Preetam Kumar

Performance Evaluation of DFT-Spread OFDM and DCT-Spread OFDM for Underwater Acoustic Communication

This paper presents a comparative study of discrete cosine transform (DCT) spread orthogonal frequency division multiplexing (OFDM) with discrete Fourier transform (DFT) spread OFDM, applied to an underwater acoustic (UWA) channel. The UWA channel is characterized as a frequency selective multipath fading channel with long delay spread. Energy compaction property of DCT works well in reducing the peak to average power ratio (PAPR) in UWA channel. It is also shown that DCT spread OFDM has lower computational complexity compared to DFT spread OFDM. The bit error rate (BER) performance has been evaluated for both the schemes. Simulation results show that DCT spread OFDM has a better performance than DFT spread OFDM in UWA channel. A novel scheme of using the null subcarriers to implement complex field repetitive coding in case of DCT spread OFDM is proposed to further reduce the BER.

Overloaded WH-Spread CI/MC-CDMA with Iterative Interference Cancellation

This paper addresses a novel Walsh-Hadamard (WH)-spread multicarrier code division multiple access (MC-CDMA) system which employs carrier interferometry (CI) codes to achieve improved bit error rate (BER) performance. In frequency selective channel, phase characteristic of CI codes ensures better estimates of received WH-spread CI/MC-CDMA signal. In this paper an iterative receiver structure is used for decoding WH-spread CI/MC-CDMA signal. Estimation of multiple access interference (MAI) becomes more reliable with time and frequency diversities of CI/MC-CDMA signal with spreading gain diversity of WH code over multipath channel. It is shown by simulation that WH-spread CI/MC-CDMA provides 2 dB gain at a BER of 1e-03 after 6th iteration with soft decision subcarrier parallel interference cancellation (SDSub-PIC) at 25% overload compared to CI/MC-CDMA.

A New Overloading Scheme for Cellular DS-CDMA using Orthogonal Gold Codes

Overloading is a scheme to accommodate more users than the spreading factor N. This is a bandwidth efficient scheme to increase the number of users in a fixed bandwidth. One of the efficient schemes to overload a CDMA system is to use two sets of orthogonal signal waveforms (O/O). The first set is assigned to the N users and the second set is assigned to the additional users. An iterative multistage detection (IMSD) technique is used to cancel interference between the two sets of users. The interference cancellation receiver uses hard decisions (HDIC) or soft decisions (SDIC) to estimate the interference. In this paper, the BER performance of a new overloading scheme using orthogonal Gold code (OG/OG) sets is evaluated with SDIC receiver. It is observed that this scheme provides 25% (16 extra users) channel overloading for synchronous DS-CDMA system in an AWGN channel, with an SNR degradation of about 0.35 dB as compared to single user bound at a BER of 10-5. We have also evaluated the overloading performance, when the two set are scrambled with set specific complex scrambling sequence. It is shown that the amount of overloading increases significantly from 25% to 63% (40 extra users) by using complex scrambling for N=64.

Performance evaluation of modified OFDM for underwater communications

Conventional orthogonal frequency division multiplexing (OFDM) can reduce errors in underwater channels but frequency selective fading can still degrade the data on carriers near deep fade regions. Recently spread-OFDM (SOFDM) which combines the properties of spread spectrum with multicarrier communication has become popular. In this paper spreading by Walsh-Hadamard (WH) codes, discrete Fourier transform (DFT), discrete cosine transform (DCT) and carrier interferometry (CI) codes have been compared for underwater acoustic communication using OFDM. The spreading technique not only helps overcome frequency-selective and multipath fading but also provides reduced peak to average power ratio (PAPR). The performance of this modified OFDM scheme is evaluated in terms of bit error rate (BER) and PAPR. Simulation results show that with CI-SOFDM it is possible to achieve a BER of 1e-4 at 15dB signal to noise ratio. It is also shown that CI-SOFDM offers best PAPR performance. To further improve the BER performance a two level orthogonal spreading by WH followed by CI-SOFDM is also evaluated for underwater communication. The proposed scheme exploits the spreading gain diversity of WH code and phase characteristic property of CI signal to achieve a further gain of 1dB at a BER of 1e-4.

Underwater acoustic sensor network for early warning generation

There is an ever felt need to create an efficient comprehensive system to tackle natural hazards such as Tsunamis, earthquakes, landslides and floods by providing a timely early warning. Underwater wireless sensor network (UWSN) seems to be one promising solution. The success of mobile wireless communication in terms of power efficiency and reliability needs to be repeated with the UWSN for fighting the havoc of nature. An integrated system for early warning generation which would provide information globally is proposed. This paper highlights the physical layer challenges in establishing a reliable, low power consuming and long life UWSN system for early warning generation.

Performance Evaluation of Orthogonal/Scrambled-Orthogonal Overloaded DS-CDMA System

Overloading is a bandwidth efficient scheme to accommodate more number of users than the spreading factor, N in a DS-CDMA system. In this paper, the performance of an overloading scheme in which the first N users are assigned Walsh-Hadamard orthogonal codes and extra users are assigned the same WH codes but overlaid by a common scrambling sequence is evaluated. The allowable load of this scheme is investigated, so that the uncoded performance with iterative multistage interference cancellation detector using hard (HDIC) and soft (SDIC) decision functions remains close to the single user bound. It is shown that this scheme provides 19% and 11% overloading with SDIC technique for N= 16 and 64 respectively.

A computationally efficient genetic algorithm for MIMO broadcast scheduling

Graphical abstractDisplay Omitted HighlightsMultiple-input multiple-output (MIMO) is suitable technique to ensure high speed data transmission as well as low delay communication networks.In MIMO, dirty paper coding (DPC) is an efficient scheme to support multiple users with optimum sum rate capacity of the system. However, DPC is a complex scheme where the user encoding sequence is important to transmit data to multiple users.An optimal exhaustive search as used in DPC is prohibited due to the extremely large size of the search space in this optimization problem.Evolutionary algorithm (genetic algorithm) can be used as an alternative for this optimization problem to reduce the complexity of the search (scheduling problem). The performance of the genetic algorithm with elitism and adaptive mutation is demonstrated to be near optimal as obtained with an exhaustive search. It has been demonstrated in this paper that GA achieves about 98-99% of system sum rate as obtained with DPC with significant reduction in time and computational complexity.The proposed BGA is able to provide the optimum solution well within the packet duration of modern wireless packet data communications. In conventional single-input single-output (SISO) systems, the capacity is limited as base station can provide service to only one user at any instant. However, multiuser (MU) multiple-input multiple-output (MIMO) systems deliver optimum system capacity by providing service to multiple users (as many as transmit antennas) simultaneously according to dirty paper coding (DPC) scheme. However, DPC is an exhaustive search algorithm (ESA) where the user encoding sequence is important to transmit data to multiple users. Exhaustive search becomes imperative as the search space grows with number of users and number of transmit antennas in the MU MIMO system. This can be treated as an optimization problem of maximizing the achievable system sum-rate. In this paper, it has been demonstrated that combined user and antenna scheduling (CUAS) with binary genetic algorithm (BGA) adopting elitism and adaptive mutation (AM) achieves about 97-99% of system sum-rate obtained by ESA (DPC) with significantly reduced computational and time complexity. It has been shown that BGA is able to find the globally optimum solution for MU MIMO systems well within the time interval of modern wireless packet data communications. However, it is interesting to observe that BGA is able to find a solution to CUAS close to the optimum value quite rapidly. In this paper, it is also shown that BGA with elitism and AM achieves higher throughput than limited feedback scheduling schemes as well.

Recent trends in multicarrier underwater acoustic communications

Underwater acoustic communication is essential in applications like remote control in the offshore oil industry, pollution monitoring in environmental systems, collection of scientific data recorded at ocean-bottom stations, disaster detection and early warning and underwater surveillance. Research on underwater wireless communication techniques plays a vital role in further exploring oceans and other marine environments. There has been an extensive growth in the volume of literature for underwater acoustic (UWA) communication but still it remains to be one of the most challenging areas of wireless communication. Over the years attention has turned on applying modified versions of multicarrier (MC) communication to underwater channel. This paper reviews the recent developments in the area of UWA communication related to multicarrier communication and particularly to orthogonal frequency division multiplexing (OFDM) with respect to applied, theoretical and simulation studies. An attempt has been made to present a compact yet exhaustive literature survey that will serve as a standard reference for researchers working in the area. Stress has been laid on the physical layer issues as it works as the basic foundation of any network. The focus areas of research activities have been identified and a summary of the ongoing activities and future trends has been presented.

Analysis of a New MIMO Broadcast Channel Limited Feedback Scheduling Algorithm with User Grouping

In this paper, we present and analyze a new multiuser multiple-input multiple-output (MU MIMO) broadcast channel scheduling algorithm with limited feedback and user grouping. Here signal-to-noise-plus-interference-ratio (SINR) feedback is used for the scheduling criteria. The algorithm deals with the following three scenarios: (a) All transmit antennas are active. (b) Receive antennas at the mobile station (MS) with maximum SINR values are assigned to the transmit antennas at the base station (BS). (c) A given receive antenna is not scheduled for more than one BS transmit antenna, which ensures user fairness. A detailed comparison of the proposed algorithm with the limited feedback antenna selection algorithms proposed by Zhang et al. and Sharif et al. is presented. Further, we have extended the proposed algorithm to heterogeneous environment, where users having similar signal-to-noise-ratio (SNR) are grouped together. Also, we evaluate the effects of sending the beamforming data (here indices of the receiving antennas) to the BS in terms of complexity of MS and feedback load ratio. A detailed analytical approach for throughput of the system is presented in this paper for the proposed algorithm. We also verify the simulation results with the analytical results.

Variable Rate Transmission Schemes for CI/MC-CDMA System

In this paper, two multirate transmission schemes have been proposed for carrier interferometry multicarrier code division multiple access (CI/MC-CDMA) system. System models for wide range of data rates are discussed based on orthogonal property of CI codes of any length. Non-contiguous CI codes have been introduced for multirate CI/MC-CDMA. Advantage of frequency diversity in non-contiguous allocation improves the performance of variable rate system over frequency selective channel. It is shown that non-contiguous allocation allows more number of very high data rate users compared to contiguous allocation with some increased complexity at iterative receiver.