It’s only recently that we’ve started to see software-defined radio (SDR) in commercial use, primarily on HF from companies such as Codan and Barrett, although it has been in amateur radio and military radio use for some time. The Wireless Innovation Forum was established in 1996 and is a non-profit international industry association dedicated to creating a revolution in wireless communications based on reconfigurable radio. Now, 20 years later, we’re starting to reap the benefits of SDR technology in commercial VHF/UHF two- way radio, as digital technology becomes more widely available at low cost.
An average SDR currently uses little analogue circuitry, typically a wideband transmitter power amplifier and front-end amplification and selectivity for VHF/UHF receive. Recently a transceiver design based entirely on digital technology, the Pizzicato, was revealed.
SDRs are flexible enough to allow wideband operation along with multimode transceive capability including spread spectrum. This latter mode allows several transceivers to operate in the same geographical location on the same frequency with very little interference, and is typically combined with appropriate error detection and correction techniques within the transceiver.
Dynamic transmitter power adjustment can typically also be employed, to lower the transmit power to the minimum necessary and hence reduce the near-far problem along with lessening interference to other users. This technique has already been used in some analogue and digital transceivers, but a SDR makes this extremely simple. Along with the transceiver circuitry, integrated software- defined antennas can also be used. These can dynamically lock on to a directional signal so that receivers can more effectively reject interference from unwanted directions.
SDRs along with software-defined antennas are also significant enablers of cognitive radio. My belief is that virtually all two- way radios in the foreseeable future will use SDR technology. In the long term, software- defined radios are also expected by SDR promoters such as the Wireless Innovation Forum to become the dominant technology in radio communications.
Cognitive radio and mesh networks
VHF and UHF spectrum for PMR is in short supply. In several areas of the UK the spectrum allocation for PMR in some bands is fully occupied, with no further exclusive channel allocations available. However, intelligent use of existing spectrum has many benefits, such as the use of trunking and community repeaters rather than exclusive channels, and adaptive base antenna systems. DMR and dPMR can also effectively double the channel use in a given allocation by allowing the possibility of two separate communication channels within each 12.5 kHz-spaced channel.
Cognitive radio (CR) is a technology that could efficiently utilise unused ‘white space’ spectrum, potentially allowing large amounts of spectrum to become available for future high bandwidth applications. White space spectrum is that which is used in some geographical areas or at certain times, but remains otherwise unused.
The long-term vision of CR technology is one in which handsets would automatically make use of under-utilised spectrum across a broad frequency range. A diagram from Ofcom’s webpage on CR appears to indicate that there are certainly many areas of the radio spectrum which are not fully utilised in different geographical areas of the country.
The ‘hidden terminal’ is also an issue with CR. This is where one radio terminal finds a frequency that is clear at its end, but at the other end of the prospective communication link the same frequency is in use. Ofcom says that the ‘hidden terminal’ problem (shown in the diagram above) must be overcome to ensure that primary users of a band are protected from interference. Ofcom adds that consideration must be given as to the most appropriate mechanisms to allow CR.
The ‘hidden terminal’ problem must be overcome if CR users are to coexist with other radio users
A study completed in 2007 by QinetiQ for Ofcom on CR addresses areas such as possible applications, regulatory issues and CR’s potential economic benefits. Although the technology holds much promise it is a radical departure from existing methods of spectrum regulation.
If Ofcom decides that there’s sufficient market demand it could make spectrum available for CR users, but it would have to consider issues such as the quality of service (QoS) that would be provided and what it would be responsible for. A network operator with an area-defined licence could allow use of its frequencies by cognitive radios via spectrum leasing.
If CR ended up being used in licence- exempt spectrum, then users would have to weigh up its benefits against the lack of quality of service and protection against inference. While a light licence based on spectrum for CR use would offer some protection against interference, Ofcom does not provide QoS thresholds for light licences.
Radio over Wi-Fi
Radio over Wi-Fi enables licence-free two-way push-to-talk or full duplex communication over an existing or a new IP/WLAN network, and allows both individual and group calls. The Icom IP Advanced Radio System is an example of this. A natural progression is systems that allow seamless roaming between PMR, cellular, and Wi-Fi networks – the Tait UnifyVoice system is one example of this. Here if a user with a smart device and a LMR radio is in an area with acceptable cellular or Wi-Fi service they will communicate using this network. If they enter an area with poor cellular or Wi-Fi coverage they will connect using traditional VHF/UHF circuitry incorporated in the terminal. This is performed seamlessly without user intervention and roaming between networks is also seamless.
Interoperability
Many radio users find interoperability with other communication systems invaluable, if not essential. In the past this has commonly been achieved by a user carrying a two-way radio, a cellphone, a data terminal and maybe a pager. A typical example is in a hotel where staff need to be made aware of an initially silent fire alarm, and to have the specific location of this automatically sent to their radio terminal by text so they may investigate before rousing guests. Likewise seamless interconnection with the hotel’s internal private automatic branch exchange.
Future PMR systems will be increasingly integrated with other communication systems, either directly or through the use of a communication gateway, inducing communication between terminals and IoT systems. SDR terminals can potentially offer this though the use of wideband multimode functionality, although this may well be in the long term and be constrained by licensing requirements. We are already seeing combined LTE and PMR terminals becoming available, although these currently require an increase in physical size and functionality. Future terminals using SDR technology will overcome this to a considerable extent.
Commercial PTT over 4G LTE
If commercial two-way radio users can tolerate a slight speech delay (typically 700 to 800 ms) PTT over Cellular (PoC) is available right now using a system called OMA PoC – Open Mobile Alliance PTT over Cellular – with a suitable app on a smartphone. Normal consumer phones can be used with such an app, which then have a ‘soft’ key on the display to be used as a PTT button. Alternatively, tougher cellphones are available that often look just like a two-way radio, complete with a push-button numeric keypad, display, rotary control, set-top antenna and side-mounted PTT. The delay of around three-quarters of a second between transmitted and received speech is only slightly longer than the typical 550 ms delay that occurs with DMR.
With PoC the user can typically create private channels and groups to allow instant group communications without boundaries. Using an in-app contact list the user can call either all group members or just select one or two of the contacts and create an ad-hoc one-to-one or smaller group. Tough PTT- equipped cellphones can be used by outdoor workers while still keeping in touch with office and management staff who have rather more fragile smartphones. This will certainly be a serious competitor to traditional PMR as 4G coverage becomes more widely available, especially with the evolving use of carrier- neutral small cells.
Mission-critical push-to-talk
On the subject of PTT over LTE, 3GPP LTE Release 12 came out last year and included proximity services, direct mode communications, group communications, and push-to-talk operation. Release 13 was frozen in March and includes mission-critical PTT. 3GPP has also (as part of Release 13) studied enhancements for LTE to use unlicensed spectrum. This includes Licensed Assisted Access (LAA) operation: the aggregation of a primary cell operating in licensed spectrum to deliver critical information and guaranteed quality of service, with a secondary cell operating in unlicensed spectrum to boost the data rate. A significant objective of this work was to ensure fair coexistence between LTE LAA and Wi-Fi. As its name implies, LTE is indeed Long-Term Evolution.
Carrier-neutral small cells
Small cells that can support more than one mobile operator have the potential to change the wireless landscape, particularly indoors. They become attractive investments for neutral host providers and tower companies. Indoor small cells could alleviate ‘dead spots’ in thousands of buildings, and neutral host providers may be ready to deploy these if they can see a return on investment. Enterprise small cells have been of interest to mobile operators for some time, but only recently have end users started to appreciate their potential value.
Small cell and distributed antenna system (DAS) networks have become an important and fast-growing complement to the deployment of new spectrum, and are becoming more common as an attractive option to increase capacity and extend coverage outdoors. Distributed network solutions are becoming increasingly popular for enterprise buildings and high traffic indoor venues. Although these systems are in their infancy they are poised for rapid growth. This is because mobile operators wish to provide improved coverage and meet ever-increasing capacity needs in key venues such as airports, shopping centres, stadiums, arenas, hotels, transit systems and other high traffic venues with dense user populations. This is certainly a future growth area to provide 4G communication coverage in otherwise unserved areas.
Bandwidth and spectrum allocation
We know that VHF and UHF spectrum is in short supply, however future two- way communication is certainly liable to require greater channel bandwidths for communication. Spread spectrum and white space operation can assist to a considerable degree here but higher two- way communication frequencies, reportedly up to 60 GHz, are likely to be required to accommodate the communication parameters expected by future users.
Many years ago we wouldn’t have dreamt that PC processor speeds of two GHz and above would be available at low cost, likewise with digital memory. The future of two-way communications is inescapably based on digital technology, and the development of faster processing speeds will allow this. But geographical coverage at such high frequencies will require a greater number of base station sites, and this will lead to an increase in terrestrial micro cells. Aerial cells are predicted to offer an increase in coverage, either though low Earth-orbiting satellites, high-altitude balloon-based systems, or drones to fulfil a temporary extra bandwidth need. Higher frequency spectrum allocations will require near line-of-sight radio coverage and the technologies discussed above are among those that will provide this.
The future We may look forward to seeing combined PMR-, Wi-Fi- and LTE-equipped cognitive SDR devices. These will be wideband multi-mode terminals, capable of being programmed for modes such as analogue FM, DMR, dPMR, LTE, TETRA, Wi-Fi and other communication modes. Carrier- neutral small cells will enhance coverage in areas where communication would otherwise be difficult or impossible. Higher frequency spectrum allocations will be employed to allow greater bandwidth and accommodate greater numbers of users, and significantly different base station situations will be required to provide service. It’s happening already, the future is visible, and it’s continually evolving.
About the author
Chris Lorek is chief technical officer at South Midlands Communications Group, a specialist comms product designer, supplier and installer.
