GUIDELINE for 2.4GHz Cell Station
allocation
1.Background
TDA wireless system adopts non-license Frequency
Hopping method to use 2.4GHz ISM band. To meet FCC requirement FH system has
unavoidable interference.
In FH system interference between CSs depends on
location of CS and PS. This needs to be taken into consideration for site planning.
This document is a guideline for site planning.
2.Technical explanation of FH system
Frame structure of TDA
wireless system
TDA wireless system adopts frame structure as
follows. Every frame has 4 downlinks (CS to PS link) slots and 4 uplinks (PS to
CS link) slots. The frame period is 10 m second. Voice signal during 10 m
second is compressed and transmitted on a slot occupied for the communication.
Frame timings of CSs are synchronized for realizing handover.
Frequency hopping system
TDA wireless system adopts Frequency Hopping method
to use 2.4GHz ISM band. FH method has to meet FCC part 15 requirement. To meet
FCC requirement FH system has unavoidable interference.
In FH system CS and PS select and use one radio
frequency among 75 frequencies every frame (10m second). The order of changing
frequency is defined as FH pattern. FH pattern is a predefined pseudo-random frequency
pattern with 75 frequencies period.
Each CS selects and uses FH pattern independently
according to FCC rule. So each CS possibly uses same frequency at the same
time. (Refer to Fig.2.)
There are possibilities of interference when CSs use
same frequency, adjacent frequencies or next adjacent frequencies. This depends
on location of CS and PS. Neighboring CS can be an interference source for the
other CS in FH system.
Mechanism of Interference
There are two kinds of channels. One is a traffic
channel and the other is a control channel.
A traffic channel is established only when PS links
CS for voice communication. A control channel keeps on broadcasting control
data for standby PSs even if during idle state of CS.
PS communicates with CS1 in slot 1 using FH pattern1.
(Refer to Fig.3.) And neighboring CS2 transmits control channel in slot1and
slot3. (Each CS has two control channels for robustness of the system.) When PS
moves closely to CS2 keeping communication with CS1, Control channel from CS2
may interfere with PS depending on FH patterns. Because each CS1 and CS2 are
synchronized and PS traffic channel and CS2 control channel occupies same slot.
And desired signal power from CS2 is much higher than that from CS1 in this
case. If CS2 does not use slot1 for traffic channel and control channel, no
interference occurs.
Coordinating FH
pattern was an idea to reduce the probability of frequency collision for each
CS. But the coordination of FH patterns in each CS is prohibited by FCC part 15
rule. Coordination means coordinating FH pattern and
FH timing for CSs by PBX in this case.
So
we could not take this approach. Unfortunately we have no other choice than
accepting frequency collision in our system.
When interference occurs, radio data errors and
causes muted voice. This muted voice may be detected as noise in some cases.
Influence of interference is subject to CS and PS location. When each cell is
located apart enough, there is no interference detected. The deeper each cell
overlaps, the wider area PS talker may hear muted voice. When each cell is
located apart enough, PS can take handover to nearer CS with ease. This
drastically reduces the possibilities of interference. Moreover muted voice is
not detectable in case of usual conversation. Muted voice is detectable In case
of continuous tone such as dial tone. Therefore interference between CS does
not impact actual use with careful site planning. Influence of interference is
also subject to the structure and material of building. Therefore it is not
easy to express in general. Please refer to 4.Allocation condition of CS for
the guideline of your site planning.
3. Influence of interference
(1) Dependence on
the desired to undesired signal ratio
Noise is subject to the desired to undesired signal
ratio (D/U).
Radio
signal level deeply depends on location of PS and CS. Please refer to Fig.4. PS
communicates with CS1. Radio signal from CS1 is desired signal for PS. On the
other hand Radio signal from CS2 is undesired signal for PS as this may
interfere to the communication between PS and CS1. When the desired to
undesired signal ratio (D/U) is high enough, no muted voice occurs even though frequency
collides. The influence of collision is negligible in this case. The further PS
moves from CS that PS communicates with, the worse interference effects.
When PS stays nearby CS that the PS does not
communicate with, muted voice is detectable. Not only frequency collision but
also adjacent frequencies influence in this case. This case should be avoided
by taking handover to nearest CS.
Fig.5 shows the
example of radio attenuation under ideal propagation condition. The influence
of interference is remarkable near undesired CS. Therefore handover is
effective to reduce the influence of interference.
In Under usual
situation muted voice is not detected easily for the following reasons.
A. Desired signal is strong enough to undesired signal
as PS links to strongest CS. Influence of interference signal is negligible.
B. When desired signal goes weak, PS takes handover to
the strongest CS before undesired signal causes muted voice.
Therefore
condition A is kept. No muted voice is detected.
(2)
3 kinds of interferences
There are 3 kinds of
interference depending on the relative position of the frequencies,
co-channel interference (frequency
collision), adjacent frequencies interference and next adjacent frequencies
interference.
Co-channel interference
(frequency collision) is caused when desired signal and undesired signal uses
same frequency. Adjacent frequencies interference is caused when desired signal
and undesired signal uses adjacent frequencies. And next adjacent frequencies
interference is explained in a similar way.
The influence of these
interferences significantly depends on D/U. Please refer to Fig.6. Radio signal has side lobes and side lobe
drops into adjacent frequencies. Side lobe is lower than main lobe, but it can
interfere adjacent frequencies when D/U is extremely low such as 30dB.
Co-channel interference is
seen in wider area than other 2 interferences, but its influence is negligible.
Next adjacent interference is seen only in limited area, but its influence is
not negligible when interference occurs. Please remember interference does not
occur always.
The
table below is a summary of 3 kinds of interference.
|
Interference
|
Critical point of D/U
(dB)
|
Influence on conversation
|
|
Co-channel
(Frequency collision)
|
9
|
Not noticeable
|
|
Adjacent frequencies
|
-20
|
Almost not noticeable
|
|
Next adjacent frequencies
|
-25
|
Noticeable
|
These
values are IDEAL value. So this should be used only for reference purpose only.
(3)
Effect of CS number
The more CS overlaps, the
higher the possibility of frequency collision becomes.
When more than 2 cells are overlapped, the
influence and possibility of interference increases in proportion to number of
overlapped cells. From our experience we recommend to limit the number of
overlap cells. More than 3 cells should not be overlapped for the quality of conversation.
4.Allocation
condition of CS
As described above
allocation of CSs is very important to reduce muted voice in FH system. Here is
the guideline of CS allocation. This does not guarantee the best performance,
but this will be helpful for installation. In this section cell is defined as
level 8 area. And level is measured in PS site survey mode.
(1) Usual allocation
|
|
|
|
|
|
|
|
|
 |
|
|
|
Handover area (overlapped over 15 ft)
Allocate CSs as PS can take handover at level 8.
|
|
(2) 
Adjacent allocation
|
|
Coverage area
is minimized,
But 6
simultaneous calls are available.
2 CSs need
to be set apart over 50ft.
CAUTION
When CS is busy,
communication in the
busy CS may experience muted voice.
Adding another CS is
effective to reduce
the possibility of muted voice.
Please refer to (4) Conditions
of Overlapping cells for the details.
|
|
Distance 50ft means the
distance PS can take handover. Each CS needs to be set apart as handover works
smoothly. It is required minimum about 50ft from CS to CS to take handover
smoothly in open space. After handover PS can receive stronger signal from new
CS.
(3)
This is a combination of
(1) and (2).
Adding CS improves
traffic condition
and this can reduce the possibility
of muted voice.
|
|
Additional CS allocation
(4)
The more CS overlaps,
the higher the possibility of frequency collision becomes. Then muted voice
becomes detectable. Therefore there are conditions for overlapping cells as
follows.
<CONDITIONS>
1.
3
cells can be overlapped. More than 3 cells should NOT be overlapped. This
may cause heavy muted noise.
2.
Each
CS needs to be set apart at least 50 ft.
3.
3
cells should overlap with small overlap such as 15ft width.
Please refer to two
samples of CS allocation under this condition.
3 cells are overlapped
in marked areas. The distances from 3 CSs are comparable in these marked
areas. The signal strengths from each CS are well balanced overall these
overlapped areas. This is good for reducing influence of interference.
When 3 CS areas are more
deeply overlapped, signal strength from 3 CSs is not balanced well. This
raises the possibility of muted voice.
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|
Conditions of overlapping
cells
Example1
Example2
