chapter 8 (part ii: 2g pcs systems

PERSONAL COMMUNICATION PERSONAL COMMUNICATION SYSTEMS: SECOND SYSTEMS: SECOND

GENERATIONGENERATION

Ian F. AkyildizIan F. Akyildiz

Broadband & Wireless Networking LaboratoryBroadband & Wireless Networking Laboratory

School of Electrical and Computer EngineeringSchool of Electrical and Computer Engineering

Georgia Institute of TechnologyGeorgia Institute of Technology

Tel: 404-894-5141; Fax: 404-894-7883 Tel: 404-894-5141; Fax: 404-894-7883

Email: [email protected]: [email protected]

Web: http://www.ece.gatech.edu/research/labs/bwnWeb: http://www.ece.gatech.edu/research/labs/bwn

2IFA’2004

Differences between AMPS Differences between AMPS and GSMand GSM

AMPS uses analog technologyAMPS uses analog technology GSM uses digital technologyGSM uses digital technology AMPS has poor performance for data transferAMPS has poor performance for data transfer Both systems use control channels to initiate Both systems use control channels to initiate

calls.calls. AMPS uses 21 control channels while GSM uses 3.AMPS uses 21 control channels while GSM uses 3. AMPS is less secure than GSMAMPS is less secure than GSM GSM has the SIM smart card which holds the GSM has the SIM smart card which holds the

user’s personal information and phone settings.user’s personal information and phone settings. In AMPS only the HLR has that data.In AMPS only the HLR has that data. Both systems support ROAMING but GSM allows Both systems support ROAMING but GSM allows

more compatibility.more compatibility.

3IFA’2004

Differences between AMPS Differences between AMPS and GSMand GSM

AMPS requires less power at the MS and at AMPS requires less power at the MS and at the BS.the BS.

GSM’s power control attempts to minimize GSM’s power control attempts to minimize radio transmission power of the MS and radio transmission power of the MS and BTS, thus, minimizes the amount of co-BTS, thus, minimizes the amount of co-channel interference.channel interference.

AMPS instead relies on the digital color AMPS instead relies on the digital color code (DCC) for that.code (DCC) for that.

AMPS has a cell radius 1.5km - 25kmAMPS has a cell radius 1.5km - 25km GSM is more flexible with cell sizes.GSM is more flexible with cell sizes.

4IFA’2004

EXAMPLES EXAMPLES

IS-41 or IS-136 (D-AMPS) uses TDMA IS-41 or IS-136 (D-AMPS) uses TDMA scheme;scheme;

(AT&T Wireless, BellSouth, Southwestern (AT&T Wireless, BellSouth, Southwestern Bell)Bell)

IS-95 uses CDMAIS-95 uses CDMA (Bell Atlantic/NYNEX, (Bell Atlantic/NYNEX, Verizon, Sprint PCS)Verizon, Sprint PCS)

GSM uses TDMAGSM uses TDMA (used worldwide; here (used worldwide; here Cingular; T-Mobile).Cingular; T-Mobile).

5IFA’2004

INTRODUCTION:INTRODUCTION:

USA: 2G SystemsUSA: 2G Systems

1G -> AMPS -> problems to serve large number of 1G -> AMPS -> problems to serve large number of users.users.

2G Systems with Digital Modulation techniques 2G Systems with Digital Modulation techniques (called Digital Cellular) achieved large (called Digital Cellular) achieved large improvements.improvements.

Late 80ies, USDC (US Digital cellular system) started Late 80ies, USDC (US Digital cellular system) started to support more users in a fixed spectrum allocation.to support more users in a fixed spectrum allocation.

US Digital Cellular System (D-AMPS:US Digital Cellular System (D-AMPS: Digital Advanced Mobile Phone System)Digital Advanced Mobile Phone System) / \/ \ / \/ \

Air InterfaceAir Interface Mobility ManagementMobility Management

IS-54IS-54 IS-41IS-41 (new version (new version IS-136IS-136))

6IFA’2004

Reference Model for Reference Model for North American SystemsNorth American Systems

The TIA Committees TR-45 The TIA Committees TR-45 and TR-46 develop and TR-46 develop performance, compatibility, performance, compatibility, interoperability, and service interoperability, and service standards.standards.– TR-45.3 TDMATR-45.3 TDMA– TR-45.5 CDMATR-45.5 CDMA

New interfaces when New interfaces when compared to GSM Model:compared to GSM Model:– DMH: data message DMH: data message

handler – collects billing handler – collects billing informationinformation

– IWF: interworking IWF: interworking function – allows an MSC function – allows an MSC to connect to other to connect to other networksnetworks

– AUX: auxiliary equipment AUX: auxiliary equipment – can connect to an MSC– can connect to an MSC

Messaging is carried out by Messaging is carried out by protocols very similar to SS-7protocols very similar to SS-7

7IFA’2004

IS-54 IS-54

IS-54 Architecture similar to AMPS, GSMIS-54 Architecture similar to AMPS, GSM in terms of MSC, BS, Mobile Stations, HLR, VLRsin terms of MSC, BS, Mobile Stations, HLR, VLRs IS-54 standardized in 1990 (Interim Standard-IS)IS-54 standardized in 1990 (Interim Standard-IS) IS-54 shares same frequencies, frequency reuse IS-54 shares same frequencies, frequency reuse

plan and base stations as AMPS so that base plan and base stations as AMPS so that base stations and subscriber units could be equipped stations and subscriber units could be equipped with both AMPS and IS 54 channels within the with both AMPS and IS 54 channels within the same piece of equipment.same piece of equipment.

Both AMPS and IS-54 cellular carriers provide new Both AMPS and IS-54 cellular carriers provide new customers with IS-54 phones and may gradually customers with IS-54 phones and may gradually replace AMPS base stations with IS-54 BSs replace AMPS base stations with IS-54 BSs channel by channel over time channel by channel over time known as D-AMPS known as D-AMPS

AMPS -AMPS - D-AMPS (IS-54) D-AMPS (IS-54)

8IFA’2004

IS-54IS-54 IS-54 uses TDMA supporting 3 full rate users or 6 half rate users IS-54 uses TDMA supporting 3 full rate users or 6 half rate users

on each AMPS channel.on each AMPS channel. 6 times more capacity than AMPS6 times more capacity than AMPS IS-54 uses the same 45 MHz FDD scheme as AMPSIS-54 uses the same 45 MHz FDD scheme as AMPSREMARK:REMARK:

* Change from Analog to Digital* Change from Analog to Digital * Temporary increase in Interference and dropped calls * Temporary increase in Interference and dropped calls in AMPS; since each BS changed over to digital; the number in AMPS; since each BS changed over to digital; the number

ofof analog channels in geographic area is decreased. analog channels in geographic area is decreased. Compatibility with AMPS:Compatibility with AMPS: * IS 54 Forward/Reverse control channels use exactly the same* IS 54 Forward/Reverse control channels use exactly the same signaling technique as AMPS.signaling technique as AMPS. * Voice Channels are 4–ary pi/4 DQPSK modulation with a* Voice Channels are 4–ary pi/4 DQPSK modulation with a channel rate 48.6 kbps.channel rate 48.6 kbps.

9IFA’2004

IS-54 IS-54 Forward and Reverse control channels use the Forward and Reverse control channels use the

same 10 kbps FSK signaling scheme as in AMPSsame 10 kbps FSK signaling scheme as in AMPSREMARK:REMARK: * IS 136 (formerly IS-54-C) includes pi/4 * IS 136 (formerly IS-54-C) includes pi/4

DQPSKDQPSK modulation for control channels.modulation for control channels. * IS-54-C * IS-54-C provides 4-ary keying instead of provides 4-ary keying instead of

FSK onFSK on control channelscontrol channels REASONS: Increase control channel data rate REASONS: Increase control channel data rate

also provide special services like paging etc..also provide special services like paging etc..

10IFA’2004

RADIO INTERFACE RADIO INTERFACE (IS-54)(IS-54)

GOAL: Smooth transition from AMPS GOAL: Smooth transition from AMPS IS-54 IS-54IS 54 designed to operate using both AMPS and IS 54 standards.IS 54 designed to operate using both AMPS and IS 54 standards.

Multiple AccessMultiple Access TDMA/FDD TDMA/FDD Modulation Modulation pi/4 DQPSK pi/4 DQPSK Channel BandwidthChannel Bandwidth 30 kHz (sam as in AMPS) 30 kHz (sam as in AMPS) Reverse Channel Freq. BWReverse Channel Freq. BW 824-894 MHz (same as in AMPS) 824-894 MHz (same as in AMPS) Forward Channel Freq. BWForward Channel Freq. BW 869-894 MHz (same as in AMPS) 869-894 MHz (same as in AMPS) FW and Reverse Channel Data RatesFW and Reverse Channel Data Rates 48.6 kbps 48.6 kbps Channel CodingChannel Coding 7 bit CRC and ½ rate convolutional coding of 7 bit CRC and ½ rate convolutional coding of

constraint length 6.constraint length 6. Users per channelsUsers per channels 3 (full rate speech coder of 7.95 3 (full rate speech coder of 7.95

kbps/user) kbps/user) 6 (half rate speech coder of 3.975 kbps/user). 6 (half rate speech coder of 3.975 kbps/user).

11IFA’2004

CHANNELS (IS-54)CHANNELS (IS-54)

Control Channels Control Channels AMPS Control AMPS Control ChannelsChannels

42 Primary AMPS Control Channels 42 Primary AMPS Control Channels PrimaryPrimary

+ IS 54 (42 additional control channels) + IS 54 (42 additional control channels) dedicated for IS 54 use only (Secondary dedicated for IS 54 use only (Secondary Control Channels)Control Channels)

12IFA’2004

DATA CHANNELS DATA CHANNELS (IS-54)(IS-54)

Digital Traffic Channel (DTC) carries user Digital Traffic Channel (DTC) carries user information (speech or user data)information (speech or user data)

Reverse DTC: carries speech from mobile Reverse DTC: carries speech from mobile to BS.to BS.

Forward DTC: carries speech from base to Forward DTC: carries speech from base to mobilemobile

13IFA’2004


Coded Digital Verification Color Code Coded Digital Verification Color Code Channel (CDVCC)Channel (CDVCC)

Slow Associated Control Channel (SACCH)Slow Associated Control Channel (SACCH) Fast Associated Control Channel (FACCH)Fast Associated Control Channel (FACCH)

14IFA’2004


Coded Digital Verification Color Code Channel (CDVCC)Coded Digital Verification Color Code Channel (CDVCC) Is a 12 bit message sent in every time slot.Is a 12 bit message sent in every time slot. (Function similar to SAT (Supervisory Audio Tone) as in AMPS(Function similar to SAT (Supervisory Audio Tone) as in AMPS allows each BS and its mobiles to confirm that they are allows each BS and its mobiles to confirm that they are

properly connected during a call.properly connected during a call. It is an 8-bit number ranging from 1 and 255 protected with 4 It is an 8-bit number ranging from 1 and 255 protected with 4

additional channel coding bits (12,8) Hamming code.additional channel coding bits (12,8) Hamming code. BS BS CDVCC values on Forward Voice Channel CDVCC values on Forward Voice Channel Each subscriber using TDMA channel must receive, decode Each subscriber using TDMA channel must receive, decode

and retransmit the same CDVCC value to BS on the reverse and retransmit the same CDVCC value to BS on the reverse voice channel.voice channel.

Handshake: If not, then time slot will be relinguished for other Handshake: If not, then time slot will be relinguished for other users.users.

15IFA’2004


Slow Associated Control Channel Slow Associated Control Channel (SACCH)(SACCH)

Sent in every time slot. Sent in every time slot. Carries various control and supervisory message Carries various control and supervisory message

between mobile and BS, e.g., power level between mobile and BS, e.g., power level changes and handoff requests.changes and handoff requests.

Also used by mobile to report results of signal Also used by mobile to report results of signal strength measurements of neighboring base strength measurements of neighboring base stationsstations

to help BS to do MAHO (Mobile Assisted to help BS to do MAHO (Mobile Assisted Handoff).Handoff).

16IFA’2004

DATA CHANNELS (IS DATA CHANNELS (IS 54)54)

Fast Associated Control Channel Fast Associated Control Channel (FACCH)(FACCH)

(Signaling Channel)(Signaling Channel) Important control or specialized traffic Important control or specialized traffic

data between BS and mobiles;data between BS and mobiles;

e.g., call release instructions, MAHO, e.g., call release instructions, MAHO, mobile status requests.mobile status requests.

The FACCH data when transmitted takes The FACCH data when transmitted takes the place of user info data within a frame.the place of user info data within a frame.

17IFA’2004

Frame Structure for IS-54Frame Structure for IS-54(Mobile to BS)(Mobile to BS)

1 Frame=1944 bits (97 symbols)= 40 ms

1 2 3 4 5 6

G R Data SYN CD DATA 6.67msec 324 bits (260 bits user data)

25 frames/sec

SDATA

G Guard Time (6 bits) (when no signal is transmitted)R Ramp Time (6 bits) (to allow transmitter to reach its full output power level)Data 16 bitsSYN 28 bitsDATA 122 bitsSACCH 12 bitsCDVCC 12 bits (helps to identify the frequency channel to which mobile is tuned)DATA 122 bits

18IFA’2004

Mobility Mobility ManagementManagement IS-41IS-41 (new version (new version

IS-136IS-136)) IS 41 <-> GSMIS 41 <-> GSM

Procedure for delivering calls to Procedure for delivering calls to mobile users in GSM very similar to IS mobile users in GSM very similar to IS 41.41.

Note that names, contents, lengths of Note that names, contents, lengths of messages may be different.messages may be different.

19IFA’2004

IS-41 Standard for MSC-IS-41 Standard for MSC-MSC interfaceMSC interface

IS-41 is primarily used in the core network IS-41 is primarily used in the core network to provide services such as automatic to provide services such as automatic roaming, authentication, intersystem roaming, authentication, intersystem handoff, short message service, etc.handoff, short message service, etc.

– All wireless network elements such as the All wireless network elements such as the MSC, HLR, VLR, EIR, and AUC, use this MSC, HLR, VLR, EIR, and AUC, use this messaging protocol to communicate among messaging protocol to communicate among themselvesthemselves

– Signaling protocol stack very similar to SS-7Signaling protocol stack very similar to SS-7– Intersystem handoff: handoff involving two Intersystem handoff: handoff involving two

BSSs controlled by different MSCs.BSSs controlled by different MSCs.

20IFA’2004

IS-41: Intersystem HandoffIS-41: Intersystem Handoff

The current MSC will request a RSS measurement from The current MSC will request a RSS measurement from the candidate MSC.the candidate MSC.

Once RSS measurements indicate the candidate MSC as Once RSS measurements indicate the candidate MSC as suitable for handoff, the two MSCs will complete the suitable for handoff, the two MSCs will complete the intersystem handoff.intersystem handoff.

Three types of handoff:Three types of handoff:– Handoff forward:Handoff forward: transfer from one MSC to another transfer from one MSC to another

MSC of a new systemMSC of a new system– Handoff backward:Handoff backward: transfer from the new MSC back to transfer from the new MSC back to

the old MSCthe old MSC– Handoff third:Handoff third: transfer from an MSC in a second system transfer from an MSC in a second system

to a MSC of a third systemto a MSC of a third system During handoff, IS-41 signaling messages will carry During handoff, IS-41 signaling messages will carry

terminal information, call information, and air-interface terminal information, call information, and air-interface information (serving and destination cells and information (serving and destination cells and channels). It also performs authentication procedures channels). It also performs authentication procedures between the two systems.between the two systems.

21IFA’2004

Differences IS-41 - Differences IS-41 - GSMGSM

When the new VLR receives the registration When the new VLR receives the registration affirmation (REGNOTaffirmation (REGNOT IS-41) from HLR, it IS-41) from HLR, it assigns a new TMSI to the terminal for the assigns a new TMSI to the terminal for the new RA.new RA.

HLR also provides new VLR with all relevant HLR also provides new VLR with all relevant subscriber profile information required for subscriber profile information required for call handling (e.g., call screening lists, etc.) call handling (e.g., call screening lists, etc.) as part of affirmation message.as part of affirmation message.

Thus, in contrast to IS-41, authentication and Thus, in contrast to IS-41, authentication and subscriber profile information are obtained subscriber profile information are obtained from both HLR and VLR and not just the HLR.from both HLR and VLR and not just the HLR.

22IFA’2004

OVERVIEWOVERVIEW IS-54 GSM IS-95IS-54 GSM IS-95

Access TechnologyAccess Technology TDMA/FDD TDMA/FDD CDMA (DS)/FDD TDMA/FDD TDMA/FDD CDMA (DS)/FDDFreq Band:Freq Band: BSBS 869-894 935-960 869-894 935-960 (1805-1880)(1805-1880) 869-894 869-894MobileMobile 824-849 890-915 824-849 890-915 (1710-1785)(1710-1785) 824-849 824-849Channel SpacingChannel Spacing 30kHz 200kHz 1250kHz 30kHz 200kHz 1250kHzModulation Modulation pi/4 DQPSK GMSK BPSK/QPSK pi/4 DQPSK GMSK BPSK/QPSKPower Max/Power Max/Average MilliwattsAverage Milliwatts 600/200 1000/125 600 600/200 1000/125 600Speech Rate (kbps Speech Rate (kbps 7.95 13 8 (variable 7.95 13 8 (variable

rate)rate)Frame DurationFrame Duration (ms) 40 4.615 20 (ms) 40 4.615 20Coding Coding ½ rate ½ rate ½ rate ½ rate ½ rate conv for ½ rate conv for

ForwardForward

convolutional conv. convolutional conv. 1/3 rate conv. for 1/3 rate conv. for Reverse; CRCReverse; CRC

DQPSK: Differential Quadratic Phase Shift Keying; QPSK: Quadrature Phase Shift Keying; GFSK: DQPSK: Differential Quadratic Phase Shift Keying; QPSK: Quadrature Phase Shift Keying; GFSK: Gaussian Freq. Shift Keying; BPSK: Binary Phase Shift Keying; GMSK: Gaussian Minimum Shift Gaussian Freq. Shift Keying; BPSK: Binary Phase Shift Keying; GMSK: Gaussian Minimum Shift KeyingKeying

23IFA’2004

IS-95IS-95

Digital Cellular:Digital Cellular:– Two different directions for the air-Two different directions for the air-

interface: interface: IS-136 standard based on TDMAIS-136 standard based on TDMA IS-95 standard based on CDMAIS-95 standard based on CDMA

– Interoperability was only possible Interoperability was only possible via dual mode telephonesvia dual mode telephones

– IS-41 standard has now evolved to IS-41 standard has now evolved to support both IS-136 and IS-95support both IS-136 and IS-95

24IFA’2004

IS-95IS-95

IS-95 is the North American digital cellular IS-95 is the North American digital cellular standard that employs CDMA as the standard that employs CDMA as the

Access MethodAccess Method as well as the as well as the Air-InterfaceAir-Interface..

It was developed by Qualcomm around 1990.It was developed by Qualcomm around 1990. CDMA/AMPS dual mode phones were produced by Qualcomm in 1994.CDMA/AMPS dual mode phones were produced by Qualcomm in 1994.

Also called cdmaOne.Also called cdmaOne.

In In Dec.Dec. 1993, the TIA published Qualcomm’s air- 1993, the TIA published Qualcomm’s air-interface specifications as the interim standard IS-95. interface specifications as the interim standard IS-95.

Formally adopted in July 1993 and revised in May 1995.Formally adopted in July 1993 and revised in May 1995.

25IFA’2004

IS-95 CDMAIS-95 CDMA

Digital AMPS increased capacity of AMPS Digital AMPS increased capacity of AMPS by factor 3by factor 3

First Code Division Multiple Access First Code Division Multiple Access (CDMA) cellular system was (CDMA) cellular system was developed in 1990, claimed to developed in 1990, claimed to increase capacity of AMPS by factor increase capacity of AMPS by factor 20.20.

26IFA’2004

IS-95 CDMAIS-95 CDMA

GOAL:GOAL: Low CostLow Cost Light-weight hand-held portable unitsLight-weight hand-held portable units Battery LifeBattery Life Spectrally efficientSpectrally efficient Low link budgetsLow link budgets Minimum number of base stationsMinimum number of base stations Excellent grade of serviceExcellent grade of service Excellent scalabilityExcellent scalability Reduction of dropped callsReduction of dropped calls Reduction of fading and poor voice qualityReduction of fading and poor voice quality

27IFA’2004

CDMACDMA

CDMA is both an CDMA is both an Access Method and an Air-InterfaceAccess Method and an Air-Interface..

Similarities:Similarities: Core fixed network infrastructureCore fixed network infrastructure GSM core network GSM core network

structurestructure

Radio resource management, mobility management, and Radio resource management, mobility management, and securitysecurity

same as in TDMA (D-AMPS) systems.same as in TDMA (D-AMPS) systems.

There are differences in terms of There are differences in terms of

* Handling the * Handling the power controlpower control * Employing * Employing soft handoff

28IFA’2004

Access Method Access Method CDMACDMA

CDMA (Code Division Multiple Access)CDMA (Code Division Multiple Access)– All terminals send on the same frequency All terminals send on the same frequency

probably at the same time and can use probably at the same time and can use the whole bandwidth of the transmission the whole bandwidth of the transmission channel channel

– Each sender has a unique random Each sender has a unique random number, the sender XORs the signal with number, the sender XORs the signal with this random numberthis random number

– The receiver can “tune” into this signal if The receiver can “tune” into this signal if it knows the pseudo random number; it knows the pseudo random number; tuning is done via a correlation functiontuning is done via a correlation function

29IFA’2004


CDMA (Code Division Multiple Access)CDMA (Code Division Multiple Access)

User data stream encoded with ½ User data stream encoded with ½ convolutional code rate, interleaved and convolutional code rate, interleaved and spread by one of 64 orthogonal spreading spread by one of 64 orthogonal spreading sequences (Walsh functions).sequences (Walsh functions).

Each MS in a given cell is assigned a different Each MS in a given cell is assigned a different spreading sequence, providing perfect spreading sequence, providing perfect separation among signals from different users.separation among signals from different users.

30IFA’2004


CDMA (Code Division Multiple Access)CDMA (Code Division Multiple Access)

To reduce interference between MSs To reduce interference between MSs which use same spreading sequence which use same spreading sequence in different cells and to provide the in different cells and to provide the desired spectral characteristic, all desired spectral characteristic, all signals in a cell are SCRAMBLED signals in a cell are SCRAMBLED using a pseudo-random sequence of using a pseudo-random sequence of length 2^15 chips.length 2^15 chips.

31IFA’2004U

ser

1

Time

Frequency

Use

r 2

Use

r n

Code

...

CDMA (Code Division Multiple CDMA (Code Division Multiple Access )Access )

32IFA’2004

CDMA (Advantages and CDMA (Advantages and Disadvantages)Disadvantages)

Advantages: Advantages: – All terminals can use the same frequency, no planning All terminals can use the same frequency, no planning

neededneeded– Huge code space compared to frequency spaceHuge code space compared to frequency space– Interference (e.g. white noise) is not codedInterference (e.g. white noise) is not coded– Forward error correction and encryption can be easily Forward error correction and encryption can be easily

integratedintegrated

Disadvantages:Disadvantages:– Higher complexity of a receiver (receiver cannot just listen Higher complexity of a receiver (receiver cannot just listen

into the medium and start receiving if there is a signal)into the medium and start receiving if there is a signal)– All signals should have the same strength at a receiverAll signals should have the same strength at a receiver

33IFA’2004

CDMA: Further CDMA: Further Advantages and Advantages and DisadvantagesDisadvantages

Advantages:Advantages:– CDMA provides an increase in system capacity when CDMA provides an increase in system capacity when

compared with the analog and TDMA systems.compared with the analog and TDMA systems.– CDMA improves quality of voice by using a better CDMA improves quality of voice by using a better

voice coder.voice coder.– CDMA has resistance to multipath and fading.CDMA has resistance to multipath and fading.– CDMA implements soft handoffs.CDMA implements soft handoffs.– CDMA has less power consumption (about 10% of CDMA has less power consumption (about 10% of

analog or TDMA phones) because of implementation analog or TDMA phones) because of implementation of power control.of power control.

– CDMA does not require frequency planning because CDMA does not require frequency planning because all cells employ the same frequency at the same all cells employ the same frequency at the same time.time.

Disadvantage:Disadvantage:– Necessity for power control and complexity.Necessity for power control and complexity.

34IFA’2004

Comparison Comparison TDMA/FDMA/CDMATDMA/FDMA/CDMA

Approach TDMA FDMA CDMA

Ideatime into disjointtime-slots, demanddriven or fixed patterns

segment thefrequency band intodisjoint sub-bands

spread the spectrum

Terminals all terminals areactive for shortperiods of time onthe same frequency

every terminal has itsown frequency,uninterrupted

all terminals can be activeat the same place at thesame moment,uninterrupted

separation

synchronization inthe time domain frequency domain

code plus specialreceivers

established, fully simple, established,robust

flexible, less frequency

handover

Dis- guard spaceneeded (multipathpropagation),synchronization difficult

inflexible,frequencies are ascarce resource

complex receivers, needsmore complicated powercontrol for senders

Comment standard in fixednetworks, togetherwith FDMA/SDMAused in manymobile networks

typically combinedwith TDMA(frequency hoppingpatterns) and SDMA(frequency reuse)

still faces some problems,higher complexity,lowered expectations; willbe integrated withTDMA/FDMA

advantages

Advantages

Signal

using orthogonal codessegment sending

digital, flexible planning needed, soft

filtering in the

35IFA’2004

Air-Interface CDMAAir-Interface CDMA

The air-interface in CDMA is not The air-interface in CDMA is not symmetrical on the forward and symmetrical on the forward and reverse channels reverse channels (separated by 45MHz)(separated by 45MHz)– One Forward Channel (1.25 MHz in 824-One Forward Channel (1.25 MHz in 824-

849 MHz bands):849 MHz bands): transmissions transmissions originate at a single transmitter (BS) originate at a single transmitter (BS) and transmissions for all users are and transmissions for all users are synchronized.synchronized.

– One Reverse Channel (1.25 MHz in 869-One Reverse Channel (1.25 MHz in 869-894 MHz bands):894 MHz bands): mobile terminals mobile terminals transmit whenever they have to.transmit whenever they have to.

36IFA’2004


Forward Channels Forward Channels (BS(BS MOBILE): MOBILE):– Pilot (1 Channel)Pilot (1 Channel): : Provides a reference signal to all MSs within a Provides a reference signal to all MSs within a

cell for demodulation. It is also used for signal cell for demodulation. It is also used for signal strength comparison. strength comparison. Determines when to Determines when to handoff.handoff.

– Synchronization (1 Channel):Synchronization (1 Channel): Used to acquire initial time synchronization. The Used to acquire initial time synchronization. The

sync message includes the system and network sync message includes the system and network identification, coding information, and the identification, coding information, and the paging channel data rate. paging channel data rate. Operates at Operates at 1200kbps.1200kbps.

37IFA’2004


Forward Channels:Forward Channels:– Paging (7 channels)Paging (7 channels) As in GSM, used to page the MS when there is an As in GSM, used to page the MS when there is an

incoming call, and to carry the control messages incoming call, and to carry the control messages for call setup. Operates at 9600,4800, 2400 bps)for call setup. Operates at 9600,4800, 2400 bps)

– Forward Traffic (63 channels)Forward Traffic (63 channels):: Carries the actual user information. Carries the actual user information. Two possible rate sets, RS1 and RS2. Two possible rate sets, RS1 and RS2. RS1 supports date rates of 9.6, 4.8, 2.4, and 1.2 RS1 supports date rates of 9.6, 4.8, 2.4, and 1.2

kbps. RS2 supports 14.4, 7.2, 3.6, and 1.8 kbps. kbps. RS2 supports 14.4, 7.2, 3.6, and 1.8 kbps. Multiplexed with power control information.Multiplexed with power control information.

38IFA’2004


Reverse Channels (MOBILE (MOBILE BS): BS):– Access (max. 32 channels):Access (max. 32 channels):

Used by the MS to transmit information to the BS such as call Used by the MS to transmit information to the BS such as call origination, response to a page and so on. origination, response to a page and so on.

FixedFixed Data Rate 4800 kbps. Random access channel user Data Rate 4800 kbps. Random access channel user uniquely identified by their long codes.uniquely identified by their long codes.

– Reverse Traffic:Reverse Traffic: Supports voice data at two rate sets: RS1 and RS2. Supports voice data at two rate sets: RS1 and RS2. It is used to send information related to the signal strength It is used to send information related to the signal strength

of the pilot and frame error rate statistics to one BS or of the pilot and frame error rate statistics to one BS or multiple BSs.multiple BSs.

It is also used to transmit control information to the BS such It is also used to transmit control information to the BS such as a handoff completion message.as a handoff completion message.

Operates on variable data rate.Operates on variable data rate.

39IFA’2004

IS-95 CDMA IS-95 CDMA ChannelsChannelsOverviewOverview

Types of ChannelsTypes of ChannelsForward Channels

Pilot

Synchronization

Paging

Traffic

Reverse channels

Access

Traffic

ApplicationSystem mon.Sync.

SignalingVoice/data

Signaling

Voice/data

40IFA’2004

IS-95 CDMA Interesting IS-95 CDMA Interesting FeaturesFeatures

Multiple users can share same Multiple users can share same frequencyfrequency

Soft capacity limit: more users raises Soft capacity limit: more users raises noise floor linearly, no absolute limit noise floor linearly, no absolute limit on number of users - performance on number of users - performance degrades gradually for all usersdegrades gradually for all users

Multipath fading is reduced by signal Multipath fading is reduced by signal spreadingspreading

41IFA’2004

IS-95 CDMA Interesting IS-95 CDMA Interesting Features (cont)Features (cont)

Spatial diversity provides soft handoff: MSC monitors signal of a user from

multiple base stations and chooses best version of signal at any time

Self-Jamming is a problem: Because spreading sequences of

different users are not exactly orthogonal– When despreading, other users can

contribute significantly to receiver decision statistic

42IFA’2004


Near-Far Problem: If power of multiple users are unequal,

the strongest received mobile signal will capture demodulator at the base station –Base stations must implement power control to ensure that each mobile within coverage area provides same signal level to base station receiver

43IFA’2004


The system can move a call from digital to analog when the call enters the coverage area of a cell that does not have CDMA capability.

The opposite does not work.

44IFA’2004

Mobility and Radio Mobility and Radio Resource Management in Resource Management in IS-95IS-95

Soft HandoffSoft Handoff– Note: Unlike channelized wireless systems that Note: Unlike channelized wireless systems that

assign different radio channels during a handoff assign different radio channels during a handoff (called a (called a hard handoffhard handoff), spread spectrum ), spread spectrum mobiles share the same channel in every cell, mobiles share the same channel in every cell, thus thus the term handoff does not mean a physical the term handoff does not mean a physical change in the assigned channel, but rather that change in the assigned channel, but rather that a different base station handles the radio a different base station handles the radio communication task.communication task.

– Refers to the process by which an MS is in Refers to the process by which an MS is in communication with multiple candidate BSs communication with multiple candidate BSs before finally deciding to communicate its traffic before finally deciding to communicate its traffic through one of them.through one of them.

45IFA’2004

Mobility and Radio Mobility and Radio Resource Management in Resource Management in IS-95IS-95

Soft HandoffSoft Handoff– Reason to implement it is based on the near-far Reason to implement it is based on the near-far

problem and the associated power control problem and the associated power control mechanism:mechanism:If an MS moves far away from a BS and continues If an MS moves far away from a BS and continues

to increase power to compensate for the near-far to increase power to compensate for the near-far problem, it might cause a lot of interference to problem, it might cause a lot of interference to MSs in neighboring cells.MSs in neighboring cells.

To avoid the above situation and to ensure that To avoid the above situation and to ensure that an MS is connected to the BS with the largest RSS an MS is connected to the BS with the largest RSS (Received Signal Strength), a soft handoff (Received Signal Strength), a soft handoff strategy is implemented.strategy is implemented.

46IFA’2004

Soft HandoffSoft Handoff

IS-95 defines three types of soft handoff:IS-95 defines three types of soft handoff:(a) (a) Softer Handoff:Softer Handoff: between two sectors of the between two sectors of the same cellsame cell

(b) (b) Soft Handoff:Soft Handoff: between two sectors of different cells between two sectors of different cells

(c) (c) Soft-Softer Handoff:Soft-Softer Handoff: includes two sectors from the same cell and includes two sectors from the same cell and

a third sector from a different cell. a third sector from a different cell.

47IFA’2004

Soft HandoffSoft Handoff A controlling BS A controlling BS

coordinates the addition or coordinates the addition or deletion of other BSsdeletion of other BSs

The primary BS uses a The primary BS uses a handoff direction message handoff direction message (HDM) to indicate the pilot (HDM) to indicate the pilot channels to be used or channels to be used or removed.removed.

At some point, the primary At some point, the primary BS is also changed after BS is also changed after handoff.handoff.

The signals from multiple The signals from multiple BSs are combined in the BSs are combined in the BSC or MSC and processed BSC or MSC and processed as a single call. This is as a single call. This is achieved by using a achieved by using a frame frame selector joinselector join message. message.

A A frame selector removeframe selector remove message is used to remove message is used to remove the old BS.the old BS.

48IFA’2004

Soft HandoffSoft Handoff

The pilot channels of each cell are involved in the The pilot channels of each cell are involved in the handoff mechanism.handoff mechanism.– Only channel not subject to power control and providing a Only channel not subject to power control and providing a

measure of the RSS.measure of the RSS. The MS maintains a list of pilot channels that it can The MS maintains a list of pilot channels that it can

hear and classifies them into:hear and classifies them into:– Active Set:Active Set: pilots continuously monitored or used by pilots continuously monitored or used by

the MS (MS has three RAKE fingers that allows it to the MS (MS has three RAKE fingers that allows it to monitor or use up to three pilots).monitor or use up to three pilots).

– Candidate Set:Candidate Set: can have at most six pilots, not in the can have at most six pilots, not in the active set but with sufficient RSS to be used.active set but with sufficient RSS to be used.

– Neighbor Set:Neighbor Set: contains pilots that belong to contains pilots that belong to neighboring cells and are intimated to the MS by a neighboring cells and are intimated to the MS by a message on the paging channel.message on the paging channel.

– Remaining Set:Remaining Set: all other possible pilots in the all other possible pilots in the system.system.

49IFA’2004

Soft Handoffs and ThresholdsSoft Handoffs and Thresholds

Whenever the strength of a pilot falls Whenever the strength of a pilot falls below a threshold, the MS starts a dwell below a threshold, the MS starts a dwell timer. timer.

Unless the pilot strength goes back Unless the pilot strength goes back above the threshold before the timer above the threshold before the timer expires, the MS will drop it from a given expires, the MS will drop it from a given set. set.

There is a trade-off in setting high and There is a trade-off in setting high and low values for these thresholds and low values for these thresholds and timers.timers.

50IFA’2004

Soft Handoffs and ThresholdsSoft Handoffs and Thresholds

If the strength of a candidate pilot If the strength of a candidate pilot channel is above the pilot detection channel is above the pilot detection threshold (T_ADD), this pilot must be threshold (T_ADD), this pilot must be added to the active set and the MS added to the active set and the MS enters a soft handoff region.enters a soft handoff region.– If T_ADD is too small, there may be If T_ADD is too small, there may be

false alarms caused by noise or false alarms caused by noise or interfering signals.interfering signals.

– If T_ADD is too large, useful pilot are If T_ADD is too large, useful pilot are not detected, and the call may be not detected, and the call may be dropped.dropped.

51IFA’2004

Handoff Thresholds in IS-Handoff Thresholds in IS-9595

52IFA’2004


1.1. As soon as the strength of the pilot exceeds As soon as the strength of the pilot exceeds T_ADD, it is transferred to the candidate set, and T_ADD, it is transferred to the candidate set, and the MS sends the new pilot strength measurement the MS sends the new pilot strength measurement to the BS that is transmitting the current pilot. to the BS that is transmitting the current pilot.

2.2. The BS sends a handoff direction message to the The BS sends a handoff direction message to the MS. The pilot is transferred to the active set.MS. The pilot is transferred to the active set.

3.3. The MS acquires a traffic channel and sends a The MS acquires a traffic channel and sends a handoff completion message.handoff completion message.

4.4. After the pilot strength falls below T_DROP, the After the pilot strength falls below T_DROP, the handoff timer is started. handoff timer is started.

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If it is still below T_DROP after the timer expires, If it is still below T_DROP after the timer expires, the MS sends another pilot strength the MS sends another pilot strength measurement to the BS associated with the pilot. measurement to the BS associated with the pilot.

The BS responds with HDM without the pilot in it. The BS responds with HDM without the pilot in it. The MS moves the pilot to the neighbor set.The MS moves the pilot to the neighbor set.

The MS then sends a handoff completion The MS then sends a handoff completion message.message.

At some point, the BS may send it a neighbor At some point, the BS may send it a neighbor update list message that no longer contains the update list message that no longer contains the pilot and it is moved into the remaining set.pilot and it is moved into the remaining set.

54IFA’2004

Power Control in Power Control in CDMACDMA

Co-channel and adjacent channel interference are Co-channel and adjacent channel interference are not the major problems in CDMA. Why?not the major problems in CDMA. Why?

The interference is from other users transmitting in The interference is from other users transmitting in the frequency band at the same time.the frequency band at the same time.

To avoid the near-far problem, it is important to To avoid the near-far problem, it is important to implement good power control.implement good power control.

Signal strength may be good in CDMA, but frames Signal strength may be good in CDMA, but frames may still be received in error because of may still be received in error because of interference.interference.

Thus, Frame Error Rate (FER)Thus, Frame Error Rate (FER) is used for power is used for power control decisions rather than the signal strength control decisions rather than the signal strength used in other PCS systems.used in other PCS systems.

55IFA’2004

Open Loop Reverse Link Open Loop Reverse Link Power Control in IS-95Power Control in IS-95

RULE:RULE: Use a transmit power that is inversely Use a transmit power that is inversely proportional to the received signal strength of proportional to the received signal strength of pilots from all BSs.pilots from all BSs.– On the access channel, the MS sends a request On the access channel, the MS sends a request

using a weak signal if the pilot is strong. using a weak signal if the pilot is strong. – An ACK may not be received because the An ACK may not be received because the

transmit power was low or because of collisions.transmit power was low or because of collisions.– If no ACK is received, a stronger signal is If no ACK is received, a stronger signal is

transmitted.transmitted.– This is repeated until a maximum power level is This is repeated until a maximum power level is

reached.reached.– The process is repeated after a back-of delay.The process is repeated after a back-of delay.

56IFA’2004

Open Loop Reverse Link Open Loop Reverse Link Power Control in IS-95Power Control in IS-95

Up to 15 attempts can be made to Up to 15 attempts can be made to obtain a traffic channel.obtain a traffic channel.

Disadvantages:Disadvantages: Assumption that Assumption that forward and reverse channels are forward and reverse channels are identical; slow response time; and identical; slow response time; and using the total power received from using the total power received from all BSs in calculating the required all BSs in calculating the required transmit power.transmit power.

57IFA’2004

Closed Loop Reverse Link Closed Loop Reverse Link Power Control in IS-95Power Control in IS-95

On Downlink channelOn Downlink channel -> A power control bit -> A power control bit is transmitted every 1.25 ms (800 times per is transmitted every 1.25 ms (800 times per second).second).– A “0” indicates the MS should increase the A “0” indicates the MS should increase the

power and a “1” indicates it should power and a “1” indicates it should decrease it.decrease it.

Every 1.25 ms, in the BS, the receiver Every 1.25 ms, in the BS, the receiver determines the received signal to determines the received signal to interference ratio by sampling it 16 times.interference ratio by sampling it 16 times.– If it is above a preset target, the MS is If it is above a preset target, the MS is

instructed to reduce its power by 1 dB.instructed to reduce its power by 1 dB.

58IFA’2004


– Inner-Loop Power Control:Inner-Loop Power Control: If below the target, the MS is instructed to If below the target, the MS is instructed to

increase its power by 1 dB. increase its power by 1 dB. – The target value controls the long-term The target value controls the long-term

frame error rate.frame error rate.

–Outer-Loop Power Control:Outer-Loop Power Control: The target is varied over time to reflect The target is varied over time to reflect

accurate values related to velocity, fading, accurate values related to velocity, fading, environment, and so on. It is reduced environment, and so on. It is reduced (increased) by x dB every 20 ms if the FER (increased) by x dB every 20 ms if the FER is small (large) enough.is small (large) enough.

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60IFA’2004

Forward Link Power Forward Link Power ControlControl

Employed to reduce inter-cell interference.Employed to reduce inter-cell interference. Wthin a cell, multiple users employ orthogonal Wthin a cell, multiple users employ orthogonal

sequences and the primary source of interference sequences and the primary source of interference is from users of other cells or from multi-path.is from users of other cells or from multi-path.

A mobile assisted power control is employed:A mobile assisted power control is employed:– The MS periodically reports the FER on the The MS periodically reports the FER on the

forward link to the BS, which will then adjust its forward link to the BS, which will then adjust its transmit power accordingly.transmit power accordingly.

A maximum transmit power is defined to avoid A maximum transmit power is defined to avoid excessive interference.excessive interference.

A minimum transmit power is defined to avoid A minimum transmit power is defined to avoid allowing voice quality to drop.allowing voice quality to drop.

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Overview of 2G SystemsOverview of 2G Systems

(S/I)(S/I)min min N N

AMPS ~18 dB 7AMPS ~18 dB 7GSM ~11 dB 4GSM ~11 dB 4IS-54 ~16 dB 7IS-54 ~16 dB 7IS-95 ~15 dB 1IS-95 ~15 dB 1

chapter 8 (part ii: 2g pcs systems

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