A government’s prime responsibility is to protect and defend its citizens. Civil protection is performed by organizations like the police, fire, and ambulance services, while the armed forces defends against military interventions. Secure, reliable, and robust wireless communications form vital aspects of each service.
Historically, the systems have migrated from analog to narrowband digital transmissions that support voice and some restricted data capabilities. The network of each service would typically have been conceived as standalone and usually unable to interoperate with any other system. Therefore, the task of coordinating—for example, police and fire services at an emergency—is fraught with difficulties.
Regrettably, it takes major incidences such as 9/11 in America, or the Sewol ferry capsizing off the Korean coast, to focus attention on the problems. Following these and other tragedies, the governments of the U.S., South Korea, and U.K. each decided that better integration between the different branches of emergency responders was an important issue to address. As such, these governments have already legislated to update their emergency service systems in light of these requirements.
The current standards of traditional land-mobile-radio (LMR) technologies such as P25 and TETRA provide voice with limited data. In contrast, commercial cellular smartphones are used by billions of people to speak, browse the internet, stream video, and view photos, demonstrating that user-friendly technology is available for much richer communications. A similar realization in the armed forces has spurred the move toward software-defined radios (SDRs) and cognitive radios.
Some countries have chosen to bite the bullet and update their systems. The plans are to base the new network for the emergency services on the LTE system, which is often called 4G. This would leverage the vast research and development work that has produced the commercial systems and bring the latest capabilities to the public-safety community. Importantly, spectrum in the U.S. has been made available in the 700-MHz band—it offers excellent propagation characteristics for penetrating buildings and providing large geographical coverage from each base station.
The 3rd Generation Partnership Project (3GPP) is a body that sets global standards for cellular telecommunications, including LTE. This specifies network technologies, including radio access, the core transport network, and service capabilities. The 3GPP requests suggestions from its members for addressing new capabilities, and then assesses and agrees on the technical details that are published as a new specification release. This gives manufacturers the confidence to build equipment with the knowledge that complying with the standards enables them to participate in a mass market and lower costs through economies of scale.
LTE employs an all-Internet-Protocol (all-IP) architecture designed for low latency and high resilience that supports interoperability of both data and Voice over LTE (VoLTE) transmissions. It also can exploit sophisticated digital encryption.
In 2015, 3GPP issued Release 12, which had a focus on technical specifications for mission-critical applications. It included a feature called Proximity Services (ProSe) that allows direct broadcast communication between nearby phones used by safety personnel in the event of a disaster taking down the network. It also includes enhancements in a capability called Multimedia Broadcast Multicast Services, which is required for push-to-talk.
The specification for Release 13 was finalized in 2016 and includes support for Mission Critical Push-to-Talk (MCPTT). This provides group calling, person-to-person calls, as well as prioritization of calls, which are all needed by first responders. It supports direct-mode voice communications, along with a "discovery" feature that lets the user know of other radios within direct-mode range. In addition, a relay capability allows an out-of-range user to hop via another LTE device to access a fixed LTE network. The work continues with Release 14 to include features such as mission-critical data and mission-critical video together with lower latency and enhanced quality of service.
The LTE specification now includes the “must-have” features for public-safety radios, but that does not mean users will switch over immediately. As an example, Germany has around 500,000 users of TETRA and just placed a sizable order for 20,000 more sets. Like many countries, Germany has made significant investments in equipment and infrastructure. Equally obvious is the fact that reliance on outdated technology could hamper its users by denying them the benefits of a broadband system.
One possible implementation strategy would be the gradual introduction of LTE as a parallel service. This would avoid the need to obsolete equipment before the end of its serviceable life, while avoiding the “big bang” of a radical overhaul.
Building Flexibility into Phones
Handset manufacturers now have a viable solution from Lime Microsystems to produce a very flexible multi-standard phone that supports both LMR and LTE technologies in a single unit. The technology would provide interoperability, which is a very important issue for public safety communications during joint-response efforts. This would allow for a hybrid or blended mix, where the phone can communicate with users of LMR equipment, as well as provide access to LTE networks.
1. The dual-transceiver LMS7002M covers all LTE, TETRA, and P25 frequencies.
Lime manufactures a range of highly integrated and low-power programmable transceivers, called field-programmable RF (FPRF) devices. These open-source devices are used in a wide range of applications that spans commercial and military communications, as well as industrial and scientific use. The dual-transceiver LMS7002M, for example, covers all LTE, TETRA, and P25 frequencies with in-system programmable frequency, gain, and bandwidth (Fig. 1). The device is compatible with digital formats such as OFDM, WCDMA, and QAM. It also supports 2×2 multiple-input, multiple-output (MIMO) operation in a single device, as well as higher levels, such as 4×4, using multiple chips.
The RF inputs to the receivers can use one of three low-noise amplifier (LNA) blocks, optimized for high-band, low-band, or wideband signals. The architecture features direct conversion, or zero-IF, which is made possible using modern semiconductor technology and astute design techniques. These are then mixed with a local oscillator (LO) and can directly yield a baseband signal.
The chip can be used in a heterodyne system, too, because the device is designed with the flexibility to break out the signal chain to use external components. Therefore, if required, the mixer is able to produce an IF of, say 10.7 MHz, which can then be further filtered and mixed down to baseband.
The receiver baseband gain and lowpass filter networks are fully programmable. The device features both analog and digital filters to provide a powerful combined capability to meet exacting requirements. The signal can be digitized using the on-board analog-to-digital converter (ADC), and is further processed to filter out any converter aliasing artifacts.
The transmit paths accept the in-phase and quadrature (I&Q) components of the signal, which is filtered by programmable digital signal processing (DSP). The data is then passed through the on-chip digital-to-analog converters (DACs) to produce an analog signal that’s further filtered and amplified before being mixed up to the required modulated RF waveform. A programmable RF gain block is used to amplify the signal prior to its output from the chip.
The transceiver has been designed to meet the demands of a wide range of applications. However, when system demands are extreme, the designer can use external components to work in conjunction with the LMS7002M.
2. Illustrated are external signal paths and bypass options for the transceiver.
For example, any of the transceiver lowpass filters can be either replaced or supplemented by external filters. In addition, designers who need higher resolution than the on-chip 12-bit ADCs can output the analog signal to external devices (Fig. 2). Any unused blocks are able to be depowered to minimize consumption.
Low power and ease of design are both enhanced by the ability to run the device from a single 1.8-V rail. The reduction in the number and complexity of external regulators saves both printed-circuit-board (PCB) space and cost, and helps enhance reliability. The device’s power consumption is typically 550 mW, which rises to 880 mW in full 2×2 MIMO mode.
The open nature of the FPRF devices and the ability to output or inject anywhere along the signal path means that it can also be used in analog systems. It’s therefore possible to consider a dual-mode handset that supports both LTE and legacy LMR using the same devices.
To date, population coverage rather than geographical coverage has driven LTE service providers. This is one aspect that must be addressed for emergency services, which may be called to operate in remote or devastated areas. Fortunately, there are several options.
The maximum transmitted power is covered by 3GPP, and so would need revised standardization first. However, LTE allows for the option of adding fixed small cells to a network. Another option afforded by LTE is to use temporary cells either on a ground vehicle (cells on wheels, known as COWs) or even held aloft by a tethered unmanned aerial vehicle (UAV) or drone. These cells can supplement poor coverage or fill in for first responders where the infrastructure has been destroyed.
3. The LimeSDR SDR card is a key component of the LimeNET open-source small-cell hardware. (Courtesy of Lime Microsystems)
The design of a specialist small cell for a limited market such as emergency services can be both complex and expensive, and at first glance would appear to be an insurmountable barrier. However, Lime has created open-source small cells under the banner of LimeNET. The open hardware units are based around SDR cards together with additional CPU processor and memory resources (Fig. 3).
The range of pre-qualified hardware features SDR units that include FPRF and programmable logic to provide the wireless subsystem. The software running on the CPU handles the higher-layer protocols. The SDR can be programmed to the frequency (Band 14 in USA, Band 28 in South Korea) and bandwidth required at any one time.
The open-source community around the world has embraced the concept of freely configurable wireless connections, and already designed numerous paid and free applications that are loaded onto an app store depository. For example, Quortus software could be used on LimeNET hardware to implement what the company terms “bring your own coverage” solutions, resulting in an LTE network that includes both data and VoLTE connectivity. The system is able to connect mobiles internally or wirelessly link to a cell tower functioning on a 2G, 3G, or LTE/4G network.
Open-source hardware and software allow the system to be easily customized by running apps on top. For instance, engineers can modify the hardware to include support for legacy analog wireless links by supplementing the FPRF with additional components. Equally, software engineers can create code that supports features such as ProSe and MCPTT, which are essential for first responders.
4. This is one of the LimeNET open-source base stations. (Courtesy of Lime Microsystems)
Similarly, in systems where the spectrum is shared with the public, the emergency-service traffic can be prioritized on a preemptive basis. Designers could conceivably modify the LimeNET to act as a gateway between analog and digital handsets, as well as provide voice interoperability to ease the transition to a fully LTE-based system (Fig. 4).
It’s a time of transition for first responders, as advances in their secure wireless systems will provide voice communication along with location tracking, messages, images, floorplans, live-video feeds, and mugshots. A flexible and open system is required to avoid expensive vendor lock-in with proprietary software, while also making it quick and easy to upgrade existing deployed infrastructure with future enhancements to LTE.
To quote from the FirstNet web, “FirstNet was created to be a force-multiplier for first responders—to give public safety 21st century communication tools to help save lives, solve crimes, and keep our communities and emergency responders safe.”