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First Generation Cellular SystemsLecture 4

1First Generation Cellular Systems1/24/2012



The era of cellular telephony as we understand it today began with the

introduction of the first generation of cellular systems (1G systems). Such

systems served mobile telephone calls via analog transmission of voice

traffic.

Despite the fact that 1G systems are considered technologically primitive

today, the fact remains that a significant number of people still use analog

cellular phones and analog cellular infrastructure is found throughout North

America and other parts of the world. Furthermore, they have found use as a

basis for the development of several second generation systems. An example

of this is D-AMPS, which is a 2G system evolving from AMPS.

This lecture describes the Advanced Mobile Phone System (AMPS) and

Nordic Mobile Telephony (NMTS) 1G cellular systems.




However, the reason why 1G systems are considered primitive is due to the

fact that they utilize analog signaling for user traffic. This leads to a number

of problems:




No use of encryption. The use of analog signaling does not permit efficient

encryption schemes. Therefore, 1G systems do not encrypt traffic. Thus, voice calls through a 1G network are subject to easy eavesdropping.

Another problem is the fact that, by listening to control channels, users

identification numbers can be stolen and used to place illegal calls,

which are charged to the user.




Inf

erior call qualities. An

alog traff

ic is easily degraded by in

terf

eren

ce, whichresults in inf erior call quality. Contrary to digital traff ic, no coding or error

correction is applied in order to combat interf erence.




Spectrum ineff iciency. In analog systems, each RF carrier is dedicated to a single

user, regardless of whether the user is active (speaking) or not (idle within the

call). This is the reason f or the ineff icient spectrum usage compared to later

generations of cellular systems.




AMPS is a representative 1G mobile wireless system developed by Bell Labs in the late

1970s and early 1980s.

It was designed to off er mobile telephone traff ic services via a number of 30 kHz

channels between the Mobile Stations (MSs) and the BSs of each cell.

These 30 kHz channels are used to carry voice traff ic. The latter is a 3 kHz signal that

is carried over the AMPS channels via analog transmission.

Advan

ced Mobile Phon

eS

ystem (AMPS

)




AMPS Frequency Allocations

The FCC made the first allocation of bandwidth for AMPS in the late 1970 in

order to enable the operation of test systems in the Chicago area.

The allocated bandwidth was in the 800 MHz part of the spectrum for a

numberof reasons:

Limited spectrum was available at lower frequencies, which are primarily

occupied either by FM radio or television systems. Lower frequencies are

sometimes used by other systems, for example, maritime systems.




Limited spectrum was available at lower frequencies, which are primarily

occupied either by FM radio or television systems. Lower frequencies are

sometimes used by other systems, for example, maritime systems.

Despite the fact that frequencies above 800 MHz are not very densely

used, allocation of frequencies in this bands for AMPS was undesirable due

to the fact that signals in those bands (e.g. several GHz) are subject to

severe attenuation either due to path loss or fading.

Such deterioration of signal qualities could not easily be handled at the

time AMPS was developed due to the fact that error correction techniquesfor an analog system like AMPS were in their infancy.

The 800 MHz band was a relatively unused band since few systems

utilized it. 9First Generation Cellular Systems1/24/2012



AMPS Channels

The operating frequency of AMPS consists of 2x25=50 MHz, which are

located in the 824849 MHzand 869894 MHz bands.

In a certain geographical region two carriers (service providers) can coexist,

with each carrier possessing 25 MHz of the spectrum (either the A or B

band).

The transmit and receive channels of each BS are separated by 45 MHz.

Both traffic channels for carrying analog voice signals and control channels

exist.

In a certain geographical area, two operators can exist and a different set of

channels is assigned to each operator.




The two channel sets, A and B, comprise channels from 1 to 333 and from 334

to 666, respectively.

Channels from 313 to 333 and from 334 to 354 are the control channels of bands

A and B, respectively.Thus, each operator has 312 voice channels and 21 control channels at its

disposal.

Each control channel can be associated with a group of voice channels, thus

each set of voice channels (either of bands A or B) can be split into groups of 16 channels, each group controlled by a different control channel.




Traffic channels (TCs) are 30-kHz analog FM channels used to serve voicetraffic.

The main traffic channels are the Forward Voice Channel (FVC) and the

Reverse Voice Channel (RVC) carrying voice traffic from the BS to the MS

and from the MS to the BS, respectively.The network assigns them to the MS upon establishment or termination of

a call.

Control channels (CCs) carry digital signaling and are used to coordinate

medium access of Mobile Stations (MSs).Specifically, each MS that is not involved in a call (idle MS) is locked onto

the strongest CC in order to receive control information. The CCs of AMPS

are summarized below:




The Forward Control Channel (FOCC). This is a dedicated continuous data

stream that is sent from the BS to the MS at 10 kbps. FOCC is a time division

multiplexed channel comprising three data streams: (a) streams A and B,

which are identified via the least significant bit of the MSs Mobile Identity

Number (described later), with bit 0 identifying stream A and bit 1 identifying

stream B and (b) the busy-idle stream, which is used to indicate the status of

the RECC (described below). The use of the busy-idle stream reduces the

possibilities of collisions on the RECC, as this might be used by more than one

MSs. The FOCC is also used by the BS to inform a MS which RVC to use for a

newly established call.

The Reverse Control Channel (RECC). This is a dedicated continuous data

stream that is sent from the MS to the BS at 10 kbps.




The Supervisory Audio Tone (SAT)

SAT is sent on the voice channels and is used in order to ensure link

continuity and enable MSs and BSs to possess information on the quality of

the link that connects them.

Both the BS and the MS send this tone on the FVC and RCC, respectively, and

the tone is added prior to the modulation of the voice signal. When a MS is

switched on or has roamed under the coverage of a new BS, it tunes to the

FOCC and reads a 2-bit field known as the SAT color code (SCC). The value of

the SCC informs the MS which SAT to expect. SAT codes are shown in Figure

1. SAT determination is performed every 250 ms and the three defined SATs

are at the following frequencies: 5.97 kHz, 6 kHz and 6.03 kHz.




Figure 1 Mapping of SATS to SCC codes.




The Signaling Tone (ST)

The ST is used to send four signals:

The request to send signal, which is used to allow the user to enter more

data on the keypad while engaged in an ongoing conversation, T;

The alert signal, which, once the MS has been alerted, is continuously sent

on the RVC until the userof the MS answers the call;

The disconnect signal, which is sent by the MS over the RVC in order to

indicatecall termination;

The handoff confirmation signal, which is sent by the MS in response to

the networks request for handoff of this MS to another BS.




Network Operations

Prior to describing some basic network operations in AMPS, we describe the

three identifier numbers used in AMPS:

The Electronic Serial Number (ESN). The ESN is a 32-bit binary string that

uniquely identifies an AMPS MS. This number is set up by the MS

manufacturer and is burned into a Read Only Memory (ROM) in an effort to

prevent unauthorized changes of this number.

The fact that this number is stored in a ROM means that the MS will become

inoperable if someone tries to rewrite the ESN. The format of an ESN is shown

in Figure 2. It comprises three fields:




Figure 2 Structure of the 32-bit ESN.




(a) part 1, comprising bits from 24 to 31; this 8-bit field is the

manufacturers code (MFR), which uniquely identifies each manufacturer;

(b) part 2 which comprises bits from 18 to 23 and has remained unusable;

and (c) part 3, which comprises bits 017, which are assigned by the

manufacturer to the MS. These bits are essentially the MSs serial number.

When a manufacturer has produced so many MSs that 18 bits are no longer

able to provide additional serial numbers for its MSs, it can apply to the

FCC for an additional MFR. Thus, it can continue to produce MSs and MSs

will be identified by a different MFR/serial numbercombination.




The System Identification Numbers (SIDs). These are 15-bit binary strings

that are assigned to AMPS systems and uniquely identify each AMPS

operator. SIDs are (a) transmitted by BSs to indicate the AMPS network they

belong to and (b) used by MSs to indicate either the AMPS network they

belong to (in cases of two collocated AMPS networks), or to determineroaming situations.

The Mobile Identification Number (MIN). This is a 34-bit string that is

derived from the MSs 10-digit telephone number.




InitializationOnce an AMPS MS is powered up, a sequence of events takes place. This

sequence is briefly described below:

Event 1. The MS receives systems parameters in order to configure itself to

useone of the two AMPS networks.

Event 2. The MS scans the 21 control channels of the selected AMPS

network to receive control messages. If a control channel with an acceptable

quality is found, this is selected.

Event 3. The MS receives a message on the control channel containing

system parameters.




Event 4. The message received in Event 3 provides the MS with informationthat is needed in order to update information that was received in possible

previous initializations. Furthermore, the MS reads the SID of the AMPS

network in this message, compares it to the SID of the network it belongs to

and when the MS is in the service area of another network, the MS canprepare for roaming operations.

Event 5.The MS identifies itself to the network by sending its MIN, ESN and

SIDS via the RECC.

Event 6.The AMPS network examines the parameters transmitted by theMS in Event 5 in order todetermine whether this MS is a roaming one or not.




Event 7.The BS verifies initialization parameters by sending a controlmessage to the MS.

Event 8.The MS enters idle state and waits for a call establishment request.

During idle mode, the MS must perform operations to (a) ensure

synchronization with the BS, (b) make the network aware of the MSslocation.




Call Setup from a MS

The procedure of placing a call from an MS can be described via a number of

events. These events are summarized below:

Event 1.The MS sends to the BS a message containing the MSs MIN, ESN and

the phone number dialed.

Event 2.The BS passes the information sent by the MS to the network for

processing.

Event 3.The BS indicates to the MS the channel number that will be used for

the voice call. Furthermore, information related to the SAT frequency to be

used is relayed to the MS.

Event 4. Both MS and BS switch to the voice channels.

Event 5.The BS sends a control message on the FVC via the SAT signal.

Event 6.The MS confirms link continuity via the SAT on the RVC.

Event 7.The call is established. 24First Generation Cellular Systems1/24/2012



Call Setup to an MS

The procedure of placing a call to an MS can be described via a number of


Event 1.The identification of the MS is passed to the BS.

Event 2.Control information, including the channel number to be used, is

conveyed to the MS.

Event 3.The MS responds by sending its MIN, ESN and other control-related

information.

Event 4.Information related to the SAT frequency to be used is relayed to

the MS.

Event 5.Both MS and BS switch to the voice channels.

Event 6.The BS sends a control message on the FVC via the SAT signal.

Event 7.The MS confirms link continuity via the SAT on the RVC.

Event 8.The call is established 25First Generation Cellular Systems1/24/2012



CallHandoff

The procedure of handoff in AMPS can be described via a number of


Event 1.The BS serving the MS notices a decrease in the MSs transmission

power.

Event 2.The BS sends a handoff measurement request to its MSC.

Event 3.The MSC instructs BSs in the neighborhood of the current BS to

perform measurements of the MSs signal strength.

Event 4.The MSC selects the best choice for a BS to serve the MS.

Event 5. The MSC allocates a traffic channel to the selected BS.




Event 6.The selected BS acknowledges the traffic channel allocation.

Event 7. The MSC sends a handoff message to the current BS.

Event 8.The current BS sends the handoff message to the MS. This

message informs the MS which traffic channel to use and the power level of

its transmission under the new BS.

Event 9.The MS confirms the current BSs message and switches to the

traffic channel.

Event 10.The MS starts scanning and eventually receives the new BSs SAT.


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