Iterative Detection of Multicode DS-CDMA Signals With Strong Nonlinear Distortion Effects

We present a blind user identification method for the direct-sequence code division multiple access (DS-CDMA) signal from a sparse signal representation perspective using an overcomplete basis composed of all possible spreading sequences. The main idea of the proposed algorithm is to combine the sparsity characteristic of the DS-CDMA signal in terms of spreading sequences with the autocorrelation and the cross-correlation characteristics of spreading sequences. Firstly, the DS-CDMA signal samples are cut into several temporal windows. Each signal of the temporal window is corresponding to a measurement of the active user's spreading sequence. Secondly, the overcomplete dictionary of spreading sequences is constructed according to the structure of spreading sequence and the support coefficient of its every atom is calculated by performing the Orthogonal Matching Pursuit (OMP) method on samples of all temporal windows. Finally, with the help of the minimum description length (MDL) criterion, the number of active users is estimated and then the spreading sequences of active users are identified by inspecting their support coefficients. Simulation results demonstrate that our proposed method is effective for both synchronous short-code (SC) DS-CDMA signal and long-code (LC) DS-CDMA signal, even with many active users and low SNR.

Quadrature amplitude modulation (QAM) constellation and Golay complementary sequences (GCSs) are usually applied in orthogonal frequency division multiplexing (OFDM) systems to obtain a higher data rate and a lower peak-to-mean envelope power ratio (PMEPR). In this paper, after a sufficient search of the literature, it was found that increasing the family size is an effective way to improve the data rate, and the family size is mainly determined by the number of offsets in the general structure of QAM GCSs. Under the guidance of this idea, we propose a new construction for -QAM GCSs through generalized Boolean functions (GBFs) based on a new description of a -QAM constellation, which aims to enlarge the family size of GCSs and obtain a low PMEPR. Furthermore, a previous construction of -QAM GCSs presented by Li has been proved to be a special case of the new one, and the family size of new sequences is much larger than those previously mentioned, which means that there was a great improvement in the data rate. On the other hand, a previous construction of 16-QAM GCSs presented by Zeng is also a special case of the new one in this paper, when . In the meantime, the proposed sequences have the same PMEPR upper bound as the previously mentioned sequences presented by Li when applied in OFDM systems, which increase the data rate without degrading the PMEPR performance. The theoretical analysis and simulation results show that the proposed new sequences can achieve a higher data rate and a low PMEPR.

Orthogonal Frequency Division Multiple Access (OFDMA) with Dynamic spectrum allocation (DSA) is able to provide a wide range of data rate requirements. This paper is focused on the design of preamble sequences in OFDMA systems with low peak-to-mean envelope power ratio (PMEPR) property in the context of DSA. We propose a systematic preamble sequence design which gives rise to low PMEPR for possibly non-contiguous spectrum allocations. With the aid of Golay-Davis-Jedwab (GDJ) sequences, two classes of preamble sequences are presented. We prove that their PMEPRs are upper bounded by 4 for any DSA over a chunk of four contiguous resource blocks.

This paper considers waveform design of orthogonal frequency division multiplexing (OFDM) signal for radar applications, which aims at mitigating the envelope fluctuation in OFDM. A novel method is proposed to reduce the peak-to-mean envelope power ratio (PMEPR), which is commonly used to evaluate the fluctuation. The proposed method is based on the tone reservation approach, in which some bits or subcarriers of OFDM are allocated for decreasing PMEPR. We introduce the coefficient of variation of envelopes (CVE) as the cost function for waveform optimization, and develop an iterative least squares algorithm. Minimizing CVE leads to distinct PMEPR reduction, and it is guaranteed that the cost function monotonically decreases by applying the iterative algorithm. Simulations demonstrate that the envelope fluctuation is significantly smoothed by the proposed method.

This letter considers waveform design of orthogonal frequency division multiplexing (OFDM) signal for radar applications, and aims at mitigating the envelope fluctuation in OFDM. A novel method is proposed to reduce the peak-to-mean envelope power ratio (PMEPR), which is commonly used to evaluate the fluctuation. The proposed method is based on the tone reservation approach, in which some bits or subcarriers of OFDM are allocated for decreasing PMEPR. We introduce the coefficient of variation of envelopes (CVE) as the cost function for waveform optimization, and develop an iterative least squares algorithm. Minimizing CVE leads to distinct PMEPR reduction, and it is guaranteed that the cost function monotonically decreases by applying the iterative algorithm. Simulations demonstrate that the envelope is significantly smoothed by the proposed method.

Golay sequences have been introduced to construct 16-QAM (quaternary amplitude modulation) code for the orthogonal frequency division multiplexing (OFDM), reducing the peak-to-mean envelope power ratio (PMEPR). As an alternative way to construct Golay sequences, the construction of 16- QAM using Rudin-Shapiro polynomials (RSP) was also reported recently in several literatures. In this paper, we develop a simple efficient construction of 16-QAM using generalized RSP, which provides high code rate and large Hamming distance while tightly controlling the PMEPR.

The peak-to-mean envelope power ratio (PMEPR) of a code employed in\northogonal frequency-division multiplexing (OFDM) systems can be reduced by\npermuting its coordinates and by rotating each coordinate by a fixed phase\nshift. Motivated by some previous designs of phase shifts using suboptimal\nmethods, the following question is considered in this paper. For a given binary\ncode, how much PMEPR reduction can be achieved when the phase shifts are taken\nfrom a 2^h-ary phase-shift keying (2^h-PSK) constellation? A lower bound on the\nachievable PMEPR is established, which is related to the covering radius of the\nbinary code. Generally speaking, the achievable region of the PMEPR shrinks as\nthe covering radius of the binary code decreases. The bound is then applied to\nsome well understood codes, including nonredundant BPSK signaling, BCH codes\nand their duals, Reed-Muller codes, and convolutional codes. It is demonstrated\nthat most (presumably not optimal) phase-shift designs from the literature\nattain or approach our bound.\n

In this paper, we consider direct sequence code division multiple access (DS-CDMA) communications, (wireless LAN in the ISM band), where the undesired DS-CDMA signals have different PN sequence lengths compared with the desired DS-CDMA signal. In this situation the conventional orthogonal matched filter (OMF) receiver's performance is poor, and so we propose a new OMF receiver to take into account these undesired signals. We show, through computer simulations, that our proposed OMF receiver will outperform the conventional OMF receiver.

We design a forest with nodes that represent orthogonal variable spreading factor (OVSF) sequences of different lengths (spreading codes of different lengths that can be used for multi-rate DS-CDMA). In addition to the non-descendant OVSF property exhibited in well-known tree-structured generation of sequences [(F. Adachi, et al., 1997) and (E.H. Dinan and B. Jabbari, 1998)], the sequences represented by the nodes of our forest have useful properties that can be used to achieve multi-channel communication (with a sequence providing a channel) with a lower total peak-to-mean envelope power ratio (PMEPR). These OSVF sequences grouped by our forest structure also have properties that facilitate symbol-by-symbol adaptation of the symbol duration in multi-rate CDMA systems. For example, certain lineages in the forest have the property that any pair of code sequences in the same lineage are shift orthogonal to each other, with the unit shift length being that of the shorter sequence. We present the forest-structured generation of the sequences (spreading codes) and their properties.

For the problem of Direct Sequence-Code Division Multiple Access (DS-CDMA) signal in traditional asynchronous single-channel, including blind estimation of the Pseudo-Noise (PN) sequence and information sequence, a method using multi-channel synchronous and asynchronous based on PARAllel FACtor (PARAFAC) is proposed. Firstly, the signal is modeled as a multi-channel receiving model, then the observed data matrix is equivalent to a factor model. Finally, the iterative least squares algorithm is applied to decomposing the parallel factor, and the information sequence and PN sequences of DS-CDMA signals are further estimated. The simulation results show that the proposed method can effectively estimate the PN sequence and information sequence of the short code DS-CDMA signal, and the estimation of 6 user PN sequences can be realized under the condition that the number of channels is 10 and the Signal-to-Noise Ratio (SNR) is –10 dB.

The design of waveforms play an important role in the performance of radar systems. Multicarrier complementary phase-coded signal has been a field of interest for many researchers due to its prominent properties. This paper aims to reduce the Peak-to-Mean Envelope Power Ratio (PMEPR) of a Multicarrier Complementary Phase-Coded (MCPC) signal consisting of P subcarriers which are phase modulated by P phase sequences. The carriers are equally separated in terms of frequency. The carrier separation is equal to the reciprocal of t <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> , which is equal to the span of each phase element, forming Orthogonal Frequency Division Multiplexing (OFDM). In this paper we propose a new approach to generate MCPC signal using Complementary Block Coding (CBC) method which has lower value of PMEPR in comparison with the conventional method.

This paper considers the problem of signal eavesdropping in direct-sequence code-division multiple-access (DS-CDMA) channels. We aim at extracting the information data symbols of all concurrent users in a DS-CDMA system without the knowledge of spreading sequences (signature waveforms), and channel state information (CSI) as well as no availability of pilot signaling (training sequence). We first develop an iterative least-squares (ILS) core procedure to blindly extract the information symbols of the concurrent users. Then, enhanced ILS (E-ILS), a procedure described herein in detail that is based on statistical analysis of repeated independent ILS processing of observation signal, is seen to offer most effective signal eavesdropping. Simulation studies show that the proposed E-ILS signal eavesdropping algorithm can achieve probability of error close to what may be attained with known spreading sequences and CSI.

Orthogonal frequency division multiplexing (OFDM) is an attractive technique for wireless communication applications. However, an OFDM signal has a large peak-to-mean envelope power ratio (PMEPR), which can result in significant distortion when transmitted through a nonlinear device, such as a transmitter power amplifier. We investigate, through extensive computer simulations, the effects of clipping and filtering on the performance of OFDM, including the power spectral density, the bit error rate, and the PMEPR. Our results show that clipping and filtering is a promising technique for the transmission of OFDM signals using realistic linear amplifiers.

A complementary M-ary orthogonal spreading (CMOS) architecture with orthogonal frequency division multiplexing (OFDM) is proposed to improve performance of bit-error rate (BER) and spectrum efficiency for high frequency (HF) communications. The CMOS-OFDM architecture is based on orthogonal complementary pairs with their central elements being zero. It is well known that complementary pairs theoretically reduce peak-to-mean envelope power ratio (PMEPR) of OFDM due to their ideal aperiodic auto-correlation properties. However, many practical OFDM applications, such as HF OFDM and IEEE 802.11, require that no data should be modulated to direct current (DC) subcarrier, which will destroy ideal properties of traditional complementary pairs and hence increase their PMEPRs. Compared with traditional complementary pairs, the proposed zero-centre complementary pairs (ZCCPs) in CMOS-OFDM architecture may maintain ideal properties and hence be more suitable for OFDM systems as long as zero element in the centre is modulated to DC subcarrier. The corresponding simulation results and numerical analyses over the HF fading channel modelled by ITU-R F.1487 are provided. Different from the previous researches on HF OFDM systems, the proposed HF CMOS-OFDM architecture using the constructed ZCCPs not only possesses excellent BER performance, but also may firmly resist PMEPR to 3 dB or near to 3 dB.

Based on the sampling-concatenating operation with any integer interval, a large number of orthogonal subsets are designed for M-ary spread spectrum (MSS) communications with orthogonal frequency division multiplexing (OFDM) modulation. Any two sequences in the designed subset are orthogonal to each other, that is, the in-phase cross-correlation (IPCC) between them is equal to zero. Compared with the previous researches, the designed multiple subsets may possess more sequences at the cost of the peak-to-mean envelope power ratio (PMEPR) and inter-subset IPCC, which will be suitable for MSS-OFDM communications where the larger number of sequences means higher data rate. We provide a simple example to show how the designed algorithm works, and the corresponding performance of PMEPR and IPCC is analyzed.