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We develop a unified performance metric and detailed analysis for hybrid automatic repeat request (HARQ) schemes based on incremental redundancy (IR) and Chase combining (CC). The general result is applicable to both symbol-based and bit-interleaved coded modulations, to HARQ processes based on rateless and fixed-rate mother codes, and IR schemes with and without a self-decodability restriction. The analysis shows that IR over CC coding gains tend to increase with the initial coding rate, but decrease with the signal-to-noise (SNR) variation between retransmissions. The gains can also be diminished when a fixed-rate mother code is used or when the self-decodability criterion is imposed. The theoretical prediction is compared with observed gains at 10% codeword error rates based on turbo-coding simulations. For scenarios with moderately varying SNR between retransmissions, the analytical model tracks actual simulation results very well. However, when the SNR varies widely and the systematic part of the turbo codeword is effectively erased, the CC scheme could, in fact, outperform some IR schemes. For these scenarios, which can be induced by fast time-varying fading or long retransmission delays, the self-decodable IR and the CC schemes prove to be more robust without much performance comprise. Finally, we discuss adaptive improvement to the conventional IR schemes based on the analytical result

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.

Cellular wireless systems have focused, to date, on system capacity, user data rates, latency and coverage as primary design criteria. Energy efficiency has recently received increased attention as another important criterion to be considered. The potential for energy savings in 3GPP LTE systems has been investigated in [1]-[3]. An important design element that determines the potential for energy efficient transmission is the structure of reference signals that need to be transmitted independent of the load in the network, such as the cell-specific reference signals (CRS) in 3GPP LTE systems. This paper discusses modifications to the reference signal structure in LTE that can enable greater energy efficiency. The time and frequency estimation performance implications of such modifications are evaluated and discussed.

We consider a new open-loop space-frequency-time coding schemes by fully sweeping the spatial diversity with additional coding. In this spatial sweeping coding (SSC) scheme, part of the channel coding is embedded in a completely randomized diversity hopping pattern, which is not synchronized between the transmitters and the receivers. An efficient max-Log-MAP demodulation algorithm of this SSC is derived. For smooth introduction of the proposal into receiver of existing systems, we further design a practical receiver structure with modular separation of the diversity combining front-end, robust efficient max-Log-MAP soft value computation and the channel decoder. Extensive numerical results are provided to investigate various aspects of the new diversity coding scheme.

We develop a physical layer abstraction to model the performance of MIMO with maximum likelihood demodulation (MLD) in a system simulation. MLD is inherently a non-linear process, and thus it is difficult to find a linear abstraction to its performance. The abstraction is based on bounds on performance obtained from the QR & QL decompositions of the channel. The accuracy of the abstraction is demonstrated using link simulations. In addition, the abstraction has low complexity. The model is used in a system simulation to evaluate the gains obtained with MLD.

This paper discusses circular buffer rate matching (CBRM) algorithms for the turbo code in the long term evolution (LTE) of the WCDMA-based air interface. To enhance performance at high code rates, systematic bit puncturing is incorporated in conjunction with the CBRM. The RM algorithm is further optimized based on the algebraic properties of the QPP interleavers and the 8-state recursive systematic convolutional code of the LTE turbo code.

The error detection performance of CRC coding in LTE with general two-level early stopping algorithms for turbo decoding is investigated. Analytical models for the probability of block error and undetected block error at the code block and transport block levels were developed. Simulations were used to verify the model for shorter CRC lengths and block sizes. The analytical models show the setting of 24-bit CRCs in LTE allows low-complexity, robust and reliable early stopping algorithms to reduce turbo decoding complexity.

Performance of conventional decoders with inaccurate channel estimation input is analyzed to construct an accurate and computationally efficient model for dynamic system simulations. The model abstracts general channel conditions experienced by a codeword into a collection of repetitive states, caused by type-I HARQ or diversity receive antennas, and non-repetitive states, caused by fast fading, type-II HARQ or OFDM transmissions. An effective SNR accounting for losses due to channel estimation errors is derived for the repetitive states. The contribution of all non-repetitive states is summarized in the normalized conditional information rate, which can then used to read off the correct codeword error rate from a look-up table. The accuracy of the proposed model is verified with extensive simulation results.

We present a new methodology for constructing accurate physical-layer (radio link) performance models for dynamic wireless network system simulations. Our approach emphasizes the consistent treatment of the residual fading effects caused by inaccurate channel estimation across slot, transmission time interval, and hybrid automatic repetition request levels. The proposed performance models can he implemented using simple manipulations of certain pre-calculated values. Numerical results are provided to illustrate the accuracy for individual models as well as for multiple models combined together.

In this paper we consider a generalization of the per-antenna-rate-control (PARC) MIMO scheme for the HSDPA option of WCDMA. The scheme, called PARC-N, allows the number of transmitted data substreams N to be less than the number of transmit antennas M and selects the best N antennas for transmission. We investigate PARC-N specifically in frequency selective fading environments. First, we develop a novel receiver structure that combines successive interference cancellation (SIC) with generalized-RAKE technology that achieves a good balance between performance and complexity. We then draw several interesting conclusions: (1) data throughput can be maximized by using N /spl les/ M, where N may be adaptively selected based on operating conditions; (2) data throughput is maximized by allocating all non-data signal power (pilots, voice, overhead, etc.) to as few transmit antennas as possible; and (3) the use of pilot subtraction at the receiver is much more beneficial in MIMO systems than in single-antenna or receive diversity systems.

Lindskog and Paulraj (see ICC-2000, June, 2000) proposed a transmit diversity scheme for dispersive channels which by utilizing two transmit antennas and only one receive antenna achieves the same diversity benefit (except for the 3 dB array gain) as can be achieved by using one transmit antenna and two receive antennas. This scheme generalizes Alamouti's (see IEEE JSAC, vol.16, no.8, p.1451-58, 1998) transmit diversity scheme (which works best for flat channels) to dispersive channels. We show how the transmit diversity (TXD) scheme of Lindskog et al. can be deployed in GPRS/FGPRS networks. A low-complexity receiver structure for demodulating Lindskog and Paulraj (LP) encoded signals is presented based on a combination of a novel pre-filter and a delayed decision feedback sequence estimation (DFSE) equalizer. This pre-filter is designed to simultaneously achieve three desired effects: (1) the effective baseband channel seen by the equalizer has half the number of taps of the original baseband channel; (2) the effective response of the baseband channel becomes minimum-phase; and (3) the effective baseband noise is whitened. An information-theoretic analysis is also presented to demonstrate the advantage of the LP scheme over other TXD schemes (e.g. over delay diversity). A burst format for transmitting LP encoded signals which is compatible with the existing GPRS/EGPRS burst format is also presented. Finally, simulations results are presented to verify the benefit of using the LP scheme in the typical urban channel of GSM/EDGE.

EDGE has enabled GSM networks to offer third generation services as part of the UMTS core network. This has been possible through two steps: the introduction of 8PSK modulation and the specification of interfaces from the GERAN (GSM/EDGE radio access network) to the UMTS core network. This paper studies further improvements to spectral efficiency and capacity for EDGE through the use of more sophisticated adaptive modulation schemes, and the use of newer coding schemes (see Nanda, S. et al., IEEE Commun. Mag., p.54-64, 2000; Ikeda, T. et al., IEEE Trans. Vehic. Tech., vol.49, p.404-12, 2000). It is possible to use theoretical predictions of outage analysis to predict the gain in performance due to the use of higher order modulations.

EDGE has enabled the GSM network to offer third generation services as part of the UMTS core network. This is due to the introduction of 8PSK modulation and the recent specification of interfaces from the GERAN to the UMTS core network. This paper studies further improvements to the spectral efficiency and capacity of EDGE through the use of more sophisticated adaptive modulation schemes based on QAM, and the use of novel turbo coding schemes. Furthermore, this work considers a novel transmit diversity scheme adapted for use in dispersive channels. We provide performance results for the enhanced system showing tremendous gains. We also use theoretical predictions of outage analysis to predict the gain in performance obtained by the use of higher order modulations and transmit diversity. The gains from theoretical predictions match our simulations almost exactly.