This thesis is concerned with the performance analysis of interference-limited large-scale
wireless networks in the setting of random node distribution. Using the tools from stochastic
geometry, we consider wireless systems that randomly located transmitters are uncoordinated and
mutually interfered with each other. Special emphasis is placed on how to combat and control the
spatial interference.
In Chapter~ref{ch3}, the first topic we address is the performance analysis of relay
cooperation for a source-destination pair embedded in the Poisson field of interferers. In the
presence of a large number of relays, some effective relays close to the source and destination
nodes need to be regionally elected and the relays far from them
shall be disregarded for relay selection process because of the
likelihood of higher outage probability. Hence, we envisage a selection guideline such that the
relays geographically
close to the source and destination are preferred to the others. It can ensure a target Quality
of-Service (QoS) as well as reduce signaling overhead and relay-selection delay. A
spatial region, called the emph{QoS region}, is obtained according to relay selection
criterions, such as random relay selection and best relay selection, and is shown to shrink as
the distance between the
source and destination increases and interfering node density increases. When the QoS region for
random relay selection is not large enough and cannot probabilistically ensure a reliable relay
therein, the best relay selection is employed since the required relay-node density and
selection range for a desired QoS can be reduced for the best relay selection. The gain of the
best relay selection with respect to the random relay selection is quantified in terms of the
relay-node density reduction and coverage extension due to selection diversity.
In the sequel, we extend the concept of the QoS region to accommodate a threshold-based `1-bit`
opportunistic feedback mechanism joining such a geographical selection region where the relay
whose effective channel gain is above a pre-determined threshold can only feedback, and thereby
establish a relation between required relay-node density and the level of feedback threshold. It
is shown that required relay-node density and selection region for a desired quality-of-service
(QoS) depend on the feedback threshold.
In Chapter~ref{ch4}, we explore the benefit of relay cooperation for maintaining coverage area
against aggregate interference from incumbent underlaid interferers. We employ a guard zone for
uplink so that non-urgent interferers are inhibited and urgent ones are admitted for
transmission with a controlled access probability as long as the primary receiver can tolerate.
Numerical results show the outage probability of the primary relay network with the guard zone
and that a desired quality-of-service (QoS) determines the access probability of urgent
interferers depending on interfering node density. Selection diversity improves $d_{max}$, which
scales like $Thetabig(sqrt{log N}big)$ and thus encourages a multi-hop relay scheme that
can fully exploit the benefit of selection diversity.
In Chapter~ref{ch5}, we build an analytical framework for analyzing a two-tier heterogeneous
cellular network (HCN) where macrocells and distributed low power cells, namely
emph{daughtercells}, are operated in a common spectrum. Due to the ad-hoc nature of
daughtercell BS deployments such as pico and femto cells, the mutual interference varies and
obviously the emph{coverage probability} behaves differently in terms of transmit powers and
densities of macrocells and daughtercells. In this paper, we employ repulsive cell activation in
the interfering daughtercell network and see the impact of a minimum separation distance between
the daughtercell BSs in terms of coverage under open access and power efficiency. The control of
the minimum separation distance plays a role in balancing cell load effectively according to
changing user density and is justified for the coexistence of low power daughtercells. The
optimal minimum separation distance in terms of user density and target per-tier user throughput
requirements is found by a numerical search based on a simple bisection method. Numerical
results show the benefit of cell repulsion in terms of increased user density support and less
emph{area power consumption}.
In Chapter~ref{ch6}, we extend the two-tier HCN to $K$-tier HCN where $K$ heterogeneous tiers
are operating in a common spectrum. Similarly to the previous case, we employ a repulsive cell
activation (or planning) in the HCN by ensuring a minimum separation distance between
interfering BSs in each tier. We also consider a emph{modified} Matern hardcore process (MHP)
for rendering a minimum separation distance between the BSs, which is realized by outweighing
random BS distribution for closed as well as open access networks. Repulsive cell activation not
only improves the coverage probability but also plays a role in balancing per-cell loads
effectively according to varying user density. Assuming a finite BS capacity in terms of a
limited number of per-cell users, we point out the importance of a emph{relative transmit power
and density control} between the tiers and propose a tier-wise density and power control by
introducing a emph{relativity} factor for configuring the HCNs distributively while satisfying per-tier user throughput constraints. Our analytical results show that employing nonzero minimum separation distances is effective in balancing cell loads and improves the coverage probabilities and average user throughput under both closed and open access.
본 논문에서는 stochastic geometry 기법을 기반으로 다수의 노드들로 구성된 네트워크에서 사용자 링크 품질을 보장하면서 효과적으로 통신을 수행하기 위한 방법을 연구하였다.
(1) Random relay network 릴레이 네트워크 환경에서는 사용자가 요구하는 outage probability 요구 조건을 만족할 수 있는 릴레이들이 존재하는 구역을 결정하고, 이 때 릴레이 선택 기법에 따라 요구되는 릴레이 밀도를 산출하였다. 릴레이 선택 지역 및 밀도에 따라 feedback threshold 를 적절하게 제어함으로써 opportunistic 하게 최적의 릴레이를 선택할 수 있다.
(2) Underlaid 간섭 환경에서 릴레이 협력 통신과 guard zone 의 성능 이득에 대해 살펴보았다. 릴레이 selection diversity 이득으로 인해 간섭을 보다 잘 견뎌내고 셀 반경이 향상되는 것을 살펴 보았다.
(3) 이종 셀룰라 망이 공존하는 환경에서는 저전력 망의 셀 간 이격거리를 제어함으로써 얻는 이득에 대한 분석을 바탕으로 사용자 전송률과 분포를 고려하여 기지국 간 최적의 최소이격거리를 확보함으로써 기지국 전력 소비를 최소화하는 기법을 제안하고 그 성능을 분석하였다.
(4) K tier 이종 셀률라 망에서 수용해야하는 사용자 밀도가 주어지고, 요구되는 사용자별 전송률을 달성해야 할 경우, 각 tier 별로 최소이격거리를 제어함으로써 주어진 목적을 달성하면서 망 간 부하 균형을 이루어는 방법을 제시하였다.
최근 GPS 가 널리 보급됨에 따라 LBS (location based service)에 대한 사용자 욕구가 증대 되고 있다. 본 논문은 stochastic geometry 기법을 대규모 무선 네트워크의 성능 분석에 활용하였고, 나아가 제공되는 노드 위치 정보가 무선 네트워크 성능 향상에 미치는 영향을 고찰하고자 한다.
