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Abstract
Virtual Cellular Networks (VCN) are an attractive type of adaptive
wireless communication architecture for propagation- and
coverage-limited environments. In this paper, we investigate the
advantages of combining a VCN and multiple receiving antennas
for the downlink resulting in a MIMO system. Several ways
to combine the signals with different levels of complexity are
presented. In the most complex case, we show that it is possible
to add coherently the contribution of each antenna in a virtual
cell whilst retaining the path diversity inherent to the VCN infrastructure.
The schemes yield several advantages: symbol diversity
is improved, path diversity is still present, antenna gain
using multiple beamformers is increased and the multipath can
be reduced. The concept is applicable to most types of single
frequency networks but it is especially well appropriate for the
60 GHz VCN/WLAN using OFDM. Simulations give a realistic
performance for QPSK, 8-PSK, and 16-QAM baseband
modulations with a 256-subcarrier OFDM using a rate 1/2–
convolutional code for a 2 × 2 VCN system. Results show a
Eb/N0 improvement of up to 7.4 dB using the singular value
decomposition method with 16–QAM compared to the SISO
coded reference.
Keywords
Virtual Cellular Network (VCN), OFDM, 60 GHz, WLAN,
Saleh-Valenzuela model, MIMO, Beamforming.
1. Introduction
1.1. Virtual Cellular Networks
Virtual Cellular Networks (VCN) use distributed access points
(AP) and Single Frequency Networks within cells in order to
form an adaptive wireless communication architecture. The
idea of VCN originates from digital broadcasting systems, such
as Digital Audio Broadcast (DAB), using single frequency networks.
The advantages of VCN described in earlier work in the
UHF band [1] were limited by the channel properties. The concept
of VCN architecture has been further developed for ubiquitous
wireless access [2, 3]. In this paper, the major motivation
for VCN is the increased path diversity at the receiver, which
was introduced in previous work [4].
Unlike conventional cellular networks, overlapping coverage
of the AP cells is desirable in order to increase path diversity.
This infrastructure is appropriate for communication systems
in which propagation and coverage are the limiting factors.
VCN is usually based on Orthogonal Frequency Division Multiplexing
(OFDM) signals sent through physically distributed
APs using the same channel and, which therefore, create a larger
cell, called a virtual cell. Since VCN uses OFDM with a large
cyclic prefix (CP), the receiver can take full advantage of the
signals from different transmitters even though the equivalent
dispersive channel is likely to have a larger time spread. In other
words, the receiver can combine and demodulate the signals in
a simple way with a minime loss of performance. Moreover,
OFDM systems allow a better margin on delays between VCN
channels with little added complexity. Therefore, as long as the
time-dispersive channels from the transmitting APs fall within
the cyclic prefix at the receiver, the OFDM signal retains its
integrity and, under some simple design conditions, the demodulation
performance should not decrease.
Since the propagation conditions are difficult, the system
should intelligently discern the APs forming a virtual cell for a
particular user. Firstly, in order to preserve capacity, we shall
not allow out-of-range APs to transmit so that power contribution
from each VCN antenna exceeds a certain received power
threshold. Secondly, a large number of APs in one virtual cell
is not necessarily desirable since the resulting channel becomes
increasingly time dispersive. Thirdly, in the case of good VCN
channel quality, we would like to combine the contributions
from each VCN AP and increase the diversity gain.
1.2. The 60 GHz frequency band
The use of VCN is particularly relevant for 60 GHz (the 59–
64 GHz unlicensed band) wireless LANs. At this frequency,
the propagation range is dramatically limited whilst the 5 GHz
bandwidth is large. Therefore, we can accept some spectral efficiency
losses in order to increase the coverage of the cells. The
59.00–59.05 GHz portion of the band will be common to all
users, for assistance in the coordination of real time use in the
remainder of the band1.
The large bandwidth available in the 60 GHz band offers a
great capacity for wireless broadband systems. For example, a
single AP in an meeting room could offer, using simple modulation,
a steady throughput of 200 Mbit/s to not less than 25
users at the same time, without sharing any resources. Unfortunately,
it is difficult to offer proper coverage