In two-tier networks comprising a conventional cellular network overlaid with shorter range hotspots (e.g. femtocells, distributed antennas, or wired relays) with universal frequency reuse, the near-far effect from cross-tier interference creates dead spots where reliable coverage cannot be guaranteed to users in either tier. Equipping the macrocell and femtocells with multiple antennas enhances robustness against the near-far problem. This work derives the maximum number of simultaneously transmitting multiple antenna femtocells meeting a per-tier outage probability constraint. Coverage dead zones are presented wherein cross-tier interference bottlenecks cellular and femtocell coverage. Two operating regimes are shown namely 1) a cellular-limited regime in which femtocell users experience unacceptable cross-tier interference and 2) a hotspot-limited regime wherein both femtocell users and cellular users are limited by hotspot interference. Our analysis accounts for the per-tier transmit powers, the number of transmit antennas (single antenna transmission being a special case) and terrestrial propagation such as the Rayleigh fading and the path loss exponents. Single-user (SU) multiple antenna transmission at each tier is shown to provide significantly superior coverage and spatial reuse relative to multiuser (MU) transmission. We propose a decentralized carrier-sensing approach to regulate femtocell transmission powers based on their location. Considering a worst-case cell-edge location, simulations using typical path loss scenarios show that our interference management strategy provides reliable cellular coverage with about 60 femtocells per cell-site.

Coverage in multi-antenna two-tier networks

In this paper, we analyze the performance of maximum ratio combining (MRC) systems with imperfect channel estimation in the presence of cochannel interference (CCI) with an arbitrary power interference-to-noise ratio (INR). The maximum combining weights are the imperfect estimates of the desired user's fading channel coefficients and are assumed to be complex Gaussian distributed. The quantified measure for estimation error is the correlation coefficient between the true fading channel coefficients and their estimates. Exact closedform expressions are derived for the probability density function(pdf) of the signal-to-interference-plus-noise ratio (SINR), as well as performance metrics including outage probability and the average symbol error probability (ASEP) for some modulation formats. Simulation results demonstrate the accuracy of our theoretic analysis.

Performance analysis of maximum ratio combining with imperfect channel estimation in the presence of cochannel interferences

This paper investigates the effects of co-channel interference on spatial diversity techniques. By analyzing distributions of post-processing signal to noise plus interference ratio (SINR) after matched filtering, we capture the performance characteristics of spatial diversity techniques in an interference limited environment. Using intuition from the theoretical analysis of a single carrier system, the performance of spatial diversity techniques in an OFDMA system with co-channel interference is also characterized. Our analytical and simulation results show that space time block code (STBC) schemes are more sensitive to co-channel interference than other spatial diversity techniques so their performance gain in a noise-limited environment can be lost in an interference-limited environment.

The Effects of Co-channel Interference on Spatial Diversity Techniques

In this paper, we analyze the performance of the uplink of multi-cellular MIMO systems in flat Rayleigh fading. There is co-channel interference from users within the same cell as well as from other cell users. I The channel model includes lognormal shadowing and path loss along with power control, resulting in a statistical model for user powers. Consistent with practical scenarios, the co-channel interference is categorized into two groups: intracell interference from users within the same cell as the desired user and intercell interference from outer cell users. We derive a compact, easily computable closed form outage probability expression in the form of finite sums. This expression allows for simpler and faster analysis of various MIMO configurations. It has been shown that using antennas on the receiver side results in better performance, since transmit diversity does not combat interference from same cell users.

Performance analysis of maximum ratio transmission based multi-cellular MIMO systems

Distributed antenna systems (DAS) are being widely considered for state-of-the art cellular communication systems to cover dead spots. Recent academic studies have shown that in addition to coverage improvements, DAS can also allow a capacity improvement. This paper provides a generalized information theoretic analysis of downlink multicell DAS for two different transmission strategies: selection diversity (where just one of the distributed antennas are used) and blanket transmission (where all antennas in the cell broadcast data). Simple repeaters are a special case of our analysis. The results show that DAS reduces other-cell interference in a multicell environment and hence significantly improves performance and capacity (by about 2-3x), with particularly large improvements for users near cell boundaries.

Capacity of multicellular distributed antenna networks

In this paper, we examine the statistical distribution of the signal to interference plus noise ratio (SINR) of the optimum combining (OC) technique in receive antenna diversity systems. The channel is modelled as flat Rayleigh fading, slowly varying and spatially independent. When the number of interferers is less than the number of antennas, our approach considers unequal power interferers and decomposes the SINR into sum of two independent terms by projection onto interference and noise orthogonal subspaces. Following the decomposition, the distributions for these projections are sought. For the case when the number of interferers is larger, approximations to the distribution are provided along with performance upper and lower bounds.

https://ieeexplore.ieee.org/iel5/7633/20837/00966365.pdf

Statistical performance analysis of optimum combining with co-channel interferers and flat Rayleigh fading

In this paper, we examine the impact of channel estimation errors on Maximum Ratio Combining (MRC) in receive diversity systems, in the presence of co-channel interferers. The channel is modelled as flat Rayleigh fading, slowly varying and spatially independent. It is assumed that the spatial combiner weights are imperfect estimates of the desired user's fading coefficients and complex Gaussian distributed. Closed form expressions for signal to interference plus noise ratio (SINR) distribution and outage probability is obtained. Using these expressions, the effect of channel estimation quality on performance is investigated.

Y. Akyildiz

Papers

Statistical performance analysis of optimum combining with co-channel interferers and flat Rayleigh fading

Maximum ratio Combining performance with imperfect channel estimates