A USRP DCS (GSM1800) RF frontend for OpenBTS

Board description

DCS Front end photo

The board is designed to be a simple RF front end when using OpenBTS in the DCS (GSM1800) band with a RFX daugther board. One of the big advantages of using it is to reduce the crosstalk between RX & TX compared to using two antennas ( RX & TX ) next to each other. With it you have a single antenna port that you can connect to a good antenna with the gain and radiation pattern you want.

The board consists of :

Simple tests

In this set of test, I used the USRP to generate a pure carrier at a known levels and various frequencies inside the GSM1800 band, both uplink and downlink, and measured the response through various paths.

All measurements in this section have been made using an Agilent E4406A, by taking the spectrum view and looking at the average amplitude at the center frequency.

LNA & RX-filter only

This is preliminary testing with just the LNA and RX filter. I didn't solder the duplexer on that test board and instead directly soldered a RG174 cable onto the pad. This allowed to inject a signal 'as-if' it came out of the RX port of the duplexer.

For each frequency, I tested at two different input power levels.

The results are presented in three columns:

ARFCN Frequency
(MHz)
Direct
(dBm)
LNA + RX-filter
(dBm)
Delta
(dB)
Downlink
512 1710.2 - 33.27 - 23.05 + 10.22
- 2.75 7.33 + 10.08
698 1747.4 - 33.87 - 21.90 + 11.97
- 3.30 8.56 + 11.85
885 1784.8 - 34.20 - 24.61 + 9.60
- 3.65 5.77 + 9.42
Uplink
512 1805.4 - 34.50 - 38.10 - 3.60
- 4.01 - 7.70 - 3.69
698 1842.4 - 35.10 - 48.34 - 13.24
- 4.57 - 17.86 - 13.29
885 1879.8 - 35.90 - 56.73 - 20.83
- 5.39 - 26.30 - 20.91

Complete frontend

Theses two sets of tests are made with the front end completely assembled. They're made with a different board than the preliminary tests above tough.

The following results are for testing the TX to RX cross-talk. A fixed carrier is generated at various points on the downlink band and sent to the TX port of the front end. Readings are then taken at the RX port, see what leaked through. The antenna port was connected to a VERT900 antenna during this test.

ARFCN Frequency
(MHz)
Input
[TX port]
(dBm)
Output
[RX port]
(dBm)
Delta
(dB)
512 1805.2 - 4.5 - 63.0 - 58.5
- 35.0 - 93.0 - 58.0
698 1842.4 - 4.9 - 72.0 - 67.1
- 35.5 < - 100 < - 64.5
885 1879.8 - 5.7 - 61.5 - 55.8
- 36.3 - 92.0 - 55.7

The following results are for testing the RX performance. A 'weak' carrier is generated in the uplink band and injected in the front end via the ANTenna port. This simulates a signal received from a phone. Readings are then taken on the RX port to see what the USRP would receive.

ARFCN Frequency
(MHz)
Input
[ANT port]
(dBm)
Output
[RX port]
(dBm)
Delta
(dB)
512 1710.2 - 33.50 - 22.05 + 11.45
698 1747.4 - 34.15 - 24.30 + 9.85
885 1784.8 - 34.70 - 25.90 + 8.8

OpenBTS tests

In these final tests, I ran a complete OpenBTS 2.6 release, running on ARFCN 885 with a 52MHz clock.

Note that to be able to view what the USRP receives, the front end RX port was connected, through a resistive power splitter to both the USRP RX2 port and the Agilen E4406A. This splitter incurred a 6.5 dB loss that you must take into account when interpretin all the mesurements below. Only the 'direct connection' measurements don't have this additional attenuation

Downlink

USRP Source [Direct connection]
3.71 dBm @ 1879.8 MHz 14.23 dBm / 300 kHz @ 1879.8 MHz
Antenna port
1.18 dBm @ 1879.8 MHz 11.32 dBm / 300 kHz @ 1879.8 MHz
Crosstalk to RX port
-57.35 dBm @ 1879.8 MHz -47.99 dBm / 300 kHz @ 1879.8 MHz

Uplink

Some measurement of uplink with a channel active. The phone was only 1 meter or so from the BTS during those tests

Channel power -30.27 dBm / 300 kHz @ 1784.8 MHz
Spectrum -41.49 dBm @ 1784.8 MHz
RF Envelope (min: - dBm  max: - dBm  avg:  - dBm) @ 1784.8 MHz
OpenBTS channel infos
	OpenBTS> chans
	TN chan      transaction UPFER RSSI TXPWR TXTA DNLEV DNBER
	TN type      id          pct    dB   dBm  sym   dBm   pct
	 2     TCH/F 1804289383  0.00    0     6    0   -48  0.00
									

For comparaisons, here's some measurement on the uplink whith the phone at the edge of the cover zone

RF Envelope (min: - dBm  max: - dBm  avg:  - dBm) @ 1784.8 MHz
OpenBTS channel infos
	OpenBTS> chans
	TN chan      transaction UPFER RSSI TXPWR TXTA DNLEV DNBER
	TN type      id          pct    dB   dBm  sym   dBm   pct
	 2     TCH/F 1804289386  0.00  -38    30    0   -98  0.00
									

A concern that was also raised in the noise generated by the USRP in the uplink band even when its transmitting in the downlink band. The following shot shows that I can't see any trace of it.

-101.13 dBm @ 1784.8 MHz (noise floor)
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