2,320 research outputs found
Rate Splitting for MIMO Wireless Networks: A Promising PHY-Layer Strategy for LTE Evolution
MIMO processing plays a central part towards the recent increase in spectral
and energy efficiencies of wireless networks. MIMO has grown beyond the
original point-to-point channel and nowadays refers to a diverse range of
centralized and distributed deployments. The fundamental bottleneck towards
enormous spectral and energy efficiency benefits in multiuser MIMO networks
lies in a huge demand for accurate channel state information at the transmitter
(CSIT). This has become increasingly difficult to satisfy due to the increasing
number of antennas and access points in next generation wireless networks
relying on dense heterogeneous networks and transmitters equipped with a large
number of antennas. CSIT inaccuracy results in a multi-user interference
problem that is the primary bottleneck of MIMO wireless networks. Looking
backward, the problem has been to strive to apply techniques designed for
perfect CSIT to scenarios with imperfect CSIT. In this paper, we depart from
this conventional approach and introduce the readers to a promising strategy
based on rate-splitting. Rate-splitting relies on the transmission of common
and private messages and is shown to provide significant benefits in terms of
spectral and energy efficiencies, reliability and CSI feedback overhead
reduction over conventional strategies used in LTE-A and exclusively relying on
private message transmissions. Open problems, impact on standard specifications
and operational challenges are also discussed.Comment: accepted to IEEE Communication Magazine, special issue on LTE
Evolutio
Downlink SDMA with Limited Feedback in Interference-Limited Wireless Networks
The tremendous capacity gains promised by space division multiple access
(SDMA) depend critically on the accuracy of the transmit channel state
information. In the broadcast channel, even without any network interference,
it is known that such gains collapse due to interstream interference if the
feedback is delayed or low rate. In this paper, we investigate SDMA in the
presence of interference from many other simultaneously active transmitters
distributed randomly over the network. In particular we consider zero-forcing
beamforming in a decentralized (ad hoc) network where each receiver provides
feedback to its respective transmitter. We derive closed-form expressions for
the outage probability, network throughput, transmission capacity, and average
achievable rate and go on to quantify the degradation in network performance
due to residual self-interference as a function of key system parameters. One
particular finding is that as in the classical broadcast channel, the per-user
feedback rate must increase linearly with the number of transmit antennas and
SINR (in dB) for the full multiplexing gains to be preserved with limited
feedback. We derive the throughput-maximizing number of streams, establishing
that single-stream transmission is optimal in most practically relevant
settings. In short, SDMA does not appear to be a prudent design choice for
interference-limited wireless networks.Comment: Submitted to IEEE Transactions on Wireless Communication
Rate Analysis of Two-Receiver MISO Broadcast Channel with Finite Rate Feedback: A Rate-Splitting Approach
To enhance the multiplexing gain of two-receiver Multiple-Input-Single-Output
Broadcast Channel with imperfect channel state information at the transmitter
(CSIT), a class of Rate-Splitting (RS) approaches has been proposed recently,
which divides one receiver's message into a common and a private part, and
superposes the common message on top of Zero-Forcing precoded private messages.
In this paper, with quantized CSIT, we study the ergodic sum rate of two
schemes, namely RS-S and RS-ST, where the common message(s) are transmitted via
a space and space-time design, respectively. Firstly, we upper-bound the sum
rate loss incurred by each scheme relative to Zero-Forcing Beamforming (ZFBF)
with perfect CSIT. Secondly, we show that, to maintain a constant sum rate
loss, RS-S scheme enables a feedback overhead reduction over ZFBF with
quantized CSIT. Such reduction scales logarithmically with the constant rate
loss at high Signal-to-Noise-Ratio (SNR). We also find that, compared to RS-S
scheme, RS-ST scheme offers a further feedback overhead reduction that scales
with the discrepancy between the feedback overhead employed by the two
receivers when there are alternating receiver-specific feedback qualities.
Finally, simulation results show that both schemes offer a significant SNR gain
over conventional single-user/multiuser mode switching when the feedback
overhead is fixed.Comment: accepted to IEEE Transactions on Communication
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