Sorour Falahati

Adaptive modulation systems for predicted wireless channels

When adaptive modulation is used to counter short-term fading in mobile radio channels, signaling delays create problems with outdated channel state information. The use of channel power prediction will improve the performance of the link adaptation. It is then of interest to take the quality of these predictions into account explicitly when designing an adaptive modulation scheme. We study the optimum design of an adaptive modulation scheme based on uncoded M-quadrature amplitude modulation, assisted by channel prediction for the flat Rayleigh fading channel. The data rate, and in some variants the transmit power, are adapted to maximize the spectral efficiency, subject to average power and bit-error rate constraints. The key issues studied here are how a known prediction error variance will affect the optimized transmission properties, such as the signal-to-noise ratio (SNR) boundaries that determine when to apply different modulation rates, and to what extent it affects the spectral efficiency. This investigation is performed by analytical optimization of the link adaptation, using the statistical properties of a particular, but efficient, channel power predictor. Optimum solutions for the rate and transmit power are derived, based on the predicted SNR and the prediction error variance.

Hybrid type-II ARQ schemes with adaptive modulation systems for wireless channels

In this paper, we propose and analyze a scheme for wireless channels which is a combination of a hybrid type-II automatic repeat request (ARQ) scheme and an adaptive modulation system (AMS). In hybrid type-II ARQ schemes, the transmission of redundancy bits for error correction is adapted to the channel variations. Additionally, in the proposed AMS, the modulation is gradually changed from a large constellation to small constellations when the channel bit error probability is high. This is done without any knowledge of the channels at the transmitter. The proposed scheme provides a significant gain in the throughput performance as compared to traditional hybrid type-II ARQ schemes, as verified by simulations.

Hybrid type-II ARQ/AMS supported by channel predictive scheduling in a multi-user scenario

A combination of radio channel predictive time slot scheduling and link layer FEC/ARQ (forward error correction/automatic repeat request) with adaptive modulation, for IP (Internet protocol) packet data is presented and evaluated through simulations. The main idea is to make use of the fast fading characteristics of the radio channel through time slot scheduling, instead of alleviating the effects with over-pessimistic channel coding. By using information about the individual data streams, together with information about future wireless channel characteristics for the different mobile hosts, it it possible to plan the transmission to achieve a good quality of service while utilizing the scarce spectrum efficiently. The simulated results confirm that out proposed system can provide a high throughput while maintaining the BER and the delay low for the majority of the users.

Adaptive trellis-coded modulation over predicted flat fading channels

We consider the optimum design of an adaptive scheme based on TCM and predicted CSI for flat Rayleigh fading channels, intended for fast link adaptation. The question of how to optimally adjust the data rate to maximize the spectral efficiency, subject to a BER constraint when imperfect CSI is taken into account, is answered. An optimum solution based on the predicted SNR and the prediction error variance is derived. The performance of the adaptive TCM scheme is illustrated by utilizing seven 4-D trellis codes based on the International Telecommunications Unions ITU-T V.34 modem standard. The results indicate that the gain in spectral efficiency by adaptive TCM is about 1-2 dB compared to adaptive uncoded M-QAM, for most of average SNRs. Finally, the performance of the adaptive TCM and in particular, its loss at high SNRs compared to adaptive uncoded M-QAM, are investigated in details.

Maximizing througput with adaptive M-QAM based on imperfect channel predictions

Uncoded adaptive M-QAM transmission over flat Rayleigh fading channels is here optimized in a novel way in the presence of channel prediction errors. The modulation rate is determined based on the predicted channel state. The modulation rate limits are adjusted by maximizing the throughput in error-free link-level frames, averaged over the pdf of the true channel state. No bit error rate constraint is imposed. This approach is appropriate when fast link-level retransmissions can be used to attain required error levels. The resulting scheme is evaluated analytically in a multiuser environment where predictive link adaptation is used in combination with a scheduling strategy that provides multiuser diversity gain. Prediction errors typical of prediction 1/3 wavelength ahead in space will then result in only 8%-12% reduction in the spectral efficiency, as compared to a case with perfectly known channels. The resulting performance is very robust with respect to the prediction error variance assumed when optimizing the rate adaptation scheme.

Coding and Resource Scheduling in Packet Oriented Adaptive TDMA/OFDMA Systems

Within the EU FP6 Integrated Project WINNER, adaptive transmission is investigated as a key technology for boosting the spectral efficiency of a new radio interface for 4G systems. Adaptive allocation and link adaptation of time-frequency chunks based on channel prediction in an OFDM-based system offers a significant potential to design a spectrally efficient system. The chunk size is typically defined based on the minimum coherence time and coherence bandwidth of the targeted channels. It is important to allow efficient channel coding and link retransmission schemes without restricting the resource scheduler, even for systems using small chunk sizes, to achieve multi-user diversity gains. In this paper we introduce some possible approaches to implement FEC coding and hybrid ARQ and analyze their interplay with resource scheduling in packet oriented adaptive TDMA/OFDMA

Convolutional coding and decoding in hybrid type-II ARQ schemes on wireless channels

We studied the performance of the hybrid type-II automatic repeat request (ARQ) system on Rayleigh fading channels. This system is based on high rate optimized rate compatible punctured convolutional (HRO-RCPC) codes combined with simple repetition coder. The performance is investigated for different packet sizes and constraint lengths of the convolutional encoder. Additionally the effect of packet size on the performance in different fading environments is studied in more detail. We have also studied the performance of the HRO-RCPC codes combined with optimized rate compatible repetition convolutional (RCRC) codes as another alternative for channel coding in hybrid type-II ARQ schemes. The simulation results show that HRO-RCPC codes combined with optimized RCRC codes with high parent code rate perform almost as good as the HRO-RCPC codes with simple repetition codes at lower parent code rate. Also a tailbiting decoder using the circular Viterbi algorithm (CVA) is examined for different hybrid type-II ARQ schemes and packet sizes. It is shown that the improvement of the performance due to tailbiting is noticeable only for short packets. For long packets, the sub-optimality of CVA decoding compared to the general Viterbi algorithm (GVA) degrades the performance considerably.

Energy Efficiency Performance of LTE Dynamic Base Station Downlink DTX Operation

Dynamic base station downlink discontinuous transmission (DTX) operations have been proposed and under discussion in the standardization body as a candidate to reduce interference in the LTE network to support rapidly increasing demand for mobile broadband services and traffic volumes [1]. In this paper, we analyze the energy efficiency improvement of the dynamic base station DTX operations from three perspectives. First, we discuss the complementary operation aspect of the dynamic base station DTX and existing LTE dormant mode. Secondly, we provide an analysis showing substantial energy efficiency benefits at all loads of dynamic base station DTX operation with near- future base station hardware components. Thirdly, we provide an analytical framework to assess the energy saving potential of the dynamic base station DTX concept for all possible future hardware improvement possibilities.

Base Station Downlink DTX Designs for Interference Mitigation in High-Performance LTE Networks

To evolve the LTE network to cope with ever and rapidly increasing demand for mobile broadband services and traffic volumes, interference avoidance designs have been identified as one major area to address in the next LTE release. In both traditional homogeneous macro network and heterogeneous network with mix of macro and small cells, intercell interference can be substantially reduced by temporarily halting unneeded downlink transmission. In this paper, we analyze several design aspects of such base station downlink discontinuous transmission (DTX) operations. We provide extensive performance evaluation results and analysis on (1) the time scale of such DTX operations; and (2) whether the DTX operations should be restricted to the small cells only. The results provide guideline for an ideal combination of these design dimensions.