Bateman, A

Marvill, JD

McGeehan, JP

English

An overview of the requirements of a scrambler for use with mobile radio equipment is presented. Details of the implementation of a scrambler are given satisfying these requirements. The scrambler is realized using general-purpose DSP technology giving the benefits of low-cost, high-volume production with the flexibility of customization and enhancement though software configuratio

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                          Bateman, A., Marvill, J. D., & McGeehan, J. P. (1992). Voice scrambling forradio, cellular and telephone systems. In Unknown. (Vol. 2, pp. 968 - 972).Institute of Electrical and Electronics Engineers (IEEE).10.1109/VETEC.1992.245266Link to published version (if available):10.1109/VETEC.1992.245266Link to publication record in Explore Bristol ResearchPDF-documentUniversity of Bristol - Explore Bristol ResearchGeneral rightsThis document is made available in accordance with publisher policies. Please cite only the publishedversion using the reference above. Full terms of use are available:http://www.bristol.ac.uk/pure/about/ebr-terms.htmlTake down policyExplore Bristol Research is a digital archive and the intention is that deposited content should not beremoved. However, if you believe that this version of the work breaches copyright law please contactopen-access@bristol.ac.uk and include the following information in your message:• Your contact details• Bibliographic details for the item, including a URL• An outline of the nature of the complaintOn receipt of your message the Open Access Team will immediately investigate your claim, make aninitial judgement of the validity of the claim and, where appropriate, withdraw the item in questionfrom public view.VOICE SCRAMBLING FOR RADIO, CELLULAR & TELEPHONE SYSTEMS A. Bateman, J. D. Masvill8z J. P. McGeehan Centre for Communications Research, University of Bristol Queens Building, University Walk Bristol BS8 lTR, U.K. Tel: +44 272 303104, Fax: +44 272 255265 Abstmct - A half.-duplex analogue speech scram- bler for radio communications pioneered by the University of Bristol and developed under li- cence to GEC-Marconi Secure Radio Systems has recently been designated by the Home Of- fice as the sole scrambler recommended for use within the UK police. It employs a novel frequency-based scrambling technique, capable of maintaining a very high level of privacy and voice quality with negligible time delay. Designed for use in the mobile environment, rugged performance is achieved with the appli- cation of forward error correction during syn- chronisataion and key variable transfer. The scrambler is implemented on a low-cost, gen- eral purpose, single-chip digital signal processor (DSP). Minimal reduction in radio battery life is achieved through power management tech- niques. The same scrambling technique has been applied to fixed and cellular telephones, in full-duplex operation. 1 Introduction The majority of existing communications links - whether fixed or wireless - do not employ security measures to deter eavesdropping. Radio transmis- sions are particularly vulnerable since cheap scan- ning radio receivers are readily available, and are commonly used to compromise cellular telephone and private mobile radio (PMR) conversations. In the UK, one of the largest users of radio communi- cations, the police, are moving to rectify the situa- tion, by adopting a speech scrambler, to make their broadcasts unintelligible to unauthorised listeners. Extensive trials have recently been undertaken by the Home Office, with only one unit meeting the requirements necessary for recommendation. This paper describes some of the techniques developed and issues addressed in the design of the speech scrambler, which is based on a prototype devel- oped at the University of Bristol. 2 Choice of Scrambling Tech- niques The most important properties of a speech scram- bler are: effectiveness in preventing readthrough quality of recovered (unscrambled) speech time delay introduced by processing bandwidth expansion level of security offered by encryption tech- nique, ruggedness of synchronisation ease of production cost The time delay issue, above any other parameter, determines the type of voice processing that can be applied in the scrambler. Conventional re-ordering 968 0-7803-0673-2/92 $3.00 1992 IEEE of speech segments in the time domain can provide a very high degree of privacy, but incurs a signifi- cant time delay penalty. This arises from the need for all speech segments in a frame to be recovered before the original voice passage can be replayed, Fig 1. In general, the longer the interleaving period for the time domain based scrambler, the higher the level of privacy afforded. Delays of several hun- dred milliseconds are typically required. For appli- cation to half duplex radio systems, where radios are keyed on and off frequently, and messages are short, the time delay experienced by these devices can lead to  operational difficulties for the users. Time Input sequence Output sequence Figure 1: Time domain scrambling To achieve a low time delay scrambler, signal ma- nipulation in the frequency domain is the most widely used approach, with techniques ranging from simple frequency inversion, to multi-band re- ordering schemes, Fig 2. To achieve compara- ble levels of privacy to those offered by time do- main techniques, the multi-band approach must be adopted, with the bands re-arranged in a rapid and suitably randomised manner. The success of a frequency domain scrambler is thus largely dic- tated by the number of bands used, the algorithm for their re-ordering, the speed of re-ordering, the precision with which the bands can be replaced, and the cost and complexity of implementation. In all of these areas it is believed that the new scrambling technique can claim a number of ad- vances. Amplitude Amplitude Normal speech. Frequency Scrambled speech. Figure 2:  Frequency domain scmmbling 3 Prototype Scrambler System Much of the improvement in the new scrambler is achieved by the use of Digital Signal Process- ing (DSP) for realisation of the scrambling process. Not only does this allow precise, fast and repeat- able frequency manipulation to be performed, but is also permits the incorporation of sophisticated synchronisation techniques to ensure very accurate and reliable reconstruction of the voice signal. 4 Crypto Engine The basic processing performed by the DSP is il- lustrated in Fig 3. One of the key blocks is the “crypto engine”, generating the stream of variables which control the pseudo-random reordering of the voice frequency. The integrity of the crypto engine is critical in achieving a high level of privacy for the scrambler. A number of connected shift register se- quence generators are used in the device, arranged 969 such that it is impossible to deduce the seed key variable from a knowledge of the output variable sequence. Figure 3: DSP processing in tmnsmit 5 Speech Manipulation Manipulation of speech based on the control vari- ables forms the next major processing block. There are two well documented methods for spec- tral reordering: The Fast Fourier Transform can be applied to obtain fiequency domain samples which can then simply be re-indexed. Alterna- tively, a filter bank can be used, similar to that em- ployed for sub-band coding, with subsequent band re-positioning by conventional phasing techniques. The latter approach is used in the new scrambler, offering a much reduced implementation overhead and processing delay than the FFT approach. 6 Synchronisation The final processing block is dedicated to achiev- ing time synchronisation between the scrambling and de-scrambling functions. With the rapid re- ordering of the speech bands, a very precise syn- chronisation process is needed. From subjective analysis of the scrambler system, it is found that a synchronisation error of less than 5 ms must be maintained in order to preserve voice quality. The initial method of synchronisation investigated involved detecting the phase reversal of a tone cen- tred in the speech band. This approach had the attraction of a very simple implementation, and was based on the knowledge that notching out a small portion of the speech energy would have a negligible effect of speech quality. The presence of the tone also served the purpose of identify- ing that the unit was operating in secure mode. When tested in the scrambler system, the tech- nique was found to work well under good signal strength conditions, but was prone to large syn- chronisation timing errors when the received sig- nal strength fell and noise energy corrupted the tone. A more significant problem was observed when the radio units were tested with repeater systems, where intermodulation between the syn- chonisation tone and speech components not only corrupted the timing reference, but also resulted in an audible distortion of the recovered speech signal. To eliminate these problems, a burst synchroni- sation scheme was adopted, based on a 1600 bps DPSK modem implemented in software. When a synchronisation update is needed, a short data burst is sent containing a known pattern which when correlated with the receiver reference pat- tern gives a precise timing reference. The timing error with this scheme was found to be less than one data bit period, (f1.25 ma). In order to ensure unique reception of the synchro- nisation word in the presence of noise, a long word length is required, however this presents a conflict with the processing overhead involved in pattern matching, and the duration of the speech blanking imposed whilst synchronisation if performed. It is also possible that when operating in poor signal conditions with high bit error rates, the synchro- nisataion word will never be successfully decoded and synchronisation will be lost. A compromise solution was reached in the final design, by using 970 a synchronisataion word length of 31 bits with the addition of BCH forward error correction. Under circumstances where synchronisation bursts were undetected due to severe signal fading, a “fly- wheeling” process was employed which maintained synchronisation based on an internal timer within the DSP. The accuracy of this flywheeling was suf- ficient to preserve voice quality for several tens of seconds before clock drift became too severe. The frequency of synchronisataion bursts for a scrambler intended for mobile radio use is deter- mined primarily by the requirement for “late en- try” operation, where users either miss an initial synchronisation word due to fading or simply being out of range, or where users switch on their equip- ment during a call. The main factor which deter- mines the minimum resync period is the subjective effect of frequent signal blanking during the data bursts. Based on customer trials, the maximum acceptable resync period was found to be three to four seconds. With the present scrambler design, this period can be altered under software control to suit specific customer needs. To further enhance the security of the scrambler, a temporary key variable, termed the “initialisation vector”, is transmitted with each resynchronisa- tion burst. This is also protected by forward error correction, and changes the re-ordering process on a frame by frame basis. Thus, even if the scram- bler sequence is deduced for a particular frame, the information is invalid for all subsequent frames. 7 Practical Implementation As mentioned at the outset, the versatility and per- formance of the scrambler is greatly enhanced by the use of digital signal processing. In the pro- totype system, there are only two main peripheral devices, the codec for analogue I/O, and EEPROM for retention of key variables and customisation in- formation. With power consumption a key factor in a scram- bler for use with portable radio equipment, the DSP chosen for the task was a first generation TMS320C17 processor from Texas Instruments. Through the use of novel algorithms, it was pos- sible to implement the entire scrambler, modem, crypto engine and error correction processing on one device, with the entire unit consuming a cur- rent when active of only 35 mA. This value never- theless is too high for many portable applications and a power down circuit was therefore added so that the device effectively switched off when the radio was in standby mode. Based on a typical 10% transmit, 10% receive and 80% standby duty cycle, the average current consumption is thus re- duced to 7 mA from a 5V supply. With the device now available with a 3.5V specification, it is an- ticipated that power consumption can be reduced accordingly. 9 Performance The University, in conjunction with GEC-Marconi Secure Radio Systems, has been a participant in a recent series of trials conducted by the UK Home Office to identify a scrambler for use by the UK police force. This trial was designed to asses a number of performance criterion including system reliability, privacy, quality, ease of use, and oper- ational range. Of the five scrambler devices put forward for the trial, the scrambler based on the Bristol University algorithm was the sole device to receive Home Office approval. 8 Application of the scrambler to full duplex cellular and fixed tele- phone systems In addition to the radio market, there is a growing requirement for scrambling on cellular and fixed telephone systems. For telephone operation, a full duplex mode of operation is required. To achieve this with a scrambler system, it is essential that all sidetone echo arising from 2:4 wire conversions at the handset and exchange is eliminated as this is 971 manifest as scrambled speech in the users earpiece. To eliminate sidetone in the telephone, a DSP based adaptive equaliser was developed, trained prior to the commencement of a call, which suc- cessfully eliminated all discernible echo. To pre- serve the subjective performance of the telephone system, an artificial, clear sidetone was generated within the DSP and passed to the users earpiece. In this application, a full duplex scrambler func- tion was realised using a single TMS320C17 DSP, with the adaptive equaliser implemented in a sec- ond identical device. 9 Performance The University, in conjunction with GEC-Marconi Secure Radio Systems, has been a participant in a recent series of trials conducted by the UK Home Office to identify a scrambler for use by the UK police force. This trial was designed to asses a number of performance criterion including system reliability, privacy, quality, ease of use, and oper- ational range. Of the five scrambler devices put forward for the trial, the scrambler based on the Bristol University algorithm was the sole device to receive Home Office approval. 10 Conclusions An overview of the requirements of a scrambler for use with mobile radio equipment has been pre- sented. Details of the implementation of a scram- bler are given satisfying these requirements, and which has recently won Home Office approval in the UK for use with the police forces. The scram- bler is realised using general purpose DSP tech- nology giving the benefits of low cost, high vol- ume production with the flexibility of customisa- tion and enhancement though software configura- tion. 972 

Voice scrambling for radio, cellular and telephone systems

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Voice scrambling for radio, cellular and telephone systems

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