47.4 Overview of systems

47.4.2 Tacs

TACS stands for Total Access Communications System, and was adapted from the AMPS standard by the UK when cellular radio was licensed for operation from 1985. The adaptation was necessary to suit European frequency allocations which were at 900MHz, with 25kHz channel spacing. This meant a reduction in frequency devi­ation and signalling speed was necessary (BS, 1990).

The signalling scheme of AMPS was retained largely unchanged, but some enhancements were introduced, particularly in the proce­dures for location registration, to make the standard more suitable for deployment in systems offering contiguous nationwide cover­age. The opportunity was also taken to introduce extra features, such as signalling of charge rate information (e.g. for payphones).

TACS was originally specified to use the full 1(X)O channels (2 x 25MHz) allocated to mobile services in Europe. However in the UK, only 600 channels (2 x 15MHz) were released by the licensing authority, the remainder being reserved for GSM. Subsequently an additional allocation of channels below the existing TACS channels was made, namely the Extended TACS (ETACS) channels, and the standard was modified accordingly.

TACS equipment availability and cost have both benefitted from the standard's similarity to AMPS, and TACS systems have been adopted by several European countries (UK, Eire, Spain, Italy, Austria and Malta), in the middle east (Kuwait, UAE and Bahrain) and the far east (Hong Kong, Singapore, Malaysia and China). In Europe, TACS is on an equal footing with NMT in terms of installed customer base. A variant of TACS (called J-TACS) has also been adopted in Japan.

47.4.3 NMT

NMT stands for Nordic Mobile Telephone (system), and was de­veloped jointly by the PTTs of Sweden, Norway, Denmark and Finland during the late 1970's/early 1980's. The system was de­signed to operate in the 450MHz band, and was later adapted to also use the 900MHz band. Although NMT was developed after AMPS, it saw commercial service before it, opening in late 1981.

NMT450 uses a channel spacing of 25kHz, speech modulation being analogue FM with a peak frequency deviation of 5kHz, the same as standard PMR practice. NMT900 also uses a frequency deviation of 5kHz, but with a 12.5kHz channel spacing to double the number of available channels, albeit with a degraded adjacent channel rejection performance which must be taken into account during frequency planning. Signalling is at 12(K) bit/s using audio fast frequency shift keying (FFSK). Error protection of the signall­ing information is by means of a Hagelbarger convolutional forward error correcting code.

NMT was designed from the outset to support international roam­ing and was first implemented with full four nation roaming in the four participating countries (Norway, Sweden, Finland and Den­mark). Since then NMT450 has been deployed in many other European countries (Austria, Spain, Netherlands, Belgium, Luxem­bourg, France, Iceland, Faroe Is., Turkey and Hungary) but due to differences in the frequency allocations in the 450MHz band be­tween countries, not all networks are fully compatible to allow roaming.

NMT900 was developed as a necessity as capacity became ex­hausted on the NMT450 networks, and has been deployed since 1987 as an overlay network in several countries, and in Switzerland as their main network.

47.4.4 C450

C450 (also known as Netz-C) was developed by Siemens during the early 1980's under the direction of the (West) German PTT, Deut­sche Bundespost. Commercial service opened in 1985 following a trial period.

C450 has a channel spacing of 20kHz, in common with other mobile services in Germany at 450MHz and speech modulation is analogue FM with a frequency deviation of 4.0kHz. Signalling for call control is transmitted at 5.28kbit/s by direct FSK. Error protec­tion of the signalling is by bit interleaving with a BCH block code backed by an acknowledgement protocol.

In addition, C450 uses continuous signalling between base station and mobile during a call, achieved by time compressing the speech in bursts of 12.5ms, each burst being compressed into 11.4ms. This process opens up slots of 1.1ms duration every 12.5ms and the signalling data is inserted into these slots and extracted by the receiver which also time expands the speech back to its original form.

This continuous signalling serves several purposes:

1. It allows the base station to send power control and handover messages to the mobile without disturbing the voice channel.

2. The data is checked for jitter, and thereby the quality of the channel can be determined in order to indicate the need for a handover.

3. The time delay between a base station transmitting a data burst and receiving the response from the mobile is measured at the base station and used to calculate the distance between them. This distance is also taken into account in handover determina­tion.

4. The data is used as a timing reference by the mobile to lock its internal clocks.

C450 contains a number of advanced features made possible by the application of current developments in technology. Although speech transmission is analogue, it can be regarded as a hybrid technology system, and several of its characteristics such as time slotted signalling channels and continuous signalling during call have been carried through into the GSM system design.

Coming later to the European scene, C450 has chiefly only served the German market, although systems are also operating in Portugal and South Africa.

47.4.5 GSM

The GSM standard was developed as a joint initiative by the mem­bers of the Conference of European Posts and Telecommunications administrations (CEPT) with the eventual aim of building a unified pan-European network, giving the user a near uniform service throughout all European countries. An added bonus of a common standard should be lower terminal equipment prices through econ­omies of scale.

Work on the standard started in 1982, and by 1987 all the basic architectural features were decided. The full Phase 1 specification was completed in 1990, but work continues on further phases incorporating new features and services. In 1987, the majority of operators participating in GSM signed a Memorandum of Under­standing (MoU) committing them to make GSM a reality by install­ing networks and opening commercial service by 1991. Since that time further operators have signed the MoU, bringing the total to date to 25.

The GSM technical standard makes full use of currently available levels of technology, incorporating features such as low bit rate speech, convolutional channel coding with bit interleaving and frequency hopping. The standard is intended to endure for many years to come.

Exercise 1 Learn the words and word combinations

frequency allocation	распределение частоты (между службами)
to comply with	подчинятся правилам
albeit	хотя
frequency deviation	девиация частоты
a compander	компандер
yield	производить, вырабатывать
shift keying	манипуляция
frequency (-shift) keying	частотная манипуляция, манипуляция сдвигом частоты
a BCH block code (Bose-Chaudhuri-Hocquengem)	блочный (-блоковый) код БХЧ (код Боуза-Чоудхури-Хок-венгема)
to abort	прерывать, прекращать
to mute	подавлять
path	маршрут (в сети передачи данных)
adjacent channel rejection	подавление помех от соседнего канала
burst	пакетный сигнал; вспышка, всплеск, выброс)
burst of signal	выброс сигнала
channel spacing	разнос каналов
handover	переключение, переход
power control	регулирование мощности
bit interleaving	чередование битов (разных сообщений при уплотнении каналов)
frequency interleaving	чередование частоты
code interleaving	кодовое перемежение
a hybrid system	дифсистема
terminal equipment	терминальное оборудование; оконечное оборудование
frequency hopping	скачкообразная перестройка частоты; перескок частоты
to endure	выдерживать испытание временем
convolution coding	сверточное кодирование
AMPS (advanced mobile phone service)	перспективная служба (радио) телефонной связи с подвижными объектами
IMTS (improved mobile telephone system)	усовершенствованная система подвижной телефонной связи
ITU (International Telecommunication Union	Международный союз электросвязи
error correcting code	код с исправлением ошибок
convolution code	сверточный код
timing reference	эталон времени
PRM (premium (tariff) – additional service when the call is partly paid by calling party, e.g. 900- services in the USA
voice circuit	тональная цепь, телефонная цепь
SAT (supervisory audio tone)	диспетчерский тональный сигнал
standard system	автономная система, функционально законченная система
TDMA (time- division multiple access	многостанционный доступ с временным разделением каналов
contiguous coverage	соприкасающиеся зоны обслуживания

Exercise 2 Read the text

Exercise 3 Find the Russian equivalents for the following English words and word combinations

interleaving	выброс сигнала
to be fully compatible with	голос; речевой сигнал
a trial period	сигнализация по общему каналу
a patchwork	развертывание, ввод в действие
burst of signal	коммутация с помощью штепсельного соединителя
deployment	уплотнение импульсных сигналов; чересстрочная развертка
frequency coverage	выдерживать испытание временем
customer	абонент
voice	передача сигналов с модуляцией на несущей
to endure	охват по частотам; частотный диапазон
carrier signaling	испытательный срок
common channel signalling	быть полностью совместимым с …
compelled signalling	принудительная передача сигналов

Exercise 4 Answer the following questions:

1	What are the four dominant cellular standards adopted in many countries?
2	Where was AMPS developed and where is it in operation now?
3	What are the key features of this standard
4	What can you tell about TACS?
5	Was NMT designed to support international roaming?
6	Does NMT operate only in the 450 MHz band?
7	Why was NMT 900 developed?
8	What do you know about speech modulation, channel spacing and a frequency deviation of C 450?
9	What purposes does the continuous signalling of C 450 serve?
10	Is C 450 widely used nowadays?
11	Whom was the GSM standard developed by?
12	When was a Memorandum of Understanding signed?
13	What is the GSM standard intended to endure for many years?

Part III (47.5.1 – 47.5.7)