1,369 research outputs found
Interference-Mitigating Waveform Design for Next-Generation Wireless Systems
A brief historical perspective of the evolution of waveform designs employed in consecutive generations of wireless communications systems is provided, highlighting the range of often conflicting demands on the various waveform characteristics. As the culmination of recent advances in the field the underlying benefits of various Multiple Input Multiple Output (MIMO) schemes are highlighted and exemplified. As an integral part of the appropriate waveform design, cognizance is given to the particular choice of the duplexing scheme used for supporting full-duplex communications and it is demonstrated that Time Division Duplexing (TDD) is substantially outperformed by Frequency Division Duplexing (FDD), unless the TDD scheme is combined with further sophisticated scheduling, MIMOs and/or adaptive modulation/coding. It is also argued that the specific choice of the Direct-Sequence (DS) spreading codes invoked in DS-CDMA predetermines the properties of the system. It is demonstrated that a specifically designed family of spreading codes exhibits a so-called interference-free window (IFW) and hence the resultant system is capable of outperforming its standardised counterpart employing classic Orthogonal Variable Spreading Factor (OVSF) codes under realistic dispersive channel conditions, provided that the interfering multi-user and multipath components arrive within this IFW. This condition may be ensured with the aid of quasisynchronous adaptive timing advance control. However, a limitation of the system is that the number of spreading codes exhibiting a certain IFW is limited, although this problem may be mitigated with the aid of novel code design principles, employing a combination of several spreading sequences in the time-frequency and spatial-domain. The paper is concluded by quantifying the achievable user load of a UTRA-like TDD Code Division Multiple Access (CDMA) system employing Loosely Synchronized (LS) spreading codes exhibiting an IFW in comparison to that of its counterpart using OVSF codes. Both system's performance is enhanced using beamforming MIMOs
Achieving Ultra-Low Latency in 5G Millimeter Wave Cellular Networks
The IMT 2020 requirements of 20 Gbps peak data rate and 1 millisecond latency
present significant engineering challenges for the design of 5G cellular
systems. Use of the millimeter wave (mmWave) bands above 10 GHz --- where vast
quantities of spectrum are available --- is a promising 5G candidate that may
be able to rise to the occasion.
However, while the mmWave bands can support massive peak data rates,
delivering these data rates on end-to-end service while maintaining reliability
and ultra-low latency performance will require rethinking all layers of the
protocol stack. This papers surveys some of the challenges and possible
solutions for delivering end-to-end, reliable, ultra-low latency services in
mmWave cellular systems in terms of the Medium Access Control (MAC) layer,
congestion control and core network architecture
Frame Structure Design and Analysis for Millimeter Wave Cellular Systems
The millimeter-wave (mmWave) frequencies have attracted considerable
attention for fifth generation (5G) cellular communication as they offer orders
of magnitude greater bandwidth than current cellular systems. However, the
medium access control (MAC) layer may need to be significantly redesigned to
support the highly directional transmissions, ultra-low latencies and high peak
rates expected in mmWave communication. To address these challenges, we present
a novel mmWave MAC layer frame structure with a number of enhancements
including flexible, highly granular transmission times, dynamic control signal
locations, extended messaging and ability to efficiently multiplex directional
control signals. Analytic formulae are derived for the utilization and control
overhead as a function of control periodicity, number of users, traffic
statistics, signal-to-noise ratio and antenna gains. Importantly, the analysis
can incorporate various front-end MIMO capability assumptions -- a critical
feature of mmWave. Under realistic system and traffic assumptions, the analysis
reveals that the proposed flexible frame structure design offers significant
benefits over designs with fixed frame structures similar to current 4G
long-term evolution (LTE). It is also shown that fully digital beamforming
architectures offer significantly lower overhead compared to analog and hybrid
beamforming under equivalent power budgets.Comment: Submitted to IEEE Transactions for Wireless Communication
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
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