248 research outputs found
Time Reversal with Post-Equalization for OFDM without CP in Massive MIMO
This paper studies the possibility of eliminating the redundant cyclic prefix
(CP) of orthogonal frequency division multiplexing (OFDM) in massive
multiple-input multiple-output systems. The absence of CP increases the
bandwidth efficiency in expense of intersymbol interference (ISI) and
intercarrier interference (ICI). It is known that in massive MIMO, different
types of interference fade away as the number of base station (BS) antennas
tends to infinity. In this paper, we investigate if the channel distortions in
the absence of CP are averaged out in the large antenna regime. To this end, we
analytically study the performance of the conventional maximum ratio combining
(MRC) and realize that there always remains some residual interference leading
to saturation of signal to interference (SIR). This saturation of SIR is
quantified through mathematical equations. Moreover, to resolve the saturation
problem, we propose a technique based on time-reversal MRC with zero forcing
multiuser detection (TR-ZF). Thus, the SIR of our proposed TR-ZF does not
saturate and is a linear function of the number of BS antennas. We also show
that TR-ZF only needs one OFDM demodulator per user irrespective of the number
of BS antennas; reducing the BS signal processing complexity significantly.
Finally, we corroborate our claims as well as analytical results through
simulations.Comment: 7 pages, 3 figure
A novel uplink multiple access scheme based on TDS-FDMA
This contribution proposes a novel time-domain synchronous frequency division multiple access (TDS-FDMA) scheme to support multi-user uplink application. A unified frame structure for both single-carrier and multi-carrier transmissions and the corresponding low-complexity receiver design are derived. Compared with standard cyclic prefix based orthogonal frequency division multiple access systems, the proposed TDSFDMA scheme improves the spectral efficiency by about 5% to 10% as well as imposes a similarly low computational complexity, while obtaining a slightly better bit error rate performance over Rayleigh fading channels
A polynomial QR decomposition based turbo equalization technique for frequency selective MIMO channels.
In the case of a frequency flat multiple-input
multiple-output (MIMO) system, QR decomposition can be
applied to reduce the MIMO channel equalization problem to
a set of decision feedback based single channel equalization
problems. Using a novel technique for polynomial matrix QR
decomposition (PMQRD) based on Givens rotations, we extend
this work to frequency selective MIMO systems. A transmitter
design based on Diagonal Bell Laboratories Layered Space Time
(D-BLAST) encoding has been implemented. Turbo equalization
is utilized at the receiver to overcome the multipath delay spread
and to facilitate multi-stream data feedback. The effect of channel
estimation error on system performance has also been considered
to demonstrate the robustness of the proposed PMQRD scheme.
Average bit error rate simulations show a considerable improvement
over a benchmark orthogonal frequency division
multiplexing (OFDM) technique. The proposed scheme thereby
has potential applicability in MIMO communication applications,
particularly for TDMA systems with frequency selective channels
Delay Alignment Modulation: Manipulating Channel Delay Spread for Efficient Single- and Multi-Carrier Communication
The evolution of mobile communication networks has always been accompanied by
the advancement of ISI mitigation techniques, from equalization in 2G, spread
spectrum and RAKE receiver in 3G, to OFDM in 4G and 5G. Looking forward towards
6G, by exploiting the high spatial resolution brought by large antenna arrays
and the multi-path sparsity of mmWave and Terahertz channels, a novel ISI
mitigation technique termed delay alignment modulation (DAM) was recently
proposed. However, existing works only consider the single-carrier perfect DAM,
which is feasible only when the number of BS antennas is no smaller than that
of channel paths, so that all multi-path signal components arrive at the
receiver simultaneously and constructively. This imposes stringent requirements
on the number of BS antennas and multi-path sparsity. In this paper, we propose
a generic DAM technique to manipulate the channel delay spread via
spatial-delay processing, thus providing a flexible framework to combat channel
time dispersion for efficient single- or multi-carrier transmissions. We first
show that when the number of BS antennas is much larger than that of channel
paths, perfect delay alignment can be achieved to transform the time-dispersive
channel to time non-dispersive channel with the simple delay pre-compensation
and path-based MRT beamforming. When perfect DAM is infeasible or undesirable,
the proposed generic DAM technique can be applied to significantly reduce the
channel delay spread. We further propose the novel DAM-OFDM technique, which is
able to save the CP overhead or mitigate the PAPR issue suffered by
conventional OFDM. We show that the proposed DAM-OFDM involves joint frequency-
and time-domain beamforming optimization, for which a closed-form solution is
derived. Simulation results show that the proposed DAM-OFDM outperforms the
conventional OFDM in terms of spectral efficiency, BER and PAPR.Comment: 16 Pages, 15 figure
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