Application of Frequency Shifting Repeaters

1 Marine Parade Central #10-09 Parkway Centre Singapore 449408 Tel: +65 6345 4908 Fax: +65 6345 1186 URL: www.comba-telecom.com

Application of Frequency Shifting Repeaters

1. Background

Radio repeaters, sometimes known as Bi-Directional Amplifiers (BDA), incorporate dual amplifiers in the uplink and downlink direction to increase the signal levels at the mobile and base stations. They are commonly used in areas with poor coverage in outdoor and indoor environment, or for coverage enhancements in areas blocked by obstacles. The basic principle involves the use of a pick up (donor) antenna to receive and amplify the radio signal from a donor base station cell, and then retransmit from an antenna (service) mounted near the area to be covered. Most repeaters employ automatic gain control (AGC) to limit intermods and spurious to a minimal level during overload. Complete local monitor function (OMT) and powerful remote repeater network administration (OMC) are available. Typical applications include coverage for conference centre, shopping mall, office building, radio shadow areas such as underground car parks, tunnels, valleys, and coverage extension in motorways.

Figure 1: Typical repeater application scenario

Some of the benefits of deploying repeaters are: Fast rollout and fast coverage enhancement leads to fast return on investment; Low build out costs with no microwave link and 2 Mbit connection; Less antennas and cable usage, and much lighter steel hardware, i.e. a strong tower is not

required. Serves as a platform for subscriber growth;

Comba Telecom Systems (S) Pte Ltd


2. Antenna Isolation Issues

While repeaters have many desirable advantages, their application in the outdoor environment is limited by the isolation between its donor and service antennas. A repeater can act as an oscillator if the signal feedback is greater than the system gain. In a typical scenario, the donor signal from the BTS is typically around -60dBm. This requires the repeater gain to be at least 90dB to achieve say 1W (30dBm) output. Depending on the level of isolation between the donor and service antennas, the amplified signal may cause positive feedback leading to oscillations in the repeater.

Figure 2: Gain and isolation in a repeater system

As a practical rule of thumb, the isolation between donor & service antenna should be at least 10 to 15 dB more than system gain. This means that to achieve a 90dB gain, the isolation between the two antennas must be at least 100dB – a difficult situation for outdoor repeater applications.

In practice, the antenna isolation can be measured by injecting a known power into one antenna (or use tracking generator function), and measure the level received by the other on a spectrum analyzer, as shown in Figure 2.

Figure 3: Isolation measurement using a tracking generator



3. Frequency Shifting Repeaters

The Comba S-9180 is a point-to-multipoint, frequency-shifting repeater (FSR) system that overcomes the antenna isolation problem in conventional repeater system. Each S-9180 can support up to 2 channel frequencies, and may be connected in parallel for higher number of channels, or cascaded to booster products for higher output power.

A typical FSR system comprises of a master unit (direct or wireless coupling) and one or more remote units. Figure 4 shows a GSM-DCS FSR utilizing a direct coupling master unit and one remote unit. The GSM signal from the BTS is coupled to the FSR master unit (S-9180-LD) and translated to a DCS (GSM1800) link signal to the remote unit (S-9180-R).

Figure 4: A point-to-point FSR system using a direct coupling main unit

The DCS signal is received by the remote unit and is translated back to GSM frequency before being used to serve the mobile units. As the link frequencies differ from the BTS frequencies, the problem of RF oscillation is eliminated. The remote unit can also deploy omni directional antenna as its service antenna without oscillation problems. A point-to-point FSR utilizing a wireless coupling master unit is shown in Figure 5.

Figure 5: A point-to-point FSR system using a wireless coupling main unit

1800MHz

900MHz

900MHz

S-9180-R

GSM BTS

S-9180-LD

S-9180-RRemote

Unit

S-9180-LWWireless Coupling

Main Unit

Internal or Ext Antenna

GSM BTS

Internal or Ext Antenna

GSM Mobile



An added advantage of a FSR system is that a point-to-multipoint system can be implemented to resolve multiple blind spots, as illustrated in Figure 6. Here, the BTS frequency, F1, is translated to a link frequency F2 and transmitted using a wide beamwidth antenna to several remote units, each covering a blind spot. The remote units, in turn, translate the F2 frequency back to the original F1 frequency.

Figure 6: A point-to-multipoint FSR system using a direct coupling main unit

The S-9180 is available as a two-channel model with three options, namely, GSM-DCS, GSM-GSM and DCS-DCS mod. All units come with built-in GSM radio modem for remote wireless control and monitoring. The remote unit also has options for integrated service antenna built into the equipment front panel, or separate RF ports for connection to external antennas. Both wireless remote and local monitor function (OMT) and powerful remote repeater network administration (OMC) are available to setup and manage the FSR system.

4. How it works?

Figure 7 shows the block diagram of a GSM-DCS wireless coupling main unit. The donor GSM frequencies Fg1 and Fg2 from the BTS is received and amplified by low noise amplifier LNA1. System gain adjustment is available in the form of variable attenuators M-ATT and C-ATT. The Frequency Shift Module performs down-conversion of the incoming RF signals to an intermediate frequency (IF) signal, where it is being filtered by a high selectivity 200 KHz bandwidth surface acoustic wave (SAW) filter. The single carrier output is then up-converted to the DCS frequency band, before it is amplified by the power amplifiers (PA) and combined using hybrid combiners. GSM channel selection is accomplished by adjustment of local oscillator fp1 while DCS channel is determined by settings on local oscillator fp1+fo1. Hence, any GSM channel can be selected and shifted to any DCS channel by setting the local oscillator frequencies.

F1

F2

F2

F2

F1

F1

F1F1

F1

F1

F1

F1

S-9180-R

S-9180-R S-9180-R

S-9180-LD

F1

GSM BTS



Figure 7: Block diagram of the FSR

Since each PA only amplifies one carrier at any one time, low intermodulation and spurious products meeting ETSI specifications can be achieved. Figure 8 shows the spurious and intermodulation levels in the GSM and DCS bands with 2x43dBm carriers at the input.

Att 20 dB*

*1 RMMAXH

A

Ref 42.5 dBm

Offset 43 dB

LVL

Center 944.5 MHz Span 5 MHz500 kHz/

*

*

UNCAL

RBW 30 kHz

VBW 1 kHz

SWT 340 ms

PRN

-50

-40

-30

-20

-10

0

10

20

30

40

SWP 1 of 1

1

Marker 1 [T1 ]

32.89 dBm

945.000000000 MHz

D1 -38.3 dBm

Date: 8.AUG.2003 09:44:35

Att 20 dB*

*1 RMMAXH

A

Ref 42.5 dBm

Offset 43 dB

LVL

Center 1.7175 GHz Span 5 MHz500 kHz/

*

*

UNCAL

RBW 30 kHzVBW 1 kHzSWT 340 ms

PRN

-50

-40

-30

-20

-10

0

10

20

30

40

SWP 1 of 1

1

Marker 1 [T1 ] 33.46 dBm 1.717000000 GHz

D1 -35.3 dBm

Date: 8.AUG.2003 09:51:11

Figure 8: Spurious and intermodulation performance in GSM and DCS bands

5. Typical Application

The low or zero isolation requirements of FSR systems makes them ideal for use in remote outdoor applications. Simple and low cost towers can be used to mount the donor and service antennas, as shown in Figure 9. The GSM-DCS master unit at the GSM site using a narrow beamwidth DCS antenna to transmit the link signal to the remote unit located at the hill top. At the remote unit, the donor Grid antenna receives the DCS link frequency and feeds to the remote unit located at the bottom of the tower. After translation back to GSM frequency, the signal is sent back up to the tower to the GSM panel antennas.



(a) At the BTS site (b) At the remote site

Figure 9: Application of FSR in remote sites

In rural areas where power supply may not be available, solar panels with battery backup can be deployed as shown in Figure 10.

Figure 10: Solar-powered repeater in rural sites


DCS Link Antenna

DCS Grid AntennaGSM Service

Antenna

GSM Panel Antennas


6. Conclusion

The use of frequency shift repeaters has increasing become an attractive option for use in rural coverage enhancement. Its requirement of zero isolation makes it suitable for use in simple tower structures where there is little separation between the donor and service antennas. Omi-directional service antennas can also be used at the remote site – something that is unheard of when using traditional repeater systems. In addition, multiple remote units are possible with a single main unit, with options for solar powered systems.


Documents

Application of Frequency Shifting Repeaters