Conventional Direct Sequence Spread Spectrum Research Articles

Multi‐user underwater acoustic communication using binary phase‐coded hyperbolic frequency‐modulated signals

Multi-user communication traditionally uses either code division multiple access (CDMA) or time-division multiple access schemes. The chirp signals resilience to multipath and the Doppler effect makes it suitable for underwater acoustic communication. This paper presents a binary phase-coded hyperbolic frequency-modulated (BPC-HFM) for multi-user underwater acoustic communication. The proposed scheme has the advantage of both linear frequency-modulated signal, which is immune to multipath and Doppler offset, while PN sequence is encoded in such a manner that a waveform possesses either a phase shift of 0 or 180 depending upon the code which results in a unique orthogonal waveform for every user. As a result, it helps to reduce multiple access interference; in addition, the time-reversal mirror technique is used to enhance the performance of the system in the high multipath environment due to the virtue of hydro-acoustic channel. Furthermore, it uses a relatively simple matched filter-based receiver. To substantiate the robustness of the proposed multi-user waveforms three types of hydro-acoustic channels using the different numbers of multiple users are chosen for simulation and the bit error rate (BER) of the system is calculated. Furthermore, the results are compared to binary phase-coded linear frequency-modulated signal and conventional Direct sequence spread spectrum CDMA.

A New Scheme for Spreading & De-spreading in the Direct Sequence Spread Spectrum Mechanism

Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS) techniques are widely used to implement code-division multiple access (CDMA) in wireless communication systems. Both DSSS and FHSS systems help reducing the effects of interference on the transmitted information making it robust against channel impairments. DSSS uses a signal bandwidth that is much broader than the information signal bandwidth. Traditionally, the wide band signal is generated by multiplying the narrowband information signal with a binary code, often designated as a spreading code, to generate the wideband signal that is transmitted. The original information signal can be recreated at the receiver by multiplying the received wideband signal by the same binary code (now designated as a de-spreading code) used to generate the wideband transmitted signal. To extract the original information signal, the spreading and de-spreading codes must be in synchronism at the receiver and amplitude match with each other. A new modification for the direct sequence spread spectrum is proposed in this paper. The mechanism introduced in this approach implicates generating the wideband signal by circularly shifting the spreading code (PN) by n places, where n represents the value of the current byte of information signal. The yielded signal is modulated using BPSK modulator before transmitting it. The original information signal is extracted at the receiver by correlating the received signal (which is actually the original spread sequence circularly shifted by n places) with a locally generated replica of the spreading code. The position of the maximum value of the cross-correlation vector represents the value of the information signal byte. The proposed configuration has been implemented using Simulink simulator and the obtained results show that its performance is identical with the conventional DSSS.

Long-range underwater acoustic communication of M-eyry spread spectrum with cyclic prefix

The Spread Spectrum is an effective transmission method for complex channels in underwater acoustic (UWA) communication because of the high correlation of pseudo-random noise (PN) sequence. In particular, the spread spectrum method is effective in an environment with a low signal-to-noise ratio. The bandwidth of UWA communication channel is limited due to the fact that sound wave attenuation in water is positively associated with frequency. The limited bandwidth directly leads to the low data rate of conventional direct-sequence spread spectrum UWA communication. M-ary spread spectrum (MSS) schemes using multiple orthogonal PN sequences were proposed as a solution to this problem. However, even in the MSS method, the performance may be limited by the delay spread characteristics in the multipath propagation channels. We propose a more robust communication method in the UWA channel by including Cyclic Prefix (CP) to achieve a more robust performance in the multipath channel. In the proposed method, a CP is inserted for every symbol. In the 160 km long-range UWA communication experiment conducted in the East Sea, the proposed method showed more reliable results than the conventional MSS method. [Work supported by the Agency for Defense Development, South Korea, Grant No. UD200010DD.]

Generalized Code Index Modulation and Spatial Modulation for High Rate and Energy-Efficient MIMO Systems on Rayleigh Block-Fading Channel

In this article, a high data rate and energy-efficient multiple-input multiple-output transmission scheme is considered by combining two popular and rational modulation techniques: spatial modulation (SM) and code index modulation-spread spectrum (CIM-SS). Since in the considered system, called generalized CIM-SM (GCIM-SM), incoming information bits determine modulated symbols, activated transmit antenna indices as well as their corresponding spreading code indices, data bits are conveyed not only by modulated symbols but also by the indices of the active antenna and spreading codes. Hence, a GCIM-SM scheme accommodates faster data rates while spending less transmission power and possessing better error performance compared to the conventional direct sequence spread spectrum (DS-SS), SM, quadrature SM (QSM), and CIM-SS systems. The mathematical expressions of the GCIM-SM system for bit error rate, throughput, energy efficiency, and the system complexity are derived to analyze the overall system performance. Besides, it has been shown via computer simulations that the GCIM-SM system has lower transmission energy, faster data transmission rate, and better error performance than DS-SS, SM, QSM, and CIM-SS systems. Performance analysis of the considered system was performed on Rayleigh block-fading channels for quadrature amplitude modulation technique.

A New IDMA System Based on CSK Modulation for Multiuser Underwater Acoustic Communications

This work proposes a novel interleave-division multiple access (IDMA) system based on cyclic shift keying (CSK) modulation, named CSK-IDMA, for multiuser underwater acoustic (UWA) communications. The CSK modulation uses a circular cyclic shift of the spreading sequence to represent the information, which can be viewed as a special $M$ -ary spread spectrum modulation. Compared with conventional direct-sequence spread spectrum (DSSS) technique, CSK overcomes the spreading gain versus data rate limitations. The proposed scheme adopts the IDMA technique to reduce the cochannel interference. At the receiver, we employ passive time reversal (PTR) technique to compress the channels and improve the signal-to-interference-noise ratio (SINR) at first. In order to perfectly incorporate with the IDMA receive structure, we propose a soft CSK demodulation which can iteratively exchange extrinsic information with a soft channel decoder to form a turbo equalization. Simulation results show the proposed CSK-IDMA scheme can offer significantly improved performance. The data processing results from a lake experiment show that the proposed scheme can support 8 users and 10 users with zero error bits for 2.3 km transmission and 2.7 km transmission, respectively. The corresponding bit error rate (BER) for 16 users are $\text{8.4} \times \text{10}^{-4}$ and $\text{9.7} \times \text{10}^{-4}$ , respectively. The data rate is 62.5 bit/s per user within 4 kHz bandwidth and spreading sequences of 256 chips. To the best of our knowledge, this is the first work which can support more than 8 users for multiuser UWA communications with a high bandwidth efficiency.

Code-Index Modulation Aided Quadrature Spatial Modulation for High-Rate MIMO Systems

In this correspondence paper, a novel code-index modulation (CIM) aided quadrature spatial modulation (QSM) scheme, called CIM-QSM, is proposed for highly spectrum and energy efficient multiple-input multiple-output (MIMO) systems. The CIM-QSM scheme transmits information over three different transmit entities: modulated symbols, activated antenna indices, and spreading code indices. Hence, a higher efficiency in terms spectrum and energy use, and better error performance compared to the conventional direct sequence spread spectrum, spatial modulation (SM), QSM, and CIM aided SS (CIM-SS) schemes are obtained by the CIM-QSM technique for MIMO communication systems. Analytical expressions for the average bit error rate, energy efficiency, and throughput of the CIM-QSM system are performed and the theoretical results are validated with Monte Carlo simulation results.

Circular Superposition Spread-Spectrum Transmission for Multiple-Input Multiple-Output Underwater Acoustic Communications

This letter proposes a circular superposition spread spectrum (CSSS) scheme for multiple input-multiple-output (MIMO) underwater acoustic (UWA) communications where the multipath channel length is long and the Doppler spread is severe. The proposed CSSS scheme divides the spreading sequence into several sub-sequences, and the circular shifting of the sub-sequences forms multiple spreading sequences. Each spreading sequence spreads different payload symbols individually, then the multiple spread symbols are summed to form the transmitted signal at one transducer. The CSSS signaling enables the receiver to apply a circularly shifted coherent RAKE to harvest the delay-spatial diversity of the MIMO UWA channels. If one of baseband symbol is known to the receiver, then the known symbol is utilized as the training symbol to estimate the time-variant channel impulse response (CIR) at every sub-sequence length. The receiver also reduces the co-channel interference (CCI) by exploiting the periodical auto-correlation property of the circular spreading sequences. Simulation results demonstrate that the proposed CSSS achieves more robust performance than the conventional direct sequence spread spectrum (DSSS) approach while reducing the training overhead for MIMO systems.

Design of Chaotic Bifurcation Parameter Modulation Scheme using Mean Value Estimation

In this paper, the bifurcation parameter of the chaotic map is estimated accurately by utilizing the ergodic properties of a chaotic dynamical signal. Binary Phase Shift Keying Ergodic Chaotic Parameter Modulation (BPSK-ECPM) scheme is used to modulate the information signal in the bifurcating parameter of the chaotic map. A mean value estimation technique is used at the receiver to retrieve the original information accurately. This method minimizes the computational complexity of the receiver; thereby, reduces the total manufacturing cost. Simulation results confirm that, in AWGN channel and at bit error rate (BER) of 10-3, BPSK-ECPM achieves gains in Eb/N0 of about 0.7dB and 5dB in comparison with conventional direct sequence spread spectrum with BPSK modulation (DS-SS BPSK) and conventional ergodic chaotic parameter modulation spread spectrum (ECPM-SS) systems respectively. The results also confirm that in Rayleigh fading channel and at BER of 10-3, BPSK-ECPM achieves gains in Eb/N0 of about 6.3dB and 1.6dB in comparison with DS-SS BPSK and ECPM-SS systems respectively, which make BPSK-ECPM an ideal candidate for the intelligent transportation system (ITS).

M-ary cyclic shift keying spread spectrum underwater acoustic communication using frequency domain energy detector

M-ary cyclic shift keying (M-CSK) method can greatly improve the communication rate of conventional direct sequence spread spectrum, but the robustness is seriously influenced by the fluctuation of sea surface and Doppler effect. As to this problem, M-ary cyclic shift keying spread spectrum (M-CSKSS) underwater acoustic communication (UAC) based on frequency domain energy detector (FDED) algorithm is proposed in this paper. Time reversal (TR) is firstly adopted to suppress multipath spread using channel impulse response (CIR) estimated from the former symbol. And then the baseband signal after TR processing goes through a FDED, decisions are made according to the output energy later. Simulation results show the robustness of this algorithm under low signal-to-noise ration and multipath environment. In a field experiment, data transmission with very low bit error rate (BER) was achieved and the communication rate was 375bits/s.

Generalized Code Index Modulation Technique for High-Data-Rate Communication Systems

In this paper, we propose a generalized code index modulation (CIM) technique for direct-sequence spread spectrum (DSSS) communication. In particular, at the transmitter, the bit stream is divided into blocks in which each block is divided into two subblocks: mapped and modulated subblocks. Thereafter, the bits within the mapped subblock are used to select one of the predefined spreading codes, which is then used to spread the modulated bits of the second subblock. In this design, using the spreading code index as an information-bearing unit increases the overall spectral efficiency of this system. At the receiver side, the spreading code index is first estimated, thus resulting in a direct estimation of mapped subblock bits. Consequently, the corresponding spreading code to this estimated index is used to despread the modulated symbol of the modulated subblock. Subsequently, mathematical expressions for bit error rate (BER), symbol error rate (SER), throughput, energy efficiency, and the system complexity are derived to analyze the system performance. Finally, simulation results show that the proposed modulation scheme can achieve a higher data rate than the conventional DSSS system, with lower energy consumption and complexity.

Coherent RAKE Receiver for CPM-Based Direct Sequence Spread Spectrum

Direct sequence spread spectrum (DSSS) using continuous phase modulation (CPM) inherits the techniques’ benefits, constant envelope, anti-interference, and spectral efficiency. To get diversity gains over a Rayleigh-fading multipath channel as in conventional direct sequence spread-spectrum binary phase shift keying (DSSS-BPSK) system, a new class of coherent RAKE receivers is proposed in this work. By introducing chip branch metric to the receiver scheme, despreading and data detection can be done meanwhile based on Maximum Likelihood Sequence Detection (MLSD). Compared to the conventional RAKE receiver which sums decision metrics symbol-by-symbol, the proposed DSSS-CPM RAKE receiver accumulates symbol branch metric increments over every phase state of multiple paths after chip phase synchronization. Numerical results show that DSSS-CPM using the synchronous despreading and demodulation algorithm has no performance loss compared to CPM system that employs MLSD algorithm under the same test conditions. Moreover, the proposed RAKE receiver outperforms conventional RAKE receiver and achieves a remarkable diversity gain of bit error rate (BER) under the Rayleigh-fading multipath channel.

Passive time reversal communication with cyclic shift keying over underwater acoustic channels

Abstract Many advanced channel estimation and equalization algorithms have been proposed for high-rate coherent underwater acoustic communications. However, these algorithms are usually computationally demanding or ineffective under low signal-to-noise ratio (SNR). Energy-efficient or reliable communications over complex underwater multipath channels are required in many applications. In this paper, passive time reversal with cyclic shift keying (CSK) spread spectrum using hyperbolic frequency modulated (HFM) waveform is proposed for reliable point-to-point underwater acoustic communication. At the transmitter, multiple bits information is represented by the cyclically shifted version of the basic HFM waveform. Therefore, the CSK scheme provides a higher data rate than conventional direct sequence spread spectrum. At the receiver, the peak position of the cyclic correlator output is estimated to recovery information. A modified passive time reversal receiver for the proposed HFM based CSK is designed to: (1) focus the multipath signal without estimating the channel impulse response to increase the received SNR; (2) and alleviate the requirement for accurate symbol synchronization. Simulation and lake experimental results verify the proposed method and show its potential for low cost and reliable underwater acoustic communication systems.

Code Index Modulation: A High Data Rate and Energy Efficient Communication System

A new scheme, called code index modulation-spread spectrum (CIM-SS), is presented in this paper. This scheme is based on direct sequence-spread spectrum (DS-SS) modulation and can achieve higher data rate than the conventional DS-SS with lower energy consumption. At the transmitter, the bit stream is divided into blocks of two bits each. For each block, only the first bit is spread by one of two spreading codes while the second bit selects the code to be used. The receiver estimates both the code and the transmitted bit. The performance of the CIM-SS is analyzed and compared to the conventional DS-SS systems.

A Novel Approach to Signal Encryption: Improved Version of Conventional Direct Sequence Spread Spectrum Scheme

A Novel Approach to Signal Encryption: Improved Version of Conventional Direct Sequence Spread Spectrum Scheme

Self-encoded spread spectrum with iterative detection under pulsed-noise jamming

Self-encoded spread spectrum (SESS) is a novel modulation technique that acquires its spreading code from a random information source, rather than using the traditional pseudo-random noise (PN) codes. In this paper, we present our study of the SESS system performance under pulsed-noise jamming and show that iterative detection can significantly improve the bit error rate (BER) performance. The jamming performance of the SESS with correlation detection is verified to be similar to that of the conventional direct sequence spread spectrum (DSSS) system. On the other hand, the time diversity detection of the SESS can completely mitigate the effect of jamming by exploiting the inherent temporal diversity of the SESS system. Furthermore, iterative detection with multiple iterations can not only eliminate the jamming completely but also achieve a gain of approximately 1 dB at 10 BER as compared with the binary phase shift keying (BPSK) system under additive white gaussian noise (AWGN) by effectively combining the correlation and time diversity detections.

A low‐complexity MSK‐based M‐ary cyclic‐shift keying transceiver for direct sequence spread spectrum system over multipath fading channels

AbstractIn this paper, a low‐complexity spread spectrum system with M‐ary cyclic‐shift keying (MCSK) symbol spreading is proposed. In addition, by using the minimum‐shift‐keying (MSK) as the chip‐level modulation, we obtain a high‐rate QPSK‐MCSK transceiver scheme which not only provides a constant‐envelop and continuous‐phase transmitted signal, but can also achieve a better performance than the conventional direct sequence spread spectrum (DSSS) system. At the transmitter, the data stream is first mapped into QPSK‐MCSK symbols in terms of orthogonal Gold code sequences, then followed by the cyclic prefix (CP) insertion for combating the interblock interference, and finally applying the MSK scheme to maintain the constant‐envelope property. The receiver first performs MSK demodulation, then CP removal, and finally the channel‐included MCSK despreading and symbol demapping. Furthermore, the single input single output (SISO) QPSK‐MCSK transceiver can be easily extended to the multiple input single output (MISO) case by incorporating the space–time block coding for high‐link quality. Simulation results show that the proposed SISO and MISO QPSK‐MCSK systems significantly outperform the conventional DSSS counterparts under the AWGN channel, and attain a more robust performance under the multipath fading channel. Copyright © 2011 John Wiley & Sons, Ltd.

Chaotic Pulse-Position Baseband Modulation for an Ultra-Wideband Transceiver in CMOS

A CMOS impulse radio transceiver that uses chaotic pulse-position modulation for ultra-wideband (UWB) communications is designed. While the conventional direct-sequence spread-spectrum (DSSS) system dither the time delay between impulses by a pseudorandom number generator, chaotic modulation dithering uses a predictable but wideband nonlinear function, resulting in significant improvements in spectral spreading. The nonlinear functions implemented in the transmitter and receiver use a tent map, requiring only a voltage-to-time and time-to-voltage converter. A R-S latch enables the parallel implementation of two consecutive states of the tent map. Due to the predictable nature of the nonlinear mapping functions, receiver synchronization time is achieved in Order (0) time, as opposed to the conventional DSSS correlating receivers. Implemented in a 0.25 μm CMOS technology, the transceiver achieves an average data rate of 25 Mbps and a BER <; 10e-9, while sending 0.4-ns wide UWB baseband impulses.

An efficient spread spectrum pulse position modulation scheme for point-to-point underwater acoustic communication

The underwater acoustic channel is a complicated and time-varying multipath channel, and many equalization algorithms have been researched and developed to overcome the difficulties for underwater acoustic communication. Unfortunately, many algorithms are computational intensive and prone to lose convergence due to their sensitiveness to different channel configurations. In this paper, a pulse position modulation (PPM) scheme is proposed, and it uses two M-sequences of low cross-correlation to transfer information, which are modulated on two orthogonal carriers. One is used as a reference sequence, and the other is shifted relative to the reference. Information is carried by the starting time difference between the two sequences in each symbol. Comparing with conventional direct-sequence spread spectrum technique, the proposed scheme is more spectral efficient. Two receiver designs are given, one takes advantages of M-sequences’ auto-correlation properties, and the other is motivated by passive phase conjugation (PPC) to take advantages of the channel. Combined with M-sequence, PPC performance is augmented without a receiving array to cover the water column, and it is far less complex than adaptive equalizers for receivers. Results from lake field trials are analyzed, and they verify potential applications of this PPM scheme.

A Novel Four Dimensional CDMA Modem for Wireless Multimedia Communications

A description and analysis of a four dimensional (4D) Direct Sequence Spread Spectrum (DSSS) modem architecture, comprising a transmitter and a receiver, are presented. The DSSS modulator employs Complex Spreading Sequences (CSS). Two integrated synchronization loops are incorporated in the DSSS receiver, namely a four dimensional Decicion-Directed Costas Carrier Recovery Loop (DD-CCRL) combined with a 4D Decision-Directed Complex Delay-Locked-Loop (CDLL). These loops are analysed and the system performance in Additive White Gaussian Noise (AWGN) verified by means of computer simulation. It is shown that the new DSSS modulation method does not suffer any losses compared to a conventional BPSK DSSS modem, whereas the multidimensional spreading technique achieves four times the data throughput rate of a BPSK DSSS system and two times the throughput of a QPSK modulation method in the same bandwidth and with identical spreading sequence lengths. The multidimensional spreading technique therefore offers extra flexibility in terms of the choice of data rate and DSSS processing gain achievable for a given DSSS transmission bandwidth without loss of performance compared to BPSK and QPSK in AWGN. Similar results are achieved in fading channels. A prototype of the new multidimensional DSSS modulation method is presently implemented with forefront Field Programmable Gate Array (FPGA) technology with a view of realising a multimedia wireless communication system for rural areas.

A CDMA overlay system using frequency-diversity spread spectrum

The frequency bands currently used by existing narrow-band users might be shared with a code-division multiple-access (CDMA) spread-spectrum (SS) overlay system. Thus, the spectral efficiency is improved, providing more capacity for future personal communications services. Since both narrow- and wide-band signals interfere with each other, an SS modulation scheme with a good resistance to narrow-hand interference results in an increased number of acceptable overlay users. We study a frequency-diversity SS modulation scheme for which optimal detection in the presence of narrow-band interference is easy to implement. The acceptable number of overlay users is evaluated and compared to that of conventional direct-sequence SS (DS-SS) modulation schemes with and without interference rejection filters. The proposed system also presents the following advantage: by suppressing transmission of replicas where narrow-band signals (NBSs) are present, the mutual interference can be avoided completely.