10-08-2013, 04:01 PM
Broadband Wireless Networks
[attachment=57064]
INTRODUCTION
Wireless communication has experienced tremendous growth in the last decade. High
data rate wireless communication is becoming increasingly important to mobile users in
corporate and public networks in the indoor environment. Although voice and low data rate
services were the first applications of cellular networks, the focus in recent years has shifted to
very high-bandwidth delivery, which is a key driver for system and network design. While
wireless local area (WLAN) access is important for data applications, mobile users still expect
reliable GSM and 3G coverage for their voice service and seamless transition as they move
indoors from an outdoor environment. Research shows that 75% of all mobile calls originate
inside buildings at work, home or public places. This has in turn led to consumer awareness and
expectation of ubiquitous coverage, and the ability to use their wireless devices anywhere.
BROADBAND WIRELESS COMMUNICATION SYSTEMS
The explosive growth of the Internet, and the success of 2G systems together with WLANs have
had a profound impact on our perception of communication. First of all, the vast majority of
users now believe in the new notion of “always on” communication. We are now living in the era
of ubiquitous connectivity or “communication anytime, anywhere, and with anything”. Secondly,
the concept of broadband communication has caught on very well. As fibre penetrates closer to
the end-user environment (Fibre to the Home/Curb/X, FTTH/C/X), wired transmission speeds
will continue to rise. Transmission speeds such at 100 Mbps (Fast Ethernet) are now beginning
to reach homes. The demand to have this broadband capacity also wirelessly has put pressure on
wireless communication systems to increase both their transmission capacity, as well as their
coverage.
CHALLENGES OF BROADBAND WIRELESS ACCESS
Figure 2.2 illustrates the configuration of narrowband wireless access systems (e.g. GSM) as we
know them today. The central office handles call processing and switching, while the Base
Stations (BS) act as the radio interfaces for the Mobile Units (MU) or Wireless Terminal Units
(WTU). The BSs may be linked to the central office through either analogue microwave links or
digital fibre optic links. Once the baseband signals are received at the BS, they are processed and
modulated onto the appropriate carrier. The radius covered by the signal from the BS is the cell
radius. All the MU/WTU within the cell, share the radio frequency spectrum. WLANs are
configured in a similar fashion, with the radio interface called the Radio Access Point (RAP).
WIRELESS OVER FIBRE TECHNOLOGY
Wireless over Fibre (WoF) systems which is also known as Radio over Fibre (RoF)
systems have attracted much interest for broadband wireless access, offering a simplified overall
system design due to the aggregation of RF signal generation and network management at a
central location. The system allows flexible network design by linking the remotely located AUs
with optical fibre. Most modern commercial office buildings already have an optical fibre
infrastructure in place for carrying the wired Gigabit Ethernet traffic. A strong case can therefore
be made that the same optical fibre infrastructure be shared with different cellular operators for
distributing the narrowband GSM and 3G as well as the broadband WLAN signals. It has been
shown that for remoting distances of more than 100m, optical fibre is preferred over coaxial
cable.
Transmitter
The use of preinstalled MMF saves cost when installing a Wireless over fibre system.
However, the highest contribution to the total subsystem cost usually comes from the optical
transmitter. Using directly modulated laser diodes, which operate uncooled, can significantly
reduce the cost of the in-building broadband wireless-over-fibre deployment. However, the key
challenge is to ensure highly linear performance is retained for the full range of temperatures in a
robust manner. Recent advances in uncooled DFB laser design for data communication
applications have resulted in greatly enhanced linearity at high temperatures.
Optical Fibre
Many of the large existing offices and public buildings already have optical fibre
installed, which is mainly MMF for carrying the Ethernet
data. Figure 3.3
shows the
different fibre types used in buildings and it can be seen that more than 90% of the fibre installed
is MMF. Much of the installed MMF comes from an age when LEDs were used as transmitters
in wired LANs and therefore have a relatively low overfilled launch (OFL) bandwidth, with 500
MHz.km being specified for MMF. A multimode fibre has a larger core diameter of 50 - 200
μm compared to a single mode fibre core diameter of
8-12μm and also has a higher numerical aperture in the range of 0.19 - 0.30. The higher
numerical aperture means a larger acceptance angle that allows more optical power to be coupled
into the fibre. The impulse esponse of a MMF can be generalized as a Gaussian response
with respect to optical power. Hence, the longer the link is, the more severe the effect is on
modal dispersion.
COMMERCIAL IMPLEMENTATIONS
Commercial products for transporting second and third generation cellular signals over
fibre are available from a number of manufacturers which either transmit the signal over single
mode fibre at the original RF, or transmit the signal over the MMF at a down-converted IF which
is selected to be within the -3 dB bandwidth of the MMF. To provide in-building cellular
coverage, the former method requires specialist and expensive fibre components. Most installed
fibres within buildings are MMF taking advantage of lower component cost. The down-
conversion method requires complicated hardware for the simultaneous transmission of a low
frequency reference tone for stabilising and locking the remote local oscillator for the remote
antenna units since up-conversion from the IF to the original RF is necessary. This adds
additional cost and complexity to the system.
[attachment=57064]
INTRODUCTION
Wireless communication has experienced tremendous growth in the last decade. High
data rate wireless communication is becoming increasingly important to mobile users in
corporate and public networks in the indoor environment. Although voice and low data rate
services were the first applications of cellular networks, the focus in recent years has shifted to
very high-bandwidth delivery, which is a key driver for system and network design. While
wireless local area (WLAN) access is important for data applications, mobile users still expect
reliable GSM and 3G coverage for their voice service and seamless transition as they move
indoors from an outdoor environment. Research shows that 75% of all mobile calls originate
inside buildings at work, home or public places. This has in turn led to consumer awareness and
expectation of ubiquitous coverage, and the ability to use their wireless devices anywhere.
BROADBAND WIRELESS COMMUNICATION SYSTEMS
The explosive growth of the Internet, and the success of 2G systems together with WLANs have
had a profound impact on our perception of communication. First of all, the vast majority of
users now believe in the new notion of “always on” communication. We are now living in the era
of ubiquitous connectivity or “communication anytime, anywhere, and with anything”. Secondly,
the concept of broadband communication has caught on very well. As fibre penetrates closer to
the end-user environment (Fibre to the Home/Curb/X, FTTH/C/X), wired transmission speeds
will continue to rise. Transmission speeds such at 100 Mbps (Fast Ethernet) are now beginning
to reach homes. The demand to have this broadband capacity also wirelessly has put pressure on
wireless communication systems to increase both their transmission capacity, as well as their
coverage.
CHALLENGES OF BROADBAND WIRELESS ACCESS
Figure 2.2 illustrates the configuration of narrowband wireless access systems (e.g. GSM) as we
know them today. The central office handles call processing and switching, while the Base
Stations (BS) act as the radio interfaces for the Mobile Units (MU) or Wireless Terminal Units
(WTU). The BSs may be linked to the central office through either analogue microwave links or
digital fibre optic links. Once the baseband signals are received at the BS, they are processed and
modulated onto the appropriate carrier. The radius covered by the signal from the BS is the cell
radius. All the MU/WTU within the cell, share the radio frequency spectrum. WLANs are
configured in a similar fashion, with the radio interface called the Radio Access Point (RAP).
WIRELESS OVER FIBRE TECHNOLOGY
Wireless over Fibre (WoF) systems which is also known as Radio over Fibre (RoF)
systems have attracted much interest for broadband wireless access, offering a simplified overall
system design due to the aggregation of RF signal generation and network management at a
central location. The system allows flexible network design by linking the remotely located AUs
with optical fibre. Most modern commercial office buildings already have an optical fibre
infrastructure in place for carrying the wired Gigabit Ethernet traffic. A strong case can therefore
be made that the same optical fibre infrastructure be shared with different cellular operators for
distributing the narrowband GSM and 3G as well as the broadband WLAN signals. It has been
shown that for remoting distances of more than 100m, optical fibre is preferred over coaxial
cable.
Transmitter
The use of preinstalled MMF saves cost when installing a Wireless over fibre system.
However, the highest contribution to the total subsystem cost usually comes from the optical
transmitter. Using directly modulated laser diodes, which operate uncooled, can significantly
reduce the cost of the in-building broadband wireless-over-fibre deployment. However, the key
challenge is to ensure highly linear performance is retained for the full range of temperatures in a
robust manner. Recent advances in uncooled DFB laser design for data communication
applications have resulted in greatly enhanced linearity at high temperatures.
Optical Fibre
Many of the large existing offices and public buildings already have optical fibre
installed, which is mainly MMF for carrying the Ethernet
data. Figure 3.3
shows the
different fibre types used in buildings and it can be seen that more than 90% of the fibre installed
is MMF. Much of the installed MMF comes from an age when LEDs were used as transmitters
in wired LANs and therefore have a relatively low overfilled launch (OFL) bandwidth, with 500
MHz.km being specified for MMF. A multimode fibre has a larger core diameter of 50 - 200
μm compared to a single mode fibre core diameter of
8-12μm and also has a higher numerical aperture in the range of 0.19 - 0.30. The higher
numerical aperture means a larger acceptance angle that allows more optical power to be coupled
into the fibre. The impulse esponse of a MMF can be generalized as a Gaussian response
with respect to optical power. Hence, the longer the link is, the more severe the effect is on
modal dispersion.
COMMERCIAL IMPLEMENTATIONS
Commercial products for transporting second and third generation cellular signals over
fibre are available from a number of manufacturers which either transmit the signal over single
mode fibre at the original RF, or transmit the signal over the MMF at a down-converted IF which
is selected to be within the -3 dB bandwidth of the MMF. To provide in-building cellular
coverage, the former method requires specialist and expensive fibre components. Most installed
fibres within buildings are MMF taking advantage of lower component cost. The down-
conversion method requires complicated hardware for the simultaneous transmission of a low
frequency reference tone for stabilising and locking the remote local oscillator for the remote
antenna units since up-conversion from the IF to the original RF is necessary. This adds
additional cost and complexity to the system.