OFDM as a possible modulation technique for multimedia applications in
the range of mm waves
Contributed by Dusan Matic
The 60 GHz band
The scarcity of spectrum and the new technical possibilities in recent years have
drawn attention all over the world to the millimeter band. It has become a hot
topic as a research area for broadband communications.
The low mm wave band from 20-60 GHz, which is nearly unused and allows
for large bandwidth applications, combines the advantages of infrared (enough
free bandwidth and UHF (good coverage). Use of the area around 60 GHz is
encouraged for the following reasons.
-
The coherence bandwidth of a 60 GHz link is several MHz.
- There is enough unused space for the multimedia needs (5 GHz). This frequency
region is not in use by any other medium, so to every user can be allocated
a large bandwidth, i.e. bit rates of an order of 155Mb/s are possible, thus
a potential exists to support broadband service access, which is especially
relevant because of the advent of the Broadband Digital Network (B-ISDN).
- Systems operating particularly in the 60 GHz frequency band can have a small
reuse distance, because of the oxygen absorption at the rate of 14 dB/km.
Usually, this is a disadvantage for many applications; however, this high
attenuation over the propagation path creates a natural barrier for co-channel
interference in the mobile cellular system. Thus, frequency reuse is easy.
- The wavelength is rather small: about 5 mm. This enables the use small
antenna's. This facilitates miniaturization of devices.
A major drawback of this frequency region is the fact that the technology
for the transceivers will be expensive in the early stages. If used indoor,
the mm wave radio channel shows adverse frequency selective multipath characteristics
due to the highly reflective indoor environment, which results in severe
signal dispersion and limits the maximum usable symbol rate. It is worth
mentioning that no definitive evidence of any hazards has been shown to
date to the general public arising from the prolonged exposure in fields
of less than 10 mW/cm2 in the mm waves.
There is a lot of fundamental investigation needed in this area, e.g.
propagation modeling for 60 GHz, effects of antenna diversity, technology
development, etc.
For the final choice between OFDM and single-carrier modulation, one
needs knowing the channel properties at 60 GHz. Measurements and models
can hardly be found in the literature. These properties have to be built
in the simulation model, which could then be used for evaluating modulation
technique candidates for the mobile multimedia communication.
