2 research outputs found
Interference mitigation and interference avoidance for cellular OFDMA-TDD networks
In recent years, cellular systems based on orthogonal frequency division multiple access â time
division duplex (OFDMA-TDD) have gained considerable popularity. Two of the major reasons
for this are, on the one hand, that OFDMA enables the receiver to effectively cope with multipath
propagation while keeping the complexity low. On the other hand, TDD offers efficient
support for cell-specific uplink (UL)/downlink (DL) asymmetry demands by allowing each cell
to independently set its UL/DL switching point (SP). However, cell-independent SP gives rise
to crossed slots. In particular, crossed slots arise when neighbouring cells use the same slot in
opposing link directions, resulting in base station (BS)-to-BS interference and mobile station
(MS)-to-MS interference. BS-to-BS interference, in particular, can be quite detrimental due to
the exposed location of BSs, which leads to high probability of line-of-sight (LOS) conditions.
The aim of this thesis is to address the BS-to-BS interference problem in OFDMA-TDDcellular
networks. A simulation-based approach is used to demonstrate the severity of BS-to-BS interference
and a signal-to-interference-plus-noise ratio (SINR) equation for OFDMA is formulated
to aid system performance analysis. The detrimental effects of crossed slot interference in
OFDMA-TDD cellular networks are highlighted by comparing methods specifically targeting
the crossed slots interference problem. In particular, the interference avoidance method fixed
slot allocation (FSA) is compared against state of the art interference mitigation approaches,
viz: random time slot opposing (RTSO) and zone division (ZD). The comparison is done based
on Monte Carlo simulations and the main comparison metric is spectral efficiency calculated
using the SINR equation formulated in this thesis. The simulation results demonstrate that
when LOS conditions among BSs are present, both RTSO and ZD perform worse than FSA for
all considered performance metrics. It is concluded from the results that current interference
mitigation techniques do not offer an effective solution to the BS-to-BS interference problem.
Hence, new interference avoidance methods, which unlike FSA, do not sacrifice the advantages
of TDD are open research issues addressed in this thesis.
The major contribution of this thesis is a novel cooperative resource balancing technique that
offers a solution to the crossed slot problem. The novel concept, termed asymmetry balancing,
is targeted towards next-generation cellular systems, envisaged to have ad hoc and multi-hop
capabilities. Asymmetry balancing completely avoids crossed slots by keeping the TDD SPs
synchronised among BSs. At the same time, the advantages of TDD are retained, which is
enabled by introducing cooperation among the entities in the network. If a cell faces resource
shortage in one link direction, while having free resources in the opposite link direction, the
free resources can be used to support the overloaded link direction. In particular, traffic can
be offloaded to near-by mobile stations at neighbouring cells that have available resources. To
model the gains attained with asymmetry balancing, a mathematical framework is developed
which is verified by Monte Carlo simulations. In addition, asymmetry balancing is compared
against both ZD and FSA based on simulations and the results demonstrate the superior performance
of asymmetry balancing. It can be concluded that the novel interference avoidance
approach is a very promising candidate t