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Reason for drop of GSM voice quality in congested network

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Jack// ani

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Hi all,

GSM network have fixed number of voice channel, only a single user utilize this voice channel.....unlike CDMA which have no such issue. Then why is there serious drop is voice quality when network is congested!! CDMA really have better voice quality in this context.....

Thanks
 

steer

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Re: GSM voice quality

Jack// ani said:
GSM network have fixed number of voice channel, only a single user utilize this voice channel.....unlike CDMA which have no such issue. Then why is there serious drop is voice quality when network is congested!! CDMA really have better voice quality in this context.....

In my area CDMA consistently has WORSE voice quality than GSM - no matter congested or not.
 

artem

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GSM voice quality

AMR codec is not widely used in GSM while it is , if i am not wrong defacto codec for WCDMA. Thus it allows less
bitrate (configurable) than half or full rate conventional codecs for GSM . That could be an issue if WCDMA network tries to solve congestion issue using less bitte coding builtin in AMR. Not all gsm phones support AMR.

Also CDMA is more vulnerable modulation than TDMA.
 

sachinlan

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Re: GSM voice quality

CDMA is an interference limited system. Capacity and quality of a CDMA system degrades when interference starts increasing. Thus when users (which cause interference) increase, the call quality will eventually degrade.

Also, CDMA uses 8K EVRC (9.6 kbps) vocoder. Rate if a vocoder indicates the rate at which the voice signal from mobile is encoded and transmitted. Thus higher the vocoder rate higher the quality. GSM vocoder rate is 13 K Full Rate (i.e. 14.4 kbps). Thus the voice quality is increased.

AMR vocoders (12.2 kbps) are supported by WCDMA and GSM but not all the operators and handsets support AMR.

Thus we can roughly say that the GSM voice quality can be better than CDMA and vice versa depending on the interference(i.e. no. of users or traffic).

But the pratical observation and experience is that CDMA voice quality is better than GSM in ideal RF conditions.
 

vfone

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Re: GSM voice quality

In GSM because is a TDMA system the voice quality is not affected by number of users in the cell.
In CDMA is different because there is more difficult to determine the cell capacity, which is dependent on the average level of interference between users.
In GSM there are other factors that affect voice quality: C/I or SNR, Signal delay due to multipath, vocoder type, etc.
 

Jack// ani

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GSM voice quality

Thanks...one other doubt left.....

When traffic increases BTS lowers down the TX power.....so that mobile unit start looking to other nearby base station, so is this probable source of interference...near end far end??
 

sachinlan

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Re: GSM voice quality

Well, BTS does not decrease its power when trafic increases. In fact when no of users increase in any given cell then no. of traffic channels assigned by that BTS increase. Thus TX power of BTS increase.

BTS will lower down the TX power when mobile is near to the cell. Moobile will tell BTS about the recieved power and tell BTS to lower down the TX power. This is called the power control and it's a very importanat aspect of CDMA systems.

Near-Far Effect:- This was the problem faced by early CDMA systems.
When no.of users closer to the BTS increase, there will be a Noise floor increase at the BTS because of the large no. of users on uplink. TO overcome this noise floor, the mobiles which are at the cell boundary will increase their TX power.If the nearby users still increase then far users will not be able to TX power after a certain limit ( MAX Tx power of a mobile which is fixed ≈+24 dBm in CDMA). Thus far users will not be able to communicate with the BTS. This is called the Near-Far effect. To overcome this problem power control is used in CDMA.

Power Control:- The purpose of power control is that the powers of signals received from all the mobiles (near and far) should be same at the BTS.

Added after 10 minutes:

To Vfone

Even though GSM is a TDMA system, when the no. of users in surrounding cells increase , the BER (bit error rate) will increase which will make the GSM voice quality poor.

Also at high traffic , BTS allocates half rate traffic channel(TCH/HR (6.5 kbps) instead of TCH/FR (13kbps)) to the users. Since vocoder rate is decreased in this case, voice quality gets affected adversely.
 

vfone

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Re: GSM voice quality

sachinlan
In the previous post I mentioned: in GSM because is a TDMA system the number of users IN THE CELL do not affect the voice quality.

Your observation about neighbor cells traffic, is part of my second remark about SNR that could be affected by external factors, including RX band noise generated by nearby GSM mobile phones, all nature of interferences, etc.
 

rojos17

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Re: GSM voice quality

In good RF conditions CDMA voice quality is slightly better than GSM, even if GSM is under EFR or AMR-FR. AMR decreases voice quality because it uses half of a voice time slot, thus, compressing more the audio.
 

Jack// ani

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Re: GSM voice quality

sachinlan said:
BTS will lower down the TX power when mobile is near to the cell. Moobile will tell BTS about the recieved power and tell BTS to lower down the TX power. This is called the power control and it's a very importanat aspect of CDMA systems.

So...do mobile unit also lower down their TX power when close to cell site?? Will I soon run out the battery when far away form the cell site?

Thanks
 

throwaway18

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GSM voice quality

GSM network have fixed number of voice channel, only a single user utilize this voice channel.

GSM uses eight timeslots per channel.

Then why is there serious drop is voice quality when network is congested!!

It dosn't. Over the last seven years I had a dozen GSM phones. I'v never noticed a drop in call quality due to the network being busy.
 

rami.masadeh

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Re: GSM voice quality

the answer of this question is in the GSM system each user use one channel and in the cell there are fixed number of channels, so that if all of the cahnnels are accupied in that cell you cant make a call so you will face a drop call, in other hand in the CDMA system all the users share the same band width not use spicific channels so let us suppose that there are 5 user use CDMA system make a call from the same cell when other 5 users come they can use the same cell but all of them will compressed in that band width so that the quality will affected but with out drop call.
 

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