1,378 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
A Novel Beamformed Control Channel Design for LTE with Full Dimension-MIMO
The Full Dimension-MIMO (FD-MIMO) technology is capable of achieving huge
improvements in network throughput with simultaneous connectivity of a large
number of mobile wireless devices, unmanned aerial vehicles, and the Internet
of Things (IoT). In FD-MIMO, with a large number of antennae at the base
station and the ability to perform beamforming, the capacity of the physical
downlink shared channel (PDSCH) has increased a lot. However, the current
specifications of the 3rd Generation Partnership Project (3GPP) does not allow
the base station to perform beamforming techniques for the physical downlink
control channel (PDCCH), and hence, PDCCH has neither the capacity nor the
coverage of PDSCH. Therefore, PDCCH capacity will still limit the performance
of a network as it dictates the number of users that can be scheduled at a
given time instant. In Release 11, 3GPP introduced enhanced PDCCH (EPDCCH) to
increase the PDCCH capacity at the cost of sacrificing the PDSCH resources. The
problem of enhancing the PDCCH capacity within the available control channel
resources has not been addressed yet in the literature. Hence, in this paper,
we propose a novel beamformed PDCCH (BF-PDCCH) design which is aligned to the
3GPP specifications and requires simple software changes at the base station.
We rely on the sounding reference signals transmitted in the uplink to decide
the best beam for a user and ingeniously schedule the users in PDCCH. We
perform system level simulations to evaluate the performance of the proposed
design and show that the proposed BF-PDCCH achieves larger network throughput
when compared with the current state of art algorithms, PDCCH and EPDCCH
schemes
Two-Layered Superposition of Broadcast/Multicast and Unicast Signals in Multiuser OFDMA Systems
We study optimal delivery strategies of one common and independent
messages from a source to multiple users in wireless environments. In
particular, two-layered superposition of broadcast/multicast and unicast
signals is considered in a downlink multiuser OFDMA system. In the literature
and industry, the two-layer superposition is often considered as a pragmatic
approach to make a compromise between the simple but suboptimal orthogonal
multiplexing (OM) and the optimal but complex fully-layered non-orthogonal
multiplexing. In this work, we show that only two-layers are necessary to
achieve the maximum sum-rate when the common message has higher priority than
the individual unicast messages, and OM cannot be sum-rate optimal in
general. We develop an algorithm that finds the optimal power allocation over
the two-layers and across the OFDMA radio resources in static channels and a
class of fading channels. Two main use-cases are considered: i) Multicast and
unicast multiplexing when users with uplink capabilities request both
common and independent messages, and ii) broadcast and unicast multiplexing
when the common message targets receive-only devices and users with uplink
capabilities additionally request independent messages. Finally, we develop a
transceiver design for broadcast/multicast and unicast superposition
transmission based on LTE-A-Pro physical layer and show with numerical
evaluations in mobile environments with multipath propagation that the capacity
improvements can be translated into significant practical performance gains
compared to the orthogonal schemes in the 3GPP specifications. We also analyze
the impact of real channel estimation and show that significant gains in terms
of spectral efficiency or coverage area are still available even with
estimation errors and imperfect interference cancellation for the two-layered
superposition system
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