Technical report on implemented digital radio link of Interactive Wireless QPSK System

Interactive Wireless QPSK System has been build for QPSK signal transmitting under the contract with Centrmedia communication operator from St.Petersburg, Russia. The system allows transmitting QPSK modulated signal with more than 32 MHz bandwidth and better than 10^-7 error rate with 95 dB system budget. The radio link can be used for point-to-point digital radio and LMDS systems. It is based on transmitter with up-conversion system and mm-wave Injection Locked Amplifier and heterodyne receiver with better than 1 dB flatness within input signal bandwidth. Experimental samples with more than 140 mW output power were built in our laboratory and tested in real outdoor urban conditions of St.Petersburg. One 32 MHz channel (Deutsche Telecom) from “Hot Bird” satellite was used as an input signal with QPSK modulation. TT1200 MPEG2-DVB Professional Decoder provided error rate detection. The system was tested in two principal configurations: point-to-point and point-to-multipoint.

Specifications for the system are presented in Tables 1, 2, 3.

Table1. Receiver Specifications.

Table 2. Transmitter Specifications.

Table 3. Common Specifications.

The Block Diagram of the Transmit-receive module is presented figure 1.

It consists of the following parts:

1.Transmitter, made on Frequency Up-converter and Output Power Amplifier
2. Receiver, made on Balanced Mixer, LO and 1800 MHz Amplifier.

The Transmitter operates using principle of the up-conversion of the input frequency. It is made on IMPATT SSB Up-converter and Local Oscillator. It converts amplified 1540 MHz input signal with QPSK modulation to 6-mm wavelength range. Bandpass Filter rejects LO and low side band (LO-1540 MHz) frequencies. IMPATT Injection Locked Amplifier provides more than 140 mW output power (saturated). The Receiver is made on Balanced Mixer with better than 6 dB Conversion Losses, Local Oscillator, 1800 MHz Low Noise and Power amplifiers.

Both Local Oscillators of the module are made using unique ELVA-1 technique of IMPATT Active Frequency Multipliers (AFM) with 6 times multiplication factor. Input signal with about 7 GHz frequency is multiplied to the desired mmw frequency.

QPSK modulated signal from a satellite was used for system testing. ALCAD UC-212 IF converter extracts one 32 MHz channel from 0.9 – 2.1 GHz input stream and converts it to 1540 MHz signal that was passed to the transmitter input. We used Deugscher Telecom channel from Hot Bird satellite. Real communication rate of the channel is 27.5 Mbps. 1800 MHz signal from the receiver output came to the TT1200 MPEG-DVB Professional Decoder that was used as the error rate detector. Measurements of the system budget were made in ELVA-1 millimetre-wave laboratory. The block diagram of the measurements is presented on figure 2. Two transceiver modules without antennas were connected to each other using waveguides and 60 dB fixed and 0 to 70 dB Variable Polarisation Attenuators. The results of the measurements are presented on figures 3. A form of the saturated spectrum of the output 1800 MHz signal proves that the output power of the transmitter is saturated. Typical spectrum for 1800MHz signal is presented on fig.4. The spectrum is measured in deep saturation conditions at maximum output power 141mW of Base Station transmitter. 

The system was tested in March 2001 in real urban conditions of St-Petersburg by ELVA-1 with the collaboration of Centrmedia communication operator. The Base Station was installed on the house-top of the high 24-floor building. Two Customer Stations were placed on two 6-floor buildings; the first one within 1 km and another one at 2 km distance from the Base Station. The 1540 MHz signal with QPSK modulation from ALCAD converter was applied to the input of the Base Station. The 1800 MHz signal from the outputs of the Customer Stations was tested using TT1200 MPEG-DVB Professional Decoder. The error rate at the output was better than 10^-6. Increasing of the error rate from <10^-7 in the tests in our laboratory to 10^-6 outdoors is determined by the conditions of the experiment and within system budget it does not depend on the distance between the Base Station and Customer Stations. The fact that error rate was 10^-6 in the real conditions compare to 10^-7 can be explained that the Base Station was installed on the building only a few meters far from the transmitting antenna of the 3 kW FM radio station.

Last development on the improvement of the Radio Link specifications

One of the most important parameters of the Radio Links which is built using the principle of the up-converting of the frequency of the input signal is the level of phase noise of the Local Oscillators. During the winter 2001 we at ELVA-1 produced and tested in the laboratory 2 new 7 GHz DROs with improved phase noise. DROs were tested together with the x13 IMPATT Active Frequency Multipliers. Block Diagram of the measurements is presented on figure 5 and the results of the measurements on figures 6, 7, 8. The 94 GHz signals from the outputs of the Frequency Multipliers were applied: the first one to the LO input of the Balanced Mixer and another one to RF input. Signal from IF output of the mixer was tested using Tektronix 494AP spectrum analyser.

Measured Phase Noise was as following:

• 89.8 dBc/Hz at 1 kHz of carrier
• 102.6 dBc/Hz at 10 kHz of carrier
• 108.6 dBc/Hz at 50 kHz of carrier

The Conclusions

1. The 42 GHz Radio Link for QPSK signal transmitting was designed and tested in real urban conditions in St.Petersburg. One 32 MHz channel from Hot Bird satellite with 27 Mbps communication rate was used for the system tests. Tests shown that the system can be used for QPSK signal transmitting with better than 10^-7 error rate in two principal configurations: point-to-point and point-to-multipoint.

2. Using of the transmitter with high output saturated power based on the IMPATT Injection Locked Amplifier allows data transmitting with better than 10^-7 error rate. That allows increasing of the system budget up to 95 dB at BER < 10^-7.

Fig. 1.
Block Diagram of the Transceiver Module

Fig. 2.
Block Diagram of the System budget measurement

Fig.3a.
Output Power of the Base Station

Fig 3b.
System Budget at BER < 10^-7.

Fig 4.
Saturated Spectrum of the output 1800 MHz signal

Fig. 5.
Block Diagram of the Phase Noise Measurement

Fig. 6
Results of the Phase Noise Measurement