Frequency Hopping
FREQUENCY HOPPING
Introduction to Frequency Hopping
Can be used to improve the quality of the network.Also can be used to
increase the capacity of the Network thereby reducing the number of sites
required for CAPACITY.The way it works,Each burst is transmitted on a
different frequency.Both mobile and base station follow the same hopping sequence
Introduction to Frequency Hopping
Fading causes quality deterioration is frequency dependent.FH diversifies the impact of fading and improves quality.The immunity to fading increases by exploiting its frequency selectivity, because using different frequencies the probability of being continuously affected by fading is reduced, so the transmission link quality is improved.This improvement is much more noticeable for slow moving mobiles.
Increased Immunity to fading
In a cellular urban environment in most cases multipath propagation will be present and, as a consequence of that, important short term variations in the received level are frequent . This is called Rayleigh fading which results in quality degradation because some of the information will be corrupted. For a fast moving mobile, the fading situation can be avoided from one burst to another because it also depends on the position of the mobile so the problem is not so serious. For a stationary one the reception may be permanently affected resulting in a very bad quality, even a drop call.Once the information is received by the mobile or the base station, the only way to cope with the disturbance produced by the fading (errors in the information bits) are the decoding and de-interleaving processes, with an effectiveness limited by the number of errors they have to deal with.
Increased Immunity to fading
Interference A result of frequency reuse & irregular terrain and sites.FH diversifies the impact of interference and improves quality.The situation of permanent interference coming from neighbour cells transmitting the same or adjacent frequencies is avoided using Frequency Hopping because the calls will spend the time moving through different frequencies not equally affected by interfering signals. This effect is called Interference Averaging.Considering a non hopping system, the set of calls on the interferer cells which can interfere with the wanted call is fixed for the duration of those calls and some calls will be found with very good quality (no interference problems) whereas some others with very bad quality (permanent interference problems).
Fading causes quality deterioration is frequency dependent.FH diversifies the impact of fading and improves quality.The immunity to fading increases by exploiting its frequency selectivity, because using different frequencies the probability of being continuously affected by fading is reduced, so the transmission link quality is improved.This improvement is much more noticeable for slow moving mobiles.
Increased Immunity to fading
In a cellular urban environment in most cases multipath propagation will be present and, as a consequence of that, important short term variations in the received level are frequent . This is called Rayleigh fading which results in quality degradation because some of the information will be corrupted. For a fast moving mobile, the fading situation can be avoided from one burst to another because it also depends on the position of the mobile so the problem is not so serious. For a stationary one the reception may be permanently affected resulting in a very bad quality, even a drop call.Once the information is received by the mobile or the base station, the only way to cope with the disturbance produced by the fading (errors in the information bits) are the decoding and de-interleaving processes, with an effectiveness limited by the number of errors they have to deal with.
Increased Immunity to fading
Interference A result of frequency reuse & irregular terrain and sites.FH diversifies the impact of interference and improves quality.The situation of permanent interference coming from neighbour cells transmitting the same or adjacent frequencies is avoided using Frequency Hopping because the calls will spend the time moving through different frequencies not equally affected by interfering signals. This effect is called Interference Averaging.Considering a non hopping system, the set of calls on the interferer cells which can interfere with the wanted call is fixed for the duration of those calls and some calls will be found with very good quality (no interference problems) whereas some others with very bad quality (permanent interference problems).
With hopping, that set of
interfering calls will be continually changing and the effect is that calls
tend to experience an average quality rather than extreme situations of either
good or bad quality (all the calls will suffer from a controlled interference
but only for short and distant periods of time, not for all the duration of the
call).This interference averaging means again spreading the raw bit errors (BER
caused by the interference) in order to have a random distribution of them
instead of bursts of errors, and therefore enhance the effectiveness of
decoding and deinterleaving processes to cope with the BER and lead to a better
value of FER.
TYPES OF Frequency HOPPING
Base Band Hopping (BBH)
The
radio units transmit always the same frequency.Number of frequencies for
hopping = Number of carriers.The radio units are always transmitting a
fixed frequency and frequency hopping is performed by moving the information
for every call among the available radio units in a cell on a per burst
basis.In reception the call is always processed by the same radio unit (the one
where the call started).The number of frequencies to hop over is limited by the
number of radio units equipped in the cell.The BCCH carrier can hop in
timeslots 1 to 7. (without power ontrol /DTX ) .
Synthesiser Frequency Hopping (SFH)
The
radio units change (retune) the frequency every burst.The call always stays in
the same radio unit. Number of frequencies for hopping > Number of
carriers.The radio units can hop over a range of different frequencies( 64 in
case of Motorola).Hybrid combiners are required in the base station (Cavity
Combiners can not be used with SFH).The BCCH carrier can never hop.For
frequency hopping operability, GSM defines the following set of parameters:
Mobile
Allocation (MA):
Set of frequencies the mobile is allowed to hop over. MA is a subset of all the
frequencies allocated by the system operator to the cell (cell allocation)
although it can be the same. Eg:- If the operator has frequencies from 1 -32,
then he can use 1-15 for BCCH and 17-32 for hopping ( MA).
Hopping Sequence Number (HSN): Determines the hopping order used in the cell. 64 different HSNs can be assigned, where HSN = 0 provides a cyclic hopping sequence and HSN = 1 to 63 provide various pseudorandom hopping sequences.
Mobile Allocation Index Offset (MAIO): Determines inside the hopping sequence which frequency the mobile starts to transmit on.
Frequency Hopping Indicator (FHI): Defines a hopping system made up by an associated set of frequencies (MA) to hop over and a hopping sequence (HSN).
Hopping Sequence Number (HSN): Determines the hopping order used in the cell. 64 different HSNs can be assigned, where HSN = 0 provides a cyclic hopping sequence and HSN = 1 to 63 provide various pseudorandom hopping sequences.
Mobile Allocation Index Offset (MAIO): Determines inside the hopping sequence which frequency the mobile starts to transmit on.
Frequency Hopping Indicator (FHI): Defines a hopping system made up by an associated set of frequencies (MA) to hop over and a hopping sequence (HSN).
RF PLANNING
INTRODUCTION
TO RF PLANNING
Designing a cellular system - particularly one that
incorporates both Macrocellular and Microcellular networks is a delicate
balancing exercise.The goal is to achieve optimum use of resources and maximum
revenue potential whilst maintaining a high level of system quality. Full
consideration must also be given to cost and spectrum allocation limitations.A
properly planned system should allow capacity to be added economically when
traffic demand increases.As every urban environment is different, so is every
macrocell and microcell network. Hence informed and accurate planning is
essential in order to ensure that the system will provide both the increased
capacity and the improvement in network quality where required, especially when
deploying Microcellular systems.RF planning plays a critical role in the Cellular design process.By doing a proper RF Planning by keeping the future
growth plan in mind we can reduce a lot of problems that we may encounter in
the future and also reduce substantially the cost of optimization.On the other
hand a poorly planned network not only leads to many Network problems , it also
increases the optimization costs and still may not ensure the desired quality.
TOOLS
USED FOR RF PLANNING
Network Planning Tool
CW Propagation Tool
Traffic Modeling Tool
Project Management Tool
Network Planning Tool
•
Planning tool is used to assist engineers in designing and optimizing
wireless networks by providing an accurate and reliable prediction of coverage, doing frequency planning automatically, creating neighbor lists etc.
•
With a database that takes into
account data such as terrain, clutter, and antenna radiation patterns, as well
as an intuitive graphical interface, the Planning tool gives RF engineers a
state-of-the-art tool to:
–
Design wireless networks– Plan network expansions
– Optimize network performance
– Diagnose system problems
• The major tools available in the market are Planet, Pegasos, Cell Cad.
• Also many vendors have developed Planning tools of their own like Netplan by Motorola, TEMS by Ericsson and so on.
Propagaton Test Kit The propagation test kit consists
of
–
Test transmitter.– Antenna ( generally Omni ).
– Receiver to scan the RSS (Received signal levels). The receiver scanning rate should be settable so that it satisfies Lee’s law.
– A laptop to collect data.
– A GPS to get latitude and longitude.
– Cables and accessories.
– Wattmeter to check VSWR.
• A single frequency is transmitted a predetermined power level from the canditate site.
• These transmitted power levels are then measured and collected by the Drive test kit. This data is then loaded on the Planning tool and used for tuning models.
• Commonly Graysons or CHASE prop test kits are used.
Traffic
Modeling Tool
•
Traffic modelling tool is
used by the planning engineer for Network modelling and dimensioning.• It helps the planning engineer to calculate the number of network elements needed to fulfil coverage, capacity and quality needs.
• Netdim by Nokia is an example of a Traffic modelling tool.
Project
Management Tool
•
Though not directly linked
to RF Design Planning, it helps in scheduling the RF Design process and also to
know the status of the project• Site database : This includes RF data, site acquisition,power, civil ,etc.
• Inventory Control
• Fault tracking
• Finance Management
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