5,038 research outputs found
Beam-searching and Transmission Scheduling in Millimeter Wave Communications
Millimeter wave (mmW) wireless networks are capable to support multi-gigabit
data rates, by using directional communications with narrow beams. However,
existing mmW communications standards are hindered by two problems: deafness
and single link scheduling. The deafness problem, that is, a misalignment
between transmitter and receiver beams, demands a time consuming beam-searching
operation, which leads to an alignment-throughput tradeoff. Moreover, the
existing mmW standards schedule a single link in each time slot and hence do
not fully exploit the potential of mmW communications, where directional
communications allow multiple concurrent transmissions. These two problems are
addressed in this paper, where a joint beamwidth selection and power allocation
problem is formulated by an optimization problem for short range mmW networks
with the objective of maximizing effective network throughput. This
optimization problem allows establishing the fundamental alignment-throughput
tradeoff, however it is computationally complex and requires exact knowledge of
network topology, which may not be available in practice. Therefore, two
standard-compliant approximation solution algorithms are developed, which rely
on underestimation and overestimation of interference. The first one exploits
directionality to maximize the reuse of available spectrum and thereby
increases the network throughput, while imposing almost no computational
complexity. The second one is a more conservative approach that protects all
active links from harmful interference, yet enhances the network throughput by
100% compared to the existing standards. Extensive performance analysis
provides useful insights on the directionality level and the number of
concurrent transmissions that should be pursued. Interestingly, extremely
narrow beams are in general not optimal.Comment: 5 figures, 7 pages, accepted in ICC 201
Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems
Burts-by-burst (BbB) adaptive high-speed downlink packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER
Effect of adjacent-channel interference in IEEE 802.11 WLANs
Frequency channels are a scarce resource in the ISM bands used by IEEE 802.11 WLANs. Current radio resource management is often limited to a small number of nonoverlapping
channels, which leaves only three possible channels in the 2.4GHz band used in IEEE 802.11b/g networks. In this paper we study and quantify the effect of adjacent channel
interference, which is caused by transmissions in partially overlapping channels. We propose a model that is able to
determine under what circumstances the use of adjacent channels is justified. The model can also be used to assist different radio resource management mechanisms (e.g.
transmitted power assignments
Multicast Multigroup Precoding and User Scheduling for Frame-Based Satellite Communications
The present work focuses on the forward link of a broadband multibeam
satellite system that aggressively reuses the user link frequency resources.
Two fundamental practical challenges, namely the need to frame multiple users
per transmission and the per-antenna transmit power limitations, are addressed.
To this end, the so-called frame-based precoding problem is optimally solved
using the principles of physical layer multicasting to multiple co-channel
groups under per-antenna constraints. In this context, a novel optimization
problem that aims at maximizing the system sum rate under individual power
constraints is proposed. Added to that, the formulation is further extended to
include availability constraints. As a result, the high gains of the sum rate
optimal design are traded off to satisfy the stringent availability
requirements of satellite systems. Moreover, the throughput maximization with a
granular spectral efficiency versus SINR function, is formulated and solved.
Finally, a multicast-aware user scheduling policy, based on the channel state
information, is developed. Thus, substantial multiuser diversity gains are
gleaned. Numerical results over a realistic simulation environment exhibit as
much as 30% gains over conventional systems, even for 7 users per frame,
without modifying the framing structure of legacy communication standards.Comment: Accepted for publication to the IEEE Transactions on Wireless
Communications, 201
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