Maximizing the Benefits of LTE Networks in Mission Critical Push-To-Talk Systems
By DVSI Engineering
Introduction
Land Mobile Radio (LMR) systems such as APCO Project 25 and Digital Mobile Radio (DMR) have a long and successful track record providing Mission Critical Push-To-Talk (MCPTT) voice services. LMR is designed specifically to provide MCPTT service in demanding environments such as public safety, manufacturing, transportation and energy industries. Land mobile radios provide outstanding range, reliability and direct user-to-user communications (i.e. “direct mode”) without the need of infrastructure. Recently, there have been efforts to interconnect LMR systems to cellular LTE networks in order to increase communication flexibility and add support for high data rate applications.
Unlike LMR, LTE networks are dependent on an infrastructure of cellular towers and were primarily developed for phone and business communications where coverage, reliability and resistance to noise or other harsh environmental factors were secondary considerations. Consequently, it is unlikely in the near term that cellular LTE networks will replace LMR for voice communication in Mission Critical PTT applications. However, given the need for LTE’s flexibility and data capabilities, it is likely that hybrid LMR-LTE networks featuring both technologies will proliferate.
The emergence of hybrid LMR-LTE networks means that both LMR (mobile radios) and LTE devices (smartphones) will need to interoperate. Unfortunately, LMR and LTE devices are not directly interoperable since their networks have been designed and built to different technical standards. LMR systems work with narrow channel bandwidths and high transmission power; that requires low data rates to support high spectral efficiency. This means that the voice coding technology, known as a vocoder, must operate with as few bits as possible. That is why the 2,450 bit per second (bps) AMBE+2™ vocoder is used by LMR systems worldwide. Whereas, LTE cellular systems are lower power/higher bandwidth, allowing them to use the AMR vocoder that operates at a minimum data rate of 4,750 bps. As far as interoperability goes, the AMR vocoder runs with too many bits to fit within the narrow channel LMR system however, an LTE cellular network can easily accommodate the low bit rate AMBE+2™ vocoder. Additionally, since most LTE devices are smartphones the low bit rate AMBE+2™ vocoder technology can simply be put on the smartphone in the form of a mobile app, thereby enabling direct voice communication between the smartphone and virtually any mobile radio used on the hybrid network.
Maximize Benefits by Avoiding Transcoding
A hybrid network that incorporates the LMR vocoder into a smartphone provides the advantages of LTE services along with better voice intelligibility,1, 2 reduced transmission delay, support of end to end voice encryption and reliable performance in high noise environments. It also reduces overall system complexity and cost by eliminating the need for the network gateway3 to transcode (i.e. perform vocoder conversion) between LMR and LTE devices.
DVSI conducted MRT tests in accordance with ANSI/ASA standard S3.2 to compare voice intelligibility for a transcoded system against one that used the LMR vocoder end-to-end.4 Intelligibility was measured in the presence of background noise typical of mission critical environments. The MRT results showed that using the AMBE+2™ vocoder end-to-end had less of an effect on voice intelligibility than transcoding from AMR to AMBE+2™. As shown in Figure 1, transcoding resulted in an average loss of more than 15% in intelligibility as compared to the scenario where the AMBE+2™ vocoder is used end-to-end. In a recent study,5 an MRT score of 75% was considered a minimal speech intelligibility threshold, and in the aforementioned tests this was only achieved in the test conditions where transcoding was not used, confirming the advantages of using the AMBE+2™ vocoder end-to-end in the hybrid LTE-LMR network.
Figure 1: Intelligibility Test Results: Transcoding vs End-to-End 2450 bps AMBE+2™ Vocoder
Communication delay and encryption are important issues related to MCPTT systems. The use of transcoding necessitates the interception of the voice data midway along the communications path, decrypting and decoding the data to regenerate the voice signal, re-encoding the voice signal with the opposite vocoder and then re-encrypting the transcoded voice data before it continues along the path to its final destination. This sequence of operations involves significant data processing which slows the data and increases communication delay.
In addition, the decryption step raises the potential for the security of the data to be compromised midstream. Numerous agencies, utilities and corporations require encrypted communication to prevent criminal intercept and protect sensitive information. The highest level of security requires the digital voice data to remain fully encrypted from the originating device on a communication link all the way to the final receiving device (commonly referred to as “end-to-end” encryption). Unfortunately, transcoding requires the data to be decrypted midstream, and thus transcoding is not viable in a hybrid LMR-LTE network that requires end-to-end encryption. The only way to deliver end-to-end encryption in a hybrid LMR-LTE network is for the AMBE+2™ vocoder to be used end-to-end in both the LMR and LTE devices. End-to-end encryption is an important requirement in many MCPTT applications and, consequently, it is vital that solutions supporting this requirement be made available. Fortunately, several LMR vendors have developed applications that allow the AMBE+2™ vocoder to be used on virtually any LTE smartphone device. The result is a hybrid LMR-LTE network that avoids transcoding, and thereby provides higher intelligibility, lower delay and support for end-to-end encryption.
Direct Mode Communications Possible
MCPTT systems were designed to provide reliable voice communications directly between two devices even when infrastructure was not available (for example at a remote site). However, LTE devices were designed to rely on cellular infrastructure for all communications, and currently there is no support for device-to-device communications when such infrastructure is not present. Given this limitation, it has been proposed to use LTE ProSe (Proximity Services) or the similar V2X (Vehicle to Everything) communication technology for this purpose. Studies have pointed to the disadvantages of using ProSe, namely its much lower range compared to current LMR devices. In one recent investigation it was determined that LTE devices using ProSe would have as little as 11% of the range of LMR in direct mode.6 Employing the low bit rate AMBE+2™ vocoder in LTE devices to avoid transcoding and support end-to-end encryption may also facilitate the use of LTE direct mode voice links by increasing their RF power per bit thereby increasing useful transmission range.
Performance in Background Noise Environments
Public safety communications often occur in conditions of high acoustic noise resulting in very low voice signal-to-noise ratio (SNR) (SNR being the primary measurement of noise severity). In low SNR conditions the voice signal can become effectively lost in the noise, lowering the voice quality and making it more difficult to communicate. While background noise is found in many public safety applications, a 2010 DVSI article8 showed that the fireground environment is one of the most challenging for communications due to the extremely high levels of background noise commonly encountered and to the specialized equipment used by many firefighters. One example of the latter is the Personal Alert Safety System (PASS) alarm system, which emits a loud alarm signal if a firefighter gets in trouble. In order to characterize the behavior of LMR and LTE devices in challenging public safety environments, DVSI conducted additional MRT tests quantifying the intelligibility of the AMR vocoder (at 4,750 bps) against the AMBE+2™ vocoder (at 2,450 bps). As shown in Figure 2, the AMBE+2™ vocoder performs as well or better than AMR despite AMR having nearly twice the bit rate (4,750 bps vs. 2,450 bps).
Figure 2: AMR and AMBE+2™ Vocoders in background noise conditions
Conclusion
LMR devices, with their rugged reliability, direct mode capability and proven ability to handle high background noise environments are likely to remain a staple in Mission Critical PTT situations. Yet the features and benefits of higher data rate applications are influencing many traditional LMR users to consider a hybrid LMR-LTE network. The availability of new PTT services over LTE requires interoperability between mobile radios and smartphones. Including the AMBE+2™ vocoder in the LTE device maximizes the benefits of this approach without losing voice intelligibility, increasing transmission delay, degradation in noisy environments and maintaining security through end-to-end encryption.
References
1 U.S. Department of Homeland Security, Public Safety Communications Technical Report DHS-TR-PSC-13-0, (2013). Objective Speech Quality Estimates for Project 25/Voice over Long Term Evolution (P25/VoLTE) Interconnections. Retrieved from:
https://www.dhs.gov/sites/default/files/publications/Objective%20Speech%20Quality%20Estimates_0.pdf
2 Evans, J. (2014). Addressing Differences in Vocoder Technologies when Interfacing TIA-102 (P25) LMR Systems to Broadband Networks. Retrieved from:
3 Gateway is used generically to refer to Media Gateways Session Border Controllers and similar devices that include media format transcoding.
4 The DVSI MRT results were very highly correlated with subsequent tests using the ITS ABC-MRT16 algorithm. Webpage of the Institute for Telecommunication Sciences ABC-MRT16 and AMC-MRT Retrieved from: https://www.its.bldrdoc.gov/resources/audio-quality-research/abc-mrt.aspx
5 Letowski, T.R. & Scharine, A.A. (2017). Correlational Analysis of Speech Intelligibility Test and Metrics for Speech Transmission ARL-TR-8227. Retrieved from:
https://www.arl.army.mil/arlreports/2017/ARL-TR-8227.pdf
6 Ericson, D. (2018). Public Safety Broadband Stakeholder Meeting Presentation. Retrieved from: https://www.nist.gov/ctl/pscr/prose
7 Webpage of the Institute for Telecommunication Sciences ABC-MRT16 and AMC-MRT Retrieved from: https://www.its.bldrdoc.gov/resources/audio-quality-research/abc-mrt.aspx
8 Hardwick, J. and Salvo, P. “Improving Digital Radio for Fireground Communications.” Public Safety Communications 76 (2010):36-40.
