During a call, signals may be interfered when being transmitted over the Um interface. Therefore, some data may be lost and the transmission quality deteriorates. To resolve this problem, the FH feature is introduced into GSM networks. With this feature, different bursts
are transmitted on different frequencies but the frequency for each burst remains unchanged. An MS changes frequencies once every Time Division Multiple Access (TDMA) frame, that is, 217 times per second. Below Figure shows the schematic drawing of FH. In the figure, f0, f1, f2, f3, and f4 indicate different frequencies; the shadowed blocks in different colors indicate speech signals of different calls.
The FH feature is applicable to TCHs, SDCCHs, and PDCHs but not applicable to BCCHs, extended BCCHs, or other common control channels.
lists are allowed in each cell, and one MA list contains a maximum of 64 frequencies. Different timeslots on the same transceiver (TRX) can use the same or different MA lists. The BSC supports three FH modes: baseband frequency hopping (BFH), radio frequency (RF) FH, and synthesized frequency hopping (SFH).
In BFH, each TRX is assigned a fixed frequency. During transmission, bursts are routed to the TRXs of the corresponding frequencies through the bus. This way, FH requires the cooperation of multiple TRXs. A single TRX cannot implement BFH. The number of frequencies available for FH cannot exceed the number of hopping TRXs. On the BCCH TRX, the timeslots other than the BCCH and extended BCCH timeslots can participate in FH to achieve FH gains. Once a TRX is faulty, all the calls that use the MA lists containing the faulty TRX are affected.
In RF FH, all the bursts of a call are transmitted on the same TRX, and the TRX determines the frequency to transmit a specific burst according to the hopping sequence number (HSN). Only one TRX is required to implement FH. In addition, the number of frequencies participating in FH is not dependent on the number of TRXs and can be greater than the number of TRXs. Each TRX can be configured to hop over a large number of frequencies to obtain the maximum FH gain. The BCCH TRX, however, cannot participate in FH.
SFH refers to an FH mode in which some TRXs adopt RF FH and other TRXs in the same cell adopt BFH. In most cases, the underlaid subcell of a co-BCCH cell works on a low frequency band (for example, the GSM900 or GSM850 band) and it is assigned a small number of frequencies; the overlaid subcell of a co-BCCH cell works on a high frequency band (for example, the DCS1800 or PCS1900 band) and it is assigned a large number of frequencies. In addition, a tight frequency reuse pattern is generally applied in the overlaid subcell, and therefore the interference is strong. In a co-BCCH cell, the TRXs in the overlaid subcell adopt RF FH, and a large number of frequencies participate in FH to obtain a great FH gain; the TRXs in the underlaid subcell adopt BFH so that a small number of frequencies can also implement FH.
FH achieves the same effects as frequency diversity and interference diversity.
In mobile telecommunications, radio signals during transmission may vary greatly in a short period due to the impact of Rayleigh fading. This variation is frequency dependent. If the spacing between two frequencies increases, the correlation between frequencies reduces, and therefore the attenuation properties become more independent. In a mobile telecommunications system, the frequency spacing of 200 kHz ensures that the attenuation properties of different frequencies are uncorrelated in most cases, and the frequency spacing of 1 MHz completely ensures that the attenuation properties of different frequencies are uncorrelated. With FH, frequencies for transmitting bursts vary from time to time. This prevents Rayleigh fading from damaging all the bursts of the same MS in the same manner and therefore improves the anti-attenuation capability of radio signals. In this sense, FH provides the same functions as frequency diversity.
When a fast-moving MS receives two consecutive bursts on the same channel, the location change of the MS is enough to eliminate the correlation of Rayleigh fading. In this case, FH does not bring great gains. For the non-moving or slow-moving MSs in a mobile network, FH is necessary and it brings about a 6.5 dB gain.
GSM is a frequency-limited system. In an area with heavy traffic, the network capacity is
limited because of the interference caused by frequency reuse. In a GSM network, the carrier-to-interference (C/I) ratio may vary greatly from call to call in different frequency reuse
patterns. The expected signal level varies with the distance and obstacle factors between MSs
and BTSs. Interference level depends to a great extent on the co- and adjacent-channel
interference in neighboring cells.
A system aims to allow as many MSs as possible to gain access. Without FH, if interference occurs on a frequency, the speech quality of MSs occupying channels on the frequency may deteriorate. With FH, interference on the frequency is spread across many MSs, that is, the interference is averaged. This improves the network performance. If the number of frequencies participating in FH is great, the impact of interference on a specific call is small, and the anti-interference capability of the network becomes strong. Interference diversity helps overcome co- and adjacent-channel interference. Therefore, when FH is enabled, a tighter frequency reuse pattern can be adopted to increase system capacity.
When a PS call uses a high-rate coding scheme, such as CS3, CS4, or MCS5 to MCS9, FH
negatively affects the call and deteriorates network performance. Therefore, do not enable FH
in such situations.
 |
| Schematic drawing of FH |
The FH feature is applicable to TCHs, SDCCHs, and PDCHs but not applicable to BCCHs, extended BCCHs, or other common control channels.
lists are allowed in each cell, and one MA list contains a maximum of 64 frequencies. Different timeslots on the same transceiver (TRX) can use the same or different MA lists. The BSC supports three FH modes: baseband frequency hopping (BFH), radio frequency (RF) FH, and synthesized frequency hopping (SFH).
In BFH, each TRX is assigned a fixed frequency. During transmission, bursts are routed to the TRXs of the corresponding frequencies through the bus. This way, FH requires the cooperation of multiple TRXs. A single TRX cannot implement BFH. The number of frequencies available for FH cannot exceed the number of hopping TRXs. On the BCCH TRX, the timeslots other than the BCCH and extended BCCH timeslots can participate in FH to achieve FH gains. Once a TRX is faulty, all the calls that use the MA lists containing the faulty TRX are affected.
In RF FH, all the bursts of a call are transmitted on the same TRX, and the TRX determines the frequency to transmit a specific burst according to the hopping sequence number (HSN). Only one TRX is required to implement FH. In addition, the number of frequencies participating in FH is not dependent on the number of TRXs and can be greater than the number of TRXs. Each TRX can be configured to hop over a large number of frequencies to obtain the maximum FH gain. The BCCH TRX, however, cannot participate in FH.
SFH refers to an FH mode in which some TRXs adopt RF FH and other TRXs in the same cell adopt BFH. In most cases, the underlaid subcell of a co-BCCH cell works on a low frequency band (for example, the GSM900 or GSM850 band) and it is assigned a small number of frequencies; the overlaid subcell of a co-BCCH cell works on a high frequency band (for example, the DCS1800 or PCS1900 band) and it is assigned a large number of frequencies. In addition, a tight frequency reuse pattern is generally applied in the overlaid subcell, and therefore the interference is strong. In a co-BCCH cell, the TRXs in the overlaid subcell adopt RF FH, and a large number of frequencies participate in FH to obtain a great FH gain; the TRXs in the underlaid subcell adopt BFH so that a small number of frequencies can also implement FH.
FH achieves the same effects as frequency diversity and interference diversity.
- Frequency diversity helps improve the anti-attenuation capability and speech quality of the entire system.
- Interference diversity helps overcome co- and adjacent-channel interference. Therefore, when FH is enabled, a tighter frequency reuse pattern can be adopted to increase system capacity.
Also read Technique Frequency Hopping in GSM Network
FH Functions in Terms of Frequency Diversity
In mobile telecommunications, radio signals during transmission may vary greatly in a short period due to the impact of Rayleigh fading. This variation is frequency dependent. If the spacing between two frequencies increases, the correlation between frequencies reduces, and therefore the attenuation properties become more independent. In a mobile telecommunications system, the frequency spacing of 200 kHz ensures that the attenuation properties of different frequencies are uncorrelated in most cases, and the frequency spacing of 1 MHz completely ensures that the attenuation properties of different frequencies are uncorrelated. With FH, frequencies for transmitting bursts vary from time to time. This prevents Rayleigh fading from damaging all the bursts of the same MS in the same manner and therefore improves the anti-attenuation capability of radio signals. In this sense, FH provides the same functions as frequency diversity.
When a fast-moving MS receives two consecutive bursts on the same channel, the location change of the MS is enough to eliminate the correlation of Rayleigh fading. In this case, FH does not bring great gains. For the non-moving or slow-moving MSs in a mobile network, FH is necessary and it brings about a 6.5 dB gain.
FH Functions in Terms of Interference Diversity
A system aims to allow as many MSs as possible to gain access. Without FH, if interference occurs on a frequency, the speech quality of MSs occupying channels on the frequency may deteriorate. With FH, interference on the frequency is spread across many MSs, that is, the interference is averaged. This improves the network performance. If the number of frequencies participating in FH is great, the impact of interference on a specific call is small, and the anti-interference capability of the network becomes strong. Interference diversity helps overcome co- and adjacent-channel interference. Therefore, when FH is enabled, a tighter frequency reuse pattern can be adopted to increase system capacity.
When a PS call uses a high-rate coding scheme, such as CS3, CS4, or MCS5 to MCS9, FH
negatively affects the call and deteriorates network performance. Therefore, do not enable FH
in such situations.
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