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Software Defined Radio: The Key to Public Safety Radios

The need to coordinate operations between different agencies such as fire, police, state and federal has become more urgent than ever. The flexibility and communications power of software defined radio is being harnessed for this daunting task.

RODGER H. HOSKING, PENTEK

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During the decade bridging the tragic events of 9/11 and the recent devastation in Japan, the world has experienced numerous disasters claiming thousands of lives and destroying massive amounts of property. Now more than ever before, firefighters, police, emergency medical teams, rescue units and other first responders are expected to act immediately, correctly and effectively to save lives and minimize damage. Major natural disasters such as hurricanes can pull in additional state, federal and military resources. Concerns over criminal or terrorist involvement add federal crime and intelligence organizations to the mission.

In nearly every case, these events are sudden and totally unpredictable in scope or nature. Assessing the many complex issues surrounding each event to quickly form a strategic plan of action is incredibly difficult and subject to revision. However, even with the best of plans, trying to coordinate local, state and federal organizations still remains one of the toughest logistical challenges, and communications is near the top of the list.

Communication systems used by the numerous organizations involved in a crisis are often incompatible, as was widely publicized and acknowledged in the 9/11 response. The selection and procurement of radio sets and the choice of frequency bands tend to be done at the local level to meet the required territorial coverage and operational needs of the particular organization, and are influenced by support for existing legacy equipment, vendor allegiances and budgetary constraints.

A nationwide public safety network of radios that can be configured to do exactly what each organization needs and yet provide the interoperability to support successful cooperation among these organizations is essential. Delivering on these features with improved levels of security, reliability and informational bandwidth requires regulatory changes for frequency allocation, government mandates for adoption of radio standards, and new development in technologies like software defined radio (Figure 1).

Figure 1
Pentek’s Model 7151 256-Channel DDC PMC module uses an advanced Virtex-5 FPGA IP core to accommodate high channel density for applications like P25 radios and GSM.

Legislative Initiatives Harness SDR Technology

In October 1989, the Association of Public Safety Communications Officials International, along with other government organizations, launched the APCO Project 25 initiative to provide a standard for public safety radios. It uses parts of the VHF and UHF bands previously allocated for analog public safety radios, and maintains compatibility with those legacy radio sets. In addition, it supports digital FM modulation for both speech and data, plus several levels of digital encryption and digital access controls for secure communications. As a result, Project 25 (or P25) represented a milestone for licensed, widespread deployment of a major communications system based on software defined radio (SDR) technology.  

SDR takes advantage of digital signal processing (DSP) techniques to replace the analog processing tasks associated with radio, such as filtering, frequency conversion, modulation and demodulation. Of course, because the electromagnetic signals propagating through the air are analog, A/D and D/A converters are an essential interface for any software radio system. The hardware used in these radios can be ASICs, processors, FPGAs, or just about any kind of digital device that can be configured to perform a digital signal processing task dictated by the user and the operational mode of the radio.

Perhaps the earliest example of a software radio function is the DDC (digital down-converter). It translates a specific signal frequency within a digitized radio band down to baseband (0 Hz) using a digital mixer and digital local oscillator, and then filters it to the required channel bandwidth using a digital low pass filter. This essential SDR element easily tunes to any of several hundred available channels with precise frequency accuracy and extremely fast switching. Using a complementary process, the DUC (digital up-converter) filters and translates a baseband channel signal up into specific channel frequency slot for transmission. 

To gain better use of the original 25 kHz analog FM channel spacing on the public safety band, Phase 1 of the P25 standard split this spacing in half to 12.5 kHz, doubling the number of channels. In Phase 2, the channel spacing halved again to 6.25 kHz (Figure 2). To handle the higher channel count, improved DDCs and DUCs became even more important.

Figure 2
SDR techniques have allowed P25 public safety radios to split the legacy 25 MHz analog FM channel into two 12.5 kHz channels (C4FM) or four 6.25 kHz channels (CQPSK) while still preserving full information capacity on each channel.

An additional important SDR function for P25 is the digital modulation scheme. In order to achieve the same information capacity across these narrower channels, a 4-level FM modulation scheme called C4FM was developed.  It handles a 4800 baud signal with 2 bits per symbol, delivering 9600 bits/s for each 12.5 kHz channel. 

While the C4FM modulation scheme required for P25 Phase 1 is now in widespread use throughout the country, the new Phase 2 requirements mandate the same channel capacity within a 6.25 kHz bandwidth. To achieve this feat, a new compatible quadrature phase shift keying (CQPSK) modulation scheme uses advanced signal processing techniques to achieve the same 9600 bits/s channel rate.

Because P25 radios must still operate with the legacy radios, the Phase 2 sets must be capable of recognizing and adapting to legacy 25 kHz analog radios and Phase 1 sets. Scanning, classifying and adapting to this diverse traffic pattern by engaging different modulation and demodulation tasks in a single radio is a perfect match for the configurable radio architectures provided by SDR.  

Another exploitation of DSP technology for P25 radios is the speech vocoder. In order to improve transmission and reception of speech over relatively low bandwidth digital channels, vocoders extract key parameters from speech including pitch, formants (resonant frequencies in the vocal tract) and fricatives (like “t” and “s” sounds). These parameters change relatively slowly compared to the bandwidth of speech itself, so the digital samples of these parameters along with error correction coding can fit well within the 9600 baud digital channel supported by P25. At the receiving end, these parameters control the frequency of a digital pitch generator, filter coefficients for the formants, and a fricative generator to reproduce the speech. The standard vocoder for P25 is the Improved Multiband Excitation (IMBE) model. This algorithm provides far superior intelligibility compared to an analog signal over the same channel bandwidth, especially in the presence of channel noise.

One major benefit of vocoders is the inherent rejection of ambient audio noise, because it does not fit the parametric model of the vocal tract. With a dual microphone handset, background noise picked up by the microphone closest to the person speaking is often the same as the noise picked up in a second microphone aimed in a different direction. By subtracting the common background noise, the first microphone signal is made clearer. Consumers are now benefiting from this same digital signal processing technique in noise-cancelling hands-free phone systems in automobiles.

Cognitive Radios

Cognitive radio is an extension of SDR that opens up important new features for public safety. The cognitive radio set uses various sensors to ascertain its environment so it can adapt its mode of operation and communicate situational information. 

For example, a cognitive radio can authenticate the user through a retina scanning camera to validate pre-authorized privileges and access rights to certain channels or modes of operation. This allows radio sets to be exchanged in case of damage and still deliver correct access rights to the new user. It also provides security against unauthorized access in case the radio is lost or stolen.

A GPS subsystem built into the radio can not only show the user his exact location, but it can also be used to transmit his location to the rest of the users. Some new P25 radios now display a graphical map of the area marked with the locations of nearby team members in one color and members of other organizations in other colors (Figure 3).

Figure 3
Handheld cognitive radios use GPS to locate each radio and automatically share their position over the air. Locations of police, fire and EMT personnel are displayed on each handheld radio.

One major objective of cognitive radios is to continuously monitor the radio frequency environment and classify the spectral terrain. In this way it can automatically adapt to interference or poor signal quality to change communication channels, modulation type and power level, as required, for best reception. This process is often referred to as dynamic channel allocation.

With a cognitive radio, the public safety communications administrator can upgrade software, change access privileges, add or delete authorized users, and even add new channels or modulation types in the field and entirely over the air, eliminating the expense and inconvenience of returning the radio sets to an upgrade facility.

LTE – The Future of Public Safety Radio

Today, there are still two significant shortcomings of public safety radio. The first is that public safety organizations have failed to take advantage of the many technical benefits of the P25 system to achieve interoperability. After over $7 billion of federal spending to improve public safety communications since 9/11, improvements in inter-organizational communications are spotty and disappointing. Much of this can be attributed to turf protection and politics.  

The second shortcoming is channel capacity. Even if multiple P25 channels are combined for extra bandwidth, transmission of photos and video and other high-speed digital traffic is either impossible or limited. 

In 2005, the FCC ordered the transition of broadcast television stations to digital broadcasting. This freed up over 100 MHz of bandwidth in the 700 MHz band, with 24 MHz reserved for public safety. Most of the remaining bandwidth was auctioned off by the FCC to commercial carriers by 2008, leaving just 10 MHz, known as the D Block. The reason for this was the controversial D Block had strings attached: the licensee must agree to relinquish use of the D Block if needed in a public safety emergency. 

The government’s rationale for this restriction was clear: let commercial carriers build up and develop this broadband infrastructure at their expense, and then let public safety have priority use of it, if necessary. It is estimated that the government could save up to $9 billion in infrastructure and build out costs if such a deal could be made. However, because of this restriction, the D Block failed to attract the minimum bid of $1.3 billion and negotiations are now underway to offer a better plan.

Nevertheless, in January 2011, the FCC announced that regardless of the funding model, the next generation of public safety radios must transition to the 700 MHz band. A major part of this band will use the Long Term Evolution (LTE) standard to provide a broadband infrastructure with plenty of bandwidth for voice, video, graphics, data bases, and access to virtually all Internet resources. 

These new, high-bandwidth public safety radios will rely even more heavily on software radio to enhance communications reliability, improve access security, streamline the delivery and sharing of information and media, and provide situational awareness for first responders so they can operate more effectively in the future.  

Pentek
Upper Saddle River, NJ.
(201) 818-5900.
[www.pentek.com].