580 research outputs found
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
Physical Layer Service Integration in 5G: Potentials and Challenges
High transmission rate and secure communication have been identified as the
key targets that need to be effectively addressed by fifth generation (5G)
wireless systems. In this context, the concept of physical-layer security
becomes attractive, as it can establish perfect security using only the
characteristics of wireless medium. Nonetheless, to further increase the
spectral efficiency, an emerging concept, termed physical-layer service
integration (PHY-SI), has been recognized as an effective means. Its basic idea
is to combine multiple coexisting services, i.e., multicast/broadcast service
and confidential service, into one integral service for one-time transmission
at the transmitter side. This article first provides a tutorial on typical
PHY-SI models. Furthermore, we propose some state-of-the-art solutions to
improve the overall performance of PHY-SI in certain important communication
scenarios. In particular, we highlight the extension of several concepts
borrowed from conventional single-service communications, such as artificial
noise (AN), eigenmode transmission etc., to the scenario of PHY-SI. These
techniques are shown to be effective in the design of reliable and robust
PHY-SI schemes. Finally, several potential research directions are identified
for future work.Comment: 12 pages, 7 figure
Cross-layer optimization of unequal protected layered video over hierarchical modulation
Abstract-unequal protection mechanisms have been proposed at several layers in order to improve the reliability of multimedia contents, especially for video data. The paper aims at implementing a multi-layer unequal protection scheme, which is based on a Physical-Transport-Application cross-layer design. Hierarchical modulation, in the physical layer, has been demonstrated to increase the overall user capacity of a wireless communications. On the other hand, unequal erasure protection codes at the transport layer turned out to be an efficient method to protect video data generated by the application layer by exploiting their intrinsic properties. In this paper, the two techniques are jointly optimized in order to enable recovering lost data in case the protection is performed separately. We show that the cross-layer design proposed herein outperforms the performance of hierarchical modulation and unequal erasure codes taken independently
Coding in 802.11 WLANs
Forward error correction (FEC) coding is widely used in communication systems to correct transmis-
sion errors. In IEEE 802.11a/g transmitters, convolutional codes are used for FEC at the physical
(PHY) layer. As is typical in wireless systems, only a limited choice of pre-speci¯ed coding rates is
supported. These are implemented in hardware and thus di±cult to change, and the coding rates are
selected with point to point operation in mind.
This thesis is concerned with using FEC coding in 802.11 WLANs in more interesting ways that are
better aligned with application requirements. For example, coding to support multicast tra±c rather
than simple point to point tra±c; coding that is cognisant of the multiuser nature of the wireless
channel; and coding which takes account of delay requirements as well as losses. We consider layering
additional coding on top of the existing 802.11 PHY layer coding, and investigate the tradeo® between
higher layer coding and PHY layer modulation and FEC coding as well as MAC layer scheduling.
Firstly we consider the joint multicast performance of higher-layer fountain coding concatenated
with 802.11a/g OFDM PHY modulation/coding. A study on the optimal choice of PHY rates with and
without fountain coding is carried out for standard 802.11 WLANs. We ¯nd that, in contrast to studies
in cellular networks, in 802.11a/g WLANs the PHY rate that optimizes uncoded multicast performance
is also close to optimal for fountain-coded multicast tra±c. This indicates that in 802.11a/g WLANs
cross-layer rate control for higher-layer fountain coding concatenated with physical layer modulation
and FEC would bring few bene¯ts.
Secondly, using experimental measurements taken in an outdoor environment, we model the chan-
nel provided by outdoor 802.11 links as a hybrid binary symmetric/packet erasure channel. This
hybrid channel o®ers capacity increases of more than 100% compared to a conventional packet erasure
channel (PEC) over a wide range of RSSIs. Based upon the established channel model, we further
consider the potential performance gains of adopting a binary symmetric channel (BSC) paradigm for
multi-destination aggregations in 802.11 WLANs. We consider two BSC-based higher-layer coding
approaches, i.e. superposition coding and a simpler time-sharing coding, for multi-destination aggre-
gated packets. The performance results for both unicast and multicast tra±c, taking account of MAC
layer overheads, demonstrate that increases in network throughput of more than 100% are possible
over a wide range of channel conditions, and that the simpler time-sharing approach yields most of
these gains and have minor loss of performance.
Finally, we consider the proportional fair allocation of high-layer coding rates and airtimes in 802.11
WLANs, taking link losses and delay constraints into account. We ¯nd that a layered approach of
separating MAC scheduling and higher-layer coding rate selection is optimal. The proportional fair
coding rate and airtime allocation (i) assigns equal total airtime (i.e. airtime including both successful
and failed transmissions) to every station in a WLAN, (ii) the station airtimes sum to unity (ensuring
operation at the rate region boundary), and (iii) the optimal coding rate is selected to maximise
goodput (treating packets decoded after the delay deadline as losses)
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