frquency planning guideline
TRANSCRIPT
7/27/2019 Frquency Planning Guideline
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Frequency Planning
Guideline
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Contents Frequency Planning Guideline ........................................................................................... 3
1. Frequency Multiplexing Technology .......................................................................... 3 2. Frequency Planning Principle ..................................................................................... 3 3. 4 x 3 Multiplexing Technology .................................................................................... 4 4. Multiple Multiplexing Pattern (MRP) .......................................................................... 5 5. Frequency Hopping (FH) ............................................................................................. 6
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Frequency Planning Guideline
1. Frequency Multip lexing Technology
Frequency multiplexing is a kind of technology commonly used in a GSM network, by which
the same frequency is applied to cover different areas. In addition, there will be a certain
distance, called co-frequency multiplexing distance between these areas using the same
frequency.
If directional antenna is used, it is recommended to adopt 4 x 3 multiplexing method. In certain
areas with heavier traffic, other multiplexing methods can be adopted according to machine
capability, such as 3 x 3 and 2 x 6. No matter which method is adopted, its basic principle is to
meet the requirements of interference protection ratio with a view to different propagation
conditions, different multiplexing methods, and multiple interference factors. They are shown
as follows:
1. Co-frequency protection ratio C/I ≥ 9 dB
2. Adjacent frequency interference protection ratio C/I ≥ -9 dB
3. 400 kHz adjacent frequency interference protection ratio C/I ≥ -41 dB
2. Frequency Planning Principle
When performing frequency planning, frequency distribution is usually achieved according to
geographic areas. Several frequency channels should be reserved for the border area, which
should be kept away from traffic hot spot or complex organizing network area. Usually
planning starts from area densely distributed with sites. For example, planning from the city’s
downtown area, to suburban area with low TRX configuration site (usually choose S111or O1
as border area). Pay attention to the situation where there are lakes or big rivers in the city. In
that case, interference by water surface reflection should be avoided. Due to irregularity of the
sites distribution, it’s hard to guarantee full use of conventional mode such as 4 x 3 or 3 x 3 in
TRX frequency within the same layer, and flexible adjustment needs to be made on basis of
actual situation.
The following rules about frequency planning need to be considered:
1. No co-located channels or adjacent channels should be distributed to one BTS site.
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2. It would be better that the frequency interval between BCCH and TCH in one single cell be
more than 400 KHz.
3. When no hopping technology is adopted, it would be better that the TCH frequency
interval in one single cell be more than 400 KHz.
4. Adjacent sites should be avoided to be configured with co-located channel. (Even though
the antenna’s main-lobes are different, strong noise will be caused by side-lobes)
5. Considering antenna height and complex wireless environment, co-located channels and
adjacent channels should be kept away from BTS sites with low inter-site distance.
6. When 1*3 reuse mode is adopted, the number of hopping frequency channels should be
configured at least twice as the hopping TRXs numbers.
7. Focus on co-channel reuse, and avoid configuring the same BCCH and BSIC in close
region.
3. 4 x 3 Multip lexing Technology
There are a variety of frequency multiplexing structures used in GSM, such as 4 x 3, 3 x 3,
and 2 x 6. Usually, regardless of the multiplexing methods adopted, limited frequencies are to
be classified into a certain number of groups. In sequence these groups will form a cluster of
frequency and be assigned to adjacent cell (shown in the following figure). According to GSM
system criteria, 4 x 3 is commonly used in various GSM networks. By this multiplexing method,
frequencies are divided into 12 groups which are assigned to 4 sites in turn. That means 3
frequency groups can be used in each station. As a result of long multiplexing distance
brought about by this frequency multiplexing method, the requirement of co-frequency
protection ratio and adjacent frequency interference protection ratio required by GSM system
can be met reliably. Therefore, it makes GSM network run in fine quality and good security.
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By this method, the full band of frequency is divided into two mutually orthogonal bands, that is,
BCCH frequency band and TCH frequency band, which are to be planned by different
multiplexing methods respectively. One method to improve system capacity is to use closer
multiplexing method. BCCH channel plays a decisive role in the process of mobile station
access and switching. Therefore, by using the frequency orthogonal to TCH frequency band,
the following benefits can be enjoyed, which will ensure the quality of BCCH channel :
1. BCCH can use 4 x 3 or higher multiplexing coefficient to ensure the quality of BCCH
channel, while TCH uses relatively close multiplexing method.
2. Separation between each layer of BCCH and TCH frequency band reduces planning
workload. Therefore, planning by layer is feasible. In addition, a section of frequency may
be reserved for micro cell.
3. BSIC decoding has nothing to do with voice channel load. BCCH frequency band and
TCH frequency band are mutually orthogonal. Therefore, the increase in TCH channel
load has little influence on BCCH channel. In addition, it does not have any impact on
BSIC decoding, and thereby improving switching performance.
4. Simplify the configuration of adjacent cell list. Some documents indicate that long
adjacent cell list will weaken switching performance, while this method can simplify
adjacent cell list, and thereby improving switching performance.
5. BCCH independently uses a segment of frequency (12 frequency points in 4 x 3 method),
and thus the adjacent cell list (composed of BCCH frequency points) can be greatly
shortened.
6. Give full play to immunity technologies to interference, such as power control and DTX.
BCCH cannot use dynamic power control and DTX and it has been transmitting signal in
the highest transmission power. Therefore, BCCH and TCH will influence the effect of
these anti-interference technologies by using the same frequency band.
7. Each layer in BCCH and TCH is comparatively independent, which is conducive to
maintenance and expansion by layer. Increasing or deleting sites or TRX in cells will not
impact pre-existing BCCH frequency planning and thus facilitating network maintenance.
5. Frequency Hopping (FH)
Frequency hopping is one kind of spread spectrum communication. It is applied to cell mobile
communication system to improve system’s countering capacity against multipath fading. In
addition, it can curb co-frequency interference on communication quality. Especially,
nowadays when spectrum resource is more and more insufficient, frequency hopping
becomes one of the most effective methods to improve spectrum utilization rate.
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GSM supports baseband frequency hopping and RF frequency hopping (also called
Synthesized Frequency Hopping). Baseband frequency hopping means that many transmitters
work on their own fixed frequency points, while on baseband signals from different channels
switched to different transmitters are sent according to frequency sequence. Baseband
frequency hopping can be easily realized. However, there are only a few frequency hopping
points as a result of the limited TRX. Frequency hopping artificial system established by ZTE
mainly supports RF frequency hopping. Baseband frequency hopping is only regarded as an
exception to RF frequency hopping (that is, the number of frequency points equals to that of
TRXs). The advantage brought by frequency hopping is mainly about the effect of Frequency
Diversity and Interference Diversity. Frequency diversity actually improves network coverage
scale, and Interference diversity increases network capacity.
The number of available frequency hopping in baseband frequency hopping is equal to that of
TRX. Therefore, it can only bring frequency diversity gain. However, now GSM operators are
more concerned about capacity as coverage is no longer a problem in most cities. RF
frequency hopping is a very effective method to solve capacity problem.
RF frequency hopping is more and more frequently applied to network planning. Frequency
diversity means its immunity ability to Rayleigh fading. Since Rayleigh fading on different
carriers is definitely irrelevance (the more frequency differential is, the less irrelevance is),
then burst distributed on different carriers will not be influenced by the same Rayleigh fading. It
counts a lot to the still and moving-at-low-speed mobile station. It is supposed to provide a
gain value of 6.5dB. However, MS moving at high speed and two successive burst in the same
channel are different in terms of timing position, which means that they are irrelevant to
Rayleigh fading. They are seldom influenced by the fading when frequency diversity provided
by low-speed frequency hopping is very little.
Under the condition that MS moves at a high speed, frequency points assigned to cells have
little impact on frequency hopping performance. While under the condition without frequency
hopping, there is frequency diversity gain of about 1 dB to 2dB. When MS moves at a low
speed (TU3), because of frequency diversity effect, the number of frequency points assigned
has significant influence on system performance. Frequency points increased by a time will
obtain gain value of about 0.2~1dB, and load rate can be increased by 10% or so.
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Interference diversity means that capability of interfering signals in other co-frequency
multiplexing cell will be curbed, that is, frequency hopping and interfering differential on the
transmission path will be provided in order to improve interference under the harshest
conditions. It enables all users to enjoy good communication quality, which is very important
for the mobile communication system with lots of users, especially critical to increase
communication traffic through augmenting frequency multiplexing rate. Usually interference
diversity effect should be provided, and the number of frequency hopping points should be no
less than 3.
If co-frequency point is 10 MHz, frequency hopping planning and capacity analysis without
frequency hopping are as follows:
The multiplexing method of BCCH is 4X3, and the multiplexing method of traffic channel is
3X3. 10 MHz has 50 frequency points. Except 1 protection frequency point and 12 BCCH
frequency points, there are 37 frequency points left. Thus, each cell can be assigned with 4
traffic frequency points ((37-1)/9), and only one frequency point is left. Then its maximum
configuration should be 5+5+5. Each cell can provide 37 channels
(1BCCH+2SDCCH+37TCH).
When RF frequency hopping technology is adopted, traffic channel can adopt 1X3 multiplexing.
In case of 50% load, each cell can provide 6 service logic frequency points. The reason why it
is called logic frequency point is that they all adopt the same 12 frequency hopping collection
((37-1)/3). Only HSN is different from MAIO, and one frequency point is left. The maximum
configuration becomes 7+7+7. It can provide 53 service traffic channels
(1BCCH+2SDCCH+53TCH) with an increase on capacity by 43%. At this time the C/I value in
more than 90% of areas will be 9dB. When DTX and distinctive ZTE fast power control
algorithm are adopted at the same time, system capacity can be improved much better. If
intelligence traffic control technology is adopted, GSM can acquire soft capacity, and gain
more system capacity by sacrificing certain voice quality in hot traffic areas.