Pilot Contamination and Mitigation Techniques in Massive MIMO Systems
(2014) EITM02 20142Department of Electrical and Information Technology
- Abstract
- A multi-antenna base station (BS) can spatially multiplex a few terminals over the same bandwidth, a technique known as multi-user, multiple-input multiple-output (MU-MIMO). A new idea in cellular MU-MIMO is the use of a large excess of BS antennas to serve several single-antenna terminals simultaneously. This so-called "massive MIMO" promises attractive gains in spectral efficiency with time-division duplex operation. Within a cell, the BS estimates the channel from mutually orthogonal reverse-link pilot sequences to formulate a receiver for the reverse link and (assuming reciprocity) a precoder for the forward link. The channel coherence is typically constrained in time as well as frequency, leading to a trade-off between the resources... (More)
- A multi-antenna base station (BS) can spatially multiplex a few terminals over the same bandwidth, a technique known as multi-user, multiple-input multiple-output (MU-MIMO). A new idea in cellular MU-MIMO is the use of a large excess of BS antennas to serve several single-antenna terminals simultaneously. This so-called "massive MIMO" promises attractive gains in spectral efficiency with time-division duplex operation. Within a cell, the BS estimates the channel from mutually orthogonal reverse-link pilot sequences to formulate a receiver for the reverse link and (assuming reciprocity) a precoder for the forward link. The channel coherence is typically constrained in time as well as frequency, leading to a trade-off between the resources spent on pilots and those available for data symbols. This pilot overhead can be reduced by reusing pilot sequences in nearby cells, however this potentially introduces interference in the channel estimation phase, the so-called "pilot contamination" effect.
In this thesis, we study the impact of pilot contamination in realistic environments and investigate schemes to mitigate it. We evaluate the mean squared error (MSE) of channel estimates in case of a plain-vanilla least-squares (LS) estimator and a minimum MSE (MMSE) estimator that exploits prior knowledge of second-order channel statistics. Next, we introduce a pilot open-loop power control (pilot OLPC) scheme to improve the SINR-fairness of received pilot signals at the BS. We evaluate the effect of relaxing the pilot reuse factor and also implement a soft pilot reuse (SPR) scheme to distribute pilot sequences efficiently. To study the trade-off between pilot and data symbols, we evaluate the achievable rate in forward link with maximum-ratio and zero-forcing precoding at the BS. We evaluate an inter-cell coordination scheme that exploits prior knowledge of all cross-channel covariance matrices to reuse pilots among spatially well-separated terminals.
We simulate a 21-cell MU-MIMO setup with up to 100-antenna BSs and up to 24 single-antenna terminals per cell in an outdoor urban macro environment. We find that pilot reuse 1 causes severe impairment of the channel estimates, which can be improved with pilot OLPC. Pilot reuse 1/3 effectively mitigates pilot contamination, and can improve the achievable rate in the forward link. SPR also mitigates contamination but with a smaller increase in pilot overhead. Inter-cell coordinated pilot allocation, implemented using a greedy approach, provides gains over random allocation only for the initial few pilots. In general, maximum ratio precoding is more robust against pilot contamination than zero-forcing. (Less) - Popular Abstract
- A multi-antenna base station (BS) can be used to improve cellular communication performance. The signal at each antenna can be designed in a way that it increases received energy at the desired terminals, and attenuates it at other locations (reducing interference). This technique can be used to serve several terminals over the same time and frequency using independent data streams, known as multi-user MIMO (MU-MIMO).
In this thesis, we investigate MU-MIMO approach for very large BS antenna arrays, also called massive MIMO. The performance of such systems depends critically on the quality of channel estimates the BS. We simulate realistic channel conditions in a multi-cell setup, which gives rise to interference during channel... (More) - A multi-antenna base station (BS) can be used to improve cellular communication performance. The signal at each antenna can be designed in a way that it increases received energy at the desired terminals, and attenuates it at other locations (reducing interference). This technique can be used to serve several terminals over the same time and frequency using independent data streams, known as multi-user MIMO (MU-MIMO).
In this thesis, we investigate MU-MIMO approach for very large BS antenna arrays, also called massive MIMO. The performance of such systems depends critically on the quality of channel estimates the BS. We simulate realistic channel conditions in a multi-cell setup, which gives rise to interference during channel estimation. We evaluate the system performance in terms of quality of BS channel estimates and the achievable data rate within a cell.
We evaluate different techniques for channel estimation, and for generating data streams from the BS to the terminals. Next, we evaluate schemes to improve the channel estimation. We conclude by noting the trade-offs involved in the various schemes and the conditions under which certain schemes might provide performance improvements. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/student-papers/record/4730439
- author
- Saxena, Vidit LU
- supervisor
- organization
- course
- EITM02 20142
- year
- 2014
- type
- H2 - Master's Degree (Two Years)
- subject
- keywords
- Multicell Systems, Massive MIMO, Pilot Contamination, LTE, Power Control, Resource Allocation, Channel Estimation, Precoding
- language
- English
- id
- 4730439
- date added to LUP
- 2014-11-19 12:34:33
- date last changed
- 2014-11-19 12:34:33
@misc{4730439,
abstract = {{A multi-antenna base station (BS) can spatially multiplex a few terminals over the same bandwidth, a technique known as multi-user, multiple-input multiple-output (MU-MIMO). A new idea in cellular MU-MIMO is the use of a large excess of BS antennas to serve several single-antenna terminals simultaneously. This so-called "massive MIMO" promises attractive gains in spectral efficiency with time-division duplex operation. Within a cell, the BS estimates the channel from mutually orthogonal reverse-link pilot sequences to formulate a receiver for the reverse link and (assuming reciprocity) a precoder for the forward link. The channel coherence is typically constrained in time as well as frequency, leading to a trade-off between the resources spent on pilots and those available for data symbols. This pilot overhead can be reduced by reusing pilot sequences in nearby cells, however this potentially introduces interference in the channel estimation phase, the so-called "pilot contamination" effect.
In this thesis, we study the impact of pilot contamination in realistic environments and investigate schemes to mitigate it. We evaluate the mean squared error (MSE) of channel estimates in case of a plain-vanilla least-squares (LS) estimator and a minimum MSE (MMSE) estimator that exploits prior knowledge of second-order channel statistics. Next, we introduce a pilot open-loop power control (pilot OLPC) scheme to improve the SINR-fairness of received pilot signals at the BS. We evaluate the effect of relaxing the pilot reuse factor and also implement a soft pilot reuse (SPR) scheme to distribute pilot sequences efficiently. To study the trade-off between pilot and data symbols, we evaluate the achievable rate in forward link with maximum-ratio and zero-forcing precoding at the BS. We evaluate an inter-cell coordination scheme that exploits prior knowledge of all cross-channel covariance matrices to reuse pilots among spatially well-separated terminals.
We simulate a 21-cell MU-MIMO setup with up to 100-antenna BSs and up to 24 single-antenna terminals per cell in an outdoor urban macro environment. We find that pilot reuse 1 causes severe impairment of the channel estimates, which can be improved with pilot OLPC. Pilot reuse 1/3 effectively mitigates pilot contamination, and can improve the achievable rate in the forward link. SPR also mitigates contamination but with a smaller increase in pilot overhead. Inter-cell coordinated pilot allocation, implemented using a greedy approach, provides gains over random allocation only for the initial few pilots. In general, maximum ratio precoding is more robust against pilot contamination than zero-forcing.}},
author = {{Saxena, Vidit}},
language = {{eng}},
note = {{Student Paper}},
title = {{Pilot Contamination and Mitigation Techniques in Massive MIMO Systems}},
year = {{2014}},
}