66 research outputs found
Precoded FIR and Redundant V-BLAST Systems for Frequency-Selective MIMO Channels
The vertical Bell labs layered space-time (V-BLAST) system is a multi-input multioutput (MIMO) system designed to achieve good multiplexing gain. In recent literature, a precoder, which exploits channel information, has been added in the V-BLAST transmitter. This precoder forces each symbol stream to have an identical mean square error (MSE). It can be viewed as an alternative to the bit-loading method. In this paper, this precoded V-BLAST system is extended to the case of frequency-selective MIMO channels. Both the FIR and redundant types of transceivers, which use cyclic-prefixing and zero-padding, are considered. A fast algorithm for computing a cyclic-prefixing-based precoded V-BLAST transceiver is developed. Experiments show that the proposed methods with redundancy have better performance than the SVD-based system with optimal powerloading and bit loading for frequency-selective MIMO channels. The gain comes from the fact that the MSE-equalizing precoder has better bit-error rate performance than the optimal bitloading method
Zero-Forcing DFE Transceiver Design Over Slowly Time-Varying MIMO Channels Using ST-GTD
This paper considers the optimization of transceivers
with decision feedback equalizers (DFE) for slowly time-varying
memoryless multi-input multi-output (MIMO) channels. The data
vectors are grouped into space-time blocks (ST-blocks) for the
spatial and temporal precoding to take advantage of the diversity
offered by time-varying channels. The space-time generalized
triangular decomposition (ST-GTD) is proposed for application
in time-varying channels. Under the assumption that the instantaneous
channel state information at the transmitter (CSIT) and
receiver (CSIR), and the channel prediction are available, we also
propose the space-time geometric mean decomposition (ST-GMD)
system based on ST-GTD. Under perfect channel prediction, the
system minimizes both the arithmetic MSE at the feedback detector,
and the average un-coded bit error rate (BER) in moderate
high signal to noise ratio (SNR) region. For practical applications,
a novel ST-GTD based system which does not require channel
prediction but shares the same asymptotic BER performance
with the ST-GMD system is also proposed. At the moderate high
SNR region, our analysis and numerical results show that all
the proposed systems have better BER performance than the
conventional GMD-based systems over time-varying channels;
the average BERs of the proposed systems are non-increasing
functions of the ST-block size
A novel structure for MMSE transceivers over slowly time-varying channels
This paper addresses the design problem of decision feedback (DF) transceiver without zero-forcing constraint over slowly time-varying narrowband multi-input multi-output (MIMO) channels. The space-time generalized triangular decomposition (ST-GTD) is applied for the design of minimum mean square error (MMSE) DF transceiver. With space-time powerloading, the proposed space-time geometric mean decomposition (ST-GMD) MMSE transceiver maximizes Gaussian mutual information over the equivalent channel seen by each space-time block. For practical applications, the causal ST-GTD MMSE transceiver which does not require channel prediction but shares the same asymptotic bit error rate (BER) performance with the ST-GMD MMSE system is also developed. In high signal to interference plus noise ratio (SINR) region, our results show that the proposed MMSE transceivers have better BER performance than the conventional GMD-based MMSE transceiver; the average BERs of the proposed systems are a non-increasing function of the ST-block size
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Channel equalization to achieve high bit rates in discrete multitone systems
textMulticarrier modulation (MCM) techniques such as orthogonal frequency division
multiplexing (OFDM) and discrete multi-tone (DMT) modulation are attractive
for high-speed data communications due to the ease with which MCM can combat
channel dispersion. With all the benefits MCM could give, DMT modulation has an
extra ability to perform dynamic bit loading, which has the potential to exploit fully
the available bandwidth in a slowly time-varying channel. In broadband wireline
communications, DMT modulation is standardized for asymmetric digital subscribe
line (ADSL) and very-high-bit-rate digital subscriber line (VDSL) modems. ADSL
and VDSL standards are used by telephone companies to provide high speed data
service to residences and offices.
In an ADSL receiver, an equalizer is required to compensate for the channel’s
dispersion in the time domain and the channel’s distortion in the frequency domain
of the transmitted waveform. This dissertation proposes design methods for linear
equalizers to increase the bit rate of the connection. The methods are amenable
to implementation on programmable fixed-point digital signal processors, which are
employed in ADSL/VDSL transceivers.
A conventional ADSL equalizer consists of a time-domain equalizer, a fast
Fourier transform, and a frequency domain equalizer. The time domain equalizer
(TEQ) is a finite impulse response filter that when coupled with a discretized channel
produces an equivalent channel whose impulse response is shorter than that of
the discretized channel. This channel shortening is required by the ADSL standards.
In this dissertation, I first propose a linear phase TEQ design that exploits symmetry
in existing eigen-filter approaches such as minimum mean square error(MMSE),
maximum shortening signal to noise ratio (MSSNR) and minimum intersymbol interference
(Min-ISI) equalizers. TEQs with symmetric coefficients can reach the
same performance as non-symmetric ones with much lower training complexity.
Second, I improve Min-ISI design. I reformulate the cost function to make
long TEQs design feasible. I remove the dependency of transmission delay in order
to reduce the complexity associated with delay optimization. The quantized
weighting is introduced to further lower the complexity. I also propose an iterative
optimization procedure of Min-ISI that completely avoids Cholesky decomposition
hence is better suited for a fixed-point implementation.
Finally I propose a dual-path TEQ structure, which designs a standard singleFIR
TEQ to achieve good bit rate over the entire transmission bandwidth, and
designs another FIR TEQ to improve the bit rate over a subset of subcarriers. Dualpath
TEQ can be viewed as a special case of a complex valued filter bank structure
that delivers the best bit rate of existing DMT equalizers. However, dual-path
TEQ provides a very good tradeoff between achievable bit rate vs. implementation
complexity on a programmable digital signal processor.Electrical and Computer Engineerin
Multihop Nonregenerative MIMO Relays - QoS Considerations
For nonregenerative multihop multiple-input multiple-output (MIMO) relay communication systems, the optimal source precoding matrix and the optimal relay amplifying matrices have been recently established for a broad class of objective functions subjecting to the transmission power constraint at each node. However, existing works do not consider any quality-of-service (QoS) constraints, which are important in practical communication systems. In this paper, we derive the optimal source and relay matrices of a multihop MIMO relay system that guarantee the predetermined QoS criteria be attained with the minimal total transmission power. In particular, we consider two types of receivers at the destination node: the linear minimal mean-squared error (MMSE) receiver and the nonlinear decision feedback equalizer (DFE) based on the MMSE criterion. We show that for both types of receivers, the solution to the original optimization problem can be upper-bounded by using a successive geometric programming (GP) approach and lower-bounded by utilizing a dual decomposition technique. Simulation results show that both bounds are tight, and to obtain the same QoS, the MIMO relay system using the nonlinear MMSE-DFE receiver requires substantially less total transmission power than the linear MMSE receiver-based system
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