10 research outputs found
Pilot Decontamination in CMT-based Massive MIMO Networks
Pilot contamination problem in massive MIMO networks operating in
time-division duplex (TDD) mode can limit their expected capacity to a great
extent. This paper addresses this problem in cosine modulated multitone (CMT)
based massive MIMO networks; taking advantage of their so-called blind
equalization property. We extend and apply the blind equalization technique
from single antenna case to multi-cellular massive MIMO systems and show that
it can remove the channel estimation errors (due to pilot contamination effect)
without any need for cooperation between different cells or transmission of
additional training information. Our numerical results advocate the efficacy of
the proposed blind technique in improving the channel estimation accuracy and
removal of the residual channel estimation errors caused by the users of the
other cells.Comment: Accepted in ISWCS 201
Time Reversal with Post-Equalization for OFDM without CP in Massive MIMO
This paper studies the possibility of eliminating the redundant cyclic prefix
(CP) of orthogonal frequency division multiplexing (OFDM) in massive
multiple-input multiple-output systems. The absence of CP increases the
bandwidth efficiency in expense of intersymbol interference (ISI) and
intercarrier interference (ICI). It is known that in massive MIMO, different
types of interference fade away as the number of base station (BS) antennas
tends to infinity. In this paper, we investigate if the channel distortions in
the absence of CP are averaged out in the large antenna regime. To this end, we
analytically study the performance of the conventional maximum ratio combining
(MRC) and realize that there always remains some residual interference leading
to saturation of signal to interference (SIR). This saturation of SIR is
quantified through mathematical equations. Moreover, to resolve the saturation
problem, we propose a technique based on time-reversal MRC with zero forcing
multiuser detection (TR-ZF). Thus, the SIR of our proposed TR-ZF does not
saturate and is a linear function of the number of BS antennas. We also show
that TR-ZF only needs one OFDM demodulator per user irrespective of the number
of BS antennas; reducing the BS signal processing complexity significantly.
Finally, we corroborate our claims as well as analytical results through
simulations.Comment: 7 pages, 3 figure
Frequency Spreading Equalization in Multicarrier Massive MIMO
Application of filter bank multicarrier (FBMC) as an effective method for
signaling over massive MIMO channels has been recently proposed. This paper
further expands the application of FBMC to massive MIMO by applying frequency
spreading equalization (FSE) to these channels. FSE allows us to achieve a more
accurate equalization. Hence, higher number of bits per symbol can be
transmitted and the bandwidth of each subcarrier can be widened. Widening the
bandwidth of each subcarrier leads to (i) higher bandwidth efficiency; (ii)
lower complexity; (iii) lower sensitivity to carrier frequency offset (CFO);
(iv) reduced peak-to-average power ratio (PAPR); and (iv) reduced latency. All
these appealing advantages have a direct impact on the digital as well as
analog circuitry that is needed for the system implementation. In this paper,
we develop the mathematical formulation of the minimum mean square error (MMSE)
FSE for massive MIMO systems. This analysis guides us to decide on the number
of subcarriers that will be sufficient for practical channel models.Comment: Accepted in IEEE ICC 2015 - Workshop on 5G & Beyond - Enabling
Technologies and Application
Advanced modulation techniques in massive mimo
dissertationOrthogonal frequency division multiplexing with cyclic pre x (CP-OFDM) is the primary modulation format in the massive multiple-input multiple-output (MIMO) literature as well as most of the current wireless communication standards. This waveform, however, su ers from a number of drawbacks. In particular, the high spectral leakage of the OFDM subcarriers, the overhead due to the CP duration, and the high peak-to-average power ratio (PAPR) are the three most important shortcomings of this waveform. In this dissertation, we study some alternative modulation formats to address the limitations of CP-OFDM, particularly in massive MIMO systems. Three di erent designs are considered to target the above drawbacks of OFDM individually. In the rst one, lter bank multicarrier (FBMC) is considered to relieve the spectral leakage problem of OFDM in massive MIMO systems. We show that in FBMC-based massive MIMO, when linear equalization methods such as MRC, ZF, and MMSE, are applied, the signal-to-interference-plus-noise ratio (SINR) does not necessarily improve as the number of BS antennas grows large and saturates at a certain deterministic level. This phenomenon results from the law of large numbers and is due to the correlation between the combiner taps and the channel coe cients. In order to resolve the saturation problem, we develop an e cient equalization method to remove this correlation. In the second design, we study the possibility of removing the CP in OFDM to increase its spectral e ciency. Again, we show that a similar saturation problem exists in OFDM without CP systems. In this part, we develop another equalization structure based on the time-reversal technique. We show that by utilizing the proposed method, higher spectral e ciency can be achieved by removing the CP overhead in massive MIMO systems. Finally, we consider single-carrier (SC) transmission in massive MIMO, to reduce PAPR. A novel receiver design based on time-reversal combining and frequency-domain equalization is proposed for this system. We show that through the proposed receiver structure, the performance of SC transmission can be enhanced in moderate to high signal-to-noise ratio (SNR) regimes
Filter Bank Multicarrier in Massive MIMO: Analysis and Channel Equalization
We perform an asymptotic study of the performance
of filter bank multicarrier in the context of massive multi-input
multi-output. We show that the effects of channel distortions,
i.e., intersymbol interference and intercarrier interference, do not
vanish as the base station (BS) array size increases. As a result,
the signal-to-interference-plus-noise ratio (SINR) cannot grow unboundedly by increasing the number of BS antennas, and is upper
bounded by a certain deterministic value. We show that this phenomenon is a result of the correlation between the multiantenna
combining tap values and the channel impulse responses between
the mobile terminals and the BS antennas. To resolve this problem,
we introduce an efficient equalization method that removes this
correlation, enabling us to achieve arbitrarily large SINR values
by increasing the number of BS antennas. We perform a thorough
analysis of the proposed system and find analytical expressions for
both equalizer coefficients and the respective SINR
Prototype filter design for FBMC in massive MIMO channels
We perform an asymptotic study on the performance of filter bank multicarrier (FBMC) in the context of massive multi-input multi-output (MIMO). We show that the signal-to-interference-plus-noise ratio (SINR) cannot grow unboundedly by increasing the number of base station (BS) antennas, and is upper bounded by a certain deterministic value. This is a result of the correlation between the multi-antenna combining tap values and the channel impulse responses between the terminals and the BS antennas. To solve this problem, we introduce a simple FBMC prototype filter design method that removes this correlation, enabling us to achieve arbitrarily large SINR values by increasing the number of BS antennas
OFDM Without CP in Massive MIMO
We study the possibility of removing the cyclic prefix (CP) overhead from orthogonal frequency division multiplexing (OFDM) in massive multiple-input multiple-output (MIMO) systems. We consider the uplink transmission, while our results are applicable to the downlink as well. The absence of CP increases the spectral efficiency in expense of intersymbol interference and intercarrier interference. It is known that in massive MIMO, the effects of uncorrelated noise and multiuser interference vanish as the number of base station antennas tends to infinity. To investigate if the channel distortions in the absence of CP fade away, we study the performance of the standard maximum ratio combining receiver. Our analysis reveals that in this receiver, there always remains some residual interference leading to saturation of signal-to-interference-plus-noise ratio. To resolve this problem, we propose using the time reversal (TR) technique. Moreover, in order to further reduce the multiuser interference, we propose a zero-forcing equalization to be deployed after the TR combining. We compare the achievable rate of the proposed system with that of the conventional CP-OFDM. We show that in realistic channels, a higher spectral efficiency is achieved by removing the CP from OFDM, while reducing the computational complexity