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GUIDELINE for 2.4GHz Cell Station allocation

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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


over 15ft

 
 

 

Simultaneous calls

3

 

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.

 
Conditions of overlapping cells

Example1


Example2

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