A. S. Madhukumar

Peak-to-average power reduction using partial transmit sequences: a suboptimal approach based on dual layered phase sequencing

A high peak-to-average power ratio (PAPR) is a major shortcoming in multicarrier systems, as it causes nonlinearity in the transmitter, degrading the performance of the system significantly. Partial transmit sequences (PTS) is one of the best methods in reducing PAPR, in which the information-bearing subcarriers are divided into M disjoint subblocks, each controlled by a phase rotation factor which brings PAPR down. Though PAPR reduction by PTS is more effective with more subblocks, there is a corresponding exponential increase in complexity. In this paper, a novel implementation of PTS is presented, in which a dual-layered approach is employed to reduce the complexity.

Capacity enhancement of a multi-user OFDM system using dynamic frequency allocation

A dynamic subcarrier allocation algorithm is developed and studied in order to improve the capacity of a multi-user OFDM system in the downlink environment. The proposed algorithm uses a decentralized approach and considers the instantaneous channel response of each user in parallel. In order to reduce the complexity of the system, the available subcarriers are divided into a number of partitions and the algorithm attempts to allocate the partition with the highest average channel gain to each user. However, situations may arise whereby two or more users attempt to select the same partition. As such, an important aspect of the algorithm is to resolve such conflicts. Based on the above allocation plan, adaptive modulation is employed for each user on the allocated partitions. The results obtained show that the proposed dynamic allocation scheme outperforms a static allocation scheme in terms of a lower BER and a higher system capacity for the same SNR. In addition, it is possible to obtain performance improvements at the expense of an increase in the system complexity by dividing the subcarriers into a larger number of partitions.

Single-carrier cyclic prefix-assisted CDMA system with frequency domain equalization for high data rate transmission

Multiple-access interference and interfinger interference limit the capacity of conventional single-carrier DS-CDMA systems. Even though multicarrier CDMA posses the advantages of conventional CDMA and OFDM, it suffers from two major implementation difficulties such as peak-to-average power ratio and high sensitivity to frequency offset and RF phase noise. A novel approach based on single-carrier cyclic prefix-assisted CDMA has been proposed to overcome the disadvantages of single-carrier CDMA and multicarrier modulation. The usefulness of the proposed approach for high-speed packet access with simplified channel estimation procedures are investigated in this paper. The paper also proposes a data-dependent pilot structure for the downlink transmission of the proposed system for enhancing pilot-assisted channel estimation in frequency domain. The performance of the proposed pilot structure is compared against the data-independent common pilot structure. The proposed system is extensively simulated for different channel parameters with different channel estimation and equalization methods and the results are compared against conventional multicarrier CDMA systems with identical system specifications.

Enhanced architecture for residue number system-based CDMA for high-rate data transmission

This paper presents an advanced architecture for residue number system (RNS)-based code-division multiple-access (CDMA) system for high-rate data transmission by combining RNS representation, phase shift keying/quadrature amplitude modulation (PSK/QAM) and orthogonal modulation. The residues obtained from a fixed number of bits are again divided into spread code index and data symbol for modulation. The modulated data symbol is spread using the indexed orthogonal codes and transmitted through a communication channel. The proposed system uses a lower number of orthogonal codes than conventional RNS-based CDMA and the performance is comparable. The computational complexity of the proposed system is compared against alternative schemes such as M-ary CDMA and conventional RNS-based CDMA. The modified system is simulated extensively for different channel conditions and the results are discussed.

A simplified transceiver structure for cyclic extended CDMA system with frequency domain equalization

In this paper, a new cyclic-extended block-spreading CDMA (CEBS-CDMA) system is proposed to combat multiple access interference (MAI) efficiently over a frequency selective fading channel for an asynchronous reverse link transmission. Attribute to the block spreading, the MAI is minimized due to the small channel variation across consecutive blocks in a typical high speed system, where the signaling block duration is very short. It suggests a simplified transceiver structure with symbol-wise efficient frequency domain equalization (FDE) to improve the performance and reduce the complexity under frequency selective fading channels. A new cell-specific scrambling code is proposed with the CEBS-CDMA system for a multi-cell environment reverse link transmission, suppressing other-cell interference (OCI) efficiently. The simulation results show that the proposed CEBS-CDMA system achieves much better performance of multiple users compared to the conventional direct sequence CDMA (DS-CDMA) and cyclic prefix-CDMA systems.

Throughput Optimization in Cooperative Communications Based on Incremental Relaying

In wireless networks, designing transmission schemes that can adapt to time-varying channel conditions is key to improving spectral efficiency. However, the realization of such schemes for multihop wireless networks is challenging, owing to the requirement of coordination among nodes. As coordination through the use of feedback between nodes incurs significant bandwidth penalty, spectral efficiency can be improved by minimizing traffic in the feedback channel. This paper proposes an adaptive rate transmission scheme using incremental-redundancy-based cooperative coding that minimizes the required feedback. By exploiting the inherent implicit feedback channel during relaying, the spectral efficiency of multihop wireless networks can be improved considerably. Rather than allocating dedicated channels to feedback the quality of information at the relay, the implicit feedback channel measures such information and determines the transmitter for the additional coded (redundancy) bits. This optimizes the throughput. The proposed scheme is thoroughly analyzed under different deployment environments. Theoretical bounds for the proposed scheme are presented and supported with results from extensive simulation studies.

MMSE detection for high data rate UWB MIMO systems

In this paper, a uniform matrix representation for ultra-wideband (UWB) multiple-input multiple-output (MIMO) systems with time-hopping spread spectrum (THSS) and direct-sequence spread spectrum (DSSS) methods is formulated. The inter symbol interference (ISI) is considered for high data rate UWB transmissions in the presence of dense multipath in an indoor environment. The bit error rate (BER) performance is evaluated based on the minimum mean square error (MMSE) detection criterion. In addition, a simplified UWB MIMO channel model is assumed for performance analysis based on the IEEE 802.15.3a channel model recommendation and the MIMO channel covariance matrix representations. Results show that a UWB MIMO system with THSS method is capable of delivering promising performance up to very high symbol rate transmission for short range. On the other hand, a UWB MIMO system with DSSS method is more vulnerable to high symbol rate transmission due to the existence of severe inter chip interference (ICI) in addition to ISI.

Power spectral density and in-band interference power of UWB signals at narrowband systems

Ultra-wideband (UWB) technology is one of the promising solutions for short-range indoor wireless communication applications. UWB systems spread the transmitted signal power over an extremely large frequency band, and the power spectral density of the signal is very low. Due to the wide bandwidth of the transmitted signal, UWB signal energy will spread over the frequency bands allocated to other radio systems. In order to evaluate the effects of UWB system to narrowband systems (such as IEEE802.11a wireless LAN) in its overlay band, it is important to understand and quantify the power spectral density (PSD) of UWB signal. In this paper, a detailed analysis on the PSD of direct sequence coded UWB (DS-UWB) signal is presented. The theoretical and simulation results reveal that the interfering DS-UWB power can be reduced by regulating UWB chip rate, pulse shape, pulse width and scrambling codes in the frequencies of victim narrowband systems.

Stackelberg Bayesian Game for Power Allocation in Two-Tier Networks

The downlink power allocation in a two-tier cellular network that consists of a macrocell network underlaid by multiple femtocell networks is addressed in this paper. This paper aims to maximize the transmission capacity of the femtocell networks while guaranteeing that the interference experienced at the macro base station does not exceed an interference constraint. We formulate a Bayesian Stackelberg game to model and analyze behaviors of macrocell and femtocell base stations (MBS and FBSs, respectively). In this game, the MBS is the leader, whereas the FBSs are the followers. The channel information between an FBS and its associated femtocell user is private information and is considered the type of the follower. The leader issues the price of interference charged to the followers first to maximize its own profit. Based on the price, the followers decide the strategies to maximize their payoffs defined as the difference between the transmission capacity and the cost of interference (CoI) paid to the leader. Using backward induction, we first analyze the follower game. The existence and uniqueness of the Bayesian Nash equilibrium (BNE) are examined, and the methods to obtain the BNE for a symmetric case are provided. Then, the leader game is analyzed. Finally, the numerical analysis is provided.

Error Detection and Correction in Communication Channels Using Inverse Gray RSNS Codes

A novel number theoretic transform called Inverse Gray Robust Symmetrical Number System (IGRSNS) is proposed for error control coding in this paper. IGRSNS is obtained by modifying Robust Symmetrical Number System (RSNS) that was proposed earlier, using Inverse Gray code property. Due to ambiguities present in each residue, RSNS has a short dynamic range (DR) compared to that in other number systems. The short DR of RSNS enables it to be effectively used for error detection without the addition of any redundant modulus as in Redundant Residue Number System. Although RSNS has a large redundant range, its detection ability is not optimal due to the Gray code property associated with it. In an attempt to overcome this limitation, we have proposed Inverse Gray coding to be combined with RSNS in increasing its effectiveness in error detection. The resulting IGRSNS thus inherits properties of both RSNS and Gray code. Analysis and simulations show that IGRSNS has a near-optimal error detection ability. The potential of IGRSNS for error correction is also investigated. Further, a novel error correction algorithm using one redundant modulus is proposed. Simulations show that the proposed algorithm performs well under all cases of single bit errors.