A Digital Signal Processing Based Ka Band Satellite Beacon Receiver

A Digital Signal Processing Based Ka Band Satellite Beacon Receiver Cornelis Jan Kikkert and Owen Patrick Kenny Electrical and Computer Engineering James Cook University Townsville, Queensland, Australia, 4811 [email protected], [email protected] Abstract—This paper describes the design of a Digital Signal Processing (DSP) based JCU-STRAP (James Cook University – Satellite Transmission Rain Attenuation Project) Ka band Satellite Beacon Receiver / Radiometer for measuring both the sky noise and the attenuation caused by rain and water vapour to satellite signals, at the same frequency and look angle. To obtain rain fade statistics, the received signal amplitude of a satellite beacon is recorded at an eight times per second rate. To maximise the dynamic range over which the receiver can stay locked onto the satellite beacon, a receiver with an ultra low phase noise and absence of spurious signals is required.

I. INTRODUCTION As the satellite transmissions at C band and Ku band become more congested, it will be necessary to use higher frequency bands for additional services. At present new services are being introduced [1] in the Ka band. The attenuation due to Rain increases as the transmission frequency increases [2]. At a 150mm/hr rain rate, which is common in the tropics for short durations, an attenuation of 20 dB per km is observed at 22GHz, while the same rain only causes a 5 dB per km attenuation in the Ku band. In addition, there is a water absorption peak at 22.2 GHz, so that high humidity, which is common in the tropics, causes additional attenuation to the received signal. To determine the minimum receiver dish size needed for a target satellite-availablity specification, the actual rain attenuation distribution should be measured at the location where the receiver is to be established. An accurate rain attenuation distribution can best be determined using satellite beacon receivers and radiometers. The JCU-STRAP team has been building and operating Satellite Beacon Recievers for many years [3-7]. II. RECEIVER DESIGN REQUIREMENTS For a satellite beacon receiving system, tracking antenna dishes cannot be used as they cause fluctuations in the received signal strength. The antenna dish size must be small, so that there is no variation due to satellite movement but large enough to give an adequate signal to noise ratio.

978-1-4244-2182-4/08/$25.00 ©2008 IEEE.

For a typical 20 GHz satellite Ka band beacon receiving system, the received signal at the back of the antenna dish is typically -110 dBm [7] and results in an expected Carrier to Noise Ratio (CNR) of 54 dB and a fade range of 48 dB. The receiver thus needs to lock onto signals less than -160 dBm. The output data rate greater than 4 samples per second is required to track accurately scintillation. III. RF DESIGN OF BEACON RECEIVER Two JCU-STRAP satellite beacon receivers, as outlined in this paper were built. One system is in operation at the Communication Research Centre (CRC) in Ottawa, Canada, to receive the 20.199 GHz Ka band beacon on the Anik F2 satellite. The other receiver at NTU in Singapore, initially to receive the 20.199798 GHz Ka band beacon on the Ipstar [8] satellite and in the future receiving the 18.9 GHz Ka band beacon on the WINDS [9] satellite. Fig. 1 shows the block diagram of the beacon receiver. It consists of a Satellite dish and Ka Band Low Noise Block Converter (LNB), which shifts the 20.199 GHz signal to 949 MHz. This signal is then transmitted to the indoor unit. The indoor unit takes the 949 MHz UHF signal and determines both the satellite beacon signal and the sky noise and sends both these results to the data logger using an RS232 port on the beacon receiver. The unit in Fig. 2, shows the indoor unit of this Ka band receiver installed at NTU in Singapore. For gain calibration of the receiver, an excess noise source is located at the antenna feed. Normally, the DSP turns this noise source ON for 10 seconds once every 30 minutes. The outdoor unit is mounted at the satellite dish and contains control circuitry for the noise source and regulated power supplies to ensure that no mains frequency related components are modulated onto the LNB output. In the receiver, any spurious signals created by the receiver should be at least 10 dB below the received sky noise, to ensure that only the beacon signal and sky noise are measured. To eliminate IM products due to different LO frequencies, one LO is used and all the other LO frequencies are harmonically related. A 54 MHz Crystal Oscillator is used for the basic LO and all the other LO frequencies are obtained by harmonic

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frequency multipicaction. The 162 MHz VHF LO is the third harmonic of 54 MHz, obtained with a helical filter. The 810 MHz UHF LO is the fifth harmonic of 162 MHz and is produced using a microstrip filter and Microwave Monolythic IC’s (MMIC). As a result, the phase noise due to the LOs in the indoor unit is less than the phase noise of the satellite beacon or the LO used in the NORSAT Ka band LNB.

IV. DIGITAL SIGNAL PROCESSING As shown in Fig. 3, the digital hardware for the beacon receiver system consists of a DSP processor, a Complex Programmable Logic Device (CPLD), an AD6600 high bandwidth analogue-to-digital converter (ADC), an AD6620 Digital-Down-Converter (DDC), a front panel LCD display, an RS232 communication port, a digital-to-analogue converter and internal and external memories. The ADSP 21369 SHARC is a 2.4GFlop DSP processor and has sufficient high-bandwidth I/O ports and direct memory access (DMA) for external RAM to perform all the DSP processing tasks required for this beacon receiver. To minimize the cost for the very small production runs of the beacon receiver, a commercial development board is used. This board also contains RS232 UARTS and audio CODECs, which are used to provide both the digital and analogue data outputs shown in Fig. 3. The development board also contains Flash Memory and RAM to allow the DSP to boot as an independent unit. The parallel expansion port on the DSP is used to connect external devices via the CPLD to ensure correct communication. External devices are accessed as memory mapped locations. The DDC internal control registers are accessed through the microcontroller interface port of the DDC. The I/Q data from the DDC is passed to the DSP for processing via the serial port of the DDC.

Figure 1. JCU-STRAP Receiver Block Diagram

Figure 3. DDC and DSP Block Diagram.

A. System Initialization

Figure 4.

Figure 2. Ka Band (Top) and Ku band [5, 6] (Bottom) JCU-STRAP receivers at NTU, Singapore.

Referring to Fig. 1, the Ka band satellite beacon signal is amplified and frequency shifted to 23 MHz. This 23 MHz signal has a 1 MHz bandwidth and is digitised using a 12 bit ADC operating at an 18 MHz sampling rate. Details of the RF design are described in [7], this paper concentrates more on the DSP aspects.

Functional Block Diagram of AD6620 DDC [10].

When the system is turned on, the DSP initializes the internal configurations and external hardware including the DDC, LCD and memories. The DDC takes the 18 Ms/sec digitized satellite beacon signal and filters and decimates it to 70.3125 k/sec data rate with a ±35.156 kHz bandwidth centered at zero frequency with in-phase (I) and quadrature (Q) components. The functional block diagram of the DDC is shown in Fig. 4. It performs the digital down conversion in three stages. The first stage consists of a second order CIC decimation filter, the second a 5th order CIC decimation filter,

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and the final stage is a 256 tap FIR filter. The decimation rates and the output scaling for each stage are controlled by setting the appropriate registers. The resulting output is a 24-bit I/Q data stream. A Numerically Controlled Oscillator (NCO) register controls frequency shifting to ensure that the satellite beacon signal is shifted to near zero frequency. At initialization, the DSP writes the required values for the scaling and decimation rates of the CIC filters and the 256 filter coefficients for FIR filter to the registers in the DDC. These registers must be set correctly to ensure that the decimated data is correct. As a result, each register is read back and verified. If all the registers are correct then a message is displayed on the front panel display indicating that the DDC has been programmed correctly, otherwise an error message is displayed. Once the external and internal devices have been initialized, the system then enters into the signal search mode. B. Signal Search Mode The purpose of the search mode is to lock onto the satellite beacon signal. This is achieved by analyzing the spectrum at different search frequencies over a ± 400 kHz range centered around 23 MHz by setting the NCO frequency in the DDC. For each search frequency a 4 k FFT is performed on the data followed by an estimation of signal and noise power. The corresponding SNR is calculated and stored in memory. The maximum SNR is examined and if greater than 20 dB, that frequency band is chosen as the one containing the beacon signal. If the maximum SNR is less than 20 dB the signal search is reinitiated, with separate second order IIR filtering being performed on each bin of the spectrum. This spectral filtering reduces noise spikes, thus preventing the system from locking onto noise spikes. Under this condition, if the maximum SNR resulting from this frequency scan is greater than 10 dB, then the corresponding frequency band is chosen as containing the beacon signal. If the maximum SNR is less than 10 dB then the system is set to radiometer mode and the NCO frequency is set to 23MHz. In this radiometer mode, only the sky noise is evaluated, so that the receiver can be used as a conventional radiometer. Once the frequency band is chosen, the NCO is set to that corresponding search frequency and the spectrum is again obtained to determine the beacon frequency location within the band. The NCO is adjusted so that the beacon signal is shifted to near zero frequency. After the beacon signal has been located, the system goes into normal operational mode. C. Normal Operational Mode The primary task of the operational mode is to form reliable estimate of the beacon signal level and the sky noise power. The DSP performs a 4 k FFT on the I/Q data every 1024 new samples, thus 68.66 FFTs are performed each second. Each successive spectrum is used to determine the signal strength, signal frequency location, and the sky noise. The FFT covers a 70.3125 kHz bandwidth. Due to the FIR filter in the DDC, only the center 50 kHz of the FFT is used for signal and noise power calculations. The satellite beacon

signal and sky noise estimates are filtered using specially designed 8th order IIR filters with a 0.5 Hz cut off frequency to enable scintillation to be investigated accurately. These filters exhibit a very fast settling time, low overshoot and a good attenuation to out of band signals. The resulting data is then decimated by 8, to produce 8.6 output samples per second. The signal power and sky noise power are displayed on the front panel, as well being sent to the data logger using an RS232 interface on the DSP board. In addition, the NCO frequency and error messages are also sent to the data logger. The DSP also performs variable coefficient IIR filtering on the individual bins of the magnitude of the spectrum obtained from the FFT. When the SNR is greater than 20 dB, the filter coefficients are set so that no filtering takes place. When the SNR is between 20 and 10 dB, the filter coefficients are varied, depending on the SNR value to provide progressively more filtering as the SNR decreases. When the SNR is less than 10 dB, the filter coefficients are set to provide the maximum amount of filtering. This filtering smoothes the noise floor and makes it easier to detect a satellite beacon signal during deep rain fades or during the search routine when the receiver is acquiring a low level signal. As a result, the receiver can reliably lock onto signals that are just 5 dB above the noise and accurately detect signals just 3 dB above the noise. Since practical satellite beacons signals are much more than 10 dB above the noise level, the receiver operates as a radiometer if during the initial search mode the maximum SNR is less than 10 dB. If the SNR level is 3 dB less than its long-term average, indicating that a rain fade is in progress, the calibration noise source is turned off. When a very deep rain fade occurs, the receiver will keep the same NCO frequency for 10 minutes, even if no signal is detected. The receiver is very stable and when the rain fade reduces, the satellite beacon will still be within the frequency range covered by the FFT. Consequently, the receiver will regain lock immediately after the signal is slightly above the noise level. Long deep fades can thus be accurately tracked. To ensure that the beacon receiver does not permanently lock onto the wrong frequency, the receiver starts a new search if the SNR is less than 10 dB for more than 10 minutes. As the satellite beacon ages and drifts in frequency, the receiver must adapt to ensure that the beacon signals are always received. The satellite beacon can drift 2 ppm per year, or 44 kHz per year. The Phase Locked LNB can drift a similar amount. As the satellite beacon or receiver LO’s drift in frequency, the NCO in the DDC is adjusted under DSP control, to ensure that the satellite beacon remains within 10 kHz of zero frequency at the output of the DDC. The excess noise source is turned on for 10 seconds once every half hour. The resulting noise level recorded by the data logger provides an accurate measure of the complete receiver gain, including the gain of the LNB. This is then used for rain fade and sky noise calibration during data analysis. To provide data with a traceable accuracy, this noise calibration is not used to modify the front panel display or data sent on the RS232 port. To ensure that no useful data is lost, the noise source is not turned on during a rain fade.

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V. RESULTS Fig. 5 shows the output spectra produced by the DSP. The bottom (blue) curve is the spectrum when a signal generator is used as an input to the ADC. The middle (cyan) curve shows the spectrum for the same input signal but with noise added such that a similar SNR as is received at CRC in Ottawa from the Anik F2 satellite. The top (red) curve shows the same input signal with noise added such that a similar SNR is obtained as is received from the Anik F2 satellite, whilst the excess noise source in ON. DDC Spectra 0 DDC

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the more thermal noise is received by the beacon receiver. Radiometers are instruments that record this received noise [6, 7]. They can be operated without a satellite beacon signal being present. It is highly desirable for a satellite beacon receiver to also record the sky noise at the same frequency and in the same direction as the satellite beacon transmission. This allows radiometer models to be improved, thus retrospectively enhancing the accuracy radiometer measurements made prior to satellite beacons being available. VII. CONCLUSION The new JCU-STRAP satellite beacon receiver described here uses ultra low phase noise oscillators and advanced DSP technology to obtain a larger dynamic range than was previously possible. A 53 dB CNR is obtained in practice. Using an excess noise source at half hourly intervals to obtain accurate gain calibrations of the receiver increases the rain fade accuracy and allows the receiver to be used as an accurate radiometer. The use of advanced DSP technology enables the receiver to have a dynamic range that is purely determined by the beacon signal and sky noise. The final data rate of more than 8 samples per second allows scintillation to be observed.

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Frequency kHz

ACKNOWLEDGEMENT

Figure 5. Spectrum of a test signal after digital demodulation.

By comparing Fig. 5 with Figs. 3 and 5 of reference [6] describing a PLL based beacon receiver, it can be seen that a significantly lower phase noise is obtained in this JCUSTRAP receiver. Some low level unwanted components due to radiation from the DSP can be seen at 7 kHz above the carrier. However, this is 15 dB below the clear sky noise level received and will not degrade the performance. A deep rain fade obtained at NTU with the JCU-STRAP receiver is shown in Fig. 6. The clear-sky signal level is 0 dB. The excess noise source is turned on by the DSP for fifteen seconds once every half hour as shown in Fig. 5 at the 83 minute time.

Figure 6. Beacon Signal and Radiometer Noise Deep Fade.

VI. RADIOMETER OPERATION Rain drops temperature range between 273K and 300K in the tropics. Each raindrop produces a small amount of thermal radiation. The higher the rain rate, the deeper the rain fade and

The authors thank the CRC in Ottawa, Canada and NTU in Singapore for providing the funding to permit these beacon receivers to be constructed. In addition, the authors thank the staff of NTU, for providing the data for Fig. 6. The authors thank Mr. Geoffrey Reid who wrote the code for the data logger and Mr. John Renehan and Mr. Lloyd Baker who assisted with the construction of the JCU-STRAP system described in this paper. REFERENCES [1]

Ka Band Launches. http://www.lyngsat.com/launches/ka.html [accessed 28 May 2007] [2] Report 719-1 “Attenuation by gases” CCIR (now ITU) 15th Plenary Assembly, Geneva, Vol 5, 1982. [3] Kikkert, C. J. “The Design of a 12 GHz Narrowband Low Noise Receiver”, 1992 Asia-Pacific Microwave Conference, Adelaide, Digest pp 809-812. [4] Kikkert C. J. Bowthorpe B. and Allan G. “Satellite Beacon Receiver Improvement using Digital Signal Processing” The fourth International Symposium on Signal Processing and its applications (ISSPA96), Gold Cost, 25-30 August 1996, pp517-520. [5] Kikkert C. J. B. Bowthorpe and J. T. Ong. “A DSP Based Satellite Beacon Receiver and Radiometer” 1998 Asia Pacific Microwave Conference, APCM98, Yokohama, Japan, 8-11 December, pp443-446. [6] Kikkert C. J. Dynamic Range Improvements of a Beacon Receiver using DSP Techniques URSI Commission F Triennium Open Symposium, 2004, Cairns, 1-4 June, 2004. ISBN 0-646-43423-3. [7] Kikkert C. J. The Design of a Ka band Satellite Beacon Receiver. Sixth International Conference on Information, Communications and Signal Processing (ICICS 2007), 10-13 Dec 2007, Singapore. [8] Thaicom 4 (IPSTAR), http://www.thaicom.net/, our satellites, [acessed 21 may 2007]. [9] Japan Aerospace Exploration Agency Wideband InterNetworking engineering test and Demonstration Satellite (WINDS) http://www.jaxa.jp/projects/sat/winds/index_e.html. [accessed 21 may 2007]. [10] Analog Devices 67MHz Digital Receive Processor AD6620 Technical Notes

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