Domain Spreading Research Articles

Joint data detection and phase recovery for downlink MC-2D-CDMA systems

The performance of a downlink synchronous MCCDMA system with joint frequency-time domain spreading (MC-2D-CDMA) is investigated in this paper. We propose a two dimensional adaptive minimum mean square error (MMSE) receiver, which works in decision-directed mode after an initial training period. A subcarrier phase tracker, which comprises a bank of phase locked-loops (PLLs), is employed in the receiver to track the fading phase variability. Furthermore, a simplified phase tracker structure is proposed to reduce the system complexity. The performance of the data detector and the behavior of the phase tracker are analyzed theorectically in this paper and are shown to match the simulation results. Both analysis and simulation indicate that the proposed system outperforms the conventional MC-DS-CDMA systems by exploiting frequency diversity and facilitating subcarrier synchronization.

Performance of spatially multiplexed MC-CDM with zero-forcing unified successive interference cancellation detection

Spatially multiplexed multicarrier code-division multiplexing (SM-MC-CDM) is a multiple-input multiple-output, orthogonal frequency division multiplexing (MIMO-OFDM) communication technique with multiple antennas used for spatial multiplexing and with frequency domain spreading on each antenna. Unified successive interference canceller (U-SIC) is an efficient detector recently introduced for SM-MC-CDM. This paper presents an analytical approach to the performance of zero-forcing (ZF) U-SIC for SM-MC-CDM communications. For a system with an equal number of transmit and receive antennas, an approximation for the probability density function of post-detection signal-to-noise ratio (SNR) is used to derive a closed-form analytical upper bound and approximations for the probability of error and ergodic capacity. It is shown that SM-MC-CDM with ZF U-SIC is able to achieve higher diversity order than that achieved by ZF and minimum mean squared error (MMSE) V-BLAST detectors used on each subcarrier of a MIMO-OFDM system with the same number of subcarriers. The diversity order obtained increases with the number of subcarriers. It is also shown that the ergodic capacity of the system decreases with increasing number of subcarriers.

Time domain spreading and frequency domain maximal ratio combining reception for frequency diversity enhancement in single carrier UWB communication systems

Ultra-wideband technology is a promising candidate for short-range broadband communication systems. Ultra-wideband channels are characterized by dense multiple paths that provide multipath diversity, also referred to as frequency diversity. Single carrier systems with frequency domain equalization have attracted much interest for high data rate communications. In this paper, frequency diversity in single carrier ultra-wideband systems is enhanced by employing simple frequency domain maximal ratio combining (FDMRC) reception with time domain complex quadratic spreading (CQS) sequences. Since ultra-wideband channels exhibit dense multiple paths and strong frequency selectivity, the proposed system can attain significant performance improvement over traditional single carrier frequency domain equalization (SC-FDE) systems. Through the comparison of the impact of the cyclic prefix length, the proposed system shows stronger robustness to inter-block interference than the conventional SC-FDE system. Compared with multicarrier systems, like orthogonal frequency-division multiplexing and multi-carrier code division multiple access, the proposed receiver complexity is reduced greatly. To accommodate multiuser communication, block spreading is further introduced to combine with the frequency diversity enhancement technique for single carrier block transmission UWB systems, realizing multiuser interference-free reception.

Bivalent domains enforce transcriptional memory of DNA methylated genes in cancer cells

Silencing of multiple cancer-related genes is associated with de novo methylation of linked CpG islands. Additionally, bivalent histone modification profiles characterized by the juxtaposition of active and inactive histone marks have been observed in genes that become hypermethylated in cancer. It is unknown how these ambiguous epigenetic states are maintained and how they interrelate with adjacent genomic regions with different epigenetic landscapes. Here, we present the analysis of a set of neighboring genes, including many frequently silenced in colon cancer cells, in a chromosomal region at 5q35.2 spanning 1.25 Mb. Promoter DNA methylation occurs only at genes maintained at a low transcriptional state and is characterized by the presence of bivalent histone marks, namely trimethylation of lysines 4 and 27 in histone 3. Chemically induced hyperacetylation and DNA demethylation lead to up-regulation of silenced genes in this locus yet do not resolve bivalent domains into a domain-wide active chromatin conformation. In contrast, active genes in the region become down-regulated after drug treatment, accompanied by a partial loss of chromatin domain boundaries and spreading of the inactive histone mark trimethylated lysine 27 in histone 3. Our results demonstrate that bivalent domains mark the promoters of genes that will become DNA methylated in adult tumor cells to enforce transcriptional silence. These bivalent domains not only remain upon drug induced gene reactivation, but also spread over adjacent CpG islands. These results may have important implications in understanding and managing epigenetic therapies of cancer.

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.

Flexible and reliable transmission techniques for scale-free UWB system

The speedy dissemination of digital consumer electronics devices within home and personal area causes increments of multimedia communication and advent of entertainment networking. This feature of communication requires variable data-rate and reliable transmission. In order to meet the various demands, in this paper, the multiband orthogonal frequency division multiplexing (MBOFDM) ultra-wideband (UWB) system by exploiting a time domain spreading factor and multiple-access scheme is proposed for variable data-rate transmission from 20kbps to 960Mbps in wireless communication. This article presents the basic structure and main functions of the variable data-rate (VR)-UWB system. With an emphasis on a proposed idea, we address the multiple-access in MB-OFDM system and apply adaptive bit allocation schemes to OFDMA system. It is shown in this article that scale-free UWB can accept multiple users and provide variable data-rate as well as support reliable transmission, while most conventional studies focus on the fixed rate transmission. The proposed system will be promising solution in ubiquitous environment to support multimedia traffic with various rates.

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.

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.

Coherent and Differential Downlink Space-Time Steering Aided Generalised Multicarrier DS-CDMA

This paper presents a generalised multicarrier direct sequence code division multiple access (MC DS-CDMA) system invoking smart antennas for improving the achievable performance in the downlink. In this contribution, the MC DS-CDMA transmitter employs an antenna array (AA) and steered space-time spreading (SSTS). Furthermore, the proposed system employs both time and frequency (TF) domain spreading for extending the capacity of the system, which is combined with a user-grouping technique for reducing the effects of multi-user interference (MUI). Moreover, to eliminate the high- complexity multiple input multiple output (MIMO) channel estimation required for coherent detection, we also propose a Differential SSTS (DSSTS) scheme. More explicitly, for coherent SSTS detection MVN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> number of channel estimates have to be generated, where M is the number of transmit AAs, V is the number of subcarriers and N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> is the number of receive antennas. This is a challenging task, which renders the low-complexity DSSTS scheme attractive.

Design of next-generation cdma using orthogonal complementary codes and offset stacked spreading

This article presents an innovative code-division multiple access system architecture that is based on orthogonal complementary spreading codes and time-frequency domain spreading. The architecture has several advantages compared to conventional CDMA systems. Specifically, it offers multiple-access-interference-free operation in AWGN channels, reduces co-channel interference significantly, and has the potential for higher capacity and spectral efficiency than conventional CDMA systems. This is accomplished by using an offset stacked spreading modulation technique followed by quadrature amplitude modulation, which optimizes performance in a fading environment. This new spreading modulation scheme also simplifies the rate matching algorithms relevant for multimedia services and IP-based applications.

Theoretical time-domain study of self-imaging properties in a multimode interference coupler

We report a theoretical and numerical study of self-imaging properties, including time domain and pulse spreading, caused by modal group-delay dispersion in generalized N x N multimode interference devices achieved by using a mode-propagation analysis and finite-difference time-domain method. It was found that the spatial self-imaging condition does not realize temporal self-imaging but lets waveforms separate whose shape depends on input position and input field distribution. Pulse spreading, which is sensitive to beam diameter, has a very large variation (420 fs) among input positions as well as rising to a very high 900 fs in response to a 21 fs and spatially Gaussian pulse for the conveniently smallest size with 10 channels.

Cyclic delay diversity based space-time/space-frequency coded MC-CDMA system

Spatial diversity techniques will inarguably play an important role in the realization of B3G wireless systems, which promise higher data transmission rates and affluent multi-media contents. In this article, we present a CDD based space time/space-frequency (ST/SF) block coded diversity transmitter for MC-CDMA system. Equipped with CDD a fraction of the total number of transmit antennas is used to extract the frequency (multipath) diversity while the remaining antennas employing ST/SF-block codes provide maximum spatial diversity. This hybrid architecture makes full use of the frequency domain spreading of MC-CDMA system and provides excellent spatial and frequency diversity which is otherwise impossible to realize with standalone CDD or block codes

Control of lateral domain spreading in congruent lithium niobate by selective proton exchange

Proton exchange was observed to increase the poling voltage for congruent lithium niobate. Patterned proton exchange was then used to control domain nucleation and inhibit broadening of reversed domains. Periodically proton exchanged samples were used to form domain gratings by electric field poling just using planar electrodes and without need for poling current control. The reversed domain gratings had a duty cycle faithfully reproducing that of the proton exchanged pattern with straight domain walls parallel to the x face, thus demonstrating that high-fidelity reversed domain patterning is possible to obtain by a relatively simple process.

Multiuser Detection Assisted Time- and Frequency-Domain Spread Multicarrier Code-Division Multiple-Access

In this contribution, we study a reduced-complexity multiuser detection aided multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) scheme, which employs both time (T)-domain and frequency (F)-domain spreading. We investigate the achievable detection performance in the context of synchronous TF-domain spread MC DS-CDMA when communicating over an additive white Gaussian noise (AWGN) channel. Five detection schemes are investigated, which include the single-user correlation based detector, the joint TF-domain decorrelating multiuser detector (MUD), the joint TF-domain MMSE MUD, the separate TF-domain decorrelating/MMSE MUD, and the separate TF-domain MMSE/decorrelating MUD. Our simulation results show that the separate TF-domain MUD schemes are capable of achieving a similar bit error rate (BER) performance to that of the significantly more complex joint TF-domain MUD schemes.

Performance Enhancement of an OFDMA/CDM-Based Cellular System in a Multi-Cell Environment

In this letter, we propose an OFDMA/CDM-based cellular system, which accommodates users in different frequency bands and multiplexes user data symbols with frequency domain spreading. The proposed system utilizes random codes to discriminate between cells and adopts the pre-equalization to enhance the performance. An efficient power allocation scheme is suggested for cellular applications with a transmit power constraint. In particular, the validity of the OFDMA/CDM-based cellular system is examined, by investigating its performance as a function of the number of multiplexed data symbols at different locations.

Performance of broadband multicarrier DS-CDMA using space-time spreading-assisted transmit diversity

In this contribution multicarrier direct-sequence code-division multiple-access (MC DS-CDMA) using space-time spreading (STS)-assisted transmit diversity is investigated in the context of broadband communications over frequency-selective Rayleigh-fading channels. We consider the issue of parameter design for the sake of achieving high-efficiency communications in various dispersive environments. Furthermore, in contrast to conventional MC DS-CDMA schemes employing time (T)-domain spreading only, in this contribution we also investigate broadband MC DS-CDMA schemes employing both T-domain and frequency (F)-domain spreading, i.e., employing TF-domain spreading. The bit-error rate (BER) performance of STS-assisted broadband MC DS-CDMA is investigated for downlink transmissions associated with the correlation based single-user detector and the decorrelating multiuser detector. Our study demonstrated that when appropriately selecting the system parameters, broadband MC DS-CDMA using STS-assisted transmit diversity constitutes a promising downlink transmission scheme. This scheme is capable of supporting ubiquitous communications over diverse communication environments without BER performance degradation.

Downlink Transmission of Broadband OFCDM Systems—Part I: Hybrid Detection

The broadband orthogonal frequency and code division multiplexing (OFCDM) system with two-dimensional spreading (time and frequency domain spreading) is becoming a very attractive technique for high-rate data transmission in future wireless communication systems. In this paper, a quasianalytical study is presented on the downlink performance of the OFCDM system with hybrid multi-code interference (MCI) cancellation and minimum mean square error (MMSE) detection. The weights of MMSE are derived and updated stage by stage of MCI cancellation. The effects of channel estimation errors and sub-carrier correlation are also studied. It is shown that the hybrid detection scheme performs much better than pure MMSE when good channel estimation is guaranteed. The power ratio between the pilot channel and all data channels should be set to 0.25, which is a near optimum value for the two-dimensional spreading system with time domain spreading factor (N/sub T/) of 4 and 8. On the other hand, in a slow fading channel, a large value of the channel estimation window size /spl gamma/N/sub T/, where /spl gamma/ is an odd integer, is expected. However, /spl gamma/=3 is large enough for the system with N/sub T/=8 while /spl gamma/=5 is more desirable for the system with N/sub T/=4. Although performance of the hybrid detection degrades in the presence of the sub-carrier correlation, the hybrid detection still works well even the correlation coefficient is as high as 0.7. Finally, given N/sub T/, although performance improves when the frequency domain spreading factor (N/sub F/) increases, the frequency diversity gain is almost saturated for a large value of N/sub F/ (i.e., N/sub F/ /spl ges/ 32).

Variable Spreading Factor-OFCDM with Two Dimensional Spreading that Prioritizes Time Domain Spreading for Forward Link Broadband Wireless Access

This paper proposes the optimum design for adaptively controlling the spreading factor in Orthogonal Frequency and Code Division Multiplexing (OFCDM) with two-dimensional spreading according to the cell configuration, channel load, and propagation channel conditions, assuming the adaptive modulation and channel coding (AMC) scheme employing QPSK and 16QAM data modulation. Furthermore, we propose a two-dimensional orthogonal channelization code assignment scheme to achieve skillfully orthogonal multiplexing of multiple physical channels. We first demonstrate the reduction effect of inter-code interference by the proposed two-dimensional orthogonal channelization code assignment. Then, computer simulation results show that in time domain spreading, the optimum spreading factor, except for an extremely high mobility case such as for the fading maximum Doppler frequency of f D = 1500 Hz, becomes SF Time = 16. Furthermore, it should be decreased to SF Time = 8 for such a very fast fading environment using 16QAM data modulation. We also clarify when the channel load is light such as C mux /SF = 0.25 (C mux and SF denote the number of multiplexed codes and total spreading factor, respectively), the required average received signal energy per symbol-to-noise power spectrum density ratio (E s /N 0 ) is reduced as the spreading factor in the frequency domain is increased up to say SF Freq = 32 for QPSK and 16QAM data modulation. When the channel load is close to full such as when C mux /SF = 0.94, the optimum spreading factor in the frequency domain is SF Freq = 1 for 16QAM data modulation and SF Freq = 1 to 8 for QPSK data modulation according to the delay spread. Consequently, by setting several combinations of spreading factors in the time and frequency domains, the near maximum link capacity is achieved both in cellular and hotspot cell configurations assuming various channel conditions.

Blind equalization in SIMO OFDM systems with frequency domain spreading

The problem of bandwidth efficient data transmission in multiple antenna orthogonal frequency division multiplex (OFDM) systems is considered. Contrary to the mono-sensor systems, the effects of frequency-selective fading may be compensated in the multiple antenna systems due to the space diversity without using a wireless overhead. A modulation scheme that improves the reliability of data transmission in fading multipath environments and performs an accurate channel equalization with no excess bandwidth neither substantial training is proposed. The new scheme exploits a frequency domain spreading along with the standard OFDM modulation. This modulation yields an improved error rate; its structure is also exploited to estimate the equalizer. The problem of reliable channel equalization in such systems is shown to be essentially different in single and multiple antenna scenarios. For the multiple antenna scenario, a new approach to robust equalization is proposed, and its performance is analyzed. The numerical study confirms the efficiency of the new method in such bandwidth-expensive applications as wireless networks.