4,243 research outputs found
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
Power Allocation Schemes for Multicell Massive MIMO Systems
This paper investigates the sum-rate gains brought by power allocation
strategies in multicell massive multipleinput multiple-output systems, assuming
time-division duplex transmission. For both uplink and downlink, we derive
tractable expressions for the achievable rate with zero-forcing receivers and
precoders respectively. To avoid high complexity joint optimization across the
network, we propose a scheduling mechanism for power allocation, where in a
single time slot, only cells that do not interfere with each other adjust their
transmit powers. Based on this, corresponding transmit power allocation
strategies are derived, aimed at maximizing the sum rate per-cell. These
schemes are shown to bring considerable gains over equal power allocation for
practical antenna configurations (e.g., up to a few hundred). However, with
fixed number of users (N), these gains diminish as M turns to infinity, and
equal power allocation becomes optimal. A different conclusion is drawn for the
case where both M and N grow large together, in which case: (i) improved rates
are achieved as M grows with fixed M/N ratio, and (ii) the relative gains over
the equal power allocation diminish as M/N grows. Moreover, we also provide
applicable values of M/N under an acceptable power allocation gain threshold,
which can be used as to determine when the proposed power allocation schemes
yield appreciable gains, and when they do not. From the network point of view,
the proposed scheduling approach can achieve almost the same performance as the
joint power allocation after one scheduling round, with much reduced
complexity
Energy-Efficient NOMA Enabled Heterogeneous Cloud Radio Access Networks
Heterogeneous cloud radio access networks (H-CRANs) are envisioned to be
promising in the fifth generation (5G) wireless networks. H-CRANs enable users
to enjoy diverse services with high energy efficiency, high spectral
efficiency, and low-cost operation, which are achieved by using cloud computing
and virtualization techniques. However, H-CRANs face many technical challenges
due to massive user connectivity, increasingly severe spectrum scarcity and
energy-constrained devices. These challenges may significantly decrease the
quality of service of users if not properly tackled. Non-orthogonal multiple
access (NOMA) schemes exploit non-orthogonal resources to provide services for
multiple users and are receiving increasing attention for their potential of
improving spectral and energy efficiency in 5G networks. In this article a
framework for energy-efficient NOMA H-CRANs is presented. The enabling
technologies for NOMA H-CRANs are surveyed. Challenges to implement these
technologies and open issues are discussed. This article also presents the
performance evaluation on energy efficiency of H-CRANs with NOMA.Comment: This work has been accepted by IEEE Network. Pages 18, Figure
Modulation-mode assignment for SVD-assisted and iteratively detected downlink multiuser MIMO transmission schemes
In this contribution we jointly optimize the number of multiple-input multiple-output (MIMO) layers and the number of bits per symbol within an iteratively-detected multiuser MIMO downlink (DL) transmission scheme under the constraint of a given fixed data throughput and integrity. Instead of treating all the users jointly as in zero-forcing (ZF) multiuser transmission techniques, the investigated singular value decomposition (SVD) assisted DL multiuser MIMO system takes the individual user's channel characteristics into account. In analogy to bit-interleaved coded irregular modulation, we introduce a MIMO-BICM scheme, where different user-specific signal constellations and mapping arrangement were used within a single codeword. Extrinsic information transfer (EXIT) charts are used for analyzing and optimizing the convergence behaviour of the iterative demapping and decoding. Our results show that in order to achieve the best bit-error rate, not necessarily all user-specific MIMO layers have to be activate
Asymptotic Analysis of SU-MIMO Channels With Transmitter Noise and Mismatched Joint Decoding
Hardware impairments in radio-frequency components of a wireless system cause
unavoidable distortions to transmission that are not captured by the
conventional linear channel model. In this paper, a 'binoisy' single-user
multiple-input multiple-output (SU-MIMO) relation is considered where the
additional distortions are modeled via an additive noise term at the transmit
side. Through this extended SU-MIMO channel model, the effects of transceiver
hardware impairments on the achievable rate of multi-antenna point-to-point
systems are studied. Channel input distributions encompassing practical
discrete modulation schemes, such as, QAM and PSK, as well as Gaussian
signaling are covered. In addition, the impact of mismatched detection and
decoding when the receiver has insufficient information about the
non-idealities is investigated. The numerical results show that for realistic
system parameters, the effects of transmit-side noise and mismatched decoding
become significant only at high modulation orders.Comment: 16 pages, 7 figure
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