Forward Traffic Channels • Forward Traffic Channels • At the end of this section, the following objectives will have been accomplished: • Understand what Forward Traffic Channels are used for, how they are generated and which are the main modulation parameters associated with them. • Introduce the concept of “Service Options”. • Understand the process of analog-to-digital signal conversion • Understand the role of the vocoders, the type of vocoders used in CDMA, and where they are physically located. • Understand the Forward Traffic Channel frame structure for both the 8 kb and the 13 kb vocoder and the purpose of the “tail bits”. • Understand the purpose of the Symbol Puncturing step applied to the modulation symbols when the 13 kb vocoder is used. • Introduce the concept of Power Control Subchannel and identify its effect on the Forward Traffic Channel bit stream. • Demonstrate how spreading and despreading work in a composite signal made of three different bit streams. • Understand the concept of “Composite I” and “Composite Q” • Understand the concept of QPSK, “I” and “Q” mapping, signal constellations, and phase transitions • Understand how the CDMA Forward Channels are demodulated and the concepts of “correlator”, “search correlator”, “finger” and “rake receiver”. • Understand the concept of Traffic Frame Staggering • Summarize the messages transmitted on the CDMA Forward Channels
Transcript
Forward Traffic Channels
• Forward Traffic Channels
• At the end of this section, the following objectives will have been accomplished:
• Understand what Forward Traffic Channels are used for, how they are generated and which are the main modulation parameters associated with them.
• Introduce the concept of “Service Options”.
• Understand the process of analog-to-digital signal conversion
• Understand the role of the vocoders, the type of vocoders used in CDMA, and where they are physically located.
• Understand the Forward Traffic Channel frame structure for both the 8 kb and the 13 kb vocoder and the purpose of the “tail bits”.
• Understand the purpose of the Symbol Puncturing step applied to the modulation symbols when the 13 kb vocoder is used.
• Introduce the concept of Power Control Subchannel and identify its effect on the Forward Traffic Channel bit stream.
• Demonstrate how spreading and despreading work in a composite signal made of three different bit streams.
• Understand the concept of “Composite I” and “Composite Q”
• Understand the concept of QPSK, “I” and “Q” mapping, signal constellations, and phase transitions
• Understand how the CDMA Forward Channels are demodulated and the concepts of “correlator”, “search correlator”, “finger” and “rake receiver”.
• Understand the concept of Traffic Frame Staggering
• Summarize the messages transmitted on the CDMA Forward Channels
CDMA Forward Traffic Channels
n Used for the transmission of user and signaling information to a specific mobile station during a call
n Maximum number of traffic channels: 64 minus one Pilot channel, one Sync channel, and 1 through 7 Paging channelsThis leaves each CDMA frequency with at least 55 traffic channelsUnused paging channels can provide up to 6 additional channelsRealistic loading will typically be about 17 subscribers when using
the 13 kb vocoder (22 when using the 8 kb vocoder)
Forward Traffic Channel
Forward Traffic Channel
Sync
Paging
Forward Traffic Cha nnel
Forward Traffic Channel
Pilot
CDMA Cell Si te
n How traffic bits are processed is determined by implementing defined Service Options
n Service Options can be requested as follows: by the mobile station upon call origination and during traffic channel
operation by the system when paging the mobile station and during traffic
channel operation Service Option type can be changed while a call is in progress
n Mobile station and base station may negotiate the service option to be provided
Forward Traffic Channel
CDMA Ce ll Site
Other Channels
Service Options8 kb vocoder13 kb vocodermobile station loopbacksfacsimilecircuit switched datapacket switched dataetc.. . . . . . .
Service Options
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64 kbs
Analog Voice Signal Sampling Quantizing
Signal Regeneration
Digital Stream 0 (DS0)
• A-law, devised by CCITT
• MU-Law, devised by BELL
Analog to Digital Conversion
Traffic Channel Vocoding
n Vocoding reduces the bit rate needed to represent speech
n Output is 20 ms frames at fixed rates Full Rate, 1/2 Rate , 1/4 Rate , 1/8 Rate, & Blank
n CRC is added to all the frames for the 13 kb vocoder, but only to the Full and 1/2 rate frames for the 8 kb vocoder
n CRC is not added to the lower rate frames in the 8 kb vocoder but that is ok because they consist mostly of background noise and have a higher processing gain
Conv ol utionalE nc odi ng
Code SymbolRepetition
Bl oc k
Interle avi ng
Data S crambli ng
P ow er ControlSubchannel
OrthogonalSpre ading
Quadra ture
Spre ading
Ba seband
Filtering
VocoderProc ess ing
Baseband Traffic to RF Section
To theConvolutional
Encoder
20 ms slices(1280 bits)
Variable RateVoice Coding
Add CRC Add 8 bitEncoder Tail
64 kbpsFrom MTX
PCM Voice
BSC
BTS
(S ymbol
Puncturing)
Variable Rate VocodingA-to-D
CONVERTER
64 kbps
VOCODER
“Codebook” Instruction(< 64 kbps)
n Speech coding algorithms (digital compression) are necessary to increase cellular system capacity
n Coding must also ensure reasonable fidelity, i.e., a minimum level of quality as perceived by the user
n Coding can be performed in a variety of ways (ex. waveform, time or frequency domain)
n Vocoders transmit parameters which control “reproduction” of voice instead of the explicit, point-by-point waveform description
Variable Rate Vocoding
n Performed by Digital Speech Processors (DSPs)
n TDMA uses VSELP encoding - fixed at 8 kbps rate
n CDMA uses QCELP - encoding a variable rate (adaptive threshold) Ranges from 13 kbps to 1 kbps (averaging 4 kbps) Takes advantages of natural pauses in speech
n Both VSELP & QCELP are Hybrid Coders which combine waveform matching & speech signal parameters
A- to- D
CO
N
VE
R
TE
R
64 kb p s
DSP QCELP VOCODER
20 ms Sample
Codebook
PitchFilter
FormantFilter
Coded ResultFeedback
Loop
Forward Traffic Channel Generation
W alshfunc tion
Powe rControl
B it
I PN
96 00 bps48 00 bps24 00 bps
12 00 bpsor
144 00 bps72 00 bps36 00 bps
18 00 bps(From Vocoder)
Convol utional
Encoding andRepetition Symbol
Puncturing
(13 k b only)
1 .2288 M cpsLong PN Code
Generati on
19.2k sps
8 00 Hz
R = 1/2
Q PN
Decimator DecimatorUser Addres s
Mas k(ESN-based)
19.2ksps
1.22 88 Mcps
Scrambling
bits symbols chips
19.2k sps
28.8ksps
CHANNEL ELEMENT
MUX
Bloc kInte rleaving
Forward Traffic Channel Modulation Parameters
Data Rate (8 kb vocoder) (13 kb vocoder)
Bits Per Second 960014400
48007200
24003600
12001800
PN Chip Rate Mega Chips Per Second1.2288 1.2288 1.2288 1.2288
Mod. Symbol Rate (8 kb) (13 kb) Mod. Symbols Per Second
1920028800
1920028800
1920028800
1920028800
Puncturing Rate (13 kb) —2/3 2/3 2/3 2/3
Mod. Symbol Rate GoingInto Block Interleaver
Mod. Symbols Per Second19200 19200 19200 19200
Es Energy per Mod. SymbolEb/2 E b/4 Eb/8 Eb/16
* each repetition of a “code symbol” is called a “modulation symbol”
Forward Traffic Channel Frame structure
TransmissionRate Total Reserved Information CRC Tail Bits
9600 192 — 172 12 8
4800 96 — 80 8 8
2400 48 — 40 — 8
1200 24 — 16 — 8
14400 288 1 267 12 8
7200 144 1 125 10 8
3600 72 1 55 8 8
1800 36 1 21 6 8
1
2
Number of Bits per FrameRateSet
Tail Bits (step 1)
00b1b2b3 0
000b1b2b3
000b1b2b3
C0,1
C1,1
Tail Bits (steps 2 & 3)
b1b2b3 0
0b1b2b3 0
0b1b2b3
C0,3 C0,2 C0,1
C1,3 C1,2 C1,1
Tail Bits (steps 2 & 3) – cont.
00b1b2b3 0
000b1b2b3
000b1b2b3
C0,1
C1,1
Tail Bits (step 4)
b2b300 b10
b1b2b3 0000
b1b2b3000
C0,4 C0,3 C0,2 C0,1
C1,4 C1,3 C1,2 C1,1
Tail Bits (steps 5 & 6)
b300 b20
b2b300 b10
b2b300
C0,5 C0,4 C0,3 • ••• C0,1
C1,5 C1,4 C1,3 • ••• C1,1
0
Tail Bits (steps 5 & 6) – cont.
000 b3
b300 b20
b3000
C0,6 C0,5 C0,4 • ••• C0,1
C1,6 C1,5 C1,4 • ••• C1,1
Forward Traffic Channel FrameInformation Bits for Multiplex Option
‘01’ 40 128 —
‘10’ 16 152 —
‘11’ — 168 —
‘00’ 80 — 88
‘01’ 40 — 128
‘10’ 16 — 152
9600
‘11’ — — 168
4800 — — — 80 — —
2400 — — — 40 — —
1200 — — — 16 — —
‘1’
‘0’
‘00’ 80 88 —
‘0’ — — 171 — —
TransmitRate
Mixed Mode(MM)
Traffic Type(TT)
Traffic Mode(TM) Primary Signaling [Secondary]
Traffic (bits per frame)
‘1’(optional)
Forward Traffic Channel FrameInformation Bits for Multiplex Option 2
‘0001’ 54 208 —
‘0010’ 20 242 —
‘0011’ — 262 —
‘0100’ 124 — 138
‘0101’ 54 — 208
‘0110’ 20 — 242
14400
‘0111’ — — 262
‘1’
‘0000’ 124 138 —
‘0’ — 266 — —
TransmitRate
Mixed Mode(MM)
Frame Mode(FM) Primary Signaling [Secondary]
Traff ic (bits per frame)
‘1000’ 20 222 20
1800
‘100’ 54 — 101
‘101’ 20 — 121
‘110’ 20 81 20
— 54 — —
‘00’ 20 32 —
‘01’ — 52 —3600
‘10’ 20 — 32‘1’
‘011’ 124 — 67‘1’
‘010’ — 121 —
‘001’ 20 101 —
‘000’ 54 67 —
‘11’ — — 52
— 20 — —
— — — 20‘1’
‘0’
‘0’
‘0’ — 124 — —
7200
Convolutional Encoding and Symbol Repetition
n Convolutional encoding Is a means of error detection/correction Results in 2 “code symbols” (or more, depending
on the “R” constant) output for each bit input
n Symbol repetition maintains a constant 19.2 Ksps output to be fed into the block interleaver
Also allows for reduction in transmit power Reduces overall noise and increases capacity
ConvolutionalEncoding
Code SymbolRepetition
BlockInterleaving
Data Scrambling
Power ControlSubchannelOrthogonalSpreadingQuadratureSpreadingBasebandFiltering
VocoderProcessing
Baseband Traff ic to RF Section
Variable RateOutput f romthe Vocoder
ConvolutionalEncoder
R=1/2 K=9
SymbolRepetition
19.2 kspsto Block
Interleaver
14.4 kbps 7.2 kbps 3.6 kbps
1.8 kbps
28.8 ksps14.4 ksps 7.2 ksps
3.6 ksps
PCM Voice
(SymbolPuncturing)
9.6 kbps
4.8 kbps2.4 kbps1.2 Kbps
19.2 ksps
9.6 ksps4.8 ksps2.4 ksps
28.8 kspsto Block
Interleaver
8 k b
13 kb
bits codesymbols
modulationsymbols
Symbol Repetition and Power Reduction
n Symbol repetition provides a constant rate to the block interleaver
n Lower rates symbols are sent at reduced power levels
n The energy per bit across all rates is identical when integrated
n Overall signal power requirement (thus noise) is reduced
Data Rate(bps)
Energy perModulation Symbol
9600
4800
2400
1200
E =E /2s b
E =E /4s b
E =E /8s b
E =E /16s b
Full Energy
1/2 Energy
1/4 Energy
1/8 Energy
Symbol PuncturingRate Set 2 (13 kb Vocoder)
n Symbol repetition maintains a constant 28.8 ksps output to puncturing section
n Symbol puncturing deletes 2 of every 6 inputs based on a six bit pattern
n Unrepeated symbols for 28.8 ksps frames are also deleted convolutional decoder in mobile station will correct these
purposeful errors
n Puncturing provides a constant 19.2 Ksps input to interleaver just like in rate set 1
This allows all other functions to remain exactly the same
PCM Voice
Convolutional
Encoding
Code SymbolRepetition
BlockI nterleav ing
Da ta Sc rambling
Powe r Control
Subcha nnel
OrthogonalSpreading
QuadratureSpreading
Bas eba ndFilteri ng
Voc oderProces sing
Baseband Traffic to RF Section
FromR=1/2 K=9
Convolutional Encoder
SymbolPuncturing
to B loc k theInterleav er
SymbolRepetition
28 .8 ks ps
(Sy mbolP uncturing)
28.8 ksps14.4 ksps 7.2 ksps 3.6 ksps
19.2 Ks ps
Block Interleaving
1 9.2 ksps
From Coding& Sy mbol
Repetition
n 20 ms symbol blocks are sequentially reordered
n Combats the effects of fast fading
n Separates repeated symbols at 4800 bps and below
Improves survivability of symbol data “Spreads” the effect of bursty interference
Input Array(Normal
Sequence)24 X 16
Output Array(ReorderedSequence)
24 X 16To Data
Scra mbl ingFunction
PCM Voice
Convolutional
Enc oding
Code SymbolRepetiti on
B lock
Interlea ving
Data Sc rambling
Powe r Control
Subchannel
OrthogonalSprea di ng
QuadratureSprea di ng
Bas eband
Fi lte ring
V oc ode r
Proce ssi ng
Baseband Traffic to RF Section
(Sy mbolPuncturing)
Data Scrambling
n The basic Long PN Code generator sequence is modified by a unique mask based on the mobile’s ESN result ing on the User Long PN Code sequence which has a unique offset
n One every 64 chips from this User Long PN Code is used to scramble the modulation symbols shuffled by the block interleaver
n The result ing stream of 0’s and 1’s is, for all practical purposes, “random”
Convoluti ona l
Encoding
Code S ymbolRe pe tition
Block
Inte rleav ing
Power Control
S ubc ha nne l
Da ta Scra mbling
Orthogona lSpreading
Qua dratureSpreading
Base ba nd
Filteri ng
Vocoder
Proces sing
Baseband Traffic to RF Section
(SymbolPunc turing)
PCM Voice
Long Codeoffs et by
use r mas k(1.22 88 Mc ps )
19 .2ks ps
Dec imator
Modul ation Symbolsfrom S ymbol Re peti tion
(19.2 ks ps )
To Power
Control
DataS crambling
Block
Inte rlea ving19.2k sps
19.2k sps
Power Control Sub Channel
n Every 1.25 ms (800 times per second) the base station estimates the received signal strength on the Reverse Traffic Channel of a particular mobile station
n Based on this estimat ion, the base stat ion determines whether that mobile station should increase or decrease its transmission power
n A “power up” (0) or “power down” (1) one-bit command is sent by the base station to that mobile station 800 times a second on the corresponding forward traff ic channel. This constitutes the “Power Control Subchannel” for that mobile station
n At the rates of Set 1, these “Power Control Bits” overwrite (puncture) TWO out of every 24 modulation symbols. At the rates of Set 2, these “Power Control Bits” overwrite (puncture) ONE out of every 24 modulation symbols
n The Power Control Bits are sent in the forward traffic channel at full power and uncoded.
Long Codeoffset by
us er ma sk(1.2 288 Mc ps)
1 9.2k sps 8 00 Hz Mux
Timi ng
P owe rControl
Bi t (80 0 bps)
MUX
Dec imator Dec imator
M odulation Sy mbolsfrom block inte rleav er
(1 9.2 Ks ps )
Sc rambled
M odula tionSy mbol
or Powe r Control B it
Da taScra mbl ing
Convolutiona l
Encoding
Code SymbolRe petiti on
Block
Inte rlea ving
Data Sc rambling
Powe r Control
Subchannel
Orthogona lSprea di ng
QuadratureSprea di ng
Bas eband
Fil te ring
V oc ode r
Proce ssing
Baseband Traffic to RF Section
(Sy mbolPunctur ing)
PCM Voice
Orthogonal Spreading
n Forward channels are distinguished from one another by the Walsh function assigned to them
n Copies of this Walsh code are supplied to the XOR mixer at a rate of 19,200 64-bit Walsh codes per second (that is, at a rate of 1.2288 Mcps)
n Each code symbol output from the Mux is XORed with each bit of the assigned Walsh function
n Result is 64 chips output for each symbol input
n Bandwidth used greatly exceeds source rate
ConvolutionalEncoding
Code SymbolRepetition
BlockInterleaving
Data Scrambling
Power ControlSubchannelOrthogonalSpreadingQuadratureSpreadingBasebandFiltering
VocoderProcessing
Baseband Traffic to RF Section
PCM Voice
(SymbolPuncturing)
800 Hz MuxTiming
PowerControl Bit(800 bps)
Wt
1.2288 Mcps
ScrambledData
19.2 Ksps
To QuadratureSpreading
Walsh Function
MUX
Direct and Inverse Walsh Code
ZEROes are represented by +1 pulses, and ONEs by -1 pulses
n RULE: “XOR the symbol with the Walsh Code, then send the result” If the symbol is “0”, each bit in the Walsh Code remains unchanged If the symbol is “1”, each bit in the Walsh Code is flipped
0 0 0 00 1 0 10 0 1 10 1 1 0
Direct Walsh CodesUsed to represent “0” modulation symbols
1 1 1 11 0 1 01 1 0 01 0 0 1
Negated Walsh CodesUsed to represe nt “1” modulation symbols
Creating a Composite Signal
uses Walsh code 0 User A
uses Walsh code 2 User B
uses Walsh code 3 User C
0
1
0
to send
to send
to send
(the direct code is sent)
(the inverse code is sent)
(the direct code is sent)
(the electrical signals are added,
chip by chip)
Modulat ionSymbols Chips
•
Example Building Blocks
0 00
0
0
0W alsh 0
W alsh 2
W alsh 3
• • • •
1
0
1
1
0
1
Example Building Block cont.
W alsh 0
W alsh 2
W alsh 3
• • • •
1 11
0
0
1
1
0
1
1
0
1
Example – Building Blocks (Walsh 0 and 2)
0
01 1
W alsh 2
0 1
0 1
W alsh 0
• • • •
Example: Spreading three Sequences
Walsh 0 A
Walsh 2 B
Walsh 3 C
1 0 0 1 0 1
0 0 1 0 0 1
1 1 0 0
• •
n To extract a Forward CDMA Code Channel from the composite signal do the following:
“XOR”, chip-by-chip, the composite signal with the Walsh code that was used to encode the information for that channel
Integrate (add up) the resulting 0’s and 1’s
If the result is close to N times1, conclude that a 1 was sent
If the result is close to N times1, conclude that a 0 was sent
If the result is close to N/2 times 0 and N/2 times 1, conclude that no signal was sent
1
XOR
Extracting a code channel from the composite signal
An Equivalent Procedure
1
XOR
1
X
Represent 0’s by +1’s and 1’s by -1’s,then multiply by (instead of XORing with) the Walsh code
Example – Despreading with Walsh Code 0 (User “A”)
Sent by “A”: 1 0 0 1 0 1
1 0 0 1 0 1
X X X X X X
= = = = = =
Example – Despreading with Walsh Code 2 (User “B”)
Sent by “B”: 0 0 1 0 0 1
0 0 1 0 0 1
X X X X X X
= = = = = =
Example – Despreading with Walsh Code 3 (User “C”)
1 1 0 0
Sent by “C”: 1 1 0 0
no signal no signal
X X X X X X
= = = = = =
Example – Despreading with Walsh Code 1 (No User)
“nothing was sent with this code”
no signal no signal no signal no signal n o signal no signal
X X X X X X
= = = = = =
Quadrature Spreading & Baseband Filtering
n The forward traffic channel is combined with two different PN sequences: “I” and “Q”
n Baseband filtering ensures the waveforms are contained within the 1.25 MHz frequency range
n The final step is to convert the two baseband signals to radio frequency (RF) in the 800 MHz or 1900 MHz range
ConvolutionalEncoding
Code SymbolRepetition
VocoderProcessing
Baseband Traffic to RF Section
PCM Voice
BlockInterleaving
Data Scrambling
Power ControlSubchannelOrthogonalSpreadingQuadratureSpreadingBasebandFiltering
(SymbolPuncturing)Wt
(Walsh Spreading)
1.2288Mcps
19.2 kspsfrom PowerControl Mux
I-Channel Pilot PN Sequence1.2288 Mcps
BasebandFilter
BasebandFilter
I
Q
I
Q
Q-Channel Pilot PN Sequence1.2288 Mcps
cos(2fct)
sin(2 fct)
Composite “I” and “Q”
n All forward channels in a partition (Sector/Cell) are combined with the same I and Q sequences
n These sequences have a particular offset that sets this partition apart from up to 511 neighboring partitions
This ensures that a mobile station does not mistakenly decode the signal from a channel with the same Walsh code from the wrong base station
n The I and Q signals for all 64 channels on each sector of a cell are added together in the analog card producing a composite I and a composite Q for each sector in the cell
n The contribution of the overhead channels (Pilot, Sync, and Paging) to the composite signal is fixed. The contribution of each Forward Traffic Channel depends on the distance of the mobile station relative to the base station
PilotChannel
SyncChannel
PagingChannel(s)
Forward Traffic Channel(s)
+
+
+
+
Walshcode
Walshcode
Walshcode
Walshcode
+
+
+
+
+
+
+
+
“Q” PN Code
“I” PN Code
Composite “I”
Composite “Q”
Quadrature Phase Shift Key (QPSK) Modulation
Q1 sin (2
fc t ) + Q2 sin (2
fc t ) = ( Q1 + Q2 ) sin (2
fc t )
I1 cos ( 2
fc t ) + I2 cos (2
fc t ) = ( I1 + I2 ) cos ( 2
fc t )
: XOR: Analog sum : Baseband x Carrier
EveryChannel
Walshcode
“Q” PN Code
“I” PN Code
Basebandfil ter
Basebandfi lter
cos ( 2 fct )
sin (2 fct )
QPSK Modulation
– cos (x) = cos (x + )
cos (x)
0
1
QPSK Modulation cont.
sin (x) = cos (x -
/ 2)
0
– sin (x) = cos (x +
/ 2)
1
cos (x)
I & Q Mapping (“I”, “Q”, or Both?)
n We could use the cos and –cos functions to represent the two states of the “I” signal (0 and 1). This is equivalent to using the cos function with phases 0 and respectively
n Or we could use the -sin and sin functions to represent the two statesof the “Q” signal (0 and 1). This is equivalent to using the cos function with phases /2 and - /2 respectively
n What would happen if we add the signal representing “I” to the signal representing “Q” in order to obtain a signal representing both “I” and “Q” simultaneously?
Hint: add the vectors for I=0, Q=0, then for I=0, Q=1, etc.
II = 0I = 1 •
Q
- /2
/2
Q = 0
Q = 1
•
I & Q Mapping (Signal Constellation)
n We have four possible cases:
when I=0 & Q=0, the state corresponds to a cos function with phase /4 when I=1 & Q=0, the state corresponds to a cos function with phase 3/4 when I=1 & Q=1, the state corresponds to a cos function with phase -3/4 when I=0 & Q=1, the state corresponds to a cos function with phase -/4
n Frames received from all mobile stations serviced by a base station arrive to that base station with delays that depend on the distance between each mobile station and the base station
n If the base station has the maximum allowed coverage radius of about 32.6 miles, the delay between the earliest and the latest frame received is not greater than 0.35 ms (0.175 ms maximum offset of the mobile timing relative to system timing plus up to 0.175 ms for the frame to arrive to the base station).
n Frame staggering spreads the demand on system processing resources and/or system interconnect links by offsetting the traffic frame transmission for different mobile stations (in the forward and reverse directions) in 1.25 ms increments relative to the system time
n The amount of frame offset for a particular mobile is indicated in a Channel Assignment Message or an Extended Handoff Direction Message
n Support is required by the mobile station, but optional by the base station
n Currently, frame staggering is not implemented (not needed)
n May degrade soft handoffs into hard CDMA-to-CDMA hard handoffs.
430 / 2 chips = 32.6 miles = 0 .175 ms each way
Forward Channel Demodulation
n IS-95A/J-STD-008 requires a minimum of four processing elements that can be independently directedThree elements must be capable of demodulating multipath
componentsOne must be a “searcher” that scans and estimates signal
