Challenge – When deploying the TETRA emergency services network Airwave Solutions needed a way to verify that coverage was available across the vast majority of the UK road network; a distance of approximately 1 million miles.
At the time the tools available to make such measurements were analogue scanning receivers capable of making 1000 measurements per second. Since Airwave had 200 channels to measure, these receivers could make five measurements on each channel per second, whereas to eliminate the effects of fading many more than this are required.
In fact when driving at 60 mph the accepted measurement rate is in excess of 70 measurements per second per channel. For 200 channels this needs a scanning rate of 14,000 channels per second – an order of magnitude faster than the best available receivers of the day.
Given the distance involved the simple solution, to drive at 4 mph, was not a practical option. The other obvious solution, 14 receivers in each vehicle would have been prohibitively expensive and cumbersome.
Approach – MAC Ltd’s solution was to apply digital signal processing to the problem. Whilst it wasn’t possible to use 14 physical receivers, by using state of the art analogue to digital conversion coupled with high-speed signal processing using the latest field-programmable gate arrays (FPGAs), it was possible to put 200 digital receivers in one box.
The result was a software-defined radio that could measure each of the 200 channels allocated to the Airwave network up to 125 times a second (an equivalent scanning rate of 25,000 channels per second). The result was the CatchAll Receiver.
The advantage of the digital approach taken by MAC Ltd went beyond the scanning rate.
With further processing the CatchAll could demodulate and decode the TETRA broadcast signals, to provide more detailed diagnostic data and could also measure signal quality to aid in the detection of co- or adjacent channel interference. Such features were not possible in analogue scanning receivers.
Outcome – Airwave deployed a fleet of drive test vehicles each equipped with a single CatchAll receiver and over the period in which it deployed the UK network it did cover the 1 million miles of A, B and C roads.
Airwave still maintains its fleet of CatchAll receivers, further enhanced by the second generation CatchAll designed for pedestrian use when measuring in buildings.
MAC Ltd sells the CatchAll to operators and installers of TETRA networks across the globe.
Now in its third generation, the CatchAll remains the benchmark for TETRA network measurement equipment and with enhanced signal processing capability will soon support measurements of LTE and 5GNR networks.
Security & Defence
High data rate radio project
Challenge – A Secure Radio manufacturer wanted to improve its high data rate MANET (mobile ad-hoc network) radio to improve export potential by improving the adaptive data rate performance, range & resilience.
Approach – After capturing the requirements, by replacing the CDMA physical layer with an OFDM physical layer to offer greater throughput, we performed extensive modelling of a coded-OFDM scheme using adaptive modulation and coding (AMC) using turbocodes.
This maximised throughput at a given SNR (Signal to Noise Ratio) and exploited higher SNRs to provide greater throughputs. The most robust lowest data rate scheme had a range matching the original PHY, but with five-time the throughput, whereas the highest data rate offered 11 times the throughput of the original PHY’s highest data rate within the same bandwidth.
The customer implemented the hardware and a linearised power amplifier, and we implemented the new PHY layer in a DSP and FPGA, with extensive testing. We also made some improvements to the MAC layer to improve the operation of the AMC. The customer made modifications to the MAC layer and above to accommodate the increased throughput of the PHY.
Outcome – The increased higher data rate and improved resilience was evident at customer trials and subsequently enabled increased large volume export orders.
The new upgrade of the Physical layer from CDMA to OFDMA – giving 5 times improvement at a low rate and 11 times improvement at the highest rate for the same bandwidth, while accommodating harsher channels. Trials of the radio were very successful with a large number of radios with the improved PHY being used by an allied country to improve their data communications.
Radio upgrade program- Legacy proprietary radio PHY conversion to SDR
Challenge -The customer had a proprietary high data rate mobile ad-hoc network (MANET) transceiver which they wanted to port to a software defined radio platform (SDR). The lower MAC and physical layers were implemented on a DSP, two FPGAs and an ASIC. There was no documentation available on the ASIC. The DSP needed to respond very quickly to signals from the FPGA. Platform specific functionality was strewn throughout the code. The software radio needed to perform identically to the original. We did not have access to the SDR ourselves for the entire programme.
Approach – After studying the capabilities of the specific SDR chosen, it became clear that the DSP-FPGA interface was too slow, so we implemented a soft processor in the FPGA fabric to perform the functions of the original radio’s DSP. As there was some spare capacity in the original radios FPGAs, our first steps were to refactor the FPGA implementations to separate waveform specific functionality from platform specific functionality. We also reverse engineered the ASIC functionality and reimplemented this functionality in the existing FPGAs, and thus were able to test and prove the refactored functionality. Using an FPGA development kit we constructed a test harness, that modelled the environment of the FPGA in SDR and allowed us to stimulate it with commands issued from MATLAB. We took the waveform specific functionality and ported it over to the new FPGA, noting not only a change of hardware generation, but also a change of FPGA vendor and hence toolsets. We then implemented the very small amount of platform specific logic needed to interface to the SDR interfaces (e.g., rate matching and FIFOs for commands). As the SDR FPGA was large, we were able to perform back-to-back testing of the modems by instantiating two of them coupled transmit-to-receive and vice versa in the FPGA, which allowed a significant amount of modem testing to be performed without access to the SDR platform. Very little support was needed when integration with the real SDR platform occurred.
Outcome – The new SDR implementation provided was proven to be completely compatible with the original radios over the air interface and has been trialled by an allied country. The new encapsulated functionality has proved straightforward to port to other DSPs and other FPGAs, and there are plans to port it to several other software radios.
Tracking & Locating
Challenge – An established Tracking company wanted to enhance their product offering to provide a second direction finding vector & improve performance & reliability. The customer wanted a new air interface for their transceivers that would allow them to be geolocated using timing measurement in addition to direction finding.
Approach – The new air interface had to provide a sensitivity similar to their previous system but carry significantly more user data. The customer provided the processing hardware, and the radio front ends; initially, it appeared that the processing requirements would exceed the capability by 2 orders of magnitude. We took the specification supplied by the customer and designed and modelled uplink and downlink physical layers to achieve the desired performance, with a view to the capabilities of the processing hardware. We then implemented this new air interface on the hardware supplied, noting that the processing resources were highly constrained, requiring several architectural iterations to reduce the complexity; the bulk of the processing was performed in an FPGA. The remote transceiver was a new design, and we advised the customer on the processing FPGA to use in this device. A close working relationship was maintained; and all code and documentation was supplied to enable ongoing support of their product with or without us in future.
Outcome – The new air interface software and firmware was able to perform the timing measurements to the required accuracy at the sensitivity specified, enabling improvements to performance for tracking & locating accuracy. The entire system is being trialled by the End customer and system user where it is performing very well.
WiMAX Forum interference study.
Challenge – Our customer wanted to enable the deployment of WiMAX in frequency bands earmarked for IMT-2000 technologies and needed a study to show that coexistence was practical.
Approach – We performed a minimum coupling loss study and Montecarlo simulations using the specific performance specifications, and measured terminal performances, to determine the likelihood of interference preventing successful operation for TDD WiMax 802.16e and FDD WCDMA, in adjacent spectrum and determined the benefits of a range of mitigation techniques to enable successful coexistence. The results were presented to; reviewed and discussed in ITU working group 8F and its successor 5D, and we contributed significantly to an ITU-R report.
Outcome – Our coexistence study was combined with another study and published as ITU-R Report M.2113. WiMAX 802.16e and subsequently became an IMT.2000 technology.