Fading, Diversity & Interference
FADING ?
• The communication between the base station and mobile station in mobile systems is mostly non-LOS.• The LOS path between the transmitter and the receiver is affected by terrain and obstructed by buildings and other objects.
• The mobile station is also moving in different directions at different speeds.
•The RF signal from the transmitter is scattered by reflection and diffraction and reaches the receiver through many non-LOS paths.
• This non-LOS path causes long-term and short term fluctuations in the form of log-normal fading and rayleigh and rician fading, which degrades the performance of the RF channel.
LONG TERM FADING
• Terrain configuration & man made environment causes long-term fading.• Due to various shadowing and terrain effects the signal level measured on a circle around base station shows some random fluctuations around the mean value of received signal strength.
• The long-term fades in signal strength, r, caused by the terrain configuration and man made environments form a log-normal distribution, i.e the mean received signal strength, r, varies log-normally in dB if the signal strength is measured over a distance of at least 40l.
• Experimentally it has been determined that the standard deviation, s, of the mean received signal strength, r, lies between 8 to 12 dB with the higher s generally found in large urban areas.
RAYLEIGH FADING
• This phenomenon is due to multipath propagation of the signal.• The Rayleigh fading is applicable to obstructed propagation paths.
• All the signals are NLOS signals and there is no dominant direct path.
• Signals from all paths have comparable signal strengths.
• The instantaneous received power seen by a moving antenna becomes a random variable depending on the location of the antenna.
RICEAN FADING
•
This phenomenon is due to
multipath propagation of the signal.
•
In this case there is a partially
scattered field.
•
One dominant signal.
•
Others are weaker.
DIVERSITY ANTENNA SYSTEMS
NEED OF DIVERSITY
• The direct path between the transmitter and the receiver is obstructed by buildings and other objects.
• Hence the signal that arrives at the receiver is either by reflection from the flat sides of buildings or by diffraction around man made or natural obstructions.
• When various incoming radiowaves arrive at the receiver antenna, they combine constructively or destructively, which leads to a rapid variation in signal strength.
• The signal fluctuations are known as ‘multipath fading’.
Multipath Propagation
• Multipath propagation causes large and rapid fluctuations in a signal.
• These fluctuations are not the same as the propagation path loss.
• Multipath causes three major things.
• Rapid changes in signal strength over a short distance or time.
• Random frequency modulation due to Doppler Shifts on different multipath signals.
• Time dispersion caused by multipath delays
• These are called “fading effects.
• Multipath propagation results in small-scale fading.
DIVERSITY TECHNIQUE
Diversity techniques have been recognised as an effective means which enhances the immunity of the communication system to the multipath fading. GSM therefore extensively adopts diversity techniques that include
CONCEPT
OF DIVERSITY ANTENNA SYSTEMS
•
Spatial and polarisation
diversity techniques are realised through antenna systems.• A diversity antenna system provides a number of receiving branches or ports from which the diversified signals are derived and fed to a receiver. The receiver then combines the incoming signals from the branches to produce a combined signal with improved quality in terms of signal strength or signal-to-noise ratio (S/N).
• The performance of a diversity antenna system primarily relies on the branch correlation and signal level difference between branches.
SPATIAL
DIVERSITY ANTENNA SYSTEMS
•
The spatial diversity antenna
system is constructed by physically separating two receiving base station
antennas.• Once they are separated far enough, both antennas receive independent fading signals. As a result, the signals captured by the antennas are most likely uncorrelated.
• The further apart are the antennas, the more likely that the signals are uncorrelated.
The types of the configuration used in
GSM networks are:
•
horizontal separation. • vertical separation.
• composite separation.
Antenna Configuration:-
Three Antenna Spatial Configuration:-
Two Antenna Spatial Configuration:-
• A single (say vertical) polarised electromagnetic wave is converted to a wave with two orthogonal polarised fields while it is propagating through scattering environment.
• It has also been found that the two fields exhibit some extent of decorrelation.
DUAL POLARISED ANTENNAS
• A dual-polarisation antenna consists of two sets of radiating elements which radiate or, in reciprocal, receive two orthogonal polarised fields.
• The antenna has two input connectors which separately connects to each set of the elements.
• The antenna has therefore the ability to simultaneously transmit and receive two orthogonally polarised fields
ADVANTAGES OF DUAL POLARISED ANTENNAS
•
The best advantage of using the dual polarisation antenna is the
reduction in the number of antennas per sector.• Reduced size of the headframe of the supporting structure
• Reduced windload and weight.
• Reduced difficulty in site acquisition and installation.
• Cost saving
– Requiring slim tower
– Requiring less installation time.
– Cost of one dual polarisation antenna is generally lower than that of two
– Single polarised antennas
• DUAL POLARISED ANTENNA CONFIGURATIONS
INTERFERENCE:-
Interference is the sum of all signal
contributions that are neither noise not the wanted signal.Interference is a
major limiting factor in the performance of cellular systems.It causes
degradation of signal quality.It introduces bit errors in the received signal.Bit
errors are partly recoverable by means of channel coding and error correction
mechanisms.The interference situation is not reciprocal in the uplink and
downlink direction.Mobile stations and base stations are exposed to different
interference situation.Another mobile in the same cell.A call in progress in
the neighboring cell.
Other base stations operating on the
same frequency.Any non-cellular system which leaks energy into the cellular
frequency band.
TYPES OF INTERFERENCE
•
There are two types of system
generated interference
– Co-channel
interference
– Adjacent
channel interference
Co-Channel Interference
•
This type of interference is the
due to frequency reuse , i.e. several cells use the same set of frequency.
•
These cells are called co-channel
cells.
•
Co-channel interference cannot be
combated by increasing the power of the transmitter. This is because an
increase in carrier transmit power increases the interference to neighboring
co-channel cells.
•
To reduce co-channel
interference, co-channel cells must be physically separated by a minimum
distance to provide sufficient isolation due to propagation or reduce the
footprint of the cell.
•
Some factors other then reuse
distance that influence co-channel interference are antenna type,
directionality, height, site position etc,
•
GSM specifies C/I > 9dB.
•
In a cellular system, when the
size of each cell is approximately the same, co-channel interference is
independent of the transmitted power and becomes a function of cell radius(R)
and the distance to the centre of the nearest co-channel cell (D).
•
Q = D / R = Ö3N
•
By increasing the ratio of D/R,
the spatial seperation between the co-channel cells relative to the coverage
distance of a cell is increased. In this way interference is reduced from
improved isolation of RF energy from the co-channel cell.
•
The parameter Q , called the
co-channel reuse ratio, is related to the cluster size.
•
A small value of Q provides
larger capacity since the cluster size N is small whereas a large value of Q
improves the transmission quality.
Adjacent-Channel Interference
•
Interference resulting from
signals which are adjacent in frequency to the desired signal is called
adjacent channel interference.
•
Adjacent channel interference
results from imperfect receiver filters which allow nearby frequencies to leak
into the passband.
•
Adjacent channel interference can
be minimized through careful filtering and channel assignments.
•
By keeping the frequency separation between each channel in a
given cell as large as possible , the adjacent interference may be reduced
considerably.
Adjacent-Channel Interference
POWER CONTROL
•
RF power control is employed to minimise
the transmit power required by MS or BS while maintaining the quality of the
radio links.
•
By minimising the transmit power
levels, interference to co-channel users is reduced.
•
Power control is implemented in
the MS as well as the BSS.
•
Power control on the Uplink also
helps to increase the battery life.
•
Power received by the MS is
continously sent in the measurement report.
•
Similarly uplink power received
from the MS by the BTS is measured by the BTS.
•
Complex algorithm evaluate this
measurements and take a decision subsequently reducing or increasing the power
in the Uplink or the downlink.
SECTORIZATION
•
For 120 degrees sectored site as
compared to an omni site almost 1/3rd interference is received in the uplink.
•
The more selective and
directional is the antenna, the smaller is the interference.
•
Reduction in interference results
in higher capacity in both links.
REPEATERS
•
Repeater units are designed to
receive signals from a donor site, amplify and rebroadcast the donor sites
signals into poor coverage areas or to extend the coverage range of the donor
site.
•
These repeater are bi-directional
and do not translate frequency and subsequently are limited in output power and
gain.
•
Repeaters provide between 50 to
80 dB of gain.
•
There are two types of repeater
band selective and channel selective.
•
Band selective repeater amplifies
a band of frequency. Hence it amplifies any frequency that falls within its
band.
•
Channel selective repeater allows
selection of a number of individual channels to amplify and rebroadcast.
•
Typically a channel selective
repeater allows selection of 2 to 4 channels.
•
If the GSM900 or DCS1800 network
incorporates frequency hopping, then only band selective repeaters should be
used.
No comments:
Post a Comment