Chapter 3 Analog Cellular Communications AMPS System

1999/4/21 Jang-Ping Sheu 3- 1

Chapter 3 Analog CellularCommunications AMPS System

• This chapter covers the original, first-generationcellular technology, focusing on AMPS(Advanced Mobile Phone System)


Architecture

• AMPS is an American National Standard

• Original AMPS is not compatible betweenoperating companies ( local services)

• Network Elements:– Mobile station, BS, Switch

• Identification Codes:– AMPS specifies several ID codes for each MH



ID Codes

• Mobile Identification Number (MIN):– a 10 digital telephone number (34 bits)

• Electronic Serial Number (ESN):– assigned permanently to each terminal (32 bits)

• Station Class Mark (SCM):– indicate the model (832 or 666 channels) and the

maximum radiated power of the terminal


ID Codes

• System Identifier (SID):– assigned by the FCC issues an SID to an operating

company (15 bits)

• Supervisory Audio Tone (SAT):– help mobile stations distinguish neighboring BSs from

one another

• Digital Color Code (DCC):– assigned by operating company to each BS

– serves a similar purpose with SAT



Radio Transmission

• Frequency Bands and Physical Channels:– The original system covered a bandwidth of 40 MHz

– The band for forward transmission is 870 - 890 MHz

– The reverse transmission is 825 - 845 MHz

– An AMPS physical channel is 30 kHz

– Total number of channel (per direction) is 20 MHz/30kHz = 666

– The carrier frequency corresponding to channel C is

– f(C) = 825 MHz + 30 C kHz (reverse direction)

– f(C) = 870 MHz + 30 C kHz (forward direction)



Frequency Bands and PhysicalChannels

• Extension of AMPS Band:– The band for forward transmission is 869 - 894 MHz

– The reverse transmission is 824 - 849 MHz

– Channel number 1 to 799 is assigned to 825 - 849 MHz(reverse transmission) and channel 991 to 1023 isassigned to band 824 - 825 MHz

– The carried frequency of the reverse channel is

f(C) = 825 MHz + 30(C-1023) kHz; 991 ≤ C ≤ 1023


Frequency Bands and PhysicalChannels

– AMPS spectrum is divided into two equal regions labeledA and B

– Two cellular operating licenses (corresponding to bands Aand B) in each geographical area.

– There are 1,466 operating licenses in the USA.

– All systems operating in A-band (416 channels) have thesame odd SIDs (least significant bit = 1)

– Among the 832 AMPS channels, there are 42 controlchannels (21 channels for each band) from 313 - 354

– All other channels (up to 395 channels) are traffic channels


Radiated Power

• Six or Eight Different Power Levels:– range from 8 dBm (6mW) to 36 dBm (4W) in steps of

4dB

– each power level is 2.5 times higher than the next lowerlevel

– Case I mobile station: power by vehicle battery hasaccess to all eight power levels

– Case II mobiles with a maximum power of 1.6 W

– Case III mobile station: handheld; range over six levelsto a maximum of 600 mW

– BS is typically 25 W per channel

What is a Decibel- dB

– Decibel is the unit used to express relative differences insignal strength.

– It is expressed as the base 10 logarithm of the ratio of thepowers of two signals:

– dB = 10 log (P1/P2)

– Logarithms are useful as the unit of measurement because

• signal power tends to span several orders ofmagnitude

• signal attenuation losses and gains can be expressed interms of subtraction and addition


Analog Signal Processing

• Compress, Pre-Emphasize, Limit, and Low PassFilter:– To maintain high signal quality and to limit adjacent

channel interference

– Human speech has a large range of amplitudes

– Loud sounds (typically vowels) is 16 times (12 dB)stronger than the energy in weak sounds (consonants)

– Compressor reduces the overall dynamic range(measured in dBs) by a factor of two and at the receiver,the expander restores the original range





Analog Signal Processing

– Pre-emphasis filter amplify high-frequency sounds (upto 3 kHz) and restore the amplitudes to their originallevel after reception

– An amplitude limiter confines the maximum excursionof frequency to 12 kHz on either side of the carrierfrequency

– A low pass filter attenuates signal components atfrequency above 3 kHz

– The notch at 6 kHz removes the signal energy at thefrequency associated with the three supervisory audiotones (SAT)




SAT and Supervisory Tone

• SAT:– Each BS has its own SAT- at 5970 Hz, 6000 Hz, and

6030 Hz

– SAT is used to distinguish the desired received signalfrom interfering signals transmitted in other cells thatuse the same physical channel

– Nearby cells with the same channel assignmentstransmit different SATs

– With a reuse factor of seven cells per cluster, the celluse the __same physical channels have a separation of4.58 (√ 21) relative to cell radius



SAT and Supervisory Tone

– The distance between two base stations using the sameSAT and the same physical channels is 7.94 (√63)

– North American digital cellular systems haveequivalent of at least 255 SATs

• Supervisory Tone:– a 10 kHz sine wave

– used for signaling similar to the on-hook/off-hookindications


Digital Signals

• AMPS digital signals are sine waves either 8 kHzabove or below the carrier

• Signal format is Manchester coded binaryfrequency at a rate of 10 kb/s

• A logical 1 is represented by a transition, in themiddle of the 100 micro second bit interval, from-8 kHz to +8kHz relative to the carrier

• A logical 0 is represented by the complementarytransition



Spectrum Efficiency

• For high-quality sound:– signal-to-interference ratio of at least 18 dB

• To meet the requirement with high probability:– most AMPS systems operates with reuse factor N = 7

– If a system with 395 traffic channels, the averagenumber of channel per cell is 395/7 ≅ 56 with N = 7

– The total spectrum assignment to an AMPS system 25MHz, the spectrum efficiency E = (395/(7 x 25)) = 2.26conversations/cell/MHz


Logical Channels

• Forward and Reverse Traffic Channels:– Used to carry user information in analog format

• Forward and Reverse Control Channels:– Physical channels 313-354

– A FOCC carries information from one BS to allterminals that do not have a call in progress

– A RECC carries information from many mobiles thatdo not have voice channels assigned (use randomaccess protocol to contend the physical channels)


Logical Channels

• Forward and Reverse Voice Channels:– To send a control message over a voice channel, the

system interrupts the flow of user information andinserts a control message, typically of duration around100 ms.



Block Codes

• Block code (n, k, dmin)– Used to Protect The Control Information

– n is the total number of transmitted bits per code word

– k is the number of information bits carried by each codeword

– dmin the minimum distance between all pairs of codeword

• ex: n = 4, k = 2, dmin = 2 (0010, 0101, 1001, 1110)

– To provide more protection against binary error, AMPSemploys an inner coder, a repetition mechanism. m isthe number of repetitions.



Block Codes

– On the FVC, the number of repetitions is 11 times

– FVC carries the handoff command is a criticalcommunication

– The FVC transmit nm = 40 x 11 = 440 bits to convey 28information bits

– The operation of the decoders is not part of the AMPSspec. Each BS and terminal manufacturer can decidewhether to operate each decoder to:

• correct two binary errors

• correct one binary error and detect up to three errors

• detect up to four binary errors with no error correction


Logical Channel Formats

• Formats:– bit sync: 1010101010

– word sync: 11100010010

– On the FOCC the synchronizing sequence appearingpredictably every 46.3 ms

– this make it an easy matter for a terminal to acquire andhold synchronism on the FOCC

– on the other three channels, transmissions take place inbursts

– on the RECC, the bit synchronizing sequence contain30 bits




– On the two voice channels, each transmission beginswith an alternating sequence of 101 bits

– The voice channels also insert a 37-bit alternatingsequence before each repetition of a BCH code word

– The other synchronization pattern common to all fourchannel is 11-bit “word sync”



• Forward Control Channel– In Fig. 3.8, word A and word B indicates that the

channel carries two multiplexed message stream

– Word A (B) carries message for even (odd) phonenumber

– The arriving information rate for each terminal is 28bits per 46.3 ms, or 604.75 b/s



• Reverse Control Channel Access Protocol:– Refer to Fig. 3.8, 42 vertical arrows each corresponds

to 1 busy/idle bit that control the random access ofmobiles to an RECC

– Before transmitting information on the RECC, aterminal examines the state of the busy/idle bits on thecorresponding FOCC

– These bits are in the idle (1) state when the BS is not inthe process of receiving information on the RECC



– A cellular terminal with information to transmitinitiates a burst in the format shown in Fig 3.10

– If the transition from idle to busy occurs too soon (lessthan 5.6 ms) the mobile station turns off its transmitterto avoid interference

– If the mobile observes no idle-to-busy transition within10.4 ms, the mobile turns off its transmitter, assumingthat the station failed to detect the beginning of theburst

– If fails to transmit an RECC message, the mobilepauses for a random time interval between 0 and 200ms




• Reverse Control Channel:– On the RECC, terminals transmit network control

information to the system in burst that convey one andfive code words

– Each code word contains a sequence of 240 bits (a 48-bit BCH sequence repeated five times)

– The 7-bit RECC transmission corresponds to a (7,2;4)block code with minimum distance dmin = 4



• Forward and Reverse Voice Channels:– To convey control information when a call is in

progress

– It interrupts user information and sends a control burstin the format of Figures 3.11 and 3.12

– Before each repetition of a 40-bit or 48-bit BCH codeword, there is a 37-bit alternating sequence followed bythe 11-bit Barker code

– This enables the BS or terminal to recover from acomplete loss of signal due to a deep fade



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Chapter 3 Analog Cellular Communications AMPS System