In recent years, GSM systems have been deployed using many base transceiver stations (BTS). However, in some areas, such as basements of buildings, tunnels and valleys, there may be no signal, and in some regions fading may occur. These problems can be solved cheaply by implementing repeaters.

Figure 1 describes succinctly the operation of GSM repeaters. The donor antenna points to and receives the signal from the BTS and feeds it to the downlink (DL) path of the repeater. The DL signal is amplified and radiated by the service antenna. On the other side, the uplink (UL) signal from the mobile station (MS) is received by the service antenna, amplified by the repeater and reradiated by the donor antenna to the BTS. Both the UL and DL signals are strengthened.

A band selective repeater, as required by the GSM standard and indicated in ETSI 300 609-4/GSM 11.26 section 7.3,¹ must meet the specifications that the net gain in both directions through the repeater shall be less than 50 dB at 400 kHz offset and greater, 40 dB at 600 kHz offset and greater, 35 dB at 1 MHz and greater, and 25 dB at 5 MHz and greater from the edge of the relevant MS or BTS transmit bands.

Following the GSM standard, the design of a band selective repeater must prevent interfering with other operators' signals while amplifying a given one. A repeater cannot be designed to comply with the above specifications by using a common RF band-pass filter unless cost is no object. The most practical method currently used is to down-convert the RF signal to an IF frequency, followed by an almost ideal SAW filter to reduce the out-of-band gain, then up-converting the signal back to RF again. Figure 2 shows the different responses of the two types of repeaters. It is obvious that the economical solution using an RF filter does not meet the ETSI 300 609-4/GSM 11.26 requirements. Figure 3 shows the diagram of a band selective repeater that meets the GSM 11.26 specifications. Table 1 shows the gain (G) and noise figure (NF) of each stage.

Noise Figure and the LNA

In a communication system having several cascaded stages, the overall noise figure for an arbitrary number of stages is given by Friis' formula²

where

F₁,F₂,F₃... = noise figure of stage 1, 2, 3...

G_a1,G_a2... = available power gain of stage 1, 2...

It is well known that the noise figure and gain of the first stage are the most important in reducing the total noise figure. If F₁ is very small and G_a1 is large enough, then the effect of the following stages can be neglected and F_total will be small. The first stage amplifiers shown are Agilent AT32011 devices, where NF = 2 dB and G = 15 dB at 1.8 GHz.

Intermodulation and the Power Amplifier

GSM 11.26 specifies that the maximum level of spurious emission and intermodulation products shall not be greater than -36 dBm (250 nW) in the 9 kHz to 1 GHz frequency band and -30 dBm (1 mW) in the 1 to 12.75 GHz frequency band. Accordingly, the intermodulation products that are generated by the active components used in the design of the entire system must be considered, especially the third intermodulation product (IM3). Amplifiers are easily saturated when the input signal level is too high. The IM3 generated then interferes with the system performance. One way to solve this problem is to add an automatic gain control (AGC) function, attenuating the input signal to insure that the IM3 does not exceed -36 dBm at the output port. The power amplifiers used are Mitsubishi MGF0913A amplifiers, which have an output power of 31 dBm P1dB at 1.8 GHz.

Step Attenuator

Generally, the gain of the repeater is not particularly constrained, but depends on the environment. When the signal is strong, a repeater with a small gain is sufficient. When the signal is weak, a high gain repeater is needed. A step attenuator, such as AA101-80 from Alpha Industries, can be inserted inside the repeater to adjust the gain of the repeater when necessary. Attention must be paid to where the step attenuator is placed. If it is inserted in front of the repeater, the strong signals are attenuated so the intermodulation performance is improved but the noise figure is increased. If the attenuator is placed at the output of the repeater, then the output power is reduced. Typically, there is no need to consider the noise figure when the level of the input signal is strong because the signal-to-noise ratio (SNR) is still high enough. Therefore, a good choice is to put the step attenuator before the AGC. In this case the attenuation can be adjusted according to the strength of the input signal.

Filters

The duplexers (DUPLEX) are three-port filters that separate the UL and DL frequencies. RF BPF1 is an RF band-pass filter to eliminate the out-of-band signals, especially the strong unwanted signals, to avoid saturating the amplifier A1. Another reason is to filter out the image frequency generated in the mixer. RF BPF2 is an RF band-pass filter used to eliminate the image frequency and local frequency of the mixer. IF LPF is a low pass filter to reject the RF signal and let only the IF (here, 70 MHz) pass through. IF SAW is the major part of the band selective repeater because the edges of the SAW filter band-pass are very steep and permit the GSM standard to be met. A SAWTEK 851555 filter with a 6.5 MHz bandwidth is used.

Measurement Results

The band selective repeater was designed with the specifications given in Table 2 . Figures 4 and 5 show the return losses at the input and output ports of the repeater. The bandwidth edges are indicated and the return losses are less than -10 dB.

Figure 6 shows the noise figure and gain of the repeater. The in-band NF is better than 8 dB. In order to protect the measuring equipment, the data was taken with a 30 dB attenuator inserted at the output, so the gain shown is only 33 dB. Figure 7 shows the two-tone test of the band selective repeater. The IM3 are less than -36 dBm, when the total output power is 15 dBm. Figure 8 shows that the out-of-band gain meets the GSM standard. Note that at 400 kHz from the band edges the gain is less than 50 dB.

Conclusion

A band selective repeater has been designed and fabricated. The measured input and output return losses are less than -10 dB, the noise figure is less than 8 dB and the output power is 15 dBm with an IM3 of -36 dBm, meeting the requirements of GSM 11.26 (the out-of-band gain also meets the GSM 11.26 standard). This repeater could be used in areas such as building basements, tunnels and valleys to improve the signals at low cost.

Acknowledgment

The authors would like to thank REMOTEK Co., Taiwan, for its help.

References

1. ETSI GSM 11.26/ETS 300 609-4.
2. C.D. Motchenbacher and F.C. Fitchen, Low Noise Electronic Design , John Wiley & Sons Inc., New York, NY 1973.

Tsang-Yen Hsieh received his master of science degree from the Institute of Optical Sciences at the National Central University of Taiwan in 1992. He may be reached via e-mail at tyhsieh@ios.ncu.edu.tw.

Nai-Chueh Wang is an assistant professor at the Institute of Optical Sciences at the National Central University of Taiwan. He may be reached via e-mail at ncwang@ios706.ios.ncu.edu.tw.