For practical multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems, it is important to develop transmit diversity schemes that are transparent to a receiver in that the receiver does not need to perform additional processing. In this paper, we examine one of the most widely-used receiver-transparent transmit diversity schemes - the cyclic delay diversity (CDD) scheme. It is shown that the CDD scheme effectively increases the delay spread of the channel and that the performance degrades under non-ideal channel estimation. As a remedy to the performance loss due to non-ideal channel estimation, a cluster-based transmit diversity scheme is proposed, which garners almost all the benefits of the CDD scheme without degrading the performance of a cluster- based channel estimation. The simulation results are provided in the realistic setting of IEEE 802.16e WiMAX systems.

We study the capacity of multicarrier transmission through a slow frequency-selective fading channel with limited feedback, which specifies channel state information. Our results are asymptotic in the number of subchannels$N$. We first assume independent and identically distributed (i.i.d.) subchannel gains, and show that, for a large class of fading distributions, a uniform power distribution over an optimized subset of subchannels, or on–off power allocation, gives the same asymptotic growth in capacity as optimal water filling, e.g.,$O(\log N)$with Rayleigh fading. Furthermore, the$O(\log N)$growth in data rate can be achieved with a feedback rate as$O(\log ^{3} N)$. If the number of active subchannels is bounded, the capacity grows only as$O(\log \log N)$with the feedback rate of$O(\log N)$. We then consider correlated subchannels modeled as a Markov process, and study the savings in feedback. Assuming a fixed ratio of coherence bandwidth to the total bandwidth, the ratio between minimum feedback rates with correlated and i.i.d. subchannels converges to zero with$N$, e.g., as$O\left (\sqrt {{ \log N}\over { N}}\right)$for Rayleigh-fading subchannels satisfying a first-order autoregressive process. We also show that adaptive modulation, or rate control schemes, in which the rate on each subchannel is selected from a quantized set, achieves the same asymptotic growth rates in capacity and required feedback. Finally, our results are extended to cellular uplink and downlink channel models.

As the standardization of Evolved-UTRA or LTE is ongoing in 3GPP, it is important to ensure the coexistence between two LTE systems or between LTE and legacy UMTS systems sharing the same frequency band. This paper is aimed at presenting a complete view of the status and challenges of the coexistence issue. The high-level methodology, UL power control, and ACLR modeling, etc. are discussed in detail. To ensure coexistence without tightening the RF requirements and hence increasing the cost for LTE, mitigation techniques like interference-aware resource allocation are proposed and shown to be effective by simulation results.

Evolved UTRA (EUTRA) is currently under study by 3GPP standards organizations as a long term evolution (LTE) for current generation universal terrestrial radio access (UTRA) networks. One of the key aspects of EUTRA wireless systems will be providing voice services in packet domain using voice over IP (VoIP). This paper provides system simulation results evaluating uplink VoIP capacity for EUTRA. Simulations indicate that if efficient control signaling and scheduling mechanisms are in place, EUTRA systems can support 80 to 130 Erlangs of VoIP traffic per sector in a 1.08 MHz spectrum allocation as part of a 5 MHz LTE carrier.

With the emergence of packet-based wireless broadband systems such as 802.16e, it is evident that a comprehensive evolution of the universal mobile telecommunications system specifications is required to remain competitive. As a result, work has begun on long term evolution (LTE) of the UMTS terrestrial radio access and radio access network aimed for commercial deployment in 2010. Goals for the evolved system include support for improved system capacity and coverage, high peak data rates, low latency, reduced operating costs, multi-antenna support, flexible bandwidth operations and seamless integration with existing systems. To reach these goals, a new design for the air interface is envisioned. This paper provides a preliminary look at the air interface for Evolved UTRA (E-UTRA) and associated key technologies required to reach its design objectives. Initial E-UTRA system performance results show a 2 to 3x improvement over a reference Rel-6 UMTS system configuration [1, 2] for both uplink and downlink.

The IEEE 802.16e system for multicast and broadcast service (MBS) is further investigated over [4]. It is shown that 802.16e networks can exploit advanced transmission and flexible resource allocation techniques to deliver high- performance MBS delivery within the constraints of the WiMAXRelease 1 system profile. A novel per-subcarrier SINR model for single frequency network (SFN) operation is proposed for the scenario in which cyclic shift transmit diversity (CSTD) - a form of delay diversity - is applied at the base station. The utility of CSTD for 802.16e MBS delivery in term of rate and coverage enhancement is investigated in detail based on the exponential effective-SINR mapping (EESM) for frame error rate (FER) prediction. In addition, an asymptotic effective SINR approach is developed for low- complexity approximation of the EESM FER mapping, and the proposed approximation is shown to be accurate for MBS simulation, while significantly reducing simulation effort.

Multi-user scheduling and OFDM techniques are well regarded as promising candidates to provide high data- rate and reliable transmissions in future communication systems. To optimally exploit multi-user diversity and frequency selectivity, exhaustive channel information is needed for every user - an impracticality given limited feedback bandwidth. Simple and efficient feedback schemes therefore become a crucial task in designing a next generation standard. In this paper, two near-optimal feedback schemes are proposed that achieve most of the multi-user scheduling benefit with very few feedback bits. It is also shown that the feedback schemes are robust to long feedback intervals. As expected, at higher Doppler spread, the performance gap using the proposed schemes becomes larger but still provides satisfactory performance.

Evolved UTRA and UTRAN is being standardized in 3GPP standard group as a long term evolution of the 3GPP radio-access technology. The goal is to achieve 2-4 times the spectral efficiency and user throughput and much smaller latency compared to HSDPA/HSUPA. Single carrier FDMA (e.g. DFT-SOFDM) is the multiple access technique of choice for uplink transmission. Interference control is one of the key elements to achieve the target spectral efficiency and user cell- edge performance requirement, especially for uplink. In this paper, interference mitigation is implemented through slow fractional power control and interference coordination through UE alignment and FDM resource allocation. Simulation results show that these techniques significantly improve uplink sector and cell edge user throughput performance.

OFDM system-level simulations require a performance estimate for each link, but it is typically infeasible to conduct simultaneous link-level simulations for multiple users within a reasonable amount of time. Therefore, it is critical to have a simple link error probability predictor that accurately models coded OFDM performance. The exponential ESM (EESM) method was shown to be a very good link error prediction method for multi-carrier systems in the case of single transmission. Here, the EESM method is extended to cover hybrid ARQ (HARQ) techniques: Chase combining, incremental redundancy (IR) or an arbitrary combination of both. Further simplifications are introduced to greatly reduce the amount of memory necessary to utilize EESM for modeling OFDM-HARQ transmissions

We evaluate the performance of reduced-rank equalizers for both single-input single-output (SISO) and multiple-input multiple-output (MIMO) frequency-selective channels. Each equalizer filter is constrained to lie in a Krylov subspace, and can be implemented as a reduced-rank multistage Wiener filter (MSWF). Both reduced–rank linear and decision-feedback equalizers (DFEs) are considered. Our results are asymptotic as the filter length goes to infinity. For SISO channels, the output mean-squared error (MSE) is expressed in terms of the moments of the channel spectrum. For MIMO channels, both successive and parallel interference cancellation are considered. The asymptotic performance in that case requires the computation of moments, which depend on shifted versions of the channel impulse response for different users. Those are also expressed in terms of the MIMO channel frequency response. Numerical results are presented, which show that near full-rank performance can be achieved with relatively low-rank equalizers.

Interference in code-division multiple access (CDMA) and multi-antenna systems can be avoided by choosing a signature (in space and/or time), which lies in the direction of least interference plus noise. If the interference statistics are unknown a priori, then the signature can be adaptively estimated with a training sequence. We present an iterative scheme for joint signature-receiver adaptation with an adaptive reduced-rank multi-stage Wiener filter (MSWF) at the receiver. We establish convergence of the iterative signature-receiver optimization scheme for a single user with fixed interference, and show that the limiting performance for any filter rank D ges 2 is the same as that obtained with a full-rank receiver. To reduce feedback requirements, we also consider joint signature-receiver adaptation with a reduced-rank signature, which is confined to a randomly chosen subspace. Numerical results are presented for both single- and multi-user (group) adaptation, which show that reduced-rank signature-receiver estimation can achieve near-optimal performance with relatively little training and low complexity

The performance of a wireless channel with multiple antennas benefits from channel knowledge at the receiver, which is typically unknown a priori. We study the capacity of a block fading multiple-input/multiple-output (MIMO) channel with a linear receiver, which is estimated from a training sequence via a least squares (LS) algorithm. Given a fixed block size, the amount of training overhead plays a key role in balancing the quality of the receiver estimate and the data transmission time. Here we study the optimal training length, which maximizes the large system MIMO capacity, i.e., the number of transmit and receive antennas go to infinity with fixed ratio. In order to obtain a meaningful limit, the training length and packet length also increase in fixed proportion to the number of antennas. We show that the optimal amount of training grows as the square root of the block size, as the block size becomes large. Furthermore, only a slight benefit is obtained from optimizing the allocation of power across training and data symbols. Numerical results show that for a fixed block length, the capacity can be increased somewhat by adding a properly chosen diagonal loading factor to the LS algorithm.

In this paper, the performance of a prototypical IEEE 802.16e network is analyzed via link and system simulations. The exponential effective SIR mapping (EESM) is used to map the instantaneous SINR of received signals to a service-specific packet erasure rate (PER), which is in turn used to assess the downlink and uplink network and user data throughput. A variety of data traffic models are considered, including web browsing (HTTP) and full buffer sessions, operating in flat-fading and frequency-selective multipath channels such as the ITU Pedestrian-B model. The downlink network performance of the prototypical IEEE 802.16e network is compared to a 3GPP UMTS high speed downlink packet access (HSDPA) system of similar physical dimensions. System simulation results for the prototypical IEEE 802.16e network considered show-when frequency-selective scheduling is not applied-that the IEEE 802.16e downlink has similar throughput performance as HSDPA for a 70/30 TDD DL/UL frame split but approximately 40%-50% higher spectral efficiency, although control channel overhead and uplink capacity limitation remain significant open issues for further study

Incremental redundancy, or Hybrid type-II ARQ (HARQ), algorithms use a combination of forward error correction and retransmissions to guarantee reliable packet data communications. In this work, we propose a HARQ algorithm that exploits received packet reliability to improve system performance. Specifically, the receiver uses the average magnitude of the log-likelihood ratios of the information bits as the packet reliability metric, which is then used to determine the sizes of subsequent retransmissions. The proposed retransmission strategy attempts to maximize user throughput while satisfying a maximum packet delay constraint. The performance of our reliability-based HARQ algorithm is evaluated in static and time-varying channels through simulations. Furthermore, analytical results on the relationship between the reliability metric, the code rate and the block error rate are presented.

An adaptive iterative (turbo) decision-feedback equalizer (DFE) for channels with intersymbol interference (ISI) is presented. The filters are computed directly from the soft decisions and received data to minimize a least-squares (LS) cost function. Numerical results show that this method gives a substantial improvement in performance relative to a turbo DFE computed from an exact channel estimate, assuming perfect feedback. Adaptive reduced-rank estimation methods are also presented, based on the multistage Wiener filter (MSWF). The adaptive reduced-rank turbo DFE for single-input/single-output channels is extended to multiple-input/multiple-output (MIMO) channels with ISI and multiple receive antennas. Numerical results show that for MIMO channels with limited training, the reduced-rank turbo DFE can perform significantly better than the full-rank turbo DFE.

Incremental redundancy, or hybrid type-II ARQ (HARQ), algorithms use a combination of forward error correction and retransmissions to guarantee reliable packet data communication. In our previous work (V. Tripathi et al. 2003), we have proposed an adaptive HARQ algorithm that exploits received codeword reliability for optimizing the size of the subsequent retransmissions. The optimization is based on two mappings: the codeword reliability to block error rate (BLER) mapping and the coding rate to codeword reliability mapping. We provide analytical approximations for these mappings for any convolutional code. Through simulations, the approximations are shown to be accurate in the range of interest.

Adaptive iterative receivers which combine multiuser decision-feedback detection with maximum a posteriori (MAP) decoding and soft feedback are presented for synchronous coded direct sequence-code-division multiple access. Both successive and parallel demodulation of users are considered. Optimal filters are derived using both minimum mean squared error and least squares (LS) criteria. The latter assumes short (repeated) spreading codes and that the users to be demodulated simultaneously transmit training sequences. The LS criterion does not require prior knowledge or estimates of spreading codes and channels. Simulation results show that the adaptive receiver can perform significantly better than the standard (soft) interference canceller, since the adaptive algorithm attempts to measure and exploit the second-order statistics between the input and output of the MAP decoder. With limited training, successive feedback and decoding performs significantly better than parallel feedback. The effect of code rate on performance is examined, and reduced-rank versions of the adaptive LS algorithms, which can reduce training overhead, are also presented.

In this paper, we consider multicarrier transmission through a frequency-selective fading channel with limited feedback. An on-off power allocation activates the set of subchannels with gains above a threshold. We model the sequence of subchannel gains as a Markov process, and give a lower bound on the feedback rate in bits per subchannel needed to specify the sequence of activated subchannels as a function of the activation threshold. Optimizing the threshold gives the same asymptotic growth in capacity as optimal water-filling as the number of subchannels N goes to infinity. If the ratio of coherence bandwidth to the total available bandwidth is fixed, then the ratio between minimum feedback rates with correlated and i.i.d. subchannels, respectively, converges to zero with N. For a sequence of Rayleigh fading subchannels, which are modeled as a first-order autoregressive process, the ratio goes to zero as O (/spl radic/logN/N) with the optimized threshold. We also consider finite-precision rate control on each subchannel, and show that the feedback rate required to specify the sequence of assigned rate levels across subchannels gives the same asymptotic increase in achievable rate with N as the (infinite-precision) on-off power allocation.

We compare the performance of iterative multi-user successive and parallel decision feedback detectors (DFDs) for code-division multiple access (CDMA). An adaptive successive-DFD (S-DFD) is first presented, which requires only a training sequence and associated timing to estimate all filter coefficients. Simulation results show that with limited training, the adaptive S-DFD performs significantly better than the adaptive parallel-DFD (P-DFD). A large system analysis of error rate for non-adaptive iterative DFDs is also presented, and shows that the S-DFD converges with fewer iterations than the P-DFD.