Frequency-domain parallel multiuser detection for quasi-constant envelope OQPSK schemes with high spectral efficiency

In this paper, we consider SC-FDE schemes (Single Carrier with Frequency-Domain Equalization) employing OQPSK (Offset Quaternary Phase Shift Keying) signals with quasi-constant envelope. To allow high spectral efficiency, different frequency channels are separated by the symbol rate, regardless of their bandwidth (which is much higher than the symbol rate for quasi-constant envelope OQPSK signals), and we consider a multi-channel frequency-domain receiver with ACI (Adjacent Channel Interference) suppression. Our performance results show that it is possible to have high spectral efficiency with quasi-constant envelope OQPSK signals. This technique is especially interesting for the uplink of future broadband wireless systems since we can employ low-cost, grossly nonlinear power amplifiers at the mobile terminals and almost all signal processing is shifted to the base station.

The paper addresses the performance analysis of pulse-shaped direct-sequence ultra-wideband (UWB) communication systems employing offset quadrature phase shift keying (OQPSK). The proposed system offers many advantages over carrier-less UWB schemes, as pulse-shaped OQPSK spreading gives better phase randomization in multi-user interference scenarios, and is also useful for minimizing large phase jumps in the modulated signals, which reduces spectral re-growth and adjacent channel interference, particularly in the presence of non-linear amplifiers. Bit error probability results are derived for fading channels with selection diversity and maximum ratio combining receivers. Numerical examples with pulse shapes based on Gaussian monocycle derivatives and modified Hermite polynomials are given to illustrate the relative performance comparisons among different chip pulse shapes and diversity combining techniques.

Constant envelope signals such as MSK, GMSK have been widely used in satellite system for their power and spectral efficiency. Shaped Offset Quadrature Phase Shift Keying (SOQPSK) belongs to excellent Continuous Phase Modulation (CPM) waveforms. With constant envelope and continuous phase, SOQPSK signal has both high power and spectral efficiency, which makes it quite suitable for satellite communication. But, satellite's high velocity expands the carrier frequency of the SOQPSK signal. To adapt it, this paper proposes a novel carrier synchronization technology based on a modified COSTAS loop, which is assisted by Fast Fourier Transformation (FFT). Firstly, FFT is used to get a rough estimation of the Doppler frequency and limit the frequency offset to a smaller range. Secondly, a modified COSTAS loop is used to track the residual frequency offset and the phase error. Simulation results show that the proposed algorithm can not only track large Doppler frequency, but also performs better than traditional COSTAS loop when it comes to track small Doppler frequency.

Similar to the orthogonal frequency division multiple access (OFDMA) system, the single-carrier frequency division multiple access (SC-FDMA) system also suffers from frequency mismatches between the transmitter and the receiver. As a result, in this system, the carrier frequency offsets (CFOs) disrupt the orthogonality between subcarriers and give rise to inter-carrier interference (ICI) and multiple access interference (MAI) among users. The authors present a new minimum mean square error (MMSE) equaliser, which jointly performs equalisation and carrier frequency offsets (CFOs) compensation. The mathematical expression of this equaliser has been derived taking into account the MAI and the channel noise. A low complexity implementation of the proposed equalisation scheme using a banded matrix approximation is presented here. From the obtained simulation results, the proposed equalisation scheme is able to enhance the performance of the SC-FDMA system, even in the presence of estimation errors.

Phase shift keying modulation approaches are widely used in the communication industry. Differential phase shift keying (DPSK) and Offset Quadrature phase shift keying (OQPSK) schemes are chosen to be investigated is multi environment channels, where both systems are designed using MATLAB Simulink and tested. Cross talk and unity of signals generated from DPSK and OQPSK are examined using Cross-correlation and auto-correlation, respectively. In this research a proposed system included improvement in bit error rate (BER) of both systems in the additive white Gaussian Noise (AWGN) channel, by using the convolutional and block codes, by increasing the ratio of energy in the specular component to the energy in the diffuse component (k) and the diversity order BER in the fading channels will be improved in both systems.

Radio over fiber is becoming an increasingly important technology for the wireless market since it introduces a higher data transmission rate and large bandwidth. In this paper, we have compared OQPSK and DPSK bidirectional radio over fiber ROF systems, where an offset quadrature phase shift keying (OQPSK) or differential phase shift keying (DPSK) signals are used for down-link and an on-off keying (OOK) signal re-modulated for up-link. Several measurements were performed including Bit Error Rate (BER) curves for uplink and downlink, RSOA gain curve and noise figure with the variation of input power and temperature for each system.

Recently, the generalisation of orthogonal frequency division multiplexing with index modulation (OFDM-IM) was proposed in the authors’ previous works. Two generalisation schemes of OFDM-IM, named as OFDM with type-1 generalised index modulation (OFDM-GIM1) and OFDM-GIM2, have been proposed and investigated. In OFDM-GIM1, the number of active subcarriers in an OFDM subblock is no longer fixed. In OFDM-GIM2, independent index modulation is performed on the in-phase and quadrature component of quadrature phase shift keying symbol of each subcarrier. Through such ways, a higher spectral efficiency than that of OFDM-IM may be achieved. In this study, the authors focus on further improving OFDM-GIM2's spectral efficiency by introducing joint I/Q index modulation where the two index modulations on the in-phase and quadrature components are no longer entirely independent. In other words, in the proposed scheme, the input bit string jointly decides the indices of active subcarriers for in-phase and quadrature components. As a result, more bits can be transmitted per OFDM frame. Computer simulation results clearly show the proposed scheme's superiority in both spectral efficiency and bit error rate performance compared with the existing OFDM-GIM2.

Shaped offset quadrature phase shift keying (SOQPSK) modulation scheme and its application in optical wavelength-division multiplexed (DWDM) transmission

Abstract
 
 Zigbee is considered to be one of the wireless sensor networks (WSNs) designed for short-range communications applications. It follows IEEE 802.15.4 specifications that aim to design networks with lowest cost and power consuming in addition to the minimum possible data rate. In this paper, a transmitter Zigbee system is designed based on PHY layer specifications of this standard. The modulation technique applied in this design is the offset quadrature phase shift keying (OQPSK) with half sine pulse-shaping for achieving a minimum possible amount of phase transitions. In addition, the applied spreading technique is direct sequence spread spectrum (DSSS) technique, which has the effect in getting better performance for the transmitter system in multipath environments. The system is developed through the use of MATLAB/Simulink to show the performance of the proposed system in response to the appliy modulation technique.
 
 Keywords: DSSS technique, IEEE 802.15.4, MSK, OQPSK, Zigbee.

The increase in the throughput of digital television and radio broadcasting (DVB) channels can be achieved due to application of signals with a compact spectrum and a relatively small peak-to-average power ratio (PAPR). The reason is the usage of traveling wave tubes (TWT) for amplifying and transmitting signals from a satellite repeater in DVB-S2X systems. At the same time, given that the bandwidth allocated for transmission should be used as efficiently as possible, a high reduction rate of out-of-band emissions level is required. The most effective solution in this direction is the transition to spectrum-economic signals, such as optimal Faster-Than-Nyquist (FTN) signals, which can provide a certain reduction rate of the out-of-band emissions level and minimum acceptable PAPR. This article proposes a method for obtaining optimal FTN pulses, which have symmetry in time domain, with specified PAPR and reduction rate of out-of-band emissions for the quadrature phase shift keying (QPSK) and offset quadrature phase shift keying (OQPSK). The possibility of synthesizing signals with OQPSK modulation is presented theoretically for the first time. Optimal FTN signals can provide PAPR reduction by at most 3 dB and outperform known root raised cosine (RRC) pulses. The simulation model adopts an architecture for quadrature generation of optimal FTN signals with OQPSK modulation with blocks for adjustable pre-amplification, clipping, and power amplification. The proposed signals can be used to increase the spectral and energy efficiencies of satellite broadcasting systems, such as DVB-S2/S2X, as well as low-rate return channels of interactive broadcasting systems with a frequency resource shortage.

This paper considers frequency-domain receiver design for OQPSK signals (Offset Quaternary Phase Shift Keying). It is shown that conventional QPSK receivers are not suitable to OQPSK schemes since they lead to significant interference between in-phase (I) and quadrature (Q) components. To overcome this problem we propose several linear receivers where there is no I-Q interference. Our receivers are suitable to OQPSK schemes, significantly outperforming frequency-domain receivers designed for QPSK modulations.

Single-carrier frequency division multiple access (SC-FDMA) is a promising alternative to orthogonal frequency division multiple access (OFDMA) for uplink wireless communications in frequency-selective fading environments. In this paper, we investigate the impact of the power amplifier (PA) on the performance of the SC-FDMA system. The suitable saturation level thresholds of the PA that can be utilized in the SC-FDMA system are studied and determined through simulations for different modulation formats. Simulation results show that the saturation level of the PA has a significant effect on the bit error rate (BER) performance of the SC-FDMA system, especially with high order modulation formats.

The paper considers a wireless transmission system in a multiple-user environment. It involves a combination of non-binary convolutional codes with Continuous-Phase Modulation (CPM) for transmission in an uplink channel using the Frequency Division Multiple Access (FDMA) method. For a more efficient available band utilization, the distances between individual system user carriers are decreased. In order to reduce the bit error rate (BER) a concatenation of CPM with convolutional codes over a ring has been used and a low-complexity iterative algorithm for inter-carrier interference (ICI) cancellation was employed on the receiver side. BER results obtained in computer simulations are presented for the proposed solution.

We consider cellular network assisted Device-to- Device (D2D) communication in an orthogonal resource sharing mode. An orthogonal Frequency Division Multiple Access (FDMA) or single-carrier FDMA system is considered, and fractional power control related to the received signal at the base station is used for both cellular and D2D transmissions. This guarantees that the interference from D2D to cellular transmissions is on tolerable level. With transceiver I/Q imbalance, there may be severe interference from cellular to D2D communication when these two happen on mirror frequency resources. This leads to an I/Q near-far effect. Signal-to-interference- plus-noise ratio statistics are investigated for D2D reception in the presence of I/Q imbalance. Closed form expressions for the average cellular interference are derived. The contribution of the I/Q imbalance is comparably high at cell edge, despite the fact that the absolute interference power decreases, when the D2D pair approaches cell edge. The I/Q near-far effect the orthogonal frequency resources given to D2D.

The huge bandwidth available at 60 GHz will allow short range wireless communication to deliver bit rates over 1 Gbps. However the design of millimeter wave analog blocks is more critical than at lower frequencies, leading to a possible high non ideality of the radio front-end (FE). A suitable air interface for low cost, low power 60 GHz transceivers should thus use a modulation technique that has a high level of immunity to FE non-idealities. In this paper, we compare the sensitivity to FE non-idealities of two promising air interfaces namely offset quadrature phase shift keying (OQPSK) with frequency domain equalization (FDE) and continuous phase modulation (CPM) with time domain equalization (TDE). Our study focus on three main analog imperfections that are likely to have the biggest impact on the overall system performance: phase noise generated by the voltage control oscillator (VCO), clipping and quantization errors caused by the analog-to-digital converter (ADC) and non-linearity in the power amplifier (PA). Results show that CPM based air interface is more robust than OQPSK-based regarding the FE non-idealities. However the latter is less complex to design and offers much higher data rate than the former in the same bandwidth.