Alan Gatherer

Performance analysis of closed-loop transmit diversity in the presence of feedback delay

In this paper, a bit-error-rate (BER) analysis for closed-loop transmit diversity in a time-selective Rayleigh fading channel containing feedback delay is presented. In the absence of feedback delay, closed-loop transmit diversity always outperforms open-loop transmit for a given transmitted signal energy. This is no longer true in the presence of feedback delay. We derive closed-form expressions of the average BER for this case assuming QPSK and BPSK signaling. The results of the analysis are instrumental for comparing closed-loop with open-loop schemes under given operating conditions. In particular, we demonstrate that, for a given transmitted energy and number of transmit antennas, open-loop outperforms closed-loop at sufficiently fast channel fading. We also show that, for a given transmitted signal energy and fading rate, closed-loop outperforms open-loop for sufficiently large numbers of transmit antennas while the total average transmitted signal energy is kept constant. For some special cases, closed-form expressions for the fading rate at which the performance of open-loop is equal to closed-loop are obtained.

Time-domain equalizer training for ADSL

In discrete multitone (DMT) a time domain equalizer is used to shorten the length of the channel to less than the length of the cyclic prefix. The weights of this equalizer are set during initialization using a training sequence. Current techniques that rely on a version of on-line frequency domain LMS show low and uncertain convergence. Here, we show that off-line techniques that estimate the channel first, have much lower complexity, while providing much better performance. We describe a solution that requires a minimum eigenvalue to be found and show that this can be done with relatively low complexity. We also describe an off-line technique based on time domain LMS.

Implementation options for WCDMA

This paper discusses the design tradeoffs and implementation options for building wideband CDMA systems. System on a chip (SOC) solutions have a range of implementation options, from processor cores to custom ASIC (or a mix of both), to satisfy the extremely challenging requirements of the digital baseband of a next generation wireless system. Additionally, good designs should have a roadmap that will take advantage of rapid improvements in process technology. This paper describes the use of coprocessors alongside DSPs to meet the demanding computational requirements of WCDMA. We show that a coprocessor based design still achieves reasonably efficient area and power use, maintains a high degree of programmability and has a good technology migration path.

Resolution-Adaptive Hybrid MIMO Architectures for Millimeter Wave Communications

In this paper, we propose a hybrid analog–digital beamforming architecture with resolution-adaptive ADCs for millimeter wave (mmWave) receivers with large antenna arrays. We adopt array response vectors for the analog combiners and derive ADC bit-allocation (BA) solutions in closed form. The BA solutions reveal that the optimal number of ADC bits is logarithmically proportional to the RF chains signal-to-noise ratio raised to the

ADC bit allocation under a power constraint for mmWave massive MIMO communication receivers

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Space-time fronthaul compression of complex baseband uplink LTE signals

power. Using the solutions, two proposed BA algorithms minimize the mean square quantization error of received analog signals under a total ADC power constraint. Contributions of this paper include 1) ADC bit-allocation algorithms to improve communication performance of a hybrid MIMO receiver, 2) approximation of the capacity with the BA algorithm as a function of channels, and 3) a worst-case analysis of the ergodic rate of the proposed MIMO receiver that quantifies system tradeoffs and serves as the lower bound. Simulation results demonstrate that the BA algorithms outperform a fixed-ADC approach in both spectral and energy efficiency, and validate the capacity and ergodic rate formula. For a power constraint equivalent to that of fixed 4-bit ADCs, the revised BA algorithm makes the quantization error negligible while achieving 22% better energy efficiency. Having negligible quantization error allows existing state-of-the-art digital beamformers to be readily applied to the proposed system.

Decision feedback equalization for Bluetooth systems

Millimeter wave (mmWave) systems operating over a wide bandwidth and using a large number of antennas impose a heavy burden on power consumption. In a massive multiple-input multiple-output (MIMO) uplink, analog-to-digital converters (ADCs) would be the primary consumer of power in the base station receiver. This paper proposes a bit allocation (BA) method for mmWave multi-user (MU) massive MIMO systems under a power constraint. We apply ADCs to the outputs of an analog phased array for beamspace projection to exploit mmWave channel sparsity. We relax a mean square quantization error (MSQE) minimization problem and map the closed-form solution to non-negative integer bits at each ADC. In link-level simulations, the proposed method gives better communication performance than conventional low-resolution ADCs for the same or less power. Our contribution is a near optimal low-complexity BA method that minimizes total MSQE under a power constraint.

Computational reduction during idle transmission in DSL modems

In this paper, we propose space-time fronthaul compression of baseband uplink LTE signals for cellular networks, in which baseband units (BBUs) support remote radio heads (RRHs) through fronthaul links. In particular, we assume massive antenna arrays in which the number of antennas in a RRH is much larger than the number of active users. The proposed method consists of two phases: dimensionality reduction phase and individual quantization phase. The key idea of the first phase is to apply principal component analysis (PCA). It performs low-rank approximation of a matrix - composed of received signals - by exploiting the correlation of the received signals across space and time. In the second phase, our method individually quantizes the dimensionality-reduced signal by applying transform coding with bit allocation to reduce the number of quantization bits. An LTE link-level simulator provides numerical results which show that the method achieves up to 8 × compression ratio for the uplink with 64 antennas and 4 active users, along with improvement in communication system performance as a result of denoising.

ANTENNA SELECTION FOR LARGE-SCALE MIMO SYSTEMS WITH LOW-RESOLUTION ADCS

Gaussian frequency shift keying modulation has been chosen as the modulation technique for the physical layer of Bluetooth. Bluetooth is a standard for low cost and low power wireless communications between various mobile devices. The optimal demodulation of a GFSK signal involves an extremely complex Viterbi decoder. Therefore designers have opted for the noncoherent detection of GFSK which uses a frequency discriminator, followed by symbol by symbol detection. We describe a decision feedback equalizer to be added after the discriminator. The DFE receiver gives gains in excess of 2 dB. We also describe how to increase the current data rate of a Bluetooth system by increasing the symbol rate and not the alphabet size.

Femtocell networks

This document describes two methods for reducing computational requirements during idle transmission in remote access systems incorporating digital subscriber line (DSL) modems, including asymmetrical DSL (ADSL) systems. These methods save processing power during idle transmission by generating an idle signal using low-complexity techniques. The generated idle signal is made spectrally compatible with xDSL systems and a nondisruptive signaling scheme is used to indicate to the far-end receiver the transition between idle to active or active to idle status. A technique is presented that modulates the phase of the pilot tone to signal status transitions to the remote receiver. The computational complexity at the receiver is reduced because full demodulation and decoding is not required to determine that an idle signal is being transmitted.