Lecture Notes 12: Digital Cellular Communications

Transcription

1 SNR Lecture Notes 2: Digital Cellular Communications Consier a cellular communications system with hexagonal cells each containing a base station an a number of mobile units Figure 5: Celluar Communication System XII- XII-2 Forwar Channel (Outboun) The link from the base station to the mobile unit Reverse Channel (Inboun) The link from the mobile to the base station Assumptions Each cell is ivie into 3 sectors an perfect isolation is possible between sectors All users employ ifferent spreaing coes Perfect power control (all fast faing (Rayleigh) an slow faing (ue to shaowing) The power receive at the mobile (or base) from ifferent users is the same Negligible thermal noise Voice Activity results in reuce interference Every cell uses the same frequency ban Interference from other cells is inclue Banwith W, Data Rate Rb Consier user A The output of the receiver matche to user A s coe sequence is X T Eb η where η accounts for the interference from all other users an b enotes the ata bit transmitte The variance of η is given by σ 2 K 3N E assuming ranom elays for the interfering users an ranom phases If on the other han we looke at the worst possible phase an elay for each of the interfering users the variance woul be K σ 2 N E The ratio of the magnitue of the output ue to the esire signal an the square root of the variance of the interference etermines the signal-to-noise ratio Assuming the worst case phases an elays E σ N K If we were not using any coing then the error probability (uner a Gaussian approximation) is given by P e b Q For other coing schemes the relation between the error probability an signal-to-noise ratio SNR XII-3 XII-

2 SNR is more complicate However, if an acceptable signal-to-noise ratio (at the output of the emoulator) is etermine from the coing scheme employe then it is possible to calculate the capacity (calls per cell) of a Direct Sequence CDMA The voice activity factor (usually taken to be /2) reuces the amount of interference by turning own the power when slower ata rates are possible (because of voice inactivity) The interference from other cells is taken to be 66% of the interference from within the cell of the user Thus the variance of the interference can be moifie to taken account the voice activity an the interference from other cells as follows The moifie signal-to-noise ratio is N K F D FW K RbD Thus the number of calls per sector K s for an output signal-to-noise ratio of SNR is The number of calls per cell is then K c is given by K s K s W R b SNR 2 D F W R b SNR 2 D F G W R b SNR 2 D F where σ 2 NF D Voice Activity Factor K ED 2 F FrequencyReuseFactor 6 Example W 25MHz R b 96bits/secon 2 SNR 6B G 3sectors/cell XII-5 XII-6 Potential Avantages of CDMA for Digital Cellular Voice Activity Users/cell for DS-CDMA = 72 users per 25 MHz per cell FDMA has a capacity of 6 channels or users per cell per 25 MHz TDMA has a capacity of 76 channels or users per cell per 25 MHz Caveat: These number are quite optimistic (especially for DS-CDMA) Many engineers believe that the capacity for CDMA is more realistically on the orer of -5 calls/cell Imperfect power control is one key factor that reuces the actual capacity couple with the fast faing which can not be compensate for by power control NoEqualizer (to eliminate intersymbol interference) OneRaio per Basestation (Front en) SoftHanoff NoGuarTime (Require by TDMA) LessFaing FrequencyManagementEliminate FrequencyReuse= Disavantages: Power Control Transition from Narrowban system to wieban system XII-7 XII-8

3 IS-95 Stanar for Cellular Transmission SpeechEncoing Network Issues ReverseLink Error Control Coing Moulation Spreaing ForwarLink Error Control Coing Moulation Spreaing Speech Encoer Voice is encoe by means of a variable rate speech encoer The possible ata rates are 86 bps, bps, 2 bps, 8 bps When operating at a lower rate users turn own the power on forwar link an gate the power off on the reverse link (to maintain a fixe E b (CRC an tail bits for the convolutional coe) the rates are 96, 8, 2 an 2 bits/secon N) an thus cause less interference for other users After a small amount of overhea The system banwith of 23MHz using pseuo-ranom spreaing-coes Multiple users occupy the whole banwith simultaneously (but with ifferent phases of a very long spreaing coe) The near-far problem typical of DS-CDMA is solve with power control XII-9 XII- Forwar Traffic Channels Network Issues Forwar CDMA Channels 23MHz channel transmitte by base station Logically there are a number of ifferent channels (using ifferent orthogonal Walsh functions on the forwar link an ifferent phases of a spreaing coe on the reverse link) besies those use for sening voice traffic These inclue the following: Pilot Chan Sync Chan Paging Ch Paging Ch 7 Traffic Ch Traffic Ch n Traffic Ch 55 W W 32 W W 7 W 8 W 63 Traffic Data Power Control Aresse by Orthogonal Walsh Coe XII- XII-2

4 Forwar Traffic Channels Pilot Channel: Transmitte on the forwar channel an use to ientify the base stations within range of the mobile The mobile keeps a list of the nearest base stations This channel is also use to provie phase synchronization for the mobile an channel gain estimates Paging Channel: Transmitte on the forwar channel an use in setting up a call to or from a mobile Transmits ata at rates of 2, 8, 96 bps Use to assign a Walsh coe (Haamar sequence) for the forwar traffic channel It is also use to ientify other neighboring base stations for the purpose of hanoff processing Forwar Traffic Channels (cont) Sync Channel: Transmitte on the forwar channel an use to bring the mobile unit into synchronization (timing) with the base Contains timing information with regar to the long coe that is use to ientify users Power Control Subchannel: Transmitte on the forwar channel The voice traffic is replace with power control bits once every 25ms or power control group to use by the mobile to increase or ecrease the transmitte power One power control bit is transmitte with uration of 2 moulation symbols or 66µs The power level for transmission of the power control bit is the same as woul be transmitte by a full rate (high power) traffic channel even when the traffic channel is transmitting at a lower power level XII-3 XII- Reverse Traffic Channels Reverse CDMA Channels 23MHz channel receive by base station Reverse Traffic Channels Access Channel: Transmitte on the reverse channel an use to alert the base to mobile initiate calls an to respon to pages (on the paging channel) It is use in a ranom access moe (Aloha) by mobiles Access Ch Access Ch n Traffic Ch Traffic Ch n Traffic Channel: Transmitte on the forwar an reverse links Use to transmit voice or ata traffic Can operate at rates of 2bps, 2bps, 8bps, an 96bps Aresse by Long Coe XII-5 XII-6

5 Data Rate Information Bits per Frame kbps kbps 2kbps 8kbps Block Diagram of Transmitter A 2/8 bit CRC for 96 an 8bps rates kbps kbps 2kbps 8kbps A 8 bit Encoer Tail kbps 8kbps 2kbps 2kbps Rate /3 Convolutional Encoer kbps Reverse Link Traffic Channel Parameter Data Rate bps Parameters Units PN Chip Rate Mcps Coe Rate /3 /3 /3 /3 Mcps I Duty Cycle percent Block Interleaver 6-ary Orthogonal Moulator Walsh chips Data Burst Ranomizer chips 2288Mcbps Coe Symbol Rate sps Moulation coe sym/mo symbol Walsh Chip Rate kcps 372kbps 2288Mcps Q Mo Symbol Duration µs Long Coe Generator PN Chips/Coe Symbol PN Chips/Mo Symbol PN Chips/Walsh Chip Long Coe Mask XII-7 XII-8 Input Output Constraint Length 9, Rate /3 Convolutional Encoer Figure 5: 2 bit CRC Encoer Switches are up for first 72 bits an own for last 2 bits + c Input Output Information Bits Coe Symbols Figure 52: 8 bit CRC Encoer Switches are up for first 72 bits an own for last 8 bits + + c c 2 XII-9 XII-2

6 Interleaver The convolutional encoer output is interleave using ifferent size interleavers For the high rate ata stream the interleaver is a 32 by 8 interleaver Symbols are written into the interleaver memory column-wise an rea out row-wise Thus if the sequence of symbols at the input to the interleaver is c,c 2,c 3, the sequence of symbols at the output of the interleaver is c c33 c 65 For the 96 bps channel the rows are rea out consecutively For the 8bps channel the rows are rea out in the following orer For the Access channel the rows are rea out in the following orer XII-2 XII-22 Interleaver For the 8 bps ata rate each symbol is repeate twice in the interleaver memory However, one of the two rows is not actually transmitte Which row is selecte is etermine from the ata bit ranomizer Similarly, for the 2 bps ata rate each symbol is repeate four times but only one of every set of four rows is actually transmitte For the 2 bps ata rate each symbol is repeate 8 times but only one of every 8 rows is selecte by the ata burst ranomizer XII-23 XII-2

7 XII-25 XII-26 Orthogonal Signals XII-27 XII-28

8 b b 2 b 7 b 72 b b 2 b 7 b 72 2 t t 7 t 8 c c 2 c 3 c c 5 c 6 c 57 c 572 c 573 c 57 c 575 c 576 c c 33 c 65 c 97 c 29 c 6 c 6 c 8 c 8 c 52 c 5 c 576 w w 2 w 6 w 68 w 6 2 Data Bits 6 Coe Bits 6 Walsh Chips XII-29 XII-3 Long Coe Shift Register Long Coe Mask The mask for the long coe epens on the channel type (traffic or access) When using the access channel the 9 high orer bits of the mask are set to The next 5 bits are set the the access channel number The next 3 bits are set to correspon to the associate paging channel Then next 6 bits are set to the base ID while the lowest 9 bits correspon to the pilot pn value for the current CDMA channel The 9 high orer bits for the mask for the long coe for the reverse traffic channel are while the low orer 32 bits are set to a permutation of the mobiles electronic serial number (ESN) XII-3 XII-32

9 s i i n q n Spreaing Each Walsh chip w i is sprea by a factor of using the long coe Then each of the chips is use for both the inphase an quarature phase channels Each of these channels is scramble accoring to the base stations short coes This scrambling is equivalent to a phase shifter as shown below Let u be the output of the long coe spreaing operation Then if we express the inphase an quarature phase signals as complex variables the output after scrambling by the short coes is s i s q s i s q v usi jusq u sjsq i ve jθ u 2ue jθ s i ss2q 2 i Since ss2j 2 i 2wehaveremoveall aspect of the scrambling function from the esire user js q e jθ After receiving the signal there is some unknown phase shift (ue to elay) in the receive signal The receive signal is r ve jθ To remove this scrambling function we must multiply by s i js q z r js q XII-33 XII-3 (-,+) Quarature-Phase Signal (+,+) Short Coes The two short coes are generate by m-sequences with feeback connections In-Phase Signal i n q n in q n 5i n in q n i n q n qn 6i 2 n q n9 qn 5 q 3 n When the shift register is in the state with zeros an one a zero is inserte to make the length of the sequence 2 5 (instea of 2 5 ) (-,-) (+,-) Each base station uses the same shift register for the short coe but the phase of the sequence is shifte by multiples of 6 chips between one base station an another base station Figure 53: Offset QPSK Constellation XII-35 XII-36

10 jθ z i Phase Shifting Network Reason for Offset QPSK on Reverse Link (Mobile-to-Base) x in x out xinzr yinzi On the link from the mobile to base battery power is a crucial issue The use of high efficiency amplifiers warrants the use of amplifiers operating in the nonlinear range If the signal is not of constant envelope or nearly constant envelope there woul be istortion to the signal when amplifie For a nonconstant envelope signal the nonlinearity can regenerate some siebans that have been filtere out by the baseban filters If stanar QPSK ha been use the signal woul be much less constant envelope (the signal going through the origin woul have significant envelope variations especially after being filtere) This woul cause significant istortion of the signal an the regeneration of the siebans On the link from the base to the mobile battery life is not an issue an only one amplifier nees to be built (for all of the signals) Thus some care can go into esigning a linear amplifier y in z r The above phase shifter oes the following computation x injy in z r jz i xinzr z i rx injy in e yinzi y out yinzr xinzi j y in z r jθ x in z i where re jz q XII-37 XII-38 Frame Structure Information Bits per Frame Data Rate 86kbps kbps 2kbps 8kbps A 2/8 bit CRC for 96 an 8bps rates kbps kbps 2kbps 8kbps A 8 bit Encoer Tail kbps 8kbps 2kbps 2kbps Rate /3 Convolutional Encoer kbps Block Interleaver Repeat I short coe generator cos 2 fct 6-ary Orthogonal Moulator Walsh chips 372kbps Data Burst Ranomizer 2288Mcps 2576 chips I Q DTc 2 Baseban Filter Baseban Filter 2288Mcps Q short coe generator sin 2 fct Long Coe Generator Long Coe Mask XII-39 XII-

11 Block Diagram of Mobile Transmitter Access Channel Reverse Link Access Channel Parameters Information Bits per Frame 88 Data Rate kbps A 8 bit Encoer Tail 96 8kbps Rate /3 Convolutional Encoer Repeat 288kbps kbps Block Interleaver Data Rate (bps) Parameters 8 Units PN Chip Rate 2288 Mcps Coe Rate /3 Mcps Coe Symbol Repetition 2 6-ary Orthogonal Moulator Walsh chips 372kbps 2288Mcps I short coe generator 2576 chips I Q D Baseban Filter Baseban Filter cos 2 fct Duty Cycle percent Coe Symbol Rate 288 sps Moulation 6 coe sym/mo symbol Moulation Symbol Rate 8 symbols/sec Walsh Chip Rate 372 kcps Q short coe generator sin 2 fct Mo Symbol Duration 2833 µs Long Coe Generator PN Chips/Coe Symbol 267 PN Chips/Mo Symbol 256 Long Coe Mask PN Chips/Walsh Chip XII- XII-2 Spreaing for Reverse Link of IS-95 Bits per Frame Data Rate 86kbps kbps 2kbps 8kbps Encoer for Reverse Link of IS-95 A 2/8 bit CRC for 96 an 8bps rates kbps kbps 2kbps 8kbps A 8 bit Encoer Tail kbps 8kbps 2kbps 2kbps Rate /3 Convolutional Encoer kbps kbps 72kbps 36kbps Repeat an Block Interleaver 576 bits/2ms 288kbps 6-ary Orthogonal Moulator Walsh chips per 2ms 372kbps Data Burst Ranomizer 2288Mcbps + Long Coe Generator I Channel Sequence + + Q Channel Sequence 2576 chips 2288Mcps Long Coe Mask XII-3 XII-

12 Frame Structure b b 2 Encoe c c 2 c 3 c c 5 c 6 Interleave c c 33 c 65 c 97 c 29 c 6 Walsh Coe Moulate Information b b 2 b 7 b 72 CRC an Tail b b 2 b 7 b 72 2 t t 7 t 8 Encoe c c 2 c 3 c c 5 c 6 c 57 c 572 c 573 c 57 c 575 c 576 Interleave c c 33 c 65 c 97 c 29 c 6 c 6 c 8 c 8 c 52 c 5 c 576 WalshCoe w w 2 w 6 w 68 w 6 w Sprea each Walsh chip with chips from the long an short coe w 6 XII-5 XII-6 Notes Reverse Channel Moulation For a 96 bps frame the ata burst ranomizer oes nothing 2 For a 8 bps frame the ata burst ranomizer removes half of the power control bit groups The ones remove epen on the state of the long coe generator in the previous speech frame 3 For a 2 bps frame the ata burst ranomizer removes three quarters of the power control bit groups Baseban Filter cos 2 f c t For a 2 bps frame the ata burst ranomizer removes seven eights of the power control bit groups 5 The set of power control groups transmitte by a 2 bps frame is a subset of that transmitte by a 2bps frame which is a subset of that transmitte by a 8bps frame Delay 2 T c Baseban Filter sin 2 f c t XII-7 XII-8

13 δ2 B Filter Characteristics for Baseban Filter Reason for Offset QPSK on Reverse Link (Mobile-to-Base) On the link from the mobile to base battery power is a crucial issue The use of high efficiency amplifiers warrants the use of amplifiers operating in the nonlinear range If the signal is not of constant envelope or nearly constant envelope there woul be istortion to the signal when amplifie For a nonconstant envelope signal the nonlinearity can regenerate some siebans that have been filtere out by the baseban filters If stanar QPSK ha been use the signal woul be much less constant envelope (the signal going through the origin woul have significant envelope variations especially after being filtere) This woul cause significant istortion of the signal an the regeneration of the siebans On the link from the base to the mobile battery life is not an issue an only one amplifier nees to be built (for all of the signals) Thus some care can go into esigning a linear amplifier Filter Requirements: δ 5B f p 59kHz f 7kHz s XII-9 XII-5 Power Control Reason for Augmenting the Short Coe The short coe is base station specific In synchronizing the system knowing the starting point of the short coe etermines the starting point of the moulation symbols since there is exactly an integer number of moulation symbols per short coe perio If there were not an integer number then the short coe synchronization woul not be sufficient for moulation symbol synchronization Reverse Link: Open Loop Analog: 85 B range, few microsecon response for suen improvement in channel, but slow power buil up when channel is poor so that close loop control can occur Close Loop: B every ms, or so, 2 B change allowe (8 Hz rate an 25 ms power control groups) Forwar Link: Approximately 5 B or 2% every 5-2 ms 6 B ynamic range XII-5 XII-52

14 System Timing The long coe an the short coe are in the state with or zeros an a single one on Jan 6, 98 at :: Universal Coorinate Time (UTC) The clock rate is 2288MHz The long coe has perio 2 2 while the short coe has perio 25 The perio of the commbination is clock ticks Notes The base stations transmissions are all reference to a system wie time scale using Global Position System time scale which is synchronous with Universal Coorinate Time (UTC) GPS an UTC iffer by the number of leap secons since January 6, 98 2 Alignment of the long coe an short coe will occur again in 37 centuries 3 The mobile attempts to synchronize to System Time base on information receive from base station transmissions XII-53 XII-5 Transmitter for Pilot, Paging an Synch Channels Notes The pilot short coe spreaing for ifferent base stations are ientical (except in the timing or sequence phase) They iffer by a multiple of 6 PN chips Thus a mobile using a single matche filter can etermine the signal strength ue to pilots signals from ifferent base stations This information is use to ecie when to hanoff to another base station XII-55 XII-56

15 Transmitter for Forwar Link Traffic Channel GSM an IS-5/36 European Mobile Communication System Global System for Mobile Communications (GSM) This is a secon generation cellular phone evelope in Europe to create a system for all Europe (replacing the analog systems in many countries) XII-57 XII-58 System Characteristics Frequency Ban Mobile Transmit Base Transmit Speech Coer Rate Information Bits/Speech Frame Speech Frame Duration Channel Encoing Overall coe rate MHz MHz 3 kbps 82 Class I 78 Class II 26 Total 2 ms 5 Class I bits protecte with 3 parity bits All Class I bits an previous parity bits protecte with rate /2 convolutional coe 26/56=57 information bits/channel bit System Characteristics (cont) Multiple Access Slots/Frame 8 Time Slot Duration Frame Duration Moulation Symbol alphabet Hop Rate Carrier Spacing TDMA/Slow Frequency Hop 5769ms 65ms GMSK B 3 T 3 Binary (ifferentially encoe) 2666 Hops/s (= Frame Rate) 2kHz XII-59 XII-6

16 Figure 5: Transmitter Block Diagram for GSM XII-6 Figure 55: Error Control Coing for GSM XII-62 Reorering 2 5 p p p p p p XII-63 Figure 56: GSM Convolutional Encoer XII-6

17 Figure 57: GSM Frame Structure Interleaving for GSM Figure 58: Interleaving for GSM XII-65 XII-66 B B B2 B3 B B5 B6 B B B B2 B3 B B5 B6 B XII-67 XII-68

18 b(t) time/tb sin(phi(t)) y(t) 5 phi(t) time/tb time/tb Figure 59: GMSK Waveform cos(phi(t)) Figure 6: GMSK Waveforms XII-69 XII-7 phi(t) time/tb IS-5/ This is a secon generation cellular phone evelope for the US market an stanarize in 99 It is very similar to PHS in Japan Figure 6: GMSK Waveforms XII-7 XII-72

19 Frequency Ban System Characteristics System Characteristics (cont) Mobile Transmit Base Transmit Speech Coer Rate Information Bits/Speech Frame 82-89MHz MHz 795kbps 77 Class I 82 Class II 59 Total Multiple Access TDMA Frame Duration ms Slots/Frame 6 Slot Duration 666ms Coe Symbols/Slot 26 Instantaneous Rate 86 kbps Speech Frame Duration 2 ms Moulation Rate 23 ksps Channel Encoing 2 Class I bits protecte with 7 parity bits Moulation DQPSK, Raise Cosine Filtere with α 35 All Class I bits an previous parity bits Symbol alphabet Quaternary (ifferentially encoe) protecte with rate /2 convolutional coe Carrier Spacing 3kHz Overall coe rate 59/26=62 information bits/channel bit XII-73 XII-7 Figure 62: Frame Structure of IS-5 Each user is assigne two of the six slots Full rate users are assigne two slots which are either slots an or 2 an 5 or 3 an 6 Half rate users are assinge one channel Thus every 3kHz channel is use by three full rate users an thus the capacity is three times that of AMPS Figure 63: Slot Format for IS-5 G= Guar Time RSVD= Reserve R= Ramp Time SACCH=Slow Associate Control Channel Data= User Information or FAACH CDVCC=Coe Digital Verification Color Coe XII-75 XII-76

20 Control Channels The Slow an Fast Associate Control Channel is use for signalling bits such as for hanoff, power control an timing The Fast Associate Control Channel is transmitte in a blank an burst moe, that is, the traffic information for a slot is replace by signalling information for that slot Color Coe The CDVCC is use to istinguish signals from ifferent cells There are 255 possible values for CDVCC which is coe with an (2,8) shortene Hamming coe for error protection Power Control Mobile must be capable of changing the power transmitte in B steps from -2BW to -3BW on comman from the Base Station Time Control Mobile must be capable of changing the time of transmission of a slot in steps of uration /2 a symbol Figure 6: Block Diagram of Encoing for IS-5 XII-77 XII-78 Figure 65: Block Diagram of Encoing for IS-5 Figure 66: Encoing for IS-5 XII-79 XII-8

21 c + Information Bits + Coe Bits c The 2-slot interleaver works as follows The even numbere bits are written into one interleaver in the even numbere locations while the o numbere bits are written into a secon interleaver These are written in column-wise filing up the first column, then the secon column an so on (The even numbere bits are enote by x an an the o numbere bits are enote by y below) The transmitte bits for a given slot are the contents of one of the interleavers rea out row-wise Figure 67: Constraint Length 6, Rate /2 Convolutional Encoer XII-8 XII-82 Interleaver for IS-5 x 26x 52x 78x x 3x 56x 82x 28x 23x y 27y 53y 79y 5y 3y 57y 83y 29y 235y 2x 28x 5x 8x 6x 32x 58x 8x 2x 236x 3y 29y 55y 8y 7y 33y 59y 85y 2y 237y x 3x 56x 82x 8x 3x 6x 86x 22x 238x 5y 3y 57y 83y 9y 35y 6y 87y 23y 239y 2x 38x 6x 9x 6x 2x 68x 9x 22x 26x 3y 39y 65y 9y 7y 3y 69y 95y 22y 27y 2x 5x 76x 2x 28x 5x 8x 26x 232x 258x 25y 5y 77y 3y 29y 55y 8y 27y 233y 259y Thus the orer of bits transmitte woul be the following Bit from current speech frame Bit 26 from current speech frame Bit 52 from current speech frame Bit 78 from current speech frame Bit from current speech frame Bit 3 from current speech frame Bit 56 from current speech frame Bit 82 from current speech frame Bit 28 from current speech frame Bit 23 from current speech frame Bit from previous speech frame Bit 27 from previous speech frame Bit 53 from previous speech frame XII-83 XII-8

22 Synchronization Sequences are shown below with the autocorrelation functions shown below Figure 68: Autocorrelation function of synchronization sequences XII-85 XII-86 Demoulation/Decoing Error in transmission can cause the CRC for the 2 most perceptually significant bits to fail When a slot is use as a FACCH the CRC will likely fail also The ecoer has six states State : CRC checks, an the receive at is use by the speech ecoer State CRC Fails: The 2 bits from the previous frame are use for the 2 most perceptually significant bits State 2 Two consecutive CRC Fails: The 2 bits from the previous correct frame are use for the 2 most perceptually significant bits State 3-6 Three consecutive CRC Fails: The 2 bits from the previous correct frame are use for the 2 most perceptually significant bits except the speech frame energy is attenuate by B is state 3, 8B in state, 2 B in state 5 an the speech frame is mute in state 6 In states 2-5 a correct CRC brings the ecoer state to In state 6 two consecutive correct CRC s bring the encoer to state XII-87