1,531 research outputs found
Asynchronous Channel Training in Multi-Cell Massive MIMO
Pilot contamination has been regarded as the main bottleneck in time division
duplexing (TDD) multi-cell massive multiple-input multiple-output (MIMO)
systems. The pilot contamination problem cannot be addressed with large-scale
antenna arrays. We provide a novel asynchronous channel training scheme to
obtain precise channel matrices without the cooperation of base stations. The
scheme takes advantage of sampling diversity by inducing intentional timing
mismatch. Then, the linear minimum mean square error (LMMSE) estimator and the
zero-forcing (ZF) estimator are designed. Moreover, we derive the minimum
square error (MSE) upper bound of the ZF estimator. In addition, we propose the
equally-divided delay scheme which under certain conditions is the optimal
solution to minimize the MSE of the ZF estimator employing the identity matrix
as pilot matrix. We calculate the uplink achievable rate using maximum ratio
combining (MRC) to compare asynchronous and synchronous channel training
schemes. Finally, simulation results demonstrate that the asynchronous channel
estimation scheme can greatly reduce the harmful effect of pilot contamination
An Analysis of Two-User Uplink Asynchronous Non-Orthogonal Multiple Access Systems
Recent studies have numerically demonstrated the possible advantages of the
asynchronous non-orthogonal multiple access (ANOMA) over the conventional
synchronous non-orthogonal multiple access (NOMA). The ANOMA makes use of the
oversampling technique by intentionally introducing a timing mismatch between
symbols of different users. Focusing on a two-user uplink system, for the first
time, we analytically prove that the ANOMA with a sufficiently large frame
length can always outperform the NOMA in terms of the sum throughput. To this
end, we derive the expression for the sum throughput of the ANOMA as a function
of signal-to-noise ratio (SNR), frame length, and normalized timing mismatch.
Based on the derived expression, we find that users should transmit at full
powers to maximize the sum throughput. In addition, we obtain the optimal
timing mismatch as the frame length goes to infinity. Moreover, we
comprehensively study the impact of timing error on the ANOMA throughput
performance. Two types of timing error, i.e., the synchronization timing error
and the coordination timing error, are considered. We derive the throughput
loss incurred by both types of timing error and find that the synchronization
timing error has a greater impact on the throughput performance compared to the
coordination timing error
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Improving NOMA Multi-Carrier Systems with Intentional Frequency Offsets
In this letter, we investigate the possible benefits of asynchrony in the frequency domain for the non-orthogonal multiple access (NOMA) schemes. Despite the common perspective that asynchrony in transmission or reception of multi-stream signals is harmful, we demonstrate the advantages of adding intentional frequency offset to the conventional power domain-NOMA (P-NOMA). We introduce two methods which add artificial frequency offsets between different sets of sub-carriers destined for different users. The first one uses the same successive interference cancellation (SIC) method as the conventional P-NOMA except that it enjoys reduced inter-user interference (IUI) between interfering sub-carriers. The second scheme adopts a precoding at the base station and a linear preprocessing scheme at the receiving user. It decomposes the broadcast channel into parallel channels circumventing the need for SIC. As a result, it fully exploits the advantages provided by the frequency asynchrony and enables the interference-free transmission to the users. The numerical results show that both methods can outperform the conventional P-NOMA
Novel multiuser detection and multi-rate schemes for multi-carrier CDMA
A large variety of services is [sic] expected for wireless systems, in particular, high data rate services, such as wireless Internet access. Users with different data rates and quality of service (QoS) requirements must be accommodated. A suitable multiple access scheme is key to enabling wireless systems to support both the high data rate and the integrated multiple data rate transmissions with satisfactory performance and flexibility. A multi-carrier code division multiple access (MC-CDMA) scheme is a promising candidate for emerging broadband wireless systems. MC-CDMA is a hybrid of orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA). The most salient feature of MC-CDMA is that the rate of transmission is not limited by the wireless channel\u27s frequency-selective fading effects caused by multipath propagation. In MC-CDMA, each chip of the desired user\u27s spreading code, multiplied by the current data bit, is modulated onto a separate subcarrier. Therefore, each subcarrier has a narrow bandwidth and undergoes frequency-flat fading. Two important issues for an MC-CDMA wireless system, multiuser detection and multi-rate access, are discussed in this dissertation.
Several advanced receiver structures capable of suppressing multiuser interference in an uplink MC-CDMA system, operating in a frequency-selective fading channel, are studied in this dissertation. One receiver is based on a so-called multishot structure, in which the interference introduced by the asynchronous reception of different users is successfully suppressed by a receiver based on the minimum mean-square error (MMSE) criterion with a built-in de-biasing feature. Like many other multiuser schemes, this receiver is very sensitive to a delay estimation error. A blind adaptive two-stage decorrelating receiver based on the bootstrap algorithm is developed to combat severe performance degradation due to a delay estimation error. It is observed that in the presence of a delay estimation error the blind adaptive bootstrap receiver is more near-far resistant than the MMSE receiver. Furthermore, a differential bootstrap receiver is proposed to extend the limited operating range of the two-stage bootstrap receiver which suffers from a phase ambiguity problem.
Another receiver is based on a partial sampling (PS) demodulation structure, which further reduces the sensitivity to unknown user delays in an uplink scenario. Using this partial sampling structure, it is no longer necessary to synchronize the receiver with the desired user. Following the partial sampling demodulator, a minimum mean-square error combining (MMSEC) detector is applied. The partial sampling MMSEC (PS-MMSEC) receiver is shown to have strong interference suppression and timing acquisition capabilities. The complexity of this receiver can be reduced significantly, with negligible performance loss, by choosing a suitable partial sampling rate and using a structure called reduced complexity PS-MMSEC (RPS-MMSEC). The adaptive implementation of these receivers yields a superior rate of convergence and symbol error rate performance in comparison to a conventional MMSEC receiver with known timing.
All the above receiver structures are for a single-rate MC-CDMA. Three novel multi-rate access schemes for multi-rate MC-CDMA, fixed spreading length (FSL), coded FSL (CFSL) and variable spreading length (VSL), have been developed. These multi-rate access schemes enable users to transmit information at different data rates in one MC-CDMA system. Hence, voice, data, image and video can be transmitted seamlessly through a wireless infrastructure. The bit error rate performance of these schemes is investigated for both low-rate and high-rate users
A Comparison of CP-OFDM, PCC-OFDM and UFMC for 5G Uplink Communications
Polynomial-cancellation-coded orthogonal frequency division multiplexing
(PCC-OFDM) is a form of OFDM that has waveforms which are very well localized
in both the time and frequency domains and so it is ideally suited for use in
the 5G network. This paper analyzes the performance of PCC-OFDM in the uplink
of a multiuser system using orthogonal frequency division multiple access
(OFDMA) and compares it with conventional cyclic prefix OFDM (CP-OFDM), and
universal filtered multicarrier (UFMC). PCC-OFDM is shown to be much less
sensitive than either CP-OFDM or UFMC to time and frequency offsets. For a
given constellation size, PCC-OFDM in additive white Gaussian noise (AWGN)
requires 3dB lower signal-to-noise ratio (SNR) for a given bit-error-rate, and
the SNR advantage of PCC-OFDM increases rapidly when there are timing and/or
frequency offsets. For PCC-OFDM no frequency guard band is required between
different OFDMA users. PCC-OFDM is completely compatible with CP-OFDM and adds
negligible complexity and latency, as it uses a simple mapping of data onto
pairs of subcarriers at the transmitter, and a simple weighting-and-adding of
pairs of subcarriers at the receiver. The weighting and adding step, which has
been omitted in some of the literature, is shown to contribute substantially to
the SNR advantage of PCC-OFDM. A disadvantage of PCC-OFDM (without overlapping)
is the potential reduction in spectral efficiency because subcarriers are
modulated in pairs, but this reduction is more than regained because no guard
band or cyclic prefix is required and because, for a given channel, larger
constellations can be used
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