Introduction
The main goal of the frequency-planning task is to increase the efficiency of the spectrum usage, keeping the interference in the network below some predefined level. Therefore it is always related to interference predictions. There are two basic approaches to solve the frequency assignment problem.
• Frequency reuse patterns
• Automatic frequency allocation
Some software’s are used with automatic frequency allocation algorithms for finding the optimum solutions. The frequency allocation is generally guided by the following information:
• Channel requirement on cell basis according to the capacity planning
• Channel spacing limitations according to BTS specification
• Quality of service requirement which is conserved to acceptable interference probability
• Traffic density distribution over the service area
• Performance of advanced system features (frequency hopping, IUO, etc….)
The frequency allocation is based on cell-to-cell interference probability estimation according to the network topology, field strength distribution and traffic load. This results in customized frequency performance of the selected radio network elements.
The starting point of automatic frequency allocation is much better, since the exact site coordinates and BTS characteristics are available. Usage of propagation model based on digital maps, we are able to obtain interference predictions very near to reality.
Frequency Reuse
A frequency used in one cell can be reused in another cell at a certain distance. This distance is called reuse distance. The advantage of digital system is that they can reuse frequencies more efficiently than the analogue ones, i.e. the reuse distance can be shorter, and the capacity increased. A cellular system is based in reuse of frequencies. All the available frequencies are divided into different frequency groups so that a certain frequency always belongs to a certain frequency group. The frequency groups together form a cluster.
“A cluster is an area in which all frequency groups are used once, but not reused.”
The frequencies can be divided into different frequency groups. This introduces the terms reuse patterns and reuse grids. The most common reuse patterns in GSM is “4/12” and “3/9”.
4/12 means that the available frequencies are divided into 12 frequency groups, which in turn are located at 4 base stations sites. This assumes that the base station has three cells connected to it. The frequency groups are often assigned a number or name such as A1, B1, C1, D1, A2,…….. D3.
3/9 means that the available frequencies are divided into 9 frequency groups located at 3 sites. Problem with C/A might appear in certain parts of a cell, arising from adjacent frequencies in neighboring cells.
Example: channel assignment of 24 frequencies in a 3/9-cell plan.
Interference calculations
The reference interference ratio is defined in GSM as the interference ratio for which the required performance in terms of frame erasure, bit error rate or residual bit error rate is met. The reference interference ratios for BS and all types MSs are the following:
• Co channel interference: C/Ic <= 9 dB
• First adjacent channel interference: C/Ia1 <= -9 dB
• Second adjacent channel interference: C/Ia2 <= -41 dB
Co channel interference
The carrier to interference (C/I) ratio at a given mobile receiver can be calculated as follows:
C/I = C / (I1 + I2 + …….. +Ik)
Where k is the number of co channel interfering cells. For regular grid case it is possible to simplify the calculations by using the popular path loss expressions
Time dispersion
Some interference effects may be caused from the reflected signals if received outside the equalizer window. This happens only when the difference between direct path and the reflection path is larger than the equalizer window (about 4.5 km) and the reflected signal is strong enough. The reflection outside the equalizer window should be regarded as an independent co channel interferer, therefore the same reference C/I <= 9 dB should be used.
Digital maps based co channel interface
From the coverage areas calculated by the help of digital maps it is quite easy to obtain the expected interference areas. Since the frequency plan is still to be done, the multiple interferences cannot be calculated. Thus the process works for every pair of BS checking the ratio between the two-signal pixels. The probability of future multiple interference can be reduced by adding some margin, say 6 dB to the reference interference ratio. If the percent of the interfered area is larger than a given predefined level (depending on the required service quality), the pair cannot operate in the same channel. The results are presented as a matrix with elements giving the minimum slowed channel difference (in this case only 0 and 1) for every pair of BSs.
Frequency hopping
Frequency hopping (FH) is changing the frequency of information signal according to a certain sequence. The transmission frequency may change at each time slot or burst and remains constant during the transmission of a burst.
FH can also decrease the overall C/I value in the network and thus improve the Quality Of Service (QOS).
Frequency hopping behavior:
Lognormal fading and Rayleigh distributed fast fading can decrease the speech quality. Rayleigh fades are the sum of a lot of reflected and phase shifted signals.
The fading at different frequencies is not the same and become more and more independent when the difference in frequency increases. With frequencies spaced sufficiently apart they can be considered completely independent (no correlation). Thus Rayleigh fading does then not damage all the bursts containing the parts of one code word in the same way.
When the ms moves of high speed the difference between its positions during the reception of two successive bursts of the same channel (i.e. at least 4,615 ms) is sufficient to decorrelate Rayleigh fading variations on the signal. In this case FH does not help except if there is interference.
The worst case is when ms is stationary or moves at slow speeds because the interleaved coding does not bring any benefit to reception. In this case FH “simulates ms movement” and thus the reception quality. This phenomenon is called frequency diversity.
In the other hand frequency hopping averages the interference directed towards each base station. Instead of a continuous interferer there are several interferers that affect only a short time each and with different intensity. Methods like power control and DTX (discontinuous transmission) affect only a single interference source and benefits can be distributed to the whole network by using FH. The gain, which comes from interference averaging, is called interference diversity.
Baseband hopping
Baseband hopping occurs between TRXs in BTS. The number of frequencies used in the hopping sequence is the same as the number of TRXs in the sector. Both random and cyclic hopping can be used.
The digital (baseband) and analogue (RF) parts of the TRX are separated from each other. The switching of TRXs is on a per timeslot basis ands enables a particular TCH to hop from one carrier to another.
Synthesized hopping
Synthesized hopping is available in configurations, which have at least 2 TRX per sector. It enables each TRX to change frequency on successive time slots, so that given carrier can hop quickly onto many different frequencies. The carrier on which the BCCH IS transmitted must remain at fixed frequency to enable the MS to measure correct signal strength. Both random and cyclic hopping can be used.
Discontinuous transmission (DTX)
The transmission is disconnected when no information flow happens in signal. This is done by lower speech encoding bit rate than when the user is effectively speaking. This low rate flow is sufficient to encode the background noise, which is generated for the listener to avoid him thinking that the connection is broken. The low rate encoding corresponds to a decreased effective radio transmission of one frame each 20 ms to one such frame each 480 ms. Typically transmission is effective 60% of the time, which decreases the interference.
In order to implement such a mechanism, the source must be able to indicate when transmission is required or not. In the case of speech, the coder must detect weather or not there is some vocal activity. This function is called Voice Activity Detection (VAD). At the reception side, the listener’s ear must not be disturbed by the sudden disappearance of noise and the decoder must therefore be able generate some Comfort noise when no signal is received.
DTX is an option controlled by the operator, and which may be used independently in the MS to BTS and in BTS to MS.
The main goal of the frequency-planning task is to increase the efficiency of the spectrum usage, keeping the interference in the network below some predefined level. Therefore it is always related to interference predictions. There are two basic approaches to solve the frequency assignment problem.
• Frequency reuse patterns
• Automatic frequency allocation
Some software’s are used with automatic frequency allocation algorithms for finding the optimum solutions. The frequency allocation is generally guided by the following information:
• Channel requirement on cell basis according to the capacity planning
• Channel spacing limitations according to BTS specification
• Quality of service requirement which is conserved to acceptable interference probability
• Traffic density distribution over the service area
• Performance of advanced system features (frequency hopping, IUO, etc….)
The frequency allocation is based on cell-to-cell interference probability estimation according to the network topology, field strength distribution and traffic load. This results in customized frequency performance of the selected radio network elements.
The starting point of automatic frequency allocation is much better, since the exact site coordinates and BTS characteristics are available. Usage of propagation model based on digital maps, we are able to obtain interference predictions very near to reality.
Frequency Reuse
A frequency used in one cell can be reused in another cell at a certain distance. This distance is called reuse distance. The advantage of digital system is that they can reuse frequencies more efficiently than the analogue ones, i.e. the reuse distance can be shorter, and the capacity increased. A cellular system is based in reuse of frequencies. All the available frequencies are divided into different frequency groups so that a certain frequency always belongs to a certain frequency group. The frequency groups together form a cluster.
“A cluster is an area in which all frequency groups are used once, but not reused.”
The frequencies can be divided into different frequency groups. This introduces the terms reuse patterns and reuse grids. The most common reuse patterns in GSM is “4/12” and “3/9”.
4/12 means that the available frequencies are divided into 12 frequency groups, which in turn are located at 4 base stations sites. This assumes that the base station has three cells connected to it. The frequency groups are often assigned a number or name such as A1, B1, C1, D1, A2,…….. D3.
3/9 means that the available frequencies are divided into 9 frequency groups located at 3 sites. Problem with C/A might appear in certain parts of a cell, arising from adjacent frequencies in neighboring cells.
Example: channel assignment of 24 frequencies in a 3/9-cell plan.
Interference calculations
The reference interference ratio is defined in GSM as the interference ratio for which the required performance in terms of frame erasure, bit error rate or residual bit error rate is met. The reference interference ratios for BS and all types MSs are the following:
• Co channel interference: C/Ic <= 9 dB
• First adjacent channel interference: C/Ia1 <= -9 dB
• Second adjacent channel interference: C/Ia2 <= -41 dB
Co channel interference
The carrier to interference (C/I) ratio at a given mobile receiver can be calculated as follows:
C/I = C / (I1 + I2 + …….. +Ik)
Where k is the number of co channel interfering cells. For regular grid case it is possible to simplify the calculations by using the popular path loss expressions
Time dispersion
Some interference effects may be caused from the reflected signals if received outside the equalizer window. This happens only when the difference between direct path and the reflection path is larger than the equalizer window (about 4.5 km) and the reflected signal is strong enough. The reflection outside the equalizer window should be regarded as an independent co channel interferer, therefore the same reference C/I <= 9 dB should be used.
Digital maps based co channel interface
From the coverage areas calculated by the help of digital maps it is quite easy to obtain the expected interference areas. Since the frequency plan is still to be done, the multiple interferences cannot be calculated. Thus the process works for every pair of BS checking the ratio between the two-signal pixels. The probability of future multiple interference can be reduced by adding some margin, say 6 dB to the reference interference ratio. If the percent of the interfered area is larger than a given predefined level (depending on the required service quality), the pair cannot operate in the same channel. The results are presented as a matrix with elements giving the minimum slowed channel difference (in this case only 0 and 1) for every pair of BSs.
Frequency hopping
Frequency hopping (FH) is changing the frequency of information signal according to a certain sequence. The transmission frequency may change at each time slot or burst and remains constant during the transmission of a burst.
FH can also decrease the overall C/I value in the network and thus improve the Quality Of Service (QOS).
Frequency hopping behavior:
Lognormal fading and Rayleigh distributed fast fading can decrease the speech quality. Rayleigh fades are the sum of a lot of reflected and phase shifted signals.
The fading at different frequencies is not the same and become more and more independent when the difference in frequency increases. With frequencies spaced sufficiently apart they can be considered completely independent (no correlation). Thus Rayleigh fading does then not damage all the bursts containing the parts of one code word in the same way.
When the ms moves of high speed the difference between its positions during the reception of two successive bursts of the same channel (i.e. at least 4,615 ms) is sufficient to decorrelate Rayleigh fading variations on the signal. In this case FH does not help except if there is interference.
The worst case is when ms is stationary or moves at slow speeds because the interleaved coding does not bring any benefit to reception. In this case FH “simulates ms movement” and thus the reception quality. This phenomenon is called frequency diversity.
In the other hand frequency hopping averages the interference directed towards each base station. Instead of a continuous interferer there are several interferers that affect only a short time each and with different intensity. Methods like power control and DTX (discontinuous transmission) affect only a single interference source and benefits can be distributed to the whole network by using FH. The gain, which comes from interference averaging, is called interference diversity.
Baseband hopping
Baseband hopping occurs between TRXs in BTS. The number of frequencies used in the hopping sequence is the same as the number of TRXs in the sector. Both random and cyclic hopping can be used.
The digital (baseband) and analogue (RF) parts of the TRX are separated from each other. The switching of TRXs is on a per timeslot basis ands enables a particular TCH to hop from one carrier to another.
Synthesized hopping
Synthesized hopping is available in configurations, which have at least 2 TRX per sector. It enables each TRX to change frequency on successive time slots, so that given carrier can hop quickly onto many different frequencies. The carrier on which the BCCH IS transmitted must remain at fixed frequency to enable the MS to measure correct signal strength. Both random and cyclic hopping can be used.
Discontinuous transmission (DTX)
The transmission is disconnected when no information flow happens in signal. This is done by lower speech encoding bit rate than when the user is effectively speaking. This low rate flow is sufficient to encode the background noise, which is generated for the listener to avoid him thinking that the connection is broken. The low rate encoding corresponds to a decreased effective radio transmission of one frame each 20 ms to one such frame each 480 ms. Typically transmission is effective 60% of the time, which decreases the interference.
In order to implement such a mechanism, the source must be able to indicate when transmission is required or not. In the case of speech, the coder must detect weather or not there is some vocal activity. This function is called Voice Activity Detection (VAD). At the reception side, the listener’s ear must not be disturbed by the sudden disappearance of noise and the decoder must therefore be able generate some Comfort noise when no signal is received.
DTX is an option controlled by the operator, and which may be used independently in the MS to BTS and in BTS to MS.
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