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... Modulation/ BPS 10-3 10-5 10-3 10-5 10-3 10-5 10-3 10-5 is introduced in Section 4.3.1, while in Section 4.4 we will evaluate the performance of a conventional DFE-aided wideband burst-by-burst adaptive coded modulation system. Specifically, in this adaptive coded modulation scheme [76] a higher-order modulation mode is employed, when the instantaneous estimated channel quality is high in order to increase the number of Bit Per Symbol (BPS) transmitted and, conversely, a more robust lower-order modulation mode is employed when the instantaneous channel quality is low, in order to improve the mean BER performance. ...
... In general fixed-mode transceivers fail to adequately counteract the time varying nature of the mobile radio channel and hence typically result in bursts of transmission errors. By contrast, in BbB adaptive schemes [76] a higher-order modulation mode is employed when the instantaneous estimated channel quality is high in order to increase the number of BPS transmitted and, conversely, a more robust lower-order modulation mode is employed when the instantaneous channel quality is low, in order to improve the mean BER performance. In the literature the performance of adaptive schemes was investigated mostly in the context of narrowband Rayleigh fading channels. ...
... In the literature the performance of adaptive schemes was investigated mostly in the context of narrowband Rayleigh fading channels. Specifically, the performance of uncoded adaptive schemes and coded adaptive schemes has been investigated for transmissions over narrowband Rayleigh fading channels in [76,[82][83][84][85][86] and [87][88][89][90][91][92][93][94], respectively. By contrast, a turbo-coded wideband adaptive scheme assisted by a DPE was investigated in [95]. ...
p>In this thesis coded modulation schemes designed for transmission over mobile wireless fading channels are proposed and investigated. Specifically, coded modulation is a bandwidth efficient scheme, where the redundancy introduced by the channel coding scheme used does not expand the required bandwidth, since the parity bits are absorbed by the extended modulated signal constellation. Coded modulation schemes were designed for transmission over both Additive White Gaussian Noise (AWGN) channels and narrowband fading channels. However, typical mobile wireless channels are dispersive, where the employment of the conventional coded modulation scheme alone may be insufficient for achieving a coding gain. In this thesis, the employment of coded modulation in the context of conventional Decision Feedback Equalisers (DFE), Radial Basis Function (RBF) based equalisers and Orthogonal Frequency Division Multiplexing (OFDM) is investigated. Furthermore, coded modulation is also being investigated in a Code-Division Multiple Access (CDMA) environment, in the context of both DFE based Multi-User Detection (MUD) as well as Genetic Algorithm (GA) assisted MUD.
Another means of mitigating the effect of wideband fading channels is employing adaptive modulation techniques. More specifically, a higher-order modulation mode is employed, when the instantaneous estimated channel quality is high in order to increase the number of bits per symbol transmitted and, conversely, a more robust lower-order modulation mode is used when the instantaneous channel quality is low, in order to improve the mean Bit Error Rate (BER) performance. In this thesis, adaptive coded modulation schemes are investigated in the context of both conventional DFE schemes and DFE based MUD aided CDMA schemes. Turbo equalisation (TEQ) is another technique of mitigating the effects of wideband fading channels. Specifically, TEQ is a joint channel equalisation and channel decoding scheme, where the equaliser is fed by both the channel outputs and by the soft decisions provided by the channel decoder. This process is then invoked in a number of iterations. In this study coded modulation schemes are also amalgamated with the proposed RBF-based TEQ and with an RBF-based reduced complexity In-phase(I)/Quadrature-phase(Q) TEQ.
Finally, coded modulation is proposed for increasing the achievable diversity gain when communicating over fading channels. Specifically, IQ-interleaved coded modulation is introduced and investigated in the context of Space Time Block Coding (STBC) schemes as well as in Rake receiver based CDMA schemes. Explicitly, IQ-interleaved coded modulation is capable of achieving a novel type of diversity, namely IQ diversity, while STBC is well known for attaining both transmit and time diversity, while the Rake receiver employed is useful for achieving multipath diversity.</p
... Hayes [111] proposed transmission power adaptation, while Cavers [112] suggested invoking a variable symbol duration scheme in response to the perceived channel quality at the expense of a variable bandwidth requirement. Since a variable-power scheme increases both the average transmitted power requirements and the level of cochannel interference [113] imposed on other users of the system, instead variable-rate Adaptive Quadrature Amplitude Modulation (AQAM) was proposed by Steele and Webb as an alternative, employing various star-QAM constellations [113,114]. variable-power AQAM [120] . However, they also found that the extra throughput achieved by the additional variable-power assisted adaptation over the constant-power, variable-rate scheme is marginal for most types of fading channels [120,122]. ...
... Adaptive modulation [13,111,113,114] attempts to provide the highest possible throughput given the current near-instantaneous channel quality, while maintaining the required data transmission integrity. ...
p>Multi-Carrier Code Division Multiple Access (MC-CDMA) exploits the joint benefits of Direct-Sequence (DS) CDMA and Orthogonal Frequency Division Multiplexing (OFDM). Hence, MC-CDMA exhibits high spectral efficiency and substantial benefits from the frequency diversity provided by frequency selective fading channels. This dissertation investigates several aspects of MC-CDMA. Firstly, the Peak-to-Average Power Ratio (PAPR) of the MC-CDMA signal is analysed. It was shown that the PAPR is characterised by the aperiodic correlation properties of the spreading sequences employed. Several orthogonal spreading codes were investigated in terms of their PAPR and it was found that the PAPR can be upper bounded by 3dB when employing orthogonal complementary codes in the context of low number of sub-carriers. Secondly, the application of adaptive modulation to MC-CDMA was studied. The optimum mode switching levels for generic adaptive modulation schemes were derived first and then, the performance of adaptive-modulation assisted MC-CDMA was analysed for transmission over various propagation scenarios. When space-time block codes were combined with adaptive-modulation assisted MC-CDMA, as expected the SNR gains of adaptive modulation over fixed-mode modems were found to decrease. Lastly, three types of reduced-complexity despreading schemes were proposed with an application to MC-CDMA in mind and the BER performance as well as the achievable complexity reduction were investigated.</p
... . (17.191) With the aid of the weight vector of Equation 17. 191 ( 17.192) • Classification/Demodulation: The above PIC and MMSE-combining steps are again followed by the classification, demodulation stage seen at the right of Figure 17.20, which obeys: 193) where the l-th user's effective channel transfer factor H (l)[i] eff is given by: ...
... The second field, where novel contributions were made, was in realms of adaptive modulation schemes. Adaptive modulation [2,193,390,392] attempts to provide the highest possible throughput given the current near-instantaneous channel quality, while maintaining the required data transmission integrity. We analysed in Section 12. 3. 3.1 the performance of adaptive modulation schemes and derived a closed form expression for the average BER and for the average Bits Per Symbol (BPS) throughput [492]. ...
Orthogonal frequency-division multiplexing (OFDM) is a method of digital modulation in which a signal is split into several narrowband channels at different frequencies. CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimising the use of available bandwidth. Multiplexing is sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end.
Multi-Carrier (MC) CDMA is a combined technique of Direct Sequence (DS) CDMA (Code Division Multiple Access) and OFDM techniques. It applies spreading sequences in the frequency domain.
Wireless communications has witnessed a tremendous growth during the past decade and further spectacular enabling technology advances are expected in an effort to render ubiquitous wireless connectivity a reality.
This technical in-depth book is unique in its detailed exposure of OFDM, MIMO-OFDM and MC-CDMA. A further attraction of the joint treatment of these topics is that it allows the reader to view their design trade-offs in a comparative context.
Divided into three main parts:
Part I provides a detailed exposure of OFDM designed for employment in various applications
Part II is another design alternative applicable in the context of OFDM systems where the channel quality fluctuations observed are averaged out with the aid of frequency-domain spreading codes, which leads to the concept of MC-CDMA
Part III discusses how to employ multiple antennas at the base station for the sake of supporting multiple users in the uplink
• Portrays the entire body of knowledge currently available on OFDM
• Provides the first complete treatment of OFDM, MIMO(Multiple Input Multiple Output)-OFDM and MC-CDMA
• Considers the benefits of channel coding and space time coding in the context of various application examples and features numerous complete system design examples
• Converts the lessons of Shannon’s information theory into design principles applicable to practical wireless systems
• Combines the benefits of a textbook with a research monograph where the depth of discussions progressively increase throughout the book
This all-encompassing self-contained treatment will appeal to researchers, postgraduate students and academics, practising research and development engineers working for wireless communications and computer networking companies and senior undergraduate students and technical managers.
... Adaptive modulation was proposed by Steele and Webb [185,186], in order to combact the time-variant fading of mobile channels. The main idea of adaptive modulation is that when the channel quality is favourable, higher-order modulation modes are employed, in order to increase the throughput of the system. ...
p>This thesis is based on the research of channel coding. First, we give an introduction to the family of conventional Bose-Chauduri-Hocquenghem (BCH) codes, characterising the performance of various BCH codes. Then we further our research into turbo codes employing BCH codes as the component codes. Various decoding algorithms are presented for turbo codes. This is followed by our simulation results, studying the effects of various parameters, affecting the performance of turbo codes.
Subsequently, another form of non-binary block codes, referred to as Redundant Residue Number System (RRNS) codes, which exhibit identical distance properties to the well-established Reed-Solomon (RS) codes, are investigated. Different bit-to-symbol mapping schemes are proposed, which result in systematic and non-systematic RRNS codes. An RRNS decoder is proposed, which accepts soft inputs and provide soft outputs. This facilitates the iterative decoding of turbo RRNS codes.
Our investigations into channel coding are also expanded to study space-time codes, which are constituted by jointly designed channel coding, modulation, transmit diversity and optional receiver diversity schemes. Specifically, combined space-time block codes and different channel codecs studied. Various simulation results are presented for space-time block codes using no channel coding. This is followed by the investigations of the bit-to-symbol mapping of the binary channel coded bits to higher modulation constellations. Finally, the performance of various channel codecs is compared by considering their estimated complexity in conjunction with space-time block coding.
In the last chapter, space-time trellis codes are then compared to the class of space-time block codes in conjunction with a range of channel codes over wideband channels. Various factors affecting the performance of space-time block codes are investigated. Finally, space-time coded adaptive Orthogonal Frequency Division Multiplexing (AOFDM) is investigated.</p
... Furthermore, to ensure the required quality of service, robust modulation schemes decrease the system throughput. Adaptive modulation has been proposed as a powerful method to maintain the desired quality of service and to maximize the transmission throughput given channel conditions [2], [3]. The basic idea of this technique is to switch between different modulation constellation sizes depending on the channel state. ...
This paper investigates an adaptive M-ary phaseshift keying (M-PSK) modulation scheme over Rayleigh flat fading channels. The data rate is adapted according to the channel state. At the receiver, the fading is estimated using pilot symbols. To cancel the channel impact, we correct the received signal by dividing it by the estimated value of the fading. So, we propose to adjust the modulation level by examining the statistics of the corrected signal. In contrast to the previous works on the adaptive M-PSK modulation techniques, our modulation switching protocol takes into account the channel estimation error variance. Moreover, we derive a new closed-form expression for the average bit error rate of the considered system.
... Steele and Webb[48]proposed adaptive modulation for exploiting the time-variant Shannonian channel capacity of fading narrowband channels, which stimulated further research at Osaka University by Sampei et al[315], at the University of Stanford by Goldsmith et al[316], by Pearce, Burr and Tozer at the University of York[317], Lau and McLeod at the University of Cambridge[318], and at Southampton University[319,320]. The associated principles can also be invoked in the context of parallel modems, as it has been demonstrated by Kalet[78], Czylwik et al[321]as well as by Chow, Cioffi and Bingham[322]. ...
Single- and Multi-carrier Quadrature Amplitude Modulation Principles and Applications for Personal Communications, WLANs and Broadcasting L. Hanzo Department of Electronics and Computer Science, University of Southampton, UK W. Webb Motorola, Arlington Heights, USA formerly at Multiple Access Communications Ltd, Southampton, UK T. Keller Ubinetics, Cambridge Technology Centre, Melbourn, UK formerly at Department of Electronics and Computer Science, University of Southampton, UK Motivated by the rapid evolution of wireless communication systems, this expanded second edition provides an overview of most major single- and multi-carrier Quadrature Amplitude Modulation (QAM) techniques commencing with simple QAM schemes for the uninitiated through to complex, rapidly-evolving areas, such as arrangements for wide-band mobile channels. Targeted at the more advanced reader, the multi-carrier modulation based second half of the book presents a research-orientated outlook using a variety of novel QAM-based arrangements.
* Features six new chapters dealing with the complexities of multi-carrier modulation which has found applications ranging from Wireless Local Area Networks (WLAN) to Digital Video Broadcasting (DVB)
* Provides a rudimentary introduction for readers requiring a background in the field of modulation and radio wave propagation
* Discusses classic QAM transmission issues relevant to Gaussian channels
* Examines QAM-based transmissions over mobile radio channels
* Incorporates QAM-related orthogonal techniques, considers the spectral efficiency of QAM in cellular frequency re-use structures and presents a QAM-based speech communications system design study
* Introduces Orthogonal Frequency Division Multiplexing (OFDM) over both Gaussian and wideband fading channels
By providing an all-encompassing self-contained treatment of single- and multi- carrier QAM based communications, a wide range of readers including senior undergraduate and postgraduate students, practising engineers and researchers alike will all find the coverage of this book attractive.
... Adaptive modulation was proposed for exploiting the time variant Shannonian channel capacity of fading channel by Steele and Webb [1]. In addition to excluding some fading subcarriers and varying the modulation mode, that is adaptive modulation, code rate can also be adapted. ...
This paper discusses the application of adaptive modulation and adaptive rate turbo coding to orthogonal frequency-division multiplexing (OFDM), to increase throughput on the time and frequency selective channel. The adaptive turbo code scheme is based on a subband adaptive method, and compares two adaptive systems: a conventional approach where a separate turbo code is used for each subband, and a single turbo code adaptive system which uses a single turbo code over all subbands. Five modulation schemes (BPSK, QPSK, 8AMPM, 16QAM, and 64QAM) are employed and turbo code rates considered are 1/2 and 1/3. The performances of both systems with high (10−2) and low (10−4) BER targets are compared. Simulation results for throughput and BER show that the single turbo code adaptive system provides a significant improvement.
In this paper, we propose a vehicular optical camera communication system that can meet low bit error rate (BER) and ultra-low latency constraints. First, we formulate a sum spectral efficiency optimization problem that aims at finding the speed of vehicles and the modulation order that maximizes the sum spectral efficiency subject to reliability and latency constraints. This problem is mixed-integer programming with nonlinear constraints, and even for a small set of modulation orders, is NP-hard. To overcome the entailed high computational and time complexity which prevents its solution with traditional methods, we first model the optimization problem as a partially observable Markov decision process. We then solve it using an independent Q-learning framework, where each vehicle acts as an independent agent. Since the state-action space is large we then adopt deep reinforcement learning (DRL) to solve it efficiently. As the problem is constrained, we employ the Lagrange relaxation approach prior to solving it using the DRL framework. Simulation results demonstrate that the proposed DRL-based optimization scheme can effectively learn how to maximize the sum spectral efficiency while satisfying the BER and ultra-low latency constraints. The evaluation further shows that our scheme can achieve superior performance compared to radio frequency-based vehicular communication systems and other vehicular OCC variants of our scheme.
It is a challenging task to improve reliable and high-speed data communication between Transmitter and Receiver. Achieving high data rate using block codes and convolution codes is still questionable through a wireless channel. Turbo codes are a class of high performance forward error correction (FEC) codes with OFDM with adaptive Modulation for 4G and LTE applications. In this paper, BER analysis is carried out for TURBO-coded OFDM data with adaptive modulation mode switching using BPSK, QPSK, 16QAM, and 64QAM. Adaptive modulation scheme is assisted by decision feedback equalizer (DFE) which controls modulation mode. Its output SNR is used as a switching metric. DFE is used to select modulation mode. It is observed that average spectral efficiency (bits per second/Hz) is much higher for adaptive modulation scheme. For higher modulation scheme average spectral efficiency is gradually increases up to 40 dB, that indicates its throughput is much higher in comparison with fixed modulation modes.
Energy Efficiency (EE) has become one of the main challenges for the development of wireless communication systems. Thus, many researchers are investigating new solutions to save power. For this reason, in this paper we apply a method based on modulation and coding scheme (MCS) for multiband orthogonal frequency division multiplexing (MB-OFDM) ultra wideband (UWB) systems. The MCS scheme is a powerful method known by its ability to improve the spectral efficiency. Our goal in this paper is to investigate the performance of MCS mechanism to diminish the transmitted power under some assumptions. Computer results show the throughput behavior in function of signal to noise ratio (SNR) for different selection of modulation and coding rate. Then, we draw the plot of bit error rate (BER) versus SNR and versus the energy per bit to noise power spectral density ratio (E b /N o ).
Within the last two decades, the amount and diversity of services provided by wireless systems has been drastically transformed. Mobile (cellular) communication, for instance, is nowadays offering a wide variety of multimedia-data services, in contrast to the limited voice and very simple data services offered in the past. In wireless local-area networks (WLAN), as another example, the ability to be on-line without needing a wired connection is not sufficient any more, and users expect to experience similar data speeds and quality of service (QoS) as with a wired connection. This has lead to a rapid increase in data-rate requirements (broadband connectivity) in the standards of new and upcoming wireless communication systems.
In this paper, we propose the transmit power controlled adaptive downlink frequency symbol spreading OFDM (TPC-AMS/FSS-OFDM) system. In the TPC-AMS/FSS-OFDM, each serial to parallel (S/P) transformed signal is spread by orthogonal spreading codes and combined in the transmitter, so the detected signals obtain the same SINR for each frequency symbol spreading block in the receiver. In this case, we can assign the same modulation level and transmit power for each frequency symbol spreading block for next transmission. Thus, the proposed system not only increases throughput performance but also reduces the total transmit power, FBI and MLI.
4G mobile communication system requires the throughput of 10–100Mbps. Adaptive modulated OFDM system is promising technique for increasing the throughput. In the pilot symbol assisted high-speed packet transmission system, the data symbol duration is generally considered to be small compared to the coherence time. However, OFDM symbol duration is longer than the symbol duration of a single carrier system, so that the packet duration of the pilot symbol assisted high speed packet transmission system is long. In this case, the change of channel conditions is too fast to be accurately estimated by channel estimator at the receiver in high Doppler frequency, so that many errors occur during demodulation, especially with the data symbols at the end of each packet. In this paper, we consider the BER at various instantaneous Eb/N0 that includes the demodulation errors in high Doppler frequency. When the coherence time is ten times longer than the duration of a single packet, the channel can be closely approximated as an AWGN channel. Otherwise, the approximation breaks down and the above-mentioned errors that occur during demodulation must be taken into consideration. In this paper, we propose the pilot symbol assisted high speed packet transmission system based on adaptive OFDM using a novel lookup table to consider the demodulated errors and evaluate the throughput performance.
The design tradeoffs of turbo-coded burst-by-burst adaptive
orthogonal frequency division multiplex (OFDM) wideband transceivers are
analyzed. We demonstrate that upon aiming for a higher throughput a
higher proportion of low-quality OFDM subcarriers has to be used for the
transmission of inherently vulnerable high-order modem modes,
transmitting several bits per subcarrier. Upon invoking turbo coding and
adjusting the modem mode switching regime near-error-free performance
can be achieved at the cost of a reduced throughput. Various blind modem
mode detection techniques have also been investigated and the most
complex channel coding trellis-based detection algorithm was found to be
the most powerful. Last, the design tradeoffs of spectral
pre-equalization have been explored and quantified. We concluded that
AOFDM provides a convenient framework for adjusting the required target
integrity and throughput both with and without turbo channel
coding
We propose a fixed-power adaptive turbo coded modulation scheme for orthogonal frequency division multiplexing (OFDM). In this paper, we first observe that turbo coded modulation provides more than 15 dB signal-to-noise-ratio (SNR) gain on uncoded OFDM in fading channels. Then we conclude that when the transmission rate is adapted to the channel adequately, adapting the power as well yields a negligible capacity gain for adaptive OFDM from numerical simulation results. An adaptive OFDM scheme based on subband adaptation is given and mode allocation algorithm based on the capacity evaluation of current subband is presented. Simulation results show that our adaptive OFDM scheme exhibits more than 7.5 dB SNR gain relative to non-adaptive turbo coded modulation. We also discuss the effect of feedback delay and channel estimation error on bit-error-rate (BER) performance of our adaptive scheme.
This paper discusses the application of adaptive modulation and adaptive rate turbo-coding to OFDM (orthogonal frequency-division multiplexing), to enable a closer approach to the Shannon capacity of the time and frequency selective channel. The adaptive turbo-code scheme is based on a subband adaptive method, and compares two adaptation algorithms: a conventional, conservative approach where modulation and code rate is chosen based on the poorest subcarrier in a subband, and an optimal approach based on a prediction of the average BER over all sub-carriers. Four modulation schemes (BPSK, QPSK, 8AMPM and 16QAM) and four code rates (1/3, 1/2, 2/3 and uncoded) are employed. Systems employing different numbers of combinations of these schemes are compared. Simulation results for throughput and BER show that 8 schemes are sufficient to approach the maximum capacity: a small reduction in throughput occurs with only 4 schemes (all 1/2 rate coded). The optimal adaptation algorithm provides a significant improvement.
In an AMS/OFDM system, base station is in control of the modulation level of each subcarrier, and then, adaptive modulated packet is transmitted from the base station to the mobile station. In this case, the mobile station is required the modulation level information (MLI) to demodulate the received packet. The MLI is generally transmitted as a data symbol, therefore, the throughput is degraded. In an OFDM, the channel response at a particular subcarrier frequency is not supposed to be totally different from its neighboring frequencies, and hence, they mast have correlation which depends on the coherence bandwidth of the channel Bc. If we could assign the same modulation level for coherently faded subcarrier block, MLI is required only one time for each subcarrier block. Moreover, we can assign the data on the empty space of pilot signals for increasing the total transmission. In this paper, we propose an adaptive subcarrier block modulation with differentially modulated pilot symbol assistance for downlink OFDM using uplink delay spread
In AMS/OFDM systems, a base station controls the modulation level of each subcarrier with feedback information (FBI), and then, adaptive modulated packets are transmitted from the base station to the mobile station. In this case, the mobile station requires modulation level information (MLI) to demodulate the received packet. The MLI is generally transmitted as a data symbol, so the throughput is degraded. To overcome this problem and increase the total throughput, in this paper, we propose frequency symbol spreading and MMSEC equalization based on an adaptive downlink OFDM system. In the proposed system, each S/P transformed signal is spread by orthogonal spreading codes and combined. This means that each subcarrier holds several superimposed S/P transformed signals with the same power rate. In this case, the frequency-selective faded subcarriers obtain the same power rate for each S/P transformed signal. Therefore, the detected signals also obtain the same SINR, and as a result, we can assign the same modulation level for each frequency symbol spreading block. Hence, the proposed system requires only one piece of FBI and MLI for each frequency symbol spreading block, as compared with conventional adaptive OFDM
Adaptive modulation schemes have been proposed to optimize Shannon's channel capacity in recent orthogonal frequency division multiplexing (OFDM) based broadband wireless standard proposals. By adapting the modulation type (effectively changing the number of bits per symbol) at the transmitter end one can improve the bit error rate (BER) during transmission at designated SNR. Blind detection of the transmitted modulation type is desirable to optimise the bandwidth available at the receivers. Hence, there is a need for an intelligent modulation classification engine at the receiver end. In this work, we evaluate some higher order statistical measures coupled with a classical Naive Bayes classifier for fast identification of adaptive modulation schemes. We also benchmark the experimental results with the optimal Maximum Likelihood Classifier, and Support Vector Machine based Classifier using the same feature set.
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 (SNR ges 13 dB), not only can satisfy the predetermined bit-error-rate (BER) requirement (BER les 10-4) but also can dynamically enhance the throughputs in the rather clean environments with high SNR values.
Adaptive modulation exploits the time-variant channel capacity
fluctuation of fading channels using a range of different modem modes
according to the equation of the transmission scheme and the results
presented in the table. This is achieved at the cost of some latency
penalty, which is quantified and mitigated by frequency hopping
In this report, an adaptive modulated orthogonal frequency division multiplexing (OFDM) scheme that provides efficient data transmission for correlated receiver antenna arrays is proposed. Previously reported adaptive modulation schemes generally select the modulation scheme based on the estimated carrier-to-noise ratio (CNR) on each subcarrier. However, in antenna array communication in addition to the CNR, the spatial fading correlation should also be considered to evaluate the system performance. Especially, when the Time Shifted Sampling (TSS) technique is applied which overcomes the spatial correlation for some subchannels of an OFDM symbol, the spatial correlation varies dramatically through subcarriers. In this work, modulation scheme is selected based on the CNR and the spatial correlation coefficient for each subcarrier when the TSS technique is employed at the receiver. Numerical results through computer simulation show that the proposed scheme outperforms the conventional systems in achieving a target BER and establishing reliable communication while having almost the same transmission rate.
Adaptive modulation exploits the channel conditions to improve spectral efficiency of the wireless communication system. Moreover, channel characteristics also depend upon Doppler shift, due to the motion of the mobile station. In this paper, a new technique of adaptive modulation is proposed which takes into account the effect of varying speed receiver. For the purpose, mapping between channel correlation and instantaneous signal-to-noise ratio is obtained based on varying speed receiver and is used to select the best suitable modulation scheme. The system performance is obtained over frequency-selective fading channel and Kalman-filter based method is used to predict the frequency domain channel coefficients. The numerical results shows the improved performance of the proposed adaptive scheme to keep required bit error rate with varying speed receiver compared to the adaptive schemes with static receiver.
Adaptive Wireless Transceivers provides the reader with a broad overview of near-instantaneously adaptive transceivers in the context of TDMA, CDMA and OFDM systems. The adaptive transceivers examined employ powerful turbo codecs, turbo equalisers and space-time codecs, equipping the reader with a future-proof technological road map. It demonstrates that adaptive transceivers are capable of mitigating the channel quality fluctuations of the wireless channel as a lower-complexity alternative to space-time coding. By contrast, if the higher complexity of multiple transmitters and multiple receiver-assisted systems is deemed acceptable, the advantages of adaptability erode.
• Provides an in-depth introduction to channel equalisers and Kalman filtering and discusses the associated complexity versus performance trade-offs
• Introduces wideband near-instantaneously adaptive transceivers and studies their performance both with and without turbo channel coding
• Describes how to optimise adaptive modulation mode switching and highlights a range of practical considerations
• Introduces neural network based channel equalisers and discusses Radial Basis Function (RBF) assisted equalisers embedded into adaptive modems supported by turbo channel coding and turbo channel equalisation
• Employs the above adaptive principles also in the context of CDMA and OFDM transceivers and discusses the pros and cons of space-time coding versus adaptive modulation
Researchers, advanced students and practising development engineers working in wireless communications will all find this valuable text an informative read.
In this paper, we study an adaptive M-PSK modulation technique with a coherent detection over Rayleigh fading channels. The data rate is adapted according to the channel state. At the receiver, the fading is estimated using pilot symbols. To cancel the channel impact, we correct the received signal by dividing it by the estimated value of the fading. To adjust the modulation level, we propose a switching protocol which takes into account the channel estimation and prediction errors.
In this paper, an adaptive quadrature amplitude modulation (AQAM) scheme for an equalized system over a selective channel is investigated. To reduce intersymbol interference (ISI), a minimum-mean-squared-error decision feedback equalizer (MMSE-DFE) with an unbiased decision rule is used. In order to select the appropriate modulation mode, the receiver estimates the MSE at the equalizer output. The estimated MSE is then sent back to the transmitter which adjusts the modulation level. A reliable feedback link between the receiver and the transmitter is required to achieve good performances.
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≥15 dB), not only can satisfy the predetermined bit error rate (BER) requirement (BER≤10-4) but also can dynamically enhance the throughputs in the rather clean environments with high SNR values.
This paper investigates an adaptive M-QAM modulation scheme with constant peak power for which the received signal is divided by the estimated channel gain. Generally, for adaptive modulation schemes, the transmitter adjusts its modulation parameters based on the received power. The receiver estimates channel conditions and sends feedback information via a return channel to the transmitter. In this paper, we consider Rayleigh flat fading channels. At the receiver the fading is estimated using pilot symbols. To cancel the channel impact, we correct the received signal by dividing it by the estimated value of the fading. So, we propose to adjust the modulation level by examining the statistics of the corrected signal. The modulation switching protocol take into account the channel estimation error variance. Since linear region of practical amplifiers is limited, when switching from one modulation scheme to an other, the peak power is kept constant.
In an AMS/OFDM system, base station is in control of the modulation level of each subcarriers and then, adaptive modulated packet is transmitted from the base station to the mobile station. In this case, the mobile station is required the modulation level information (MLI) to demodulate the received packet. The MLI is generally transmitted as a data symbol therefore the throughput is degraded. Moreover, it is necessary to have some transmission delay times and the processing time to make an adaptive modulation command (AMC) using feedback information (FBI). With the FBI delay and processing time, the system performance might be degraded. To reduce these problems, in this paper, we propose a differential modulated pilot symbol assisted adaptive OFDM for reducing the MLI.
This document provides a system analysis for hitless flexible data rate adapted to the received power variation with fixed spectrum. In our investigation, we are interested in digital line-of-sight (LOS) microwave links. The channel fading state, the grade of service and the adaptive modulation scheme are examined. Essentially, the flexible communication scheme consists in modifying, from one frame to the next, the modulation level used to communicate on a wireless link. The radio thus provides for significant capacity gains if traffic streams requiring different quality of service are being communicated. Most significant scenarios are considered and quantified to obtain a realistic analysis of the grades of services allocated to each traffic stream. We show as an example, that a 25 Mbit/s carrier grade link (99.9995% availability) may be modified into the sum of a 25 Mbit/s carrier grade link with a 100 Mbit/s data link of 99.9% availability.
In this paper, two adaptive orthogonal frequency division multiplexing (OFDM) schemes employing rate-compatible turbo coded modulation with power variable subband-by-subband and symbol-by-symbol are proposed. Our object is to reduce the overall transmit power under target bit-error-rate (BER) for constant throughput OFDM transmissions. Simulation results indicate that our adaptive scheme with power variable over symbols exhibits a more than 5 dB signal-noise-ratio (SNR) gain in contrast to non-adaptive turbo coded modulation, and about 1-2 dB SNR gap from our scheme with power variable over subbands. We also discuss the effect of the amount of subbands on our adaptation performance.
An adaptive turbo coded modulation orthogonal frequency division multiplexing (OFDM) schemes with power variable over subcarriers, variable over subbands, and constant over subcarriers is proposed. Our object is to improve the overall throughput under target bit-error-rate (BER) for OFDM transmissions. Simulation results indicate that our fixed power adaptation scheme exhibits a more than 7dB signal-noise-ratio (SNR) gain relative to nonadaptive turbo coded modulation, and about 2dB additional gain can be achieved with our variable power adaptation. We also discuss the effect of the amount of subbands on throughput performance in our adaptive scheme.
Several adaptive transmission schemes have been proposed to improve the performance of wireless networks, with adaptive modulation and adaptive error control receiving a lot of attention in the last years. We compare the performances of adaptive forward error correction (FEC), adaptive automatic repeat request (ARQ) and adaptive modulation schemes. We have analysed a hybrid scheme, that uses adaptive FEC together with adaptive modulation, to improve the performance and decrease the complexity of the error correction code.
Several adaptive transmission schemes have been proposed to improve the performance of wireless networks. In this paper we have analysed three criteria to compute the switching points in a wireless ATM system with adaptive modulation, Then, we have proposed two hybrid approaches using adaptive modulation and channel coding to improve the performance of the wireless link.
Adaptive modulation can achieve channel capacity gains by adapting
the number of bits per transmission symbol on a burst-by-burst basis, in
harmony with channel quality fluctuations. This is demonstrated for
target bit error rates of 1 and 0.01%, respectively, in comparison to
conventional fixed modems. However, the achievable gains depend strongly
on the prevalent interference levels and hence interference cancellation
is invoked on the basis of adjusting the demodulation decision
boundaries after estimating the interfering channel's magnitude and
phase. Using given modem-mode switching levels and with the aid of
interference cancellation, target BERs can be maintained over
slow-fading channels for a wide range of channel signal-to-noise ratios
(SNR) and signal-to-interference ratios (SIR)
A novel, uneven protection phase shift keying technique is
proposed for the encoding of the required modulation scheme in an
adaptive modem arrangement, which exhibits an improved performance in
comparison to previously proposed schemes. The performance is derived
numerically and a system dependent optimisation is presented. A benefit
of 5 dB is achieved in a Rayleigh channel
The complexity robustness, image and speech quality as well as
packet multiplexing issues of a re-configurable multi-media mobile
communicator are addressed. The proposed moderate complexity
motion-compensated discrete cosine transform (DCT) based image
communicator provides an image peak signal-to-noise ratio (PSNR) of 38
dB at an average bit rate of about 25 kbits/s. The speech codec used is
a low-complexity 32 kbit/s CCITT G721 standard scheme. Bandwidth
efficient 16 or 64-level quadrature amplitude modulation (QAM) combined
with embedded low-complexity binary Bose-Chaudhuri-Hocquenghem (BCH)
forward error correction (FEC) coding is deployed. The 2O-slot packet
reservation multiple access (PRMA) scheme used supports an extra 2.4
kbit/s low-rate data channel for each speech user, in addition to
providing 5-6 videophone channels. The ADPCM/DCT/BCH/16-QAM and
ADPCM/DCT/BCH/64-QAM schemes provide nearly unimpaired speech and image
quality for channel SNRs in excess of 30 dB and 38 dB,
respectively
A range of adaptive orthogonal frequency division multiplex (AOFDM) video systems are proposed for interactive communications over wireless channels. The proposed constant target bit-rate subband-adaptive OFDM modems can provide a lower bit error rate than a corresponding conventional OFDM modem. The slightly more complex switched or time-variant target bit rate AOFDM modems can provide a balanced video quality performance, across a wider range of channel signal-to-noise ratios, maintaining the best video performance. Upon invoking the technique advocated, irrespective of the channel conditions experienced, the transceiver achieves always the best possible video quality by automatically adjusting the achievable bit rate and the associated video quality in order to match the channel quality experienced. This is achieved on a near-instantaneous basis under given propagation conditions in order to cater for the effects of path-loss, fast-fading, slow-fading, dispersion, etc. Furthermore, when the mobile is roaming in a hostile outdoor propagation environment, typically low-order, low-rate modem modes are invoked, while in benign indoor environments predominantly the high-rate, high source-signal representation quality modes are employed.
The interference levels that may be expected for a range of conventional cellular and microcellular clusters used in personal communication networks (PCNs) and personal communication systems (PCSs) are discussed. Simulation results show that for conventional size cells, 32- and 64-level quadrature amplitude modulation (QAM) schemes are preferred for bit error rates (BER) above 1*10/sup -2/, while 4PSK or variable-rate QAM schemes are better for lower BER. For microcells with communications at both 900 MHz and 1.8 GHz, four- or six-cell clusters are advocated, depending on the SNR expected. Based on the expected signal-to-noise ratio (SNR) and interference levels, it is argued that variable-rate QAM schemes are superior to the other modulation schemes considered. For low BERs 4PSK may often provide the best performance, whereas for high BERs, particularly when the SNR is high, 32- and 64-level star QAM are the most suitable.< >
Adaptive modulation is proposed by many researchers for future generation wireless networks based on multi-carrier transmission technologies. In this paper, we propose and investigate some blind mechanisms at the receiver of the adaptive modulation based coded OFDM-CDMA systems in Rayleigh fading channels for the enhancement of the systems in terms of both the practical implementation and data throughput capacity. In this respect, blind techniques to estimate the channel impulse response and modulation scheme are proposed and investigated and their effect on the BER performance of the systems are also analyzed. A subspace based algorithm is proposed to estimate the channel impulse response blindly. This estimate is then used to coherently demodulate the OFDM-CDMA signals and adaptively switches the modulation schemes for varying channel conditions. Furthermore, an algorithm to estimate the modulation scheme at the receiver is also proposed. Results show error free estimation of modulation scheme for a maximum of 7 active users in the system at a channel condition of 20 dB. Simulation results show that in some situations, the systems based on blind channel estimation show little degradation in terms of the BER performance compared to the pilot based scheme. However, the advantages of blind estimation schemes far outweigh this degradation.
Adaptive modulation based MC-CDMA systems can play a vital role in future generation consumer communication electronics. Adaptive modulation, combined with MC-CDMA based transmission technology, is a promising way to increase the data rate that can be reliably transmitted over the wireless radio channels. For 4G wireless networks, which demand very high data rate up to 100 Mbits/s with the constraints limiting higher data rate being severe ISI due to multipath and limited spectrum, such kind of adaptive modulation based multi-carrier systems applied to a wide-area environment, can achieve very large average user throughputs. In this paper, adaptive modulation based M-ary PSK, M-ary QAM, M-ary CPM, M-ary MHPM and GMSK systems applied to a turbo coded MC-CDMA system in a Rayleigh fast fading channel environment have been investigated and the BER performance of all these digital modulation techniques have been compared. Results of the comparative study indicate that the continuous phase modulation schemes like CPM, MHPM, and GMSK gives better performance as compared to PSK and QAM schemes. At most of the time, the MHPM systems outperforms both GMSK and CPM. The PSK and QAM based systems perform well till the number of users are around 10. As a whole, the adaptive MHPM system is found to give the optimum performance among the considered digital modulation schemes for the MC-CDMA system in a 4G environment.
OFDM is one of the promising modulation candidates for a fourth
generation broadband mobile communication system because of its
robustness against intersymbol interference (ISI). The adaptive
modulation scheme is also an efficient scheme to increase the
transmission rate by changing the channel modulation scheme according to
the estimated channel state information. Since its implementation
depends on the channel environment of the system and control period by
using feedback information, this paper presents an evaluation for the
effects of various modulation scheme combinations, target BER, Doppler
frequency, and various adaptation intervals as control period on the
performance of adaptive OFDM. We also propose a predicted feedback
information scheme which increases the adaptation interval using the
predicted power estimation in order to reduce the transmission time of
feedback information from receiver to transmitter. Computer simulation
results show that the case with BPSK, QPSK and 16QAM modulation
combination at target BER 10-2 achieves 2Mbit/s improvement
over other combination cases in high Doppler frequency. On the other
hand, at target BER 10-3, the case with BPSK, QPSK, 8PSK and
16QAM modulation combination achieves 3Mbit/s improvement compared to
the case of target BER 10-2. It is also shown that the
predicted feedback information scheme effectively reduces the
transmission time of feedback information from the receiver to
transmitter
Quadrature amplitude modulation (QAM) schemes which vary the
number of modulation levels in accordance with the mobile radio fading
channel variations are investigated. Important parameters considered are
the fading rate and the block size used. We describe how the adaptive
QAM modems can be employed and consider their use in a DECT-like TDD
packet structure. System performance in the presence of cochannel
interference is also considered. Simulations show that the variable rate
system has about 5 dB improvement in channel SNR over a fixed 16-level
QAM system for BER's between 10-2 and 10-5 and
channel SNR's between 25 and 40 dB
Lee (1989) has produced equations describing spectrum efficiency
for modulation schemes operating in cellular environments. The present
authors consider the performance of a range of multilevel modulation
schemes when these spectrum efficiency equations are applied. They then
proceed to adapt these efficiency equations for use in microcells before
considering the performance of multilevel modulation within this
environment. The results show that the preferred modulation scheme
depends on the BER required, with 4 level PSK being preferred in
conventional cells, and variable 16 level QAM, or 32 or 64 level QAM
being suitable in microcells, particularly when a high SNR is available
The achievable performance of channel coded space-time trellis (STT) codes and space-time block (STB) codes transmitted over wideband channels is studied in the context of schemes having an effective throughput of 2 bits/symbol (BPS) and 3 BPS. At high implementational complexities, the best performance was typically provided by Alamouti's unity-rate G2 code in both the 2-BPS and 3-BPS scenarios. However, if a low complexity implementation is sought, the 3-BPS 8PSK space-time trellis code outperforms the G2 code. The G2 space-time block code is also combined with symbol-by-symbol adaptive orthogonal frequency division multiplex (AOFDM) modems and turbo convolutional channel codecs for enhancing the system's performance. It was concluded that upon exploiting the diversity effect of the G2 space-time block code, the channel-induced fading effects are mitigated, and therefore, the benefits of adaptive modulation erode. In other words, once the time- and frequency-domain fades of the wideband channel have been counteracted by the diversity-aided G2 code, the benefits of adaptive modulation erode, and hence, it is sufficient to employ fixed-mode modems. Therefore, the low-complexity approach of mitigating the effects of fading can be viewed as employing a single-transmitter, single-receiver-based AOFDM modem. By contrast, it is sufficient to employ fixed-mode OFDM modems when the added complexity of a two-transmitter G2 scheme is affordable.
A set of optimum mode-switching levels is derived for a generic constant-power adaptive-modulation scheme based on a closed-form expression of the average bit error ratio (BER) and the average bits-per-symbol (BPS) throughput of the adaptive-modulation scheme. This results in a constant BER, variable-throughput arrangement. The corresponding BPS throughput performance and the achievable signal-to-noise ratio (SNR) gain are investigated for the optimum mode-switching assisted constant-power adaptive-modulation schemes employing various diversity schemes, including maximal ratio combining (MRC) receive-antenna diversity, a two-dimensional RAKE receiver, as well as transmit-diversity aided space-time (ST) coding, when communicating over various fading scenarios. The BPS throughput of our constant-power adaptive quadrature amplitude modulation (AQAM) scheme approaches the throughput of variable-power variable-rate AQAM within 1 dB. However, the achievable throughput gain of the adaptive-modulation scheme, in comparison to conventional fixed-mode modems, is substantially reduced as the diversity order of the receiver is increased. Hence, adaptive modulation constitutes a lower complexity alternative to multiple-transmitter and receiver-based systems when considering the range of techniques that can be used for mitigating the effects of the channel-quality fluctuations imposed by wireless channels.
Adaptive modulation can achieve channel capacity gains by adapting
the number of bits per transmission symbol on a burst-by-burst basis, in
harmony with channel quality fluctuations. This is demonstrated in the
paper for target bit error rates of 1 and 0.01%, respectively, in
comparison to conventional fixed modems. However, the achievable gains
depend strongly on the prevalent interference levels and hence
interference cancellation is invoked on the basis of adjusting the
demodulation decision boundaries after estimating the interfering
channel's magnitude and phase. Using the modem-mode switching levels of
Table X and with the aid of interference cancellation, target BERs of 1
and 0.01% can be maintained over slow-fading channels for a wide range
of channel signal-to-noise ratios (SNR) and signal-to-interference
ratios (SIR)
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