For both outdoor and indoor wireless systems there is an increasing demand of high spectral efficiency at a very low cost and power consumption. In this context, MIMO wireless system adopting spatial multiplexing offer a way of increasing the spectral efficiency of the system. In order to fully exploit this capacity non linear MIMO detectors such as maximum likelihood detectors are required. However, when high order modulation schemes are applied, the complexity of this kind of detector becomes prohibitive for a practical implementation. As a solution to this problem, low complexity maximum likelihood detectors such as sphere detectors are appealing as a low complexity solution for high spectral efficiency transmission. Although sphere decoding provides a lower complexity solution than a classical ML detector, its complexity still remains unpredictable and exponentially dependent on channel propagation conditions. This variability in complexity makes the implementation of sphere decoders not practical. In this paper, a new approach for constraining the ML search space is proposed which provides a predictable upper bound for complexity, hence facilitating its implementation. Moreover, the new approach for computing the constrained search space significantly reduces the complexity of the detection while offering scalability in terms of performance and complexity. Simulation results in a cellular system demonstrate the scalability of our detector and the performance/complexity trade-off that it enables.

ML and near-ML MIMO detectors have attracted a lot of interest in recent years. However, almost all of the reported implementations are delivered in ASIC or FPGA. Our contribution is to co-optimize the near-ML MIMO detector algorithm and implementation for parallel programmable base-band architectures, such as DSPs with VLIW, SIMD or vector processing features. Although for hardware the architecture can be tuned to fit algorithms, for programmable platforms the algorithm must be elaborately designed to fit the given architecture, so that efficient resource-utilizations can be achieved. By thoroughly analyzing and exploiting the interaction between algorithms and architectures, we propose the SSFE (selective spanning with fast enumeration) as an architecture-friendly near-ML MIMO detector. The SSFE has a distributed and greedy algorithmic structure that brings a completely deterministic and regular dataflow. The SSFE has been evaluated for coded OFDM transmissions over 802.11n channels and 3GPP channels. Under the same performance constraints, the complexity of the SSFE is significantly lower than the K-Best, the most popular detector implemented in hardware. More importantly, SSFE can be easily parallelized and efficiently mapped on programmable baseband architectures. With TI TMS320C6416, the SSFE delivers 37.4 - 125.3 Mbps throughput for 4x4 64 QAM transmissions. To the best of our knowledge, this is the first reported near-ML MIMO detector explicitly designed for parallel programmable architectures and demonstrated on a real-life platform.

The improvement of spectral efficiency comes at the cost of exponential increment of signal processing complexity [1]. Hence, the energy-efficiency of baseband has recently turned out to be the bottleneck when deploying advanced air interfaces such as that in 4G. We advocate the scalable baseband design as a system level technique to aggressively optimize the average computation-load and associated energy-consumption. The key technique is to dynamically scale the baseband processing itself to the user requirement, the environment, the platform, etc. In this paper, we present the scalable design and VLIW processor based implementation of the OFDMA modulator, which is one of the most energy consuming parts of OFDMA and MIMO- OFDMA transmitters (in IEEE 802.16e , 3GPP LTE, etc.). Our work enables the OFDMA modulator to scale the modulation- accuracy and computation-load, so that the OFDMA modulator can dynamically reconfigure and work with minimal number of operations, whereas the required modulation-accuracy is still firmly guaranteed. Our work brings significant reductions in the average computation-load and associated energy-dissipation on real-life programmable platforms. Specifically, when the user is working with 16QAM and 1/2 coding rate (Turbo Coding) in a half-loaded 8-user system, the proposed scheme reduces 84% of the cycle-count and the associated energy-consumption on TI TMS320C6713, whereas the resulted Relative Constellation Error (RCE) is still lOdB better than the required RCE in IEEE 802.16e specifications.

Nowadays a lot of effort is spent on developing OFDM-based inexpensive wireless transceivers. Direct-conversion radio frequency transceivers are appealing because they avoid costly IF analog components. This kind of transceivers imply analog RF I/Q separation. The mismatch between the analog components in the in-phase and quadrature branches introduces an unwanted in-band interference. Unfortunately, OFDM-based systems are very sensitive to I/Q mismatch, mostly when high order modulation schemes are applied. A digital compensation of this unwanted effect is required. In this paper, we developed an iterative method for estimating and compensating the transmitter/receiver frequency-dependent I/Q imbalance jointly with the propagation channel in the frequency domain. The new estimation technique resorts to the iterative Expectation- Maximization algorithm in order converge to the ML estimation for both channel and IQ imbalance coefficients in presence of unknown transmitted data symbols. Although the the estimation may be perform in the absence of known pilot symbols, the algorithm provides flexibility in terms of pilot sequence, allocation and load in order to improve its convergence. An advanced low complexity equalizer is proposed which compensates for the frequency-dependent IQ imbalance. Simulation results show optimal performance even in the absence of any pilot symbol and large IQ mismatches. The proposed technique enables the system to achieve high SNRs.

In order to meet the data rate and quality-of-service (QoS) requirements of the future cellular systems, new air interfaces are currently under development. In this paper, we compare two air interfaces of particular interest for the uplink: cyclic-prefix code-division multiple access (CP-CDMA) proposed in the literature as an evolution of direct-sequence code-division multiple access (DS-CDMA) because it enables the low complexity equalization of the multipath channel in the frequency domain, and single-carrier frequency-division multiple access (SC-FDMA), recently proposed in the long term evolution of the 3GPP standard because it enables the easy separation of the users in the frequency domain. We demonstrate analytically that SC-FDMA is a special case of CP-CDMA, in which the CDMA codes have been optimized to minimize the symbol estimation mean square error (MSE) under a constraint of received power. Numerical results show that SC-FDMA outperforms significantly CP-CDMA at high user loads. The transmit power necessary to fulfill the received power constraint is higher in case of SC-FDMA than in case of CP-CDMA when the carrier sub-sets are allocated randomly to the users, and lower when the carrier sub-sets are allocated in an optimized way.

New air interfaces are currently being developed to meet the high spectral efficiency requirements of the emerging wireless communication systems. In this context, OFDM is considered as a promising air interface candidate for both indoor and outdoor communications. Besides spectral efficiency and power consumption, the production cost of the transceiver should also be optimized. Direct-conversion radio frequency receivers are appealing because they avoid costly intermediate frequency hardware. However, they imply analog IQ separation, introducing a phase and amplitude mismatch between the I and Q branches. A communication system based on OFDM is sensitive to synchronization errors, such as CFO, and to front-end non-idealities, such as IQ imbalance. The goal of this paper is to use the iterative EM algorithm to acquire jointly the CFO and the IQ imbalance. The solution relies on a repetitive preamble and does not require the knowledge of the propagation channel. Based on a second order approximation of the likelihood function, the complexity of the EM algorithm is significantly reduced. The algorithm is shown to perform extremely well: the estimates of the CFO and of the IQ imbalance converge to their ML estimate after less than 3 iterations. While the CFO estimate is robust against variations of the SNR, the IQ imbalance estimate accuracy is reduced at values of the SNR below 10 dB and above 35 dB.

Nowadays a lot of effort is spent on developing OFDM- based inexpensive wireless transceivers. Direct-conversion radio frequency transceivers are appealing because they avoid costly IF analog components. This kind of transceivers imply analog RF I/Q separation. The mismatch between the analog components in the in-phase and quadrature branches introduces an unwanted in-band interference. Unfortunately, OFDM-based systems are very sensitive to I/Q mismatch, mostly when high order modulation schemes are applied. A digital compensation of this unwanted effect is required. In this paper, we developed a method for compensating the transmitter/receiver frequency-dependent I/Q imbalance jointly with the propagation channel in the frequency domain. Based on a low complexity ML channel estimator, a pilot design criterion is derived for channel and IQ imbalance coefficients estimation. An advanced equalizer is proposed which compensates for the frequency-dependent IQ imbalance. Both steps, estimation and compensation, are low cost in terms of implementation complexity. The proposed algorithm enables the system to achieve high SNRs.

Single-carrier code-division multiple access (SC-CDMA), also named cyclic-prefix CDMA in the literature, is a promising air interface for the uplink of the 4G cellular wireless communication systems. It enables the high capacity intrinsically offered by CDMA by making the equalization of the multipath channels and the mitigation of the resulting interference possible at a low complexity. This paper proposes a new air interface that combines SC-CDMA with space-time block coding (STBC) across multiple transmit antennas in order to make the link more robust. Contrary to existing air interfaces that perform the STBC at the chip level, making them only applicable to the downlink, the STBC is performed at the symbol level, making it also applicable to the uplink. In order to optimally detect the different antenna and user signals, a linear joint detector optimized according to the minimum mean square error (MMSE) criterion is designed. By exploiting the cyclic properties of the channel matrices, the complexity of the joint detector is significantly reduced. Furthermore, it is shown analytically that the inter-antenna interference is canceled out at the output of the first stage of the linear MMSE joint detector, consisting of a matched filter. By space-time coding the signal through two antennas at each transmit mobile terminal, a significant gain in signal-to-noise-ratio can be achieved. However, the spatial diversity gain of the proposed system is limited by the multiuser interference (MUI), that is increasing with the user load. Higher complexity non-linear receivers are needed to better compensate the MUI and still benefit from the spatial diversity at high user loads.

In fast mobile environments, the reception of orthogonal frequency domain modulation (OFDM) signals with tight sub-carrier spacing is challenged by the inter-carrier interference (ICI). This ICI must be compensated for by a suitable equalizer design, which requires the estimation of a larger number of parameters than for static environments. In addition, ICI also degrades the quality of the channel estimation. We propose an analytical approach to estimate the frequency domain channel matrix that relies on its nearly banded structure and a first order Taylor approximation of the channel variation. An additional constraint of this work is to accommodate OFDM systems with large number of sub-carriers and scattered pilots, such as used in DVB-T and DVB-H broadcast. We provide simulation results to demonstrate the performance of the proposed channel estimation method.

Nowadays OFDM is combined with advanced multiple access and multiple antenna techniques in order to improve the link capacity. In this context, a lot of effort is spent on developing inexpensive wireless OFDM-based receivers. Direct-conversion radio frequency receivers are appealing because they avoid costly IF filters. This kind of receivers implies analog RF I/Q separation introducing an unwanted in-band image interfearence encedue to the mismatch between the in-phase and quadrature branches. Unfortunately, systems combining OFDM with multiple antennas and multiple access techniques are very sensitive to I/Q mismatch, mostly when high order modulation schemes are applied. A digital compensation of this un-wanted effect is required. In this paper, we propose a method for estimating jointly the frequency dependent I/Q imbalance and the propagation channel in the frequency domain. First the channel is estimated based on a pilot optimized to suppress the interference caused by I/Q imbalance. Second I/ C imbalance is estimated and compensated relying on the estimated channel. Both steps are low cost in terms of implementation complexity. Finally, it is shown that the performance of the channel estimate is not degraded by I/Q imbalance. The proposed algorithm enables the system to work at high SNRs.

In the context of 4G wireless mobile systems with direct conversion receivers, an new algorithm for carrier frequency offset (CFO) and IQ-imbalance estimation and compensation at the receiver is proposed. All processing for estimation and compensation of both CFO and IQ-imbalance is done in the time domain. Because the CFO is estimated while taking into account the presence of IQ-imbalance, a very precise estimate of the CFO can be obtained, even in the presence of large IQ-imbalance. The algorithm uses a repetitive low peak-to-average preamble sequence. At the transmitter, in order to improve the estimation result, an artificial CFO is digitally super-imposed on a part of the preamble sequence. The new algorithm yields better results than existing CFO estimation algorithms for SNRs where the IQ imbalance is dominant over the Gaussian noise. The structure of the preamble also allows an effective IQ-imbalance estimation.

New air interfaces are currently being developed to meet the high spectral efficiency requirements of the emerging wireless communication systems. Multicarrier code-division multiple access (MC-CDMA) is seen as a promising candidate for the fourth-generation (4G) cellular communication systems because it can interestingly deal with the multipath propagation at a low processing complexity. Besides spectral efficiency and power consumption, the production cost of the transceiver should also be optimized. Direct conversion radio frequency (RF) receivers are appealing because they avoid costly intermediate frequency (IF) filters. However, they imply RF IQ separation, introducing a phase and amplitude mismatch between the I and Q branches. A communication system based on MC-CDMA is sensitive to synchronization errors and front-end non-idealities because it uses a long symbol duration. The goal of this paper is to evaluate the impact of the carrier frequency offset, the sampling clock offset, and the IQ imbalance on the MC-CDMA downlink system performance, considering a receiver based on channel tracking designed to cope with high mobility conditions. It is demonstrated that part of the effects is compensated by the channel estimation and an expression of the variance of the remaining symbol estimation error is provided. For the cellular system and the target performance considered in this paper, specifications are defined on the non-idealities. The results are validated with bit-error rate simulations

Single-carrier code division multiple access (SC-CDMA), also named cyclic-prefix CDMA in the literature, is a promising air interface for the uplink of the 4G cellular wireless communication systems. It enables the high capacity intrinsically offered by CDMA by making the equalization of the multipath channels and the mitigation of the resulting interference possible at a low complexity. This paper proposes a new air interface that combines SC-CDMA with space time block coding (STBC) across multiple transmit antennas in order to robustify the link. Contrary to existing air interfaces that perform the STBC encoding at the chip level, making them only applicable to the downlink, the STBC encoding is performed at the symbol level, making it also applicable to the uplink. In order to optimally detect the antenna and user signals, a joint detector optimized according to the minimum mean square error (MMSE) criterion is designed. By exploiting the cyclo-stationarity of the channels, the complexity of the joint detector is significantly reduced. It is analytically shown that the inter-antenna interference is canceled out at the output of the first stage of the linear MMSE joint detector, consisting of a matched filter. Assuming typical outdoor channels, a gain of 4 dB signal-to-noise-ratio (SNR) is achieved by the use of two transmit antennas over the single antenna system.

New air interfaces are currently being developed to meet the high requirements of the emerging wireless communication systems. In this context, MC-CDMA is seen as a promising candidate for the 4G cellular communication systems since it can interestingly deal with the multipath propagation. The goal of this paper is to evaluate the impact of the carrier frequency offset, the sampling clock offset and IQ imbalance on the MC-CDMA downlink system performance, considering a receiver based on channel tracking designed to cope with high mobility conditions. It is demonstrated that part of the effects is compensated by the channel estimation and an expression of the variance of the remaining error is provided. For the setup considered in this paper, mostly IQ imbalance degrades the performance. On the other hand, the impact of sample clock offset is negligible with respect to the one of carrier frequency offset.