Signal Processing in OFDM
The telecommunications industry
faces the problem of providing telephone services to rural areas,
where the customer base is small, but the cost of installing a wired
phone network is very high. One method of reducing the high
infrastructure cost of a wired system is to use a fixed wireless radio
network. The problem with this is that for rural and urban areas,
large cell sizes are required to get sufficient coverage. This
presents extra problems as there are long delay times in multi-path
Currently Global System
for Mobile telecommunications (GSM) technology is being applied to
fixed wireless phone systems. However, GSM uses time division multiple
access (TDMA), which has a high symbol rate leading to problems with
multi-path causing inter-symbol interference.
OFDM/COFDM allows many
users to transmit in an allocated band, by subdividing the available
bandwidth into many narrow bandwidth carriers. Each user is allocated
several carriers in which to transmit their data. The transmission is
generated in such a way that the carriers used are orthogonal to one
another, thus allowing them to be packed together much closer than
standard frequency division multiplexing (FDM). This leads to OFDM/COFDM
providing a high spectral efficiency.
is an emerging field, which has seen enormous growth in the last
several years. The huge uptake rate of mobile phone technology,
Wireless Local area Networks (WLAN) and the exponential growth of the
Internet have resulted in an increased demand for new methods of
obtaining high capacity wireless networks.
Most WLAN systems
currently use the IEEE802.11b standard, which provides a maximum data
rate of 11 Mbps. Newer WLAN standards such as IEEE802.11a and
HiperLAN2 are based on OFDM technology and provide a much higher data
rate of 54 Mbps. However systems of the near future will require WLANs
with data rates of greater than 100 Mbps, and so there is a need to
further improve the spectral efficiency and data capacity of OFDM
systems in WLAN applications.
For cellular mobile applications,
we will see in the near future a complete convergence of mobile phone
technology, computing, Internet access, and potentially many
multimedia applications such as video and high quality audio. In fact,
some may argue that this convergence has already largely occurred,
with the advent of being able to send and receive data using a
notebook computer and a mobile phone. Although this is possible with
current 2G (2nd Generation) Mobile phones, the data rates provided are
very low (9.6 kbps – 14.4 kbps) and the cost is high.
The goal of third and
fourth generation mobile networks is to provide users with a high data
rate, and to provide a wider range of services, such as voice
communications, videophones, and high speed Internet access. The
higher data rate of future mobile networks will be achieved by
increasing the amount of spectrum allocated to the service and by
improvements in the spectral efficiency. OFDM is a potential candidate
for the physical layer of fourth generation mobile systems. Much
research is going on techniques for improving the spectral efficiency
of OFDM systems applied in WLAN and mobile networks.
OFDM for two-way
communications or for multi-user applications include wireless modems,
Wireless Local area Networks (WLAN's), Wireless Local Loop (WLL),
mobile phones, and mobile high speed internet.