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TFTS is a cellular system that uses direct radio links to ground stations GS that are connected to the fixed network to provide public communication services for air passengers see Figur

Other Public Mobile Radio Systems

Air–Ground Communication

In 1993 ETSI RES 5 submitted a standard for the Terrestrial Flight Tele-phone System (TFTS), specifying the radio interface and the interfaces to public telecommunications networks At the same time the European Airlines Electronic Committee (EAEC) specified the airline equipment and interfaces

to cabin facilities Commercial operations began in 1994 In July 1994, after inviting international tenders, the Ministry of Post and Telecommunications granted a licence to DeTeMobil for the operation of TFTS DeTeMobil was to supply radio coverage to all airspace up to an altitude of 4500 m The service was available by 1996

Thirteen network operators in Europe have signed an MoU for the intro-duction of TFTS and an agreement on a cooperation with the major European airlines in order to resolve related commercial, organizational, technical and operational issues [1, 2]

TFTS is a cellular system that uses direct radio links to ground stations (GS) that are connected to the fixed network to provide public communication services for air passengers (see Figure 4.1)

There are three types of ground stations differentiated by area covered (cell) and related transmitter power:

• En-route (ER) GS for altitudes from 13 to 4.5 km, with cell radii up to

240 km

• Intermediate (I) GS for altitudes below 4.5 km, with cell radii up to

45 km

• Airport (AP) GS, with cell radii of 5 km

Handover between areas is part of the system According to WARC’92, two 5 MHz wide bands have been specified for operation of the TFTS:

• 1670–1675 MHz for uplink (ground-to-air)

Mobile Radio Networks: Networking and Protocols Bernhard H Walke

Copyright © 1999 John Wiley & Sons Ltd ISBNs: 0-471-97595-8 (Hardback); 0-470-84193-1 (Electronic)

h

ER, MAX

INT, MAX

43000 ft (14 km)

15000 ft (4.6 km)

EN-ROUTE GS INTERMEDIATE GS

AIRPORT GS

Figure 4.1: Coverage areas and ground stations

Channel

Channel

#1

#1

#2

#2

#164

#164

Uplink

Downlink

Figure 4.2: TFTS channel map

• 1800–1805 MHz for downlink

The system offers automatic dialled connections to PSTN/ISDN without any limitation on target subscribers, with the same quality of service as cus-tomary in PLMNs

In addition to speech, data services such as facsimile, data transfer at 4.8 kbit/s and DTMF signalling are supported Calls from the ground to an aircraft are only allowed to be made for operational purposes or for paging The user is billed directly by (credit) card for services used

Each 5 MHz band is divided into 164 FDM channels (each 30.45 kHz wide); see Figure 4.2

Each FDM channel transmits at 44.2 kbit/s gross On the uplink this capacity is divided into 17 time channels based on the TDM method, and on

4.1 Airline Telephone Network for Public Air–Ground Communication 303

Traffic

Channel

#1

#2

#3

#4

TDMA Frame (80 ms)

4.706 ms

Traffic/Control/

Specific Data Sync

Guard

208 bit (4.706 ms)

Figure 4.3: Frames and time slots

the downlink into 17 time channels based on the TDMA method Each FDM channel carries four voice channels

According to Figure 4.3, 17 time slots are combined into a frame of 80 ms duration, and 20 frames form a superframe of the duration of 1.6 s Each time slot contains 208 bits and has a duration of 4.706 ms

Voice signals are digitally coded into blocks of 192 bits and transmitted at 9.6 kbit/s in time slots A 9.6 kbit/s voice channel occupies 4 of the 17 time slots of an FDM channel; the 17th slot is used for network control

As soon as voice codecs are available for a 4.8 kbit/s transmission rate, the number of voice channels will be doubled Data services at 4.8 kbit/s require

2 time slots per frame, therefore each FDM channel carries 8 data channels

Each aircraft has transmitting and receiving facilities (transceiver ), which can

be tuned selectively to one of the different FDM channels Four communica-tions can be carried out on the same FDM channel at the same time Ground stations can transmit to different aircrafts simultaneously (on different time channels) on each one of their FDM channels

Signals are transmitted digitally with linear π/4-DQPSK (Differential Quadrature Phase Shift Keying) modulation, and require a simple non-coherent receiver

GS GS GS GS

PSTN/ISDN/PSPDN

Figure 4.4: Architecture of a complete TFTS network

Handover can be initiated by the mobile or the ground station, and is controlled by signal quality, distance and flight state The particular ground station selected as a target is the one towards which the mobile station is moving

The distance between mobile and ground stations is estimated on the basis

of signal propagation delay time This information also determines the net-work synchronization for the ground stations capable of receiving Ground stations are linked to the fixed network through ground switching centres (GSC) (see Figure 4.4)

The GSC has responsibility for all the ground stations linked to it, and its tasks include mobility management, connection establishment to mobile sub-scribers, handover control and dynamic frequency management The TFTS fixed network additionally contains three management components, namely:

• Operations and maintenance centre (OMC)

• Network management centre (NMC)

• Administration centre for billing (AC)

The MoU group produced a coordinated introduction plan for the TFTS ground network to enable the system to be introduced throughout Europe This effort required a cooperation between telecommunications network oper-ators and airlines

4.2 The US Digital Cellular System (USDC) 305

1 7

10 11

5

12

13 14

15 16

19

20 21 22

17 18

24 25

26

27 28

29

30

31 32

33 34

35 36 42

40

6

Figure 4.5: Cellular coverage through en-route ground stations in Europe

En-route ground stations are spaced approximately 380 km apart according

to a hexagonal grid, with a nominal range of approximately 240 km, which cannot be exceeded for signal propagation reasons (see Figure 4.5)

Cochannel ground stations are planned at a distance of 760 km, and neigh-bouring channel cells at a distance of at least 600 km Cell planning is more difficult compared with terrestrial cellular networks because of the need to incorporate flying altitudes

During the 1980s there was an impressive increase in the number of subscribers

to the public cellular mobile radio network in the USA Because approval for the installation of new base stations and antennas is expensive and difficult to obtain in larger cities, only a portion of this increased need for capacity could

be accommodated through a reduction in cell sizes A permanent solution turned out to be the development of a digital system capable of coping with increased capacity without the need for new base stations

In March 1988 the Telecommunication Industries Association (TIA) set

up the TR-45.3 subcommittee to develop the standard for a cellular digital system This digital system, the American Digital Cellular System (ADC),

Mobile Station

MS

Base Station BS

E A Um

Sm

B

Visitor Location

AC

PSTN

ISDN

H

F Ai

Di

C

Centre Authentication

Public Switched

Register VLR

Visitor Location Register VLR

Home Location HLR Register

Equipment Identity Register EIR MSC

Mobile Switching Centre

Mobile Switching MSC Centre

Telephone Network

Figure 4.6: Functional architecture of the USDC system

was to support and be compatible with the existing analogue mobile radio network, the American Mobile Phone System (AMPS); see [3] The digital system operates in the frequency range of the analogue AMPS system at the same time, which allows individual channels to change over gradually to digital technology A characteristic of this system is that terminal equipment can be used for analogue as well as for digital operation (dual-mode) In addition to increased capacity, the ADC standard enables the introduction

of new services, such as authentication, a data service and a short-message service, which were not supported by AMPS

In 1990 the digital standard was accepted by industry as Interim Stan-dard 54 (IS-54) The North American digital system with the architecture illustrated in Figure 4.6 is now called US Digital Cellular (USDC) In ad-dition, a number of standards have been accepted by FCC for the Personal Communication System PCS 1900 market, e.g., IS-134

The USDC system uses the 824–849 MHz frequency band for transmission between mobile station and base station (uplink), and in the reverse direc-tion (downlink) the 869–894 MHz band The duplex separadirec-tion between the transmit and receive frequency is therefore 45 MHz The frequency bands are divided into FDM channels with a 30 kHz bandwidth, thereby providing

832 frequency carriers

4.3 CDMA Cellular Radio According to US-TIA/IS-95 307

Slot 1 Slot 2 Slot 3 Slot 4 Slot 5 Slot 6

20 ms

6.7 ms Figure 4.7: Structure of a TDMA frame in the USDC system with half-rate chan-nels

The modulation technique used is π/4-DQPSK (Differential Quadrature Phase Shift Keying), a four-level scheme that, although it produces higher spectral efficiency than GMSK, places a heavy demand on the linearity of the output amplifier In addition, for optimal detection at the receiver in-put, filters with a transmission function capable of describing the root of the Nyquist transmission function are required, and this is something that in-expensive filters can only approximate In contrast to GMSK, π/4-DQPSK contains different amplitude components The eight different phase states in π/4-DQPSK modulation are all in one circle, but the four allowed phase tran-sitions from one phase to another do not run in the circle This means that not only the phase but also the amplitude is covered in the specifications for modulation

Like the GSM system, the USDC system operates in time-division multi-plexing (TDM) and multiple access (TDMA) mode, albeit with three voice channels being transmitted over one carrier The length of the TDMA frame

is 20 ms and is divided into three time slots each of 6.7 ms duration The modulation data rate per FDM channel (3 time slots, 30 kHz) is 48.6 kbit/s After the development and introduction of a half-rate codec, a TDMA frame will contain six time slots (see Figure 4.7) [5]

The USDC system uses a VSELP speech codec (Vector Sum Excited Linear Prediction) which, compared with GSM, results in lower source rates With

a full-rate codec, voice coding together with error-protection coding produces

an overall transmission rate of 13 kbit/s, whereas the total rate on the SACCH

is 0.6 kbit/s

US-TIA/IS-95

TIA Interim Standard 95 was developed by QUALCOMM Unlike IS-54, which guarantees compatibility of a digital system with analogue, the IS-95 standard defines a CDMA transmission system It includes the lowest three levels of the OSI reference model The transmission system of the LEO sys-tem Globalstar will be based on the IS-95 standard with modifications (see Section 14.3.3) The physical layer is described below However, only the

modulators have been standardized but not the demodulators; these can be specified by the manufacturer

Forward-link uses coherent QPSK modulation in which transmitter and re-ceiver must be phase-synchronized for demodulation Walsh sequences are used for channel separation (see Section 2.6.4) A short PN sequence is used for each in-phase and quadrature-phase for the spreading A long PN sequence individually assigned to the user is used for the traffic channel Demodulation

is carried out through a pilot tone that is also transmitted

4.3.1.1 Modulator

Figure 4.8 shows the modulator for the forward link A number of physical channels are available for establishing a connection The first thing that must

be carried out when a mobile station is switched on is synchronization Phase synchronization and frame synchronization are achieved through the trans-mission of a pilot tone The network synchronization is then carried out over the synchronization channel This involves transmitting the paging channel data rate and power control information Data for channel allocation is sent over the paging channel Information is transmitted over the traffic channel

Pilot channel The all-one Walsh sequence W0is combined with a short code and transmitted to the modulator With a value set of (0, 1) the two codes are added modulo 2, or with a bipolar (−1, 1) approach they are multiplied The Walsh sequences are the lines of the Hadamard matrix, and are formed according to the following recursion:

H1= 0 and H2N = HN HN

HN H¯N



(4.1)

in which N must be a power of two and ¯HN is the negation of HN The next two matrices are formed in the same way:

H2= 0 0













All Walsh sequences of the same matrix are orthogonal to each other The IS-95 standard uses 26 = 64 Walsh sequences The Globalstar system will probably use 27= 128 sequences

In IS-95 the short code is formed with two irreducible polynomials (the polynomials 121 641 and 117 071 are primitive Note that because code se-quences can be produced with a polynomial and its reciprocal polynomial,

4.3 CDMA Cellular Radio According to US-TIA/IS-95 309

9600 bit/s 4800 bit/s 1200 bit/s Paging code

Power control 19.2 ksymb/s

Traffic channel 9600 bit/s 4800 bit/s 2400 bit/s 1.200 bit/s

Convolutional coder Convolutional coder

Subscriber code

Long code Long code

Xor Xor Xor Xor Xor Xor

Access-Channel Information Paging-Channel Mask

0

0

41

110001111

Traffic-Channel Mask

Permuted Serial Number

41

110001100

Modulo 2 Addition

Figure 4.9: Long code generator

only one polynomial is given in the tables [7]) In IS-95 the grade is n = 15;

in the Globalstar system the grade will probably be n = 17

The polynomials for the in-phase components and the quadrature-phase components in IS-95 are

PQ = x15+ x12+ x11+ x10+ x6+ x5+ x4+ x3+ 1 (4.4) The short code is the same for the whole system In Globalstar a code mis-alignment (different mismis-alignment in the shift register) is used to provide unique identification of the gateway, the satellite and the beam

The Walsh sequence is spread with the short code at a 1.23 MHz clock-pulse rate over the entire bandwidth and QPSK-modulated

Synchronization channel The synchronization channel produces data flow

at a rate of 1200 bit/s The data is channel-coded with a (R = 1/2, K = 9) convolutional coder, then interleaved and combined with the Walsh sequence

W32 The signal is then spread with the short code and QPSK-modulated Paging channel Data is channel-coded with a (R = 1/2, K = 9) convolu-tional coder, then interleaved and spread with a long code For the channel separation the signal is combined with the Wp Walsh sequence allocated to the paging channel The signal is then spread with the short code and QPSK-modulated Figure 4.9 shows the structure of a long-code generator

4.3 CDMA Cellular Radio According to US-TIA/IS-95 311

This involves setting up a shift register with 42 delay elements, with the outputs linked by a 42-bit long mask The outputs are added modulo 2 and generate the long code

Traffic channel The vocoder (standardized in accordance with IS-96), which

is capable of producing different data rates as required, delivers the data to the channel coder and the interleaver Each user has a personal secret key number which forms part of the long code mask for the traffic channel The long code

is linked to the output of the interleaver On this basis, power control data and traffic channel data are alternatively spread using a user Walsh sequence

Wu The data flow is combined with the short code and QPSK-modulated 4.3.1.2 Power Control on the Forward-Link

In IS-95 power control is carried out in a closed loop on the forward-link This requires a periodic reduction in the transmitted power of the base station The reduction continues until the user notices an increase in the frame error ratio The user then sends a command for the power to be increased The measurement increments of power control are relatively small and in the area

of 0.5 dB The dynamics covers an area of ±6 dB The power changes occur every 20 ms

Non-coherent orthogonal 64-correlated Walsh modulation is used in the return-link in IS-95 This modulation can be interpreted as FSK modula-tion with the Walsh sequences corresponding to different frequencies

The long code is used here for the channel separation In forward-link Walsh sequences are used for channel separation, whereas here the Walsh sequences are used for modulation The data is spread with the short code and transmitted using QPSK

4.3.2.1 Modulator

There are two physical channels: an access channel and a traffic channel, differentiated only by the long code mask Figure 4.10 shows the modulator for the return-link of the traffic channel

Access channel A base station receives access requests on the access channel First a preamble of three frames of 96 zeros per frame is transmitted Then the user’s long code is transmitted The data rate is always 4.8 kbit/s Another eight bits containing only zeros are added after each net data frame The data

is channel-coded with a (R = 1/3, K = 9) convolutional coder, scrambled by the interleaver and modulated orthogonally The long-code generator (n = 41), which is combined with a paging mask, and the short-code generator