Adaptive Modulation System Research Articles

THROUGHPUT RETURNS DUE TO ADAPTIVE MODULATION TECHNIQUES

Adaptive modulation is a powerful technique to increases the throughput of a wireless network by adjusting the modulation scheme to the channel status. We investigate the throughput returns due to the employments of adaptive modulation techniques (MPSK:M-ary Phase Shift Keying or MQAM:M-ary Quadrature Amplitude Modulation). In this project, the Bit Error Rate (BER) performance and spectral efficiency of a coded adaptive modulation technique over a multipath fading channel (no fading, flat fading and dispersive fading) will be implemented. By using simuation, we find out that the SNR increased because higher modulation technique is use. While SNR decrease when the lower modulation technique is use. For this case, when SNR>8dB, bit rate is 54Mbps, the 64QAM ¾ is apply and when SNR<1, bit rate is 6Mbps, the BPSK ½ is implement. This results in good data rates in the system. Therefore, Adaptive modulation systems offer a significant throughput and/or power gain compared to non-adaptive transmission, particularly when the channel conditions are slowly time varying and a reliable feedback channel is available.

Modified 24 kb/s ADPCM System

This paper presents new modified 24 kb/s Adaptive Differential Pulse Code Modulation (ADPCM) system. The modified ADPCM uses 4-bit quantizer with sampling rate of 6,000 sample/s, while, the standard ADPCM uses 3-bit quantizer with sampling rate of 8,000 sample/s. The purpose of this modification is to reduce the nonlinear distortion introduced by ADPCM when QAM signal is sent over it. The performances of standard and modified ADPCM are studied using QAM signal at data rate of 9.6 kb/s. The simulation results show that the performance of modified ADPCM is better than standard ADPCM.

Performance Evaluation of Adaptive Modulation System over Mobile Satellite Channels

The performance of an adaptive five-mode M-ary quadrature amplitude modulation (QAM) system under constant power constraint over time-varying land mobile satellite channel is evaluated in this paper. The mathematically-tractable closed-form expressions of performance for outage probability, bit error rate (BER) and average spectral efficiency are deduced by a simple Rician channel model in which the amplitude of the line-of-sight path is represented by the Nakagami distribution. Two adaptive modulation schemes (AMS) are compared, one with the optimal switching thresholds to maximize the average throughput under the constraint of average BER and another with fixed thresholds defined by the given BER tolerance for each modulation. Numeric simulation results show that the M -QAM AMS with optimum switching levels offers performance gains of ~2dB compared to fixed mode at given average spectrum efficiency, and nearly 0.3bps higher average throughput for different Rician K-factor.

Data-Aided Noise Reduction for Long-Range Fading Prediction in Adaptive Modulation Systems

The long-range prediction (LRP) of fading signals enables adaptive transmission methods for rapidly varying mobile radio channels encountered in vehicular communications, but its performance is severely degraded by the additive noise and interference. A data-aided noise reduction (DANR) method is proposed to enhance the accuracy of fading prediction and to improve the spectral efficiency of adaptive modulation systems enabled by the LRP. The DANR includes an adaptive pilot transmission mechanism, robust noise reduction (NR), and decision-directed channel estimation. Due to improved prediction accuracy and low pilot rates, the DANR results in higher spectral efficiency than previously proposed NR techniques, which rely on oversampled pilots. It is also demonstrated that DANR-aided LRP increases the coding gain of adaptive trellis-coded modulation (ATCM). Finally, for low-to-medium signal-to-noise ratio (SNR) values, we show that LRP-enabled adaptive modulation performs better for realistic reflector configurations than for the conventional Jakes model (JM) data set.

Transmission of Visual Data over Wireless Fading Channel in Real-Time Systems Based on Superposition Coding Scheme

Real-time visual applications are among the most important requirements in the next generation wireless communication systems. In these applications, the transmitted data comprise different layers with different importance levels based on their influence on the reception quality. Furthermore, the real-time transmission nature of these applications makes them sensitive to data losses and transmission delay. To address these issues, an efficient superposition adaptive modulation and coding system, for the optimal system performance, is proposed in this paper. The proposed system switches its modulation and coding scheme adaptively to select the suitable modulation order and coding rate that best match with the instantaneous channel condition. The channel state information is estimated in receiver and fed back to transmitter. In such method, better performances in both data rate and bit error rate (BER) can be attained. Here, the source data are divided into different priority layers with different importance. Each layer bit stream is sent with specific error protection level against channel corruption. The highest error protection level is assigned to the highest priority layer, and vice versa. The modulated bit streams of all layers are then superimposed together and transmitted via Rayleigh fading channel. At the receiver side, a specific multi-stage decoding receiver is used to reconstruct the source data which demodulates the layers in the order of their priorities. Simulation results show that the proposed system provides up to 18 dB SNR and 46 % data rate gains, respectively, compared to the traditional BPSK scheme at BER of 10−4.

Adaptive Modulation Using Neuro-Fuzzy (N-F) Controller for OFDM System

As demand for high quality transmission increases, improving spectrum efficiency and error performance in wireless communication systems are important. OFDM is a multi-carrier modulation technique with densely spaced sub-carriers that has gained a lot of popularity among the broadband community in the last few years. One of the promising approaches to next generation communication systems are adaptive OFDM (AOFDM). Fixed modulation systems uses only one type of modulation scheme (or order), so that either performance or capacity should be compromised but in adaptive modulated systems change modulation scheme (or order) depending on instantaneous Signal to Noise Ratio (SNR) to attain superior performance and capacity compared to fixed modulated systems. Neuro-fuzzy controller combines advantages of fuzzy logic and neural networks. Neuro-fuzzy controller provides automatic adaption procedure to fuzzy logic controller. Neural networks requires sufficient prior knowledge to be initialized but neuro-fuzzy systems doesn't requires any prior knowledge to be initialized and is efficient compared to fuzzy logic and neural networks. In this paper we propose an adaptive modulated OFDM system using neuro-fuzzy controller. The proposed system is simulated in MATLAB and compared with existing systems, simulation results shown significant improvement in systems performance.  Index Terms—Adaptive modulation, OFDM, Neuro-Fuzzy controller

Multi-Edge-Type Low-Density Parity-Check Codes for Bandwidth-Efficient Modulation

A method of designing low-density parity-check codes for bandwidth-efficient high-order modulation is proposed. A multi-edge-type LDPC code ensemble is used to improve the correspondence between modulation bit-channel capacity and bit-level protection in a bit-interleaved coded modulation system. A key innovation is the development of a multi-dimensional extrinsic information transfer (EXIT) vector field technique for the analysis and design of multi-edge-type codes. A condition, sufficient for the decoder to converge, is derived on the multi-dimensional EXIT vector field, and this condition is used as a constraint in code optimization. Code designs indicate that the proposed method produces codes matching the performance of codes designed using best available methods in ensemble threshold, and is capable of achieving identical finite-length error rates with shorter block-lengths. In addition to simplified design complexity, the resulting codes allow for low-complexity implementation and rate-adaptivity, and hence are well-suited for adaptive modulation systems.

Survey of Adaptive Modulation Scheme in MIMO Transmission

Adaptive modulation (AM) technique may make more promote in the execution of wireless communication systems through adaptively conformity transmitter parameters to fading channels; therefore, it has been taken as one of the key physical techniques in 3GPP Evolved Universal Terrestrial Radio Access (E-UTRA). This paper offers a general overview of the adaptive modulation scheme in wireless multiple-input multiple-output (MIMO) systems. Study a set of properties by which adaptive modulation systems are evaluated, and then apply this valuation method to survey a number of existing systems, also, discuss modulation schemes and channel modeling in detail since it is utilized in most current system or solutions, as well survey some MIMO models which are included in adaptive modulation activities in order to reproduce and corresponding MIMO system ratings. Comprehensive execution comparisons including, BER analysis, average spectral efficiency (ASE), modulation schemes, and channel model are presented.

Performance of orthogonal STBC-MIMO with variable-power adaptive modulation and delayed feedback in Nakagami fading channels

A variable-power (VP) adaptive modulation (AM) scheme with space-time block code (STBC) is developed in MIMO systems, and the corresponding performance is investigated in Nakagami fading channels. The optimum switching thresholds are derived so that the spectrum efficiency (SE) under an average power and a target bit error rate (BER) constraint is maximized. By the switching thresholds, average BER and SE are further derived. As a result, closed-form expressions are obtained. Besides, the exact and approximate BER expressions of VP AM and constant-power (CP) AM systems with delayed feedback are respectively derived. Simulation results show that the theoretical SE and BER can match the corresponding simulation well. The results indicate that VP-AM with STBC provide higher spectrum efficiency than the CP counterparts, and has slighter bite error rate performance degradation when the normalized delay is less than 0.01.

A simplified bit metric calculation method for high-order PSK

This paper proposes a simplified bit metric calculation method for high-order PSK signal with Gray-mapping. This method calculates the bit metric recursively by the symmetry of Gray code. Analysis and simulation results show that the proposed method can significantly reduce the computational complexity and maintain similar performance compared to ML and Max-Log bit metric. Additionally, this method generates bit metrics of PSK signals with different modulation order in the same way, so it can be applied to adaptive modulation system.

Performance of variable-power adaptive MQAM with transmit antenna selection and delayed feedback in Nakagami Fading channel

In this paper, the performance of MIMO systems with variable-power (VP) adaptive MQAM and transmit antenna selection (TAS) over Nakagami-m fading channel 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. It is shown that the Lagrange multiplier in the constrained SE optimization will be unique if the existence condition for MIMO system with TAS is satisfied. A practical iterative algorithm based on Newton's method for finding the Lagrange multiplier is proposed. By using the switching thresholds, closed-form expressions of the SE and average BER are obtained. Besides, BER expressions with delayed feedback are derived for VP and constant-power (CP) systems, respectively. With these expressions, the impact of feedback delay on the BER performance is effectively assessed. Computer simulation for SE and BER shows that the theoretical analysis and simulation are in good agreement. The results show that the VP adaptive modulation (AM) system with TAS provides better SE than the CP counterpart and VP AM system with space-time coding, and has slight BER performance degradation for the normalized time delay less than 0.01.

Does Fast Adaptive Modulation Always Outperform Slow Adaptive Modulation?

Link adaptation techniques are important modern and future wireless communication systems to cope with quality of service fluctuations in fading channels. These techniques require the knowledge of the channel state obtained with a portion of resources devoted to channel estimation instead of data and updated every coherence time of the process to be tracked. In this paper, we analyze fast and slow adaptive modulation systems with diversity and non-ideal channel estimation under energy constraints. The framework enables to address the following questions: (i) What is the impact of non-ideal channel estimation on fast and slow adaptive modulation systems? (ii) How to define a proper figure of merit which considers both resources dedicated to data and those to channel estimation? (iii) Does fast adaptive always outperform slow adaptive techniques? Our analysis shows that, despite the lower complexity and feedback rate, slow adaptive modulation (SAM) can achieve higher spectral efficiency than fast adaptive modulation (FAM) in the presence of energy constraint, diversity, and non-ideal channel estimation. In addition, SAM satisfies bit error outage requirements also in FAM-denied region.

Decision Feedback 신호의 자기 상관 기반 SNR 추정 방법을 적용한 적응 변조 시스템

적응 변조(Adaptive Modulation: AM) 방식은 시간적으로 공간적으로 바뀌는 채널의 상태에 적합한 변조 방식을 적응적으로 할당함으로써, 시스템의 효율을 높이는 중요한 통신 방식이다. 고정 변조 방식은 시간에 따라 신호 대 잡음비(SNR: Signal-to-Noise Ratio)가 변하는 채널에서 BER(Bit Error Rate) 성능이 변한다. 하지만 적응변조 방식은 모든 채널 상태의 SNR에 대하여 일정한 평균 BER 성능을 유지하므로 채널의 상태가 수시로 변하는 통신 환경에서 시스템의 성능을 확보한다. 이를 위해서 무엇보다도 정확하고, 빠르게 신호 대 잡음비를 추정할 수 있는 간단한 SNR 추정 방법이 요구된다. 본 논문에서는 효과적인 적응 변조를 위하여 SNR 추정 성능이 적응 변조 시스템에 미치는 영향을 평균 BER과 평균 데이터 처리율(throughput)을 통하여 분석한다. 또한, 본 논문에서는 decision feedback 신호의 자기 상관 기반의 SNR 추정 방법 및 기존의 SNR 추정 방법들을 적응 변조시스템에 적용하여 각 변조 레벨 변환 점에서 SNR 추정 성능에 따라 결정되는 적응 변조 시스템의 성능 변화를 확인한다. Decision feedback 신호의 자기 상관 기반 SNR 추정 방법은 M-QAM(Quadrature Amplitude Modulation) 신호에서도 비교적 안정적인 추정 성능을 보이기 때문에 적응 변조 시스템에서 다른 SNR 추정 방법들에 비해 변조 레벨 변환 점에서 성능 열화를 줄인다. Adaptive modulation(AM) is an important technique to increase the system efficiency, in which transmitter selects the most suitable modulation mode adaptively according to channel state in the temporary and spatially varying communication environment. Fixed modulation on channels with varying signal-to-noise ratio(SNR) is that the bit-errorrate(BER) probability performance is changing with the channel quality. An adaptive modulation scheme can be designed to have a BER which is constant for all channel SNRs. The correct as well as fast and simple SNR estimation is required essentially for this adaptive modulation. In order to operate adaptive modulation system effectively, in this paper, we analyze the effect of SNR estimation performance to it through the average BER and data throughput. Applying SNR estimation based on auto-correlation of decision feedback signal and others to adaptive modulation system, we also confirm performance degradation or improvement of its which is decided by SNR estimation error at each transition point of modulation level. Since SNR estimation based on auto-correlation of decision feedback signal shows stable estimation performance for various quadrature amplitude modulation(QAM) comparatively, this can be reduced degradation than others at each transition point of modulation level.

Characterization of DC Offset on Adaptive MIMO Direct Conversion Transceivers

The effect of DC offset on multi-input multi-output (MIMO) direct transceivers with adaptive modulation (AM) is discussed in this paper. A variable-rate variable-power (VRVP) AM system with perfect channel state information (P-CSI) at both the transmitter and receiver in a MIMO scenario is considered. The DC offset is modeled as a zero mean complex Gaussian distributed random variable. By this modeling of the DC offset, the analytical expression for degraded bit error rate (BER) is derived. To derive this analytical expression, we establish a reasonable approximation. The good agreement between the analytical and simulation results shows that the approximation is valid and confirms the accuracy of the analytical expressions. Moreover, an approach to improve the degraded BER in these systems is introduced. For this purpose, we introduce a design for AM MIMO systems that takes account of DC offset and its effectiveness is confirmed. Throughput analysis for the AM MIMO system in the presence of DC offset is presented in this paper too. An analytical expression for throughput is derived and approximated to a simpler equation. At last, throughput results are compared to the simulation outcomes.

Adaptive technologies for hybrid ad-hoc/cellular network architecture

Ad-hoc networking was brought into the scene of cellular communications on the assumption that transmissions over short links relax background interference and improve the overall performance. Ad-hoc Networking Over UMTS Protocol (ANOUP) was introduced to improve on the cellular performance. However, this improvement was shown to be limited by excessive interference and scheduling throughput delay. To enhance ANOUP performance, consideration is given to combining ANOUP's transmission strategy with adaptive wireless techniques. In this paper we focus on adaptive modulation and adaptive smart antenna system. Our results show that throughput rate can be doubled using developed cross-layer protocol design.

Throughput improvement of STS-based MC DS-CDMA system with adaptive modulation

The throughput enhancement of space-time spreading (STS)-based multicarrier direct sequence code division multiple access (MC DS-CDMA) system is investigated in this paper. Adaptive modulation is performed to improve the data throughput of the system in Rayleigh fading channel. In this contribution, an analytical approach is proposed to compute a new expression for the bit error rate (BER) performance of the STS-based MC DS-CDMA system for QAM data. The other contribution of the paper is deriving a new analytical expression for the throughput enhancement and the BER performance of the adaptive modulated STS-based MC DS-CDMA system over Rayleigh fading channel. The novel expressions presented in this paper bring out the facility of evaluation of the proposed system with no need for more calculations and simulations.

Position-aided mobile communications

Capacity limits of position aware adaptive mobile communication systems exploiting channel state information (CSI) at the transmitter (CSIT) are presented. The position aware adaptive communication systems are benchmarked against state-of-the-art adaptive modulation and coding (AMC) systems that use only feedback CSI or additionally predict CSIT. Numerical results show that position aware adaptive systems achieve large capacity gains compared to state-of-the-art AMC systems for medium to large feedback delays.

Elimination of idle tones by a second order 2-bit adaptive sigma delta modulation system

We investigate the operation and implementation aspects of a second order 2-bit adaptive sigma---delta modulation system that offers complete elimination of the idle tones without dithering as needed by various other adaptive and simple sigma delta schemes. Performance comparisons made by computer simulation indicate that this advantageous feature together with the enhanced performance stability is due to the combination of both "memory" and "look-ahead" estimation of the employed step-size adaptation algorithm.

Elimination of Idle Tones by a 2-Bit Adaptive Sigma-Delta Modulation System

The operation of a first-order 2-bit adaptive sigma-delta modulation system is described and discussed in this paper. The system operation is based on the combination of both “memory” and “look-ahead” estimation in the employed step-size adaptation algorithm of the basic quantizer. In comparison to simple systems and other adaptive sigma-delta systems, computer simulation results show that these features of the described system are responsible for the high SNR values and the extended dynamic range achieved for AC signals as well as the noise power reduction of almost 10 dB and the complete elimination of the idle tones for DC signals. However, such an advantageous performance requires the least possible multiplicative error accumulation, and this cannot be achieved without analog circuits of the highest possible accuracy.

Adaptive modulation for SVD-based MIMO systems with outdated CSI

The performance of singular value decomposition (SVD)-based multiple-input multiple-output (MIMO) with adaptive modulation (AM) is dependent on the accuracy of the channel state information (CSI) at the transmitter and the receiver. However, for a time-varying channel, the CSI obtained at the transmitter or the receiver is incorrect owing to an inherent delay or error. A typical system employing SVD suffers degradation in capacity when outdated CSI is used to transmit data. This article first addresses the adaptive modulation MIMO system designed under perfect CSI assumption. Based on that, two scenarios of outdated CSI are investigated and analyzed. The effect of imperfect CSI on the system performance is focused on and a simple post processing SNR (pSNR) adjustment design is proposed on AM system in the two scenarios for reducing degradation in average spectral efficiency (ASE). Simulation results show that these designs provide good performance in an adaptive way, and enable the adaptive MIMO system considerably more to be robust against CSI imperfection.