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We propose in this letter the time-based receive transformation (TRT) to suppress the inter-block interference (IBI) in the single-input-single-output (SISO) system with faster-than-Nyquist (FTN) signaling under the multi-path channel. For the trailing-zeros based FTN signaling or the OFDM based FTN signaling, the achievable rate benefits from the increased symbol rate but is degraded by the increased number of symbols that multi-path inter-symbol interference (ISI) affects. We show that the TRT based FTN signaling can completely eliminate the effect of IBI so that its achievable rate benefits from both the increased symbol rate and the increased number of symbols that multi-path ISI affects. We also show that the achievable rate for the TRT based FTN signaling is larger than that for the trailing-zeros based FTN signaling and the OFDM based FTN signaling, and this advantage increases with the ratio of the symbol rate to the Nyquist rate.

A novel method based on constellation structure is proposed to identify PSK and QAM modulation of different orders, in the slow and flat fading channel. The proposed method does not require training for threshold optimization and considers carrier frequency, symbol rate, and phase offset unknown. The symbol rate is estimated using the spectrum of the instantaneous phase of the complex baseband signal. Carrier frequency offset (CFO) is estimated and corrected from the downconverted signal and downsampled to the estimated symbol rate for extraction of constellation points. The phase offset is determined based on the symmetrical structure of constellation. The features extracted using k-medoids are used for classification of the final modulation scheme. Results show that the proposed algorithm outperforms some existing classifiers and offers lower computational complexity compared to algorithms based on subtractive clustering.

Light-emitting diode (LED)-based communications, such as visible light communications and infrared communications, are candidate techniques to provide short-range and high-speed data transmission. In this letter, $M$ -ary pulse amplitude modulation (M-PAM), used as a high bandwidth efficiency scheme, is compared with three well-known optical orthogonal frequency-division multiplexing (OFDM) schemes. Considering the bandwidth limit and constrained peak transmitted power characteristics of LEDs, a bit loading algorithm and a single-tap equalizer with an optimized modulation index are used for the optical OFDM schemes tested. To reduce the inter-symbol interference caused by the bandlimited channel, an optimized pulse shape and a minimum mean squared error equalizer are applied to the M-PAM system. From numerical results, M-PAM can provide a substantially higher data rate than OFDM for bandlimited channels. When the channel bandwidth is ample compared with the symbol rate, optical OFDM outperforms M-PAM.

In this letter, we propose a space–time coding construction for the two parallel blocks free-space optical communication with four transmitting lasers and at least two receiving apertures. The proposed coding scheme has a rate of two symbols per channel use and achieves the full spatial and temporal diversity. It extends the previously quadratic field compositum (QFC) constructed for a single block transmission. Compared with other replication coding schemes, the extended QFC code has a higher symbol rate, and then, a lower constellation order is required to achieve the same target data. As a consequence, the effective signal-to-noise ratio at the receiver side is enhanced, which in turn reduces the packet error rate.

In this letter, a robust nondata-aided (NDA) carrier tracking method is proposed for low-density parity-check (LDPC) coded systems with symbol rate sampling. The proposed method has a novel and hybrid carrier tracking loop combined with the frequency-locked loop (FLL), increasing/decreasing sampling rate blocks, phase-locked loop (PLL), and modified symbol decision method, which is capable of improving estimation accuracy of the frequency offsets and phase offsets at low signal-to-noise ratios (SNRs). When applied to the $(4096,3072)$ LDPC coded system, numerical simulations and comparisons are provided to validate the effectiveness of the proposed carrier tracking method in the case of relatively low SNRs.

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.

This letter deals with symbol sequence detection of multi-receiver signals based on the maximum-likelihood (ML) criterion. A novel iterative scheme is proposed to estimate the symbols without synchronization or alignment of timing and phase. Instead of the accurate delay compensation and optimal samples recovery, the detection of symbols employs the direct calculation of the matched filter outputs, eliminating the need for separate synchronizer and combiner. Since the received samples at each branch are viewed as incomplete observations, the joint ML estimation of symbols can be obtained in an iterative way by means of the expectation-maximization algorithm. The proposed scheme is compared with conventional signal combining schemes and joint ML synchronizers by simulation results. It is shown that the proposed detector enables significant improvement on the bit-error rate for weak received signals.

A novel and flexible means of symbol rate adaptation for MIMO systems is presented and analyzed. Constellations of L Grassmannian subspaces are used to precede a length-M symbol vector, resulting in a symbol rate of M/L. The result is a practical and versatile method of trading between diversity and multiplexing gains at operational signal-to-noise ratios.

This letter describes an improved method to mitigate interference when the cyclic prefix is not sufficient for coded orthogonal frequency-division multiplexing (OFDM) systems. The proposed method, based on the residual intersymbol interference cancellation (RISIC), efficiently restores the cyclicity of the ith received symbol by adding the weighted (i+1)th received symbol to the ith received symbol. Simulation results show that the proposed scheme outperforms the conventional RISIC in the symbol error rate even when the channel impulse response is longer than a half of the symbol duration.

We propose an adaptive minimum variance equalizer, which is able to recover the transmitted symbols even in the presence of severe cochannel interference (CCI) or adjacent-channel interference (ACI). Under the assumption that the interference exhibits a different symbol rate from the desired signal, the proposed equalizer can mitigate CCI/ACI without requiring knowledge of the channel impulse response of the interference.

We investigate the sensitivity of a downlink MC-DS-CDMA system to carrier frequency offsets. It is shown that the resulting interference power depends only on the ratio of the frequency offset to the carrier spacing. Hence, for given a frequency offset and system bandwidth, the MC-DS-CDMA performance degradation is independent of the spreading factor but increases with the number of carriers; equivalently, for given frequency offset and symbol rate, the performance degradation increases with the ratio of the number of carriers to the spreading factor.

This letter presents simulation results for the application of turbo coding to an OFDM system with diversity. First, results are presented for convolutional and Reed-Solomon codes. It is shown that even with short constraint lengths, convolutional codes have the potential to outperform Reed-Solomon codes, provided that sufficient precision is used in the soft decoder. We then evaluate the performance of turbo codes under slow fading conditions and study the effects of varying codeword size. Increasing codeword size theoretically provides better interleaving between the two component codes. However, this advantage is less clear when the fading rate is significantly lower than the symbol rate, which is typical of the high-data-rate systems considered here. Under such conditions, the advantage of using two component convolutional codes in turbo codes is limited. A single convolutional code with a long constraint length may be a better choice.

A novel blind algorithm is proposed for joint carrier phase/Doppler shift least-squares (LS) estimation for burst transmission of m-PSK modulated data. The algorithm can be extended to include Doppler rate estimation. The performance of the algorithm is contrasted against that of known algorithms, and it is shown to be advantageous particularly for short bursts and large frequency offsets. The signal-to-noise ratio penalty with respect to the ideal estimation case is shown to be below 1 dB for a frequency error as large as 0.1 of the symbol rate and bursts as short as 20 symbols.