Photonics Components for Next Generation 60GHz Home Area Networks
Using photonic components and OFDM modulation techniques,
IPHOBAC has successfully demonstrated bidirectional transmission of more than 1.25
Gb/s over a radio-over-fibre system composed of a central station, 50 m of
optical fibre, a remote antenna, an indoor radio propagation path of up to 15m
and a mobile station. A maximum data rate of 2.88 Gb/s is achieved with the
same system if radio propagation multi path effects are avoided.
 Radio-over-Fibre techniques and phonics components are key
technologies for the conception of the next generation of ultra broad band wireless
home area networks (UBB-HAN) based on the upcoming IEEE802.15.3c standard operating
in the 59-66 GHz band where 7 GHz of unlicensed spectrum bandwidth enables multigigabits
datarates.
The study takes place in the larger context of designing future networks
for end-user’s homes. It is a necessity for telecom operators to deploy UBB-HAN
at their customers premises to allow for the forecasted growth in bandwidth
needs and follow the natural evolution of current access networks (towards
FTTH). Special care has to be devoted on increased available data rates but as
well on increased Quality of Service (QoS) and mobility within the home. FTTH
Access equipments deployed today (e.g. Gigabit Passive Optical Network G-PON [1])
can currently transmit symmetric rates around 100 Mbps and are well
capable of transmitting several gigabits per second to the end-user but, from
an operator point of view, it would be impossible to market an access bandwidth
greater than what the home network can handle. Moreover, the multiplication and
diversification of applications and terminals increases the pressure on
requirements for large data rates and better QoS on the home network. Based on
usage studies, it is foreseen that future UBB-HAN will required data rate >
1 Gb/s and round trip time <1ms.
Terminal mobility is another key requirement for Next generation
UBB-HAN. WiFi today has enabled a very flexible usage of the HAN and Strategy
Analytics, in February 2007, forecasted that by 2010, 80% of HANs will be
wireless. The most promising technology is described in the pre-standard
IEEE802.15.3c. It exploits the 60GHz spectrum and allows several gigabits per
second to be transferred up to around 10 m, creating small high speed
radio cells referred to as Wireless Personal Area Networks (WPAN). The
properties of 60 GHz radio waves combined with the limited coverage imply the
deployment of multiple radio access points within a single house to obtain a
complete coverage. Note that this is true for any wireless end-connectivity
providing similar data rates. For this reason, the fact that 60 GHz radio waves
do not propagate through walls is a strong advantage of this technology,
leading to an ease of deployment of this multi-cell network since inter-cell
interference is limited, and thus frequency reuse is facilitated.
In order to demonstrate how Radio-over-Fibre technologies and photonics
components allow the above requirements to be met, we have set-up an
experimental hybrid wireless-optical communication link using commercially
available components.
The experimental link is representative of a communication between a
fixed Central Station (CS) and a mobile End-Device (ED) used in one of the
rooms of the house. The ED communicates wirelessly with a local wireless access
point (WAP) situated in quasi line of sight on the ceiling or one of the walls
of the room. The different WAPs are linked to the CS though an optical fibre
network. Two configurations have been evaluated: one is representative of the
uplink while the other is representative of the downlink (See Fig.1). The
signal waveform is generated by an AWG at system input and sampled by a
real-time scope at system output. Signal Modulation and demodulation are
processed off-line. The optical link is composed of a Mach-Zehnder Modulator
fed by a CW laser, 50m of single mode fibre and a fast PIN photodiode and the
radio link is composed of 2 horn antennas located up to 15m away from one
another in a corridor, thus taking multi path effects into account.
 
Figure 1: Schematic of the downlink architecture configuration (left) and
uplink configuration (right)
The modulation format chosen was a triple-band OFDM signal having a
total bandwidth of 1.6GHz and centred at 60.7GHz. The OFDM modulation scheme
presents the advantage of being spectrally efficient and robust to the
frequency selective fadings due to multipath propagation. It is therefore widely
used in indoor high data rate systems and is one of the modulation scheme
currently proposed for standardization in the IEEE802.15.3c group.
However the timeline of these works has not permitted us to use an
IEEE802.15.3c compliant OFDM signal for transmission in the proof of concept.
Instead, we have used a modified version of the UWB ECMA-368 radio interface
translating its centre frequency to the 60 GHz band and allowing the
modulation of the carriers to vary from BPSK to 32-QAM. To be closer to the
IEEE802.15.3c specification, we transmitted simultaneously three contiguous UWB
sub-bands allowing the total RF bandwidth of the signal to reach 1.6 GHz
which was, considering our tests equipments, as close as we could achieve to
the 2.16 GHz of the IEEE802.15.3c group.
Performances are evaluated in terms of EVM. For a given
measurement, depending on the achieved EVM on each OFDM band, the best
constellation type for the modulation of the sub-carriers of each band is
chosen so as to keep a BER below 10-5. The total achievable data
rate for a BER<10-5 is plotted versus distance for both uplink
and downlink in figure 2.
Figure 2: Data rate as a function of the propagation distance assuming that
the best constellation (QPSK to 32QAM) is used on each OFDM sub-band depending
on the measured EVM with the constraint of having a BER<10-5.
Up to 15m and even in the worst case, a datarate of 1.25 Gb/s is
achieved. For distances less than 1 m, 8-QAM modulation can be used on all
bands, leading to a total data rate of 2.88 GBps in full duplex (See figure
2). Between 5 and 10 m, the data rate decreases because QPSK has to be
used on at least one of the sub-bands. The spread between the maximum and minimum
data rate for a given distance increases with radio propagation distance. This
is due to propagation frequency selective fading produced by multi-path
interferences in the corridor. The datarate achievable can thus vary significantly
between two close locations. OFDM modulation is particularly well adapted to
the application of equalization techniques in order to mitigate effects of
multi-path fadings, however, these are beyond the scope of this study and where
not used here. For more than 10 meters, the limitation of the system
performance is attributable to the phase noise of the TX and RX.
It is interesting to underline that the present system is never limited
by the photonics components used in the Radio-over-Fibre link. If the
performance needs to be increased, enhancements should be achieved on the phase
noise of the TX and RX or on the mitigation of the radio propagation
impairments.
Future studies in the frame of IPHOBAC will consist in evaluating applicability
and performance of the photonic components developed by the project partners
for the design of UBB-HAN. In this scope, interesting IPHOBAC photonics
components include dual-mode and mode locked laser, photodiodes and photomixers
as well as reflective electro-optic transceivers.
|
