Superposition-based adaptive modulated space time block coding for MIMO-OFDM systems

An underwater acoustic multimedia communication (UWAMC) system is proposed with 2400 transmission modes according to time-varying multipath underwater acoustic (UWA) channel conditions. The orthogonal variable spreading factor (OVSF) scheme and Gold sequence (GS) scramble code are integrated into multi-input multi-output UWAMC system based on orthogonal frequency-division multiplexing to achieve the quality of service of multimedia transmission in the UWA channel. Binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) adaptive modulation, direct mapping (DM) or space–time block code (STBC) transmission strategies, convolution channel code with rate 1/2 and 1/3, and a power assignment mechanism were adopted in the proposed system. Simulation results show that the bit error rate (BER) and power saving ratio (PSR) performance of the STBC strategy with transmission diversity is superior to that of the DM strategy without transmission diversity, and the performance of the BERs and PSRs of the transmission scheme with the GS scramble code surpasses that of the scheme without the code. The performance of the BERs and PSRs of BPSK modulation with a channel code rate of 1/3 is better than that of BPSK modulation with a channel code rate of 1/2, and the performances of BERs and PSRs of BPSK modulation with a channel code rate of 1/3 are better than that of QPSK modulation with a channel code rate of 1/3. As the length of the OVSF codes increases, the UWAMC system’s BERs decrease, and its PSRs increase. The UWAMC system can achieve either maximum transmission speed or maximum transmission power efficiency.

Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation scheme widely used in 4G. It is effective for reducing the effects of multipath delay in wireless communication. For OFDM system, total delay spread (Td) is less than symbol period (T); it can be combined with antenna arrays at transmitter and receiver (MIMO) to increase diversity gain called as MIMO-OFDM system. MIMO-OFDM improves system capacity on time-variant and frequency-selective channels. The quality of service provided by a system greatly depends upon the correct selection of modulation scheme. In this paper, analysis and simulation of OFDM-MIMO system with different digital modulation schemes like BPSK, 8-PSK, QPSK, and QAM is performed using Alamouti space–time block codes (STBC). STBC uses spatial and time dimensions for error control unlike traditional methods that uses only time dimension. A comparison study shows that for a given data rate and channel conditions, OFDM-MIMO using STBC with BPSK modulation is better compared with other digital modulation techniques in terms of error performance. OFDM-MIMO using STBC with BPSK modulation is further analyzed and simulated for various antenna configurations. It is observed that as numbers of antennas at transmitter and receiver side are increased, the system diversity increases and gives better performance in terms of BER.

Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission scheme. OFDM transmits data by using a large number of narrow bandwidth carriers. In an OFDM transmission system, each subcarrier is attenuated individually under the frequency-selective and fast fading channel if the same fixed transmission scheme is used for all OFDM subcarriers thus it results in highest attenuation and hence poor performance. The purpose of this paper is to introduce the adaptive modulation to get an understanding of the differences between fixed and adaptive modulations schemes. In this work adaptive modulation is implemented by dividing whole subcarriers into blocks of adjacent subcarriers. Based on calculated average instantaneous signal to noise (SNR) same modulation scheme is applied to all subcarriers of same block. Here average bit error rate (BER) performance of OFDM system under fixed modulation and adaptive modulation is observed. Average BER performance of these modulation techniques is observed with various inverse fast Fourier transform (IFFT) size and using simpler adaptive quadrature amplitude modulation (QAM) schemes. The simulation results show that BER performance of OFDM system using adaptive modulation is better than fixed modulation.

We introduce a new CDMA system architecture for combining space time block coding and adaptive sectorization using an electronically tunable reflectarray. The advantage of this architecture is that it allows both adaptive sectorization and space time coding to be implemented with only one base station transmit chain and amplifier. Tunable reflectarrays enable adaptive beam patterns to be synthesized from a single RF signal. It is shown that space time block codes can be implemented with these arrays though a combination of baseband processing and manipulation of the reflectarray phase response. Simulations illustrate that space time coding implemented using reflectarrays will significantly improve CDMA forward link performance, even when reflectarray imperfections are taken into account.

In this paper, we propose two adaptive schemes where either the modulation mode or the space-time block code (STBC) pattern is changed for a space-time coded system with transmit antenna selection. Simulation results show that the improvements in terms of capacity, data throughput and BER can be obtained over the nonadaptive schemes

In this letter, an orthogonal space-time (ST) block coding (STBC) technique for continuous phase modulation (CPM) is proposed and combined with a frequency domain equalizer (FDE) to obtain high performance gains over frequency-selective channels. The new method maintains the constant envelope and phase continuity of the CPM waveforms perfectly by using appropriate tail symbols during the addition of cyclic prefix and interblock transitions and, therefore, has no negative impact on the spectral efficiency. Because the STcoded CPM blocks remain orthogonal, FDE is applied as in the case of single antenna transmissions without any increase in the computational load. Simulations corroborate the performance improvement offered by STBC while also preserving the desired bandwidth efficiency of CPM.

Orthogonal frequency division multiplexing (OFDM) is the contemporary technology adopted for digital audio/video broadcasting as well as wireless local-area and metropolitan-area networks. Since the wireless multimedia services often have different quality-of-service requirements and their performance is sensitive to the channel conditions, the conventional fixed OFDM modulation scheme might not be a satisfactory solution nowadays. In this paper, we introduce a novel pilot-free adaptive modulation scheme, which is bandwidth-efficient and allows variable data rates, for the future robust OFDM systems. We design a number of modulation modes in a combination of different constellation sizes and different polynomial cancellation coding methods (PCC) to combat the crucial intercarrier interference problem. Instead of estimating the channel quality based on the overhead pilot symbols, we propose to directly estimate the signal-to-noise ratio (SNR) without using any pilot. Besides, our scheme offers more modulation modes than some other existing adaptive modulation methods which are simply based on different constellation sizes. According to the Monte Carlo simulations, the empirical results show that our adaptive modulation scheme, in most channel conditions (SNR/spl ges/15 dB), not only can satisfy the predetermined bit error rate (BER) requirement (BER/spl les/10/sup -4/) but also can dynamically enhance the throughputs in the rather clean environments with high SNR values.

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Orthogonal frequency-division multiplexing (OFDM) is the ubiquitous contemporary technology adopted for digital audio/video broadcasting, as well as wireless local and metropolitan area networks. Since the wireless multimedia services often have different quality-of-service requirements and their performance is sensitive to the channel conditions, the conventional fixed OFDM modulation scheme might not be a satisfactory solution nowadays. In this paper, we introduce a novel pilot-free adaptive modulation scheme, which is bandwidth efficient and allows variable data rates, for the future robust OFDM systems. We design a number of modulation modes in a combination of different constellation sizes and different polynomial cancellation coding (PCC) methods to combat the crucial intercarrier interference (ICI) problem. Instead of estimating the channel quality based on the overhead pilot symbols, we propose to directly estimate the signal-to-noise ratio (SNR) free of using any pilot. Moreover, our scheme offers more modulation modes than some other existing adaptive modulation methods, which are simply based on different constellation sizes. According to the Monte Carlo simulations, the empirical results show that our adaptive modulation scheme, in most channel conditions <formula formulatype="inline"><tex>$(\hbox{SNR} \geq 13\ \hbox{dB})$</tex></formula>, not only can satisfy the predetermined bit-error-rate (BER) requirement <formula formulatype="inline"><tex>$(\hbox{BER} \leq 10^{-4})$</tex></formula> but also can dynamically enhance the throughputs in the rather clean environments with high SNR values. </para>

In this paper, a novel combination scheme of hybrid ARQ and adaptive modulation and coding (AMC) with space-time block coding (STBC) and turbo codes for wireless packet data transmission is proposed. Performance of the proposed HARQ/AMC system is investigated and compared in terms of frame error rate (FER) and throughput.

We consider a combined space-time (ST) block coding and direct-sequence code division multiple access (DS/CDMA) scheme for downlink transmissions over multipath fading channels, in which ST block coding is performed at the chip level, i.e. after code spreading. For this scheme, we develop low complexity and low decoding delay linear ST single-user detection algorithms. In addition, we demonstrate that by exploiting the signal structure imposed by the chip-level ST block coding, blind channel multipath estimation (with only a scalar ambiguity) is feasible, even with the assignment of a single code vector to each user.

The blind classification of space-time block codes (STBCs) is a key research issue related to cognitive radio. There have been numerous studies attempting to classify STBCs, but the blind classification of STBCs with transmission impairments merits further research. The novel blind classification algorithm proposed in this paper exploits the fourth-order cyclostationarity of STBCs to classify them with a single receiver antenna in the presence of transmission impairments. The proposed algorithm relies on the fourth-order cyclic moment as a discriminating feature and employs the distance between adjacent spectral lines to classify the transmitted STBCs. As opposed to other methods, this algorithm does not require a priori information regarding the modulation scheme, channel coefficients, or noise power; consequently, it is very well-suited to non-cooperative scenarios. According to rough estimation of symbol duration and carrier frequency offset, the distance between adjacent spectral lines remains almost the same with value dependent on the transmitted STBCs. Simulations indicate that the proposed method performs well even at low signal-to-noise ratios (SNRs).

An adaptive sub-carrier modulation in OFDM transmission is an effective scheme in order to increase transmission rate because it is possible for OFDM to determine a modulation scheme of each sub-carrier independently according to its channel state information. However, in this case, it is necessary to transmit allocation information of modulation from the transmitter to the receiver and this leads to degradation of performance. In this paper, an adaptive modulation scheme for OFDM that makes possible less information transmission from the transmitter to the receiver for allocation information is proposed. Moreover, sub-carrier power control is combined in order to improve performance further. As the result of theoretical analysis and simulation, better performance of the proposed scheme was confirmed.

The performance analysis of a space–time block coded multiple input multiple output system with variable-power (VP) adaptive modulation (AM) over Rician fading channels for imperfect channel state information is presented. The optimum switching thresholds for attaining maximum spectrum efficiency (SE) subject to a target bit error rate (BER) and an average power constraint are derived. In the derivation, a very tight BER expression of quadrature amplitude modulation (QAM) is used to develop a new power adaptation scheme that can fulfil the target BER even at low signal-to-noise ratio (SNR) and achieve higher SE than the scheme using the BER upper bound commonly adopted in the literature. The existence and uniqueness of the Lagrange multiplier used in the constrained optimisation are investigated. It is shown that the Lagrange multiplier is unique when it exists. By using the switching thresholds, the authors obtain closed-form expressions of SE and average BER of the system. Computer simulation shows that the theoretical analysis is in good agreement with simulation. The results show that the proposed VP-AM scheme provides better SE than the constant-power counterparts and the VP-AM scheme using the commonly used BER bound.

Multiple input multiple output (MIMO) techniques are widely employed in modern wireless systems. The most prominent approaches to MIMO communication are space-time coding (STC) and spatial multiplexing (SM). Less studied is a hybrid technique called adaptive MIMO switching (AMS), which may be employed by fixing the transmission rate and selecting between space-time block coding and SM transmission modes based on instantaneous channel conditions to achieve the most efficient link. This study presents a hardware implementations of a selector algorithm and a detector employing it. The detector is designed for a 2×2 MIMO system and was implemented on an Altera Cyclone II FPGA. In addition, two transmission protocols with decision feedback from the receiver to the transmitter via a low-rate feedback channel are introduced and the feasibility of switching for high-data-rate transmissions is discussed. © 2012 The Institution of Engineering and Technology.

Due to the half-duplex transmission nature of cooperative protocols, almost all cooperative diversity systems suffer from low spectral efficiency. In general, adaptive modulation can offer a good tradeoff between spectral efficiency and bit-error-probability (BEP) performance of wireless communication systems. In this paper, we propose two adaptive modulation schemes for the cooperative system with detect-and-forward relaying that simultaneously exploit the cooperative diversity gain and improve the spectral efficiency of the system. In the first scheme, which is similar to conventional selection relaying, the relay decides independently whether to forward the source messages to the destination or not by comparing the instantaneous received signal-to-noise ratio (SNR) with a predetermined fixed threshold, which is independent of the instantaneous modulation scheme. In contrast, in the second scheme, the relay and the destination jointly decide whether the relay will participate in the second phase of the protocol or remain silent. This second scheme can be viewed as a combination of selection and incremental relaying and achieves higher spectral efficiency in comparison with its nonincremental counterparts. We evaluate the performance of the protocols in terms of the end-to-end BEP, the average spectral efficiency, and the outage probability for both identically and nonidentically distributed Rayleigh fading environments. In our analysis, we consider M-ary phase-shift keying ( M-PSK) modulation, M -ary pulse-amplitude modulation (M-PAM), and M-ary quadrature-amplitude modulation ( M-QAM) schemes and examine the impact of error propagation from the relay to the destination on the end-to-end performance of the system in detail. Moreover, we derive a closed-form expression for the optimal relaying threshold that effectively mitigates error propagation and minimizes the end-to-end BEP of the system. The high-SNR analysis demonstrates that both proposed schemes provide full diversity.