Error-Control Coding and Convolutional Codes
2.4 Applications of Convolutional Codes
In terms of actual applications of coding techniques to date, convolutional codes are perhaps the most popular choice in practice. This is mainly due to the existence of an optimum decoding algorithm (i.e., the VA) that can be easily implemented for either soft er hard-decisions, thus making convolutional coding a more economical approach in practice than competing block coding techniques for similar levels of error-correction. Typical applications of convolutional codes are satellite communications, mobile communications, voice-band data communications and digital broadcasting systems. To date, the realisation of the coding part of these applications uses a DSP (digital signal processor) or a digital ASIC (application specific integrated circuit) solution. The choice depends mainly on the data rate requirements.
2.4.1 Applications in Satellite Communications
Convolutional codes found their first application in early deep-space communications where the available power from the spacecraft is very restricted. Examples of deep-space missions include the NASA (National American Space Agency) Pioneer, Voyager and Gallileo missions [21]. A few years later, error-control convolutional coding was applied to digital satellite communication systems not only to overcome power limitation, but also to operate in bandwidth and interference-limited transmission channels [40]. Tables 2.1 and 2.2 summarise the convolutional coding parameters employed in some of
INMARSAT (International Maritime Satellite Organisation) and INTELSAT
(International Telecommunication Satellite Organisation) serviees, respectively.
T able 2.1 Convolutional coding parameters employed in INMARSAT.
Standard Service R K Data rate (kbit/s) Realisation
INMARSAT B Voice - 3/4 1 16 ASIC/DSP
Data 1/2 - 1 9.6 Shore-mobile telex 1/2 - 1 6 Mobile-shore telex 1/2 - 1 24 INMARSAT-C Data 1/2 - 1 0.6 DSP Shore-mobile telex 1/2 - 1 0.6 Mobile-shore telex 1/2 - 1 0.6
INMARSAT-M Voice - 3/4 1 6.4 ASIC/DSP
Data 1/2 - 1 2.4
INMARSAT -Aero Voice 1/2 - 1 2.4-9.6 ASIC/DSP
Data 1/2 - 1 0.6-10.5
High-speed data Mobile-shore 1/2 - 1 56 ASIC
Table 2.2 Convolutional coding parameters employed in INTELSAT.
Service R K Data rate (kbit/s) Realisation
INTELSAT-IBS'^ 1/2 3/4 1 64-8448 ASIC
INTELS AT-IDRt 1/2 3/4 1 64-44736 ASIC
* INTELSAT Business Service, i INTELS AT-I n termed i ate Data Rate.
2.4.2 Applications in Mobile Communications
In mobile communications, the available power and bandwidth are also very restricted. The transmitted signal is subject to multipath fading, and the size of the mobile transceiver is also restricted. Therefore, recent design and development of digital cellular mobile radio systems make extensive use of error-eontrol coding (convolutional and/or block coding) to enhance the system performance. Examples of cellular radio systems employing convolutional coding (see Table 2.3) include the European GSM (Group Spécial Mobile) system and the North American IS-95 (interim standard 95) and IS-136 (interim standard 136) systems [41]. The GSM and IS -136 are bandwidth-limited systems whereas the IS-95 is a power-limited system.
2.4.3 Applications in Voice-Band Data Communications
Channel coding techniques are not useful for transmission over some bandwidth-limited channels such as the general-switched telephone network (GSTN). In the GSTN the
T ab le 2.3 Convolutional coding parameters for various digital radio systems.
Standard Data type R K Frame size Frame rate Realisation
GSM Voice 1/2 - 5 189 bits 50 Hz DSP
Data - 9.6 1/2 57/61 5 244 bits 50 Hz
Data - 4.8 1/3 - 5 152 bits 50 Hz
Data - 2.4 1/6 - 5 76 bits 50 Hz
IS-95 Fwd voice 1/2 - 9 192 bits 50 Hz DSP
Rev. voice 1/3 - 9 192 bits 50 Hz
IS-136 Voice 1/2 - 6 89 bits 50 Hz DSP
FACCH* 1/4 - 6 65 bits 50 Hz
*Fast associated control channel.
bandwidth typically extends from 300Hz to 3.4kHz and so conventional error-control coding techniques are not suitable since channel bandwidth expansion here is not an option. However, it is possible to achieve significant values of CG over conventional M- ary modulation techniques without compromising bandwidth efficiency by employing TCM techniques [26]. TCM schemes employ M-ary modulation techniques in combination with trellis coding and on the AWGN channel CG values of more than 6dB over uncoded M-ary modulation are possible. Such improvements are obtained without bandwidth expansion or reduction of the effective information rate as would be required by conventional error-control coding techniques. In TCM trellis codes are optimised for
maximum Euclidean free distance, rather than for maximum and redundancy is
introduced by expanding the modulation signal alphabet to avoid channel bandwidth expansion.
For over a decade now, TCM schemes have been adopted by the ITU-T (International Telecommunication Union - Telecommunication Standardisation Sector) for use in new generation high-speed voice-band data modems over the GSTN. Examples of such modems include the ITU-T Recommendations V.32, V.32 bis and V.34. A summary of their key specification parameters is given in Table 2.4.
Table 2.4 ITU-T standards for high-speed voice-band data modems.
Standard R States Modulation scheme Data rate (kbit/s) Realisation
V.32 2/3 8 2-D TMC/32-CR 4.8 or 9.6 DSP
V.32 bis 2/3 8 2-D TMC/128-CR 14.4 DSP
V.34 2 /3 , 3 /4 ,4 /5 16, 32, 64 4-D TMC/960-QAM 2&8 DSP
2.4.4 Applications in Digital Broadcasting
Sound and television broadcasting are undergoing a revolutionary change from the established analogue to digital standards, where digital signal processing techniques promise vast improvements in quality and variety of services. Two important innovations of the video and audio broadcasting era are the development of the DVB (Digital Video Broadcasting) [35] and DAB (Digital Audio Broadcasting) [42] systems that are currently being implemented around the world. The DVB-S (satellite), the DVB-T (terrestrial) and the DAB systems employ convolutional coding as part of their FEC (see Table 2.5).
Table 2.5 Convolutional coding parameters for digital audio and video broadcasting.
Standard R K Data rate (Mbit/s) Realisation
DVB-S 1/2 2 /3 , 3 /4 , 5 /6 , 7/8 1 25-50 ASIC
DVB-T 1/2 2 /3 , 3 /4 , 5 /6 , 7/8 7 Variable ASIC