Multiple users compete for a common resource like bandwidth to communicate
data in interference networks. Existing approaches in dealing with interference
limit the rate of communication due to paucity of shared resources. This limitation
in the rate gets more glaring as the number of users in the network increases.
For example, existing wireless systems either choose to orthogonalize the users
(for example, Frequency Division Multiple Access (FDMA) systems or Code Division
Multiple Access (CDMA) systems) or treat interference as Gaussian noise at
the receivers. It is well known that these approaches are sub-optimal in general.
Orthogonalization of users limit the number of available interference-free channels
(known as degrees of freedom, abbreviated as DoF) and treating interference as
noise means that the receiver cannot make use of the structure in the interfering
signals. This motivates the need to analyze alternate transmit and decoding
schemes in interference networks.
This thesis mainly analyzes transmit schemes that use linear precoding for
various configurations of interference networks with some practical constraints
imposed by the use of finite input constellations, propagation delays, and channel
state availability at the transmitters. The main contributions of this thesis are
listed below.
Achievable rates using precoding with finite constellation inputs in Gaussian
Interference Channels (GIC) is analyzed. A metric for finding the approximate
angle of rotation to maximally enlarge the Constellation Constrained (CC) capacity
of two-user Gaussian Strong Interference Channel (GSIC) is proposed. Even as
the Gaussian alphabet FDMA rate curve touches the capacity curve of the GSIC,
with both the users using the same finite constellation, we show that the CC
FDMA rate curve lies strictly inside the CC capacity curve at high powers. For a
K-user MIMO GIC, a set of necessary and sufficient conditions on the precoders
under which the mutual information between between relevant transmit-receive
pairs saturate like in the single user case is derived. Gradient-ascent based algorithms
to optimize the sum-rate achieved by precoding with finite constellation
inputs and treating interference as noise are proposed.
For a class of Gaussian interference networks with general message demands,
identified as symmetrically connected interference networks, the expected sumspectral efficiency (in bits/sec/Hz) is shown to grow linearly with the number
of transmitters at finite SNR, using a time-domain Interference Alignment (IA)
scheme in the presence of line of sight (LOS) channels.
For a 2×2 MIMO X-Network with M antennas at each node, we identify spacetime
block codes that could be coupled with an appropriate precoding scheme to
achieve the maximum possible sum-DoF of 4M
3 , for M = 3, 4. The proposed
schemes are shown to achieve a diversity gain of M with SNR-independent finite
constellation inputs. The proposed schemes have lower CSIT requirements
compared to existing schemes.
This thesis also makes an attempt to guarantee a minimum throughput when
the zero-interference conditions cannot be satisfied in a wireline network with three
unicast sessions with delays, using Precoding Based Network Alignment (PBNA).
Three different PBNA schemes namely PBNA with time-varying local encoding
coefficients (LECs), PBNA using transform approach and time-invariant LECs,
and PBNA using transform approach and block time-varying LECs are proposed
and their feasibility conditions analyzed.
| Identifer | oai:union.ndltd.org:IISc/oai:etd.ncsi.iisc.ernet.in:2005/3190 |
| Date | January 2014 |
| Creators | Ganesan, Abhinav |
| Contributors | Rajan, B Sundar |
| Source Sets | India Institute of Science |
| Language | en_US |
| Detected Language | English |
| Type | Thesis |
| Relation | G26390 |
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