spreading codes and thus introduces time domain MCI, its effect on system performance is evaluated accurately. It is also shown that it is expected to use time domain spreading factor NT of 8 for a large range of Doppler frequencies, and frequency domain spreading factor as large as possible, especially in high signal to noise ratio. Moreover, in order to improve performance, the pilot channel should occupy around 20% to 25% of the total transmitted power for a large range of Doppler.

MIMO-OFCDM systems with joint iterative detection and optimal power allocation

This paper investigates the orthogonal frequency and code division multiplexing (OFCDM) systems with multiple input multiple output multiplexing (MIMO-OFCDM) and multicode transmission. Combining the iterative detection in the space domain and the hybrid multi-code interference (MDI) cancellation and minimum mean square error (MMSE) detection in the frequency domain, a joint iterative detection is proposed, which enables space and frequency diversity gains to be jointly exploited. Moreover, using a two-dimensional (2-D) averaging channel estimation algorithm, a close form expression is derived for the optimal power allocation between the pilot and all data channels that achieves the best system performance. It is shown that the optimal power ratio mainly depends on the channel estimation algorithm, the number of transmit antennas as well as the number of pilot and data symbols in a packet, but is not sensitive to the changes in signal-to-noise ratio (SNR) and diversity gains. Simulations are conducted to verify the derived optimal power ratio and study the performance of the proposed joint detection algorithm. It is shown that considerable improvement can be obtained when the number of loops in the joint iterative detection increases. Moreover, the system performance is enhanced significantly when the frequency domain spreading factor, N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">F</sub> , increases.

Downlink Transmission of Broadband OFCDM Systems-Part V: Code Assignment

To provide high speed mobile services, orthogonal multi-code transmission is employed in the orthogonal frequency and code division multiplexing (OFCDM) system. However, in a wireless frequency selective channel either with or without Doppler shift, code orthogonality is distorted and multi-code interference (MCI) is caused. The variance of the MCI is dependent on which codes to be used. This paper investigates the code assignment in the OFCDM system, aiming to mitigate the MCI. The code assignment can be explored in terms of the code distance defined in this paper. Generally, the codes with large code distances to each other should be assigned with high priority. Moreover, to avoid large interference from the powerful code-multiplexed pilot channel, the time spreading code adjacent to the pilot code in the code tree (Adachi et al., 1998) is suggested not be used when the time domain spreading factor is large. The performance of the proposed non-sequential code assignment scheme is analytically evaluated. It has been shown that the system performance can be improved significantly by the non-sequential scheme under various channel conditions, especially when a low to medium system load is assumed.

Analysis of Phase Shift Tolerance in a Coherent OCDM System Using FBG Phase En/Decoders

We previously reported, for a coherent optical code division multiplexing (OCDM) system using fiber Bragg grating (FBG) phase en/decoders, that the signals exhibited phase shift tolerance for a difference between the light source wavelength and the Bragg wavelength of an FBG phase en/decoder when the two signals were multiplexed. However, the phase conditions of only the central wavelength among the ones forming a pulse were analyzed. For a more specific consideration, we calculated the phase of each wavelength forming the decoded pulse. In this report, the measured and calculated reflecting properties were compared and the reliability of the method was confirmed. We calculated the phase conditions of the decoded pulses and clarified the phase characteristics with regard to the phase modulation and the interference between pulses overlapped during decoding. For the realization of an asynchronous access, the FBG phase en/decoders should be designed so that the spreading time is the inverse of 2×(transmission rate).

Performance analysis of a threshold-based group-adaptive modulation scheme with adaptive subcarrier allocation in OFCDM systems

This paper proposes an adaptive modulation algorithm for orthogonal frequency and code division multiplexing (OFCDM) system to increase the spectral efficiency without sacrificing the BER performance under different spreading factors. The proposed algorithm is used with an adaptive subcarrier allocation technique which assigns users to subcarriers to minimize the overall BER of the system. A fixed threshold is used to switch between modulation levels depending on the estimated SINR in each group. A spectral efficiency of 3.2 bits per symbol is obtained for a target BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . BCH (511, 385) coding with rate 3/4 used to accommodate a lower target BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> yields a spectral efficiency of 2.8 bits per symbol. The proposed algorithm provides an increase in spectral efficiency than using BPSK only, without increasing the total transmit power.

Receiver simplification in synchronous communication systems: simulation study

This paper presents a simulation study for code-division multiplex system over time varying channels. The scheme investigated here uses a precoding technique in the transmitter depending on the channel information, so that no need for any signal processing in the receiver rather than testing against a certain threshold. This coding technique minimizes the effect of intersymbol interference (ISI). The data itself is transmitted in the form of blocks of data symbols separated by blocks of no signal. The appropriate length of each data block depends on the channel parameters expected and the allowed complexity. The impulse response is required to be known at the transmitter which is the requirement for all systems that employ coding at the transmitter.

OFCDM: a promising broadband wireless access technique

Future mobile communication systems aim to provide extremely high-speed data transmission, especially in the downlink. The broadband orthogonal frequency- and code-division multiplexing system with two-dimensional spreading (time and frequency domain spreading) is becoming a very promising technique for highspeed wireless communications due to its advantages over OFDM. This article presents the basic structure and main functions of the OFCDM system. A nonsequential code assignment scheme is introduced. The novel detection method for the OFCDM, called hybrid multi-code interference cancellation and minimum mean square error detection, is described. The application of advanced techniques to the OFCDM, such as turbo coding and MIMO, is also discussed. It is shown in this article that OFCDM is superior to OFDM.

Performance Analysis of Phase-coding Based on Superstructured Fiber Bragg Grating

The performance of phase encoding and decoding based on phase shift superstructured fiber Bragg grating (SSFBG) in the optical code division multiplexing (OCDM) system was investigated and analyzed. The correlation characteristics for phase shift SSFBG based en/decoder were evaluated by considering input pulse width, grating refractive index modulation amplitude and wavelength mismatch between encoder grating and decoder grating. The impact of fiber dispersion on OCDM transmission over single-mode fiber (SMF) is also estimated. The theoretical results show that the degradation of coding performance is obvious under the impact of increasing of input pulse width, encoder-decoder wavelength mismatch and fiber dispersion, and there is a contradiction between correlation property and insertion loss of SSFBG en/decoder, we need to select moderate refractive index modulation amplitude of SSFBG grating.

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.