Repeater for cellular communication

[amirke]

Hi all,

I am designing a Cellular amplifier/repeater.

My architecture will be as :

RF signal -> Duplexer -> SAW filter -> LNA -> PA (with gain control).
This is simple approach.

But there is something else. Architecture that involves modulator (and VCO) and a demodulator right in the middle of the RX path :
RF signal -> Duplexer -> LNA -> demodulator ->amp -> SAW filter -> modulator -> amp -> PA (with gain control).

Can some recommend me what is the advantge in the second approach ?

Thanks

 

[vfone]

The first approach is a basic repeater, which just receive the RF signal from the air, amplify it, and send back to the air.
The second one is more complicated, the RF signal is received, is demodulated, modulated, RF signal is amplified, and sent to the air.
The second approach can re-translate the RF signal to a different frequency that was received, and in this way can use more TX power than first approach (TX frequency is different than RX frequency)

There is a third option, simpler than both above. To use a passive link antennas, usually back-to-back high gain antennas placed on a proper location.

 

[amirke]

Thanks for your answer.

The third option is not good for me since I need it for indoor use and I need maximum receiver sensitivity.

Regarding the second method, I have few question :
1) Does it gives better Rx sensitivity ? I mean, will it manage to receive lower RF signals (in power, dBm) then the simple architecture ?

2) In a repeater, you have to transmit the data in the same frequency that you received it.
Therefor no need to change frequency during this transition ? correct ?

3) How does this method determine what is the correct frequency in order to de-modulate the signal ?
For example, PCS1900 is from 1930.2–1989.8MHz. What will be the frequency to demodulate the data ?

 

[davenn]

Demodulating the signal only causes the project to become orders of magnitude more difficult than is needed

The first approach of just a plain repeater is the best way to go if back to back antennas are not an option
The thing is you are probably still going to use at least 1 yagi antenna anyway which will be pointing to the closest
cell tower
I can envision a lot of design problems ahead for you as you try to keep the TX out of the RX when the system is operating across the same freq band. You are going to want to keep TX power levels to an absolute minimum and still provide a reliable signal within the building

Cheers
Dave

 

[amirke]

So what is the advantage in De-modulating the signal and amplifying him before modulating and sending him again?
I saw couple of system that take this approach.

 

[davenn]

So what is the advantage in De-modulating the signal and amplifying him before modulating and sending him again?
I saw couple of system that take this approach.

There's no real advantage that I'm aware of, some one else may comment. As I said earlier, the HUGE disadvantage is it makes the project so much more complex.

Since my previous post, I have been trying to think of ways that you can approach the TX and RX isolation probs.
Separating the outdoor TX/RX from the indoor TX/RX will help initially. I'm not totally up on the workings of how the bands on the cell system are used, I have mainly worked with standard repeater systems.

Maybe someone can tell us if the cellular system TX and RX and top and bottom parts of say the 1800MHz band or is say... 1800MHz used for handset RX and the 800-900MHz band used for handset TX, or visa versa. I'm not sure. If they use a split between the 2 bands then TX /RX isolation will be very easy.

Let's see if some one can answer that question
In the mean time I will see what answers google has

Cheers
Dave

---------- Post added at 09:52 ---------- Previous post was at 09:35 ----------

What country are you in, amirke ??

Is PCS1900 even used whe you are? It seems to be a north American thing

D

 

[amirke]

I am from Israel.
I develop a system to work in north America.

 

[amirke]

So what do you think is the best way to separate and isolate TX from RX in cell frequency ?

 

[E Kafeman]

For separating GSM uplink and downlink frequencies exist fitting SAW filter, so that part is relative easy.
Amplification without echo cancellation can not be higher then isolation between UL and DL antennas.
To avoid local echo in active repeaters is frequency shift common. As a part of information to establish a call is a communication channel request sent. This channel information must be translated to the frequency shifted channel in the repeater, as well as power control information and station ID, sent from base to hand set. That is why information must be demodulated and corrected.
Traffic Control Channel communication and a lot of other information must also be translated by the repeater. A common alternative for cheap repeaters placed in rural areas, is to communicate with very low local output power, with coverage less then 1 meter and also send control information to handset to communicate with low power level. Frequency shift is then not needed.

 

[davenn]

After doing some reading last nite.
If it was standard GSM where the TX and RX split is between 900MHz and 1800MHz then filtering is really easy

PCS uses much closer band spacing.....
This site gives a goog freq spacing that would allow for reasonably easy filtering..... http://tk.com/wireless/1900pcs.html

The split is between 1800 and 1900 MHz? If the TX power levels were high. Say > 1 W, some care would be needed to ensure the TX didnt desense the RX


Dave

 

[E Kafeman]

Block Total Spectrum Lower Sub-band Upper Sub-band
Block A 30 MHz 1850-1865 MHz 1930-1945 MHz
Block D 10 MHz 1865-1870 MHz 1945-1950 MHz
Block B1 10 MHz 1870-1875 MHz 1950-1955 MHz
Block B2 10 MHz 1875-1880 MHz 1955-1960 MHz
Block B3 10 MHz 1880-1885 MHz 1960-1965 MHz
Block E 10 MHz 1885-1890 MHz 1965-1970 MHz
Block F 10 MHz 1890-1895 MHz 1970-1975 MHz
Block C1 10 MHz 1895-1900 MHz 1975-1980 MHz
Block C2 10 MHz 1900-1905 MHz 1980-1985 MHz
Block C3 10 MHz 1905-1910 MHz 1985-1990 MHz
Block G* 10 MHz 1910-1915 MHz 1990-1995 MHz

G= Reserved for future use.

Uplink: 1850 - 1910 MHz
Guard band: 1910-1920 MHz
Unlicensed PCS 1920-1930 <-
Downlink: 1930 - 1990 MHz

No RX/TX overlapping
Band plan is regulated by 3GPP.
Specific band plan for US: https://www.3gpp.org/ftp/tsg_ran/TSG_RAN/TSGR_46/Docs/RP-091201.zip

 

[amirke]

Thanks guys,

Now I am worried from echo from UL and DL antennas.
This device is for home use.
So one antenna is indoor and one is outdoor, using good shielded cable.
I understand the need for frequency shift , in order to overcome the echo effect, but it does complicates my design. I don't want to re-build the RX message in different frequency. Thus,
1) How can I improve my design in order to make the best isolation between the two antenna's ?


And About Separation the up-link from down-link:
1) SAW filters are OK. Where in the uplink and downlink path should I insert them ? ( Duplexer ? LNA, PA )
2) How many SAW filters should I put ? What is the recommended filter order ?
3) Does SAW filters, or any other filters can replace the Duplexer ? Could it be better than a Duplexer ?
4) Does The idea of two separate PCB's ( one for downlink and one for uplink) is a good idea ? if it is, with which connector to use ?

 

[amirke]

Some one ?

 

[E Kafeman]

1. Normally SAW filters before each LNA and PA. Depends mainly on how much gain you have in each stage. Typical chain: Ext.antenna->SAW->LNA->SAW->PA->Int.antenna.
Verify that SAW out of band attenuation is good enough, so that TX power from uplink PA not can saturate downlink LNA and that uplink is enough suppressed in downlink chain, so that self oscillation is avoided.
2. Number of SAW's depends on total gain and need of suppression.
3. For a simple repeater, uplink PA and downlink LNA need to be combined if you want to use a single external antenna. Same for indoor antenna. For that do you need some kind of combiner or power splitter or use separate antennas.
4. Better isolation with separate PCB's maybe. Connector type is not important. Solder coax cable directly at PCB if you want.

 

[amirke]

Great. Thanks for your answer, Kafeman.

One more question, Do I need to insert a SAW filter between a Duplexer and a LNA ?
Since a Duplexer is made out of SAW filters.

Duplexer is causing a -2dB insertion loss.
A SAW will contribute the same amount of loss (-2dB, maybe a bit less).
Together, almost -4dB before the LNA.

What do you think ?

 

[E Kafeman]

No, there is no need to have dual SAW filters, if you get enough TX/RX isolation with a duplexer.