Wireless Future - Telenor

In order to meet the ever-present demand for
more capacity, basically two different approaches
can be taken:
a) Take into use new, previously unused parts of
the radio spectrum;
b)Improve the spectrum utilisation, i.e. increase
the number of transferred bits/s per bandwidth
unit (Hz).
Both approaches are being followed in current
research activities. In the case of new spectrum,
the bands around 40 and 60 GHz have been
given great attention in recent years. Improvements
in spectrum utilisation can be achieved by
improving the physical and link layers of the
radio systems and in some cases also the network
layer. In the following sections we will
look briefly into some promising emerging technologies.
Use of 40 and 60 GHz Bands
Use of frequency spectrum in the millimetre
wave range is considered a promising way to
realise high datarate wireless systems because
of the large amount of available spectrum in
these bands. The bands of greatest importance
lie in the 40 and 60 GHz ranges.
In the 40 GHz range, CEPT has allocated the
spectrum from 40.5 to 43.5 GHz, a total of
3 GHz bandwidth, to so-called multimedia
wireless systems (MWS) [20].
In the frequency range around 60 GHz, oxygenabsorption
of radio waves is significant resulting
in a higher propagation attenuation. Realising
that this opens up for a high frequency reuse factor
in radio systems, CEPT has recommended
that frequencies in the range 54.2 – 66.0 GHz
are used for terrestrial fixed and mobile systems
[21].
The 60 GHz range was selected by the MBS
project [5] run under the European RACE 2 programme.
The MBS system used the bands 62 –
63 GHz and 65 – 66 GHz, divided into 34 (outdoor)
or 17 (indoor) subchannels (carriers),
using 4-OQAM (offset quadrature amplitude
modulation) or 16-OQAM. Data rates up to
155 Mbit/s could be provided using more than
one carrier. The system was designed for small
cells (max 500 m radius) and required generally
line-of-sight between the base station and mobile
terminal. To support fast moving mobiles (cars,
trains), advanced handover algorithms were
required.
In the follow up ACTS project SAMBA [22] frequencies
in the 40 GHz band were used, with
a target bit rate of 34 Mbit/s using OQPSK mod-
Telektronikk 1.2001
ulation. Again, the system was designed for fast
moving mobile terminals.
Ideas from the MBS project were also adopted
by the ACTS MEDIAN project [23]. This time,
the focus was on WLAN type scenarios (i.e. low
mobility) using the 60 GHz band. Using OFDM
type modulation, the target was to be able to provide
data rates up to 155 Mbit/s.
No standards or commercial products have
emerged so far, most likely due to the immaturity
of the technology and lack of a real market
demand. The only known standardisation work
going on in this area is by the Japanese MMAC
project, where a “Ultra High Speed Wireless
LAN” is being specified for the 60 GHz band
with a target bit rate of 156 Mbit/s [24]. It is to
be expected, however, that work in this area
gains new momentum when the market for highrate
radio systems matures.
From a broadcast and fixed access viewpoint,
there is still significant activity on systems for
the 40 GHz band. The IST EMBRACE project
aims at designing a low cost LMDS type fixed
radio access system [25]. The system is designed
with a downlink capable of supplying several
Mbit/s for both broadcast channels as well as
high speed communication data and an uplink
for virtual no return (such as 64 kbit/s ISDN) up
to several Mbit/s for high quality contribution
and data transfer.
Advanced Coding and Modulation
While the 2G systems, like GSM, marked the
introduction of digital radio technology into cellular
telephony, 3G is the great breakthrough for
CDMA technology, pioneered by the American
2G IS-95 system. CDMA has many desirable
properties, one of the most prominent being the
frequency reuse factor of 1, i.e. the possibility to
use the same frequency in all cells.
At the edge of 4G, the arguably hottest technology
is OFDM – orthogonal frequency division
multiplexing. OFDM has already been introduced
into digital broadcasting systems like
DAB and DVB-T, and is selected for next generation
WLAN systems like HIPERLAN/2, IEEE
802.11a and MMAC High Speed Wireless LAN.
OFDM possesses excellent properties in dispersive
radio channels, i.e. with high delay spreads,
eliminating the need for complex equalisers.
Another important trend is the use of adaptive
modulation. Radio communication is characterised
by time-varying and complex radio channels.
Traditionally, radio systems have been
designed for worst-case conditions, the aim
being to keep the outage probability below a certain,
low value. By allowing the modulation to
11