3 research outputs found
Future cellular systems: fundamentals and the role of large antenna arrays
In this thesis, we analyze the performance of three promising technologies being
considered for future fifth generation (5G) and beyond wireless communication systems,
with primary goals to: i) render 10-100 times higher user data rate, ii) serve 10-100
times more users simultaneously, iii) 1000 times more data volume per unit area, iv)
improve energy efficiency on the order of 100 times, and iv) provide higher bandwidths.
Accordingly, we focus on massive multiple-input multiple-output (MIMO) systems and
other future wireless technologies, namely millimeter wave (mmWave) and full-duplex
(FD) systems that are being considered to fulfill the above requirements.
We begin by focusing on fundamental performance limits of massive MIMO systems
under practical constraints such as low complexity processing, array size and limited
physical space. First, we analyze the performance of a massive MIMO base station
(BS) serving spatially distributed multi-antenna users within a fixed coverage area.
Stochastic geometry is used to characterize the spatially distributed users while large
dimensional random matrix theory is used to achieve deterministic approximations of
the sum rate of the system. We then examine the deployment of a massive MIMO
BS and the resulting energy efficiency (EE) by considering a more realistic set-up of a
rectangular array with increasing antenna elements within a fixed physical space. The
effects of mutual coupling and correlation among the BS antennas are incorporated
by deriving a practical mutual coupling matrix which considers coupling among all
antenna elements within the BS. Accordingly, the optimum number of antennas that
can be deployed for a particular antenna spacing when EE is considered as a design
criteria is derived. Also, it is found that mutual coupling effect reduces the EE of the
massive system by around 40-45% depending on the precoder/receiver used and the
physical space available for antenna deployment.
After establishing the constraints of antenna spacing on massive MIMO systems
for the current microwave spectrum, we shift our focus to mmWave frequencies (more
than 100GHz available bandwidth), where the wavelength is very small and as a result
more antennas can be rigged within a constrained space. Accordingly, we integrate
the massive MIMO technology with mmWave networks. In particular, we analyze the
performance of a mmWave network consisting of spatially distributed BS equipped with
very large uniform circular arrays (UCA) serving spatially distributed users within a
fixed coverage area. The use of UCA is due to its capability of scanning through both
the azimuth as well as elevation dimensions. We show that using such 3D massive
MIMO techniques in mmWave systems yield significant performance gains. Further,
we show the effect of blockages and path loss on mmWave networks. Since blockages are
found to be quite detrimental to mmWave networks, we create alternative propagation
paths with the aid of relays. In particular, we consider the deployment of relays in
outdoor mmWave networks and then derive expressions for the coverage probability
and transmission capacity from sources to a destination for such relay aided mmWave
networks using stochastic geometric tools. Overall, relay aided mmWave transmission
is seen to improve the signal to noise ratio at the destination by around 5-10dB with
respect to specific coverage probabilities.
Finally, due to the fact that the current half duplex (HD) mode transmission only
utilizes half the spectrum at the same time in the same frequency, we consider a multiuser
MIMO cellular system, where a FD BS serves multiple HD users simultaneously.
However, since FD systems are plagued by severe self-interference (SI), we focus on the
design of robust transceivers, which can cancel the residual SI left after antenna and
analog cancellations. In particular, we address the sum mean-squared-errors (MSE)
minimization problem by transforming it into an equivalent semidefinite programming
(SDP) problem. We propose iterative alternating algorithms to design the transceiver
matrices jointly and accordingly show the gains of FD over HD systems. We show that
with proper SI cancellation, it is possible to achieve gains on sum rate of up to 70-80%
over HD systems