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1 WCDMA Transmitter
1 WCDMA Transmitter
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WCDMA Radio Workshop
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Objectives
Upon completion of this chapter the student should be able to:
Explain the basic structure of a WCDMA transmitter
List the different error protection schemes used in WCDMA
Explain how spreading is performed
Explain how scrambling is performed
Explain how channels are added in the downlink
Explain the principle of Quadrature Phase Shift Keying (QPSK)
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CONTENTS
INTRODUCTION..............................................................................6
ERROR PROTECTION....................................................................7
CYCLIC REDUNDANCY CHECK (CRC)...................................................7
FORWARD ERROR CORRECTION (FEC)................................................8
INTERLEAVING..........................................................................................9
SERIAL TO PARALLEL CONVERSION.......................................10
SPREADING..................................................................................11
ORTHOGONAL CODES (OC)..................................................................12
SCRAMBLING...............................................................................15
SCRAMBLING CODE GENERATION......................................................15
SPREADING AND SCRAMBLING EXAMPLE..............................18
CHANNEL ADDITION....................................................................19
Modulation......................................................................................20
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INTRODUCTION
A simplified diagram of the WCDMA downlink transmitter from an RBS is shown in Figure 1-1 below.
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RBS WCDMA Transmitter
Pulse shaping
Pulse shaping
+
Cos (ωt)
- Sin (ωt)
Real
&
Imag
split
Σ
Cch,SF,m
Serial
to
Parallel
Sdl,nI
Q
Error Protection
I + jQ
G1
CH1 +
Gn
CHn
Error Protection
Spreading ScramblingChannel addition
Modulation
bit rate (kbps) symbol rate (ksps) chip rate (3.84 Mcps) RF (2 GHz)
I
Q
Figure 1-1 RBS WCDMA Transmitter
A larger picture of this can be found at the back of this book.
The data arriving from the RNC goes through the five processes listed below:
Error Protection
Spreading
Scrambling
Channel Addition
Modulation
These processes are explained in this chapter.
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ERROR PROTECTION
To cope with bit errors introduced typically during the transmission across the air interface, the data is subjected to the following error protection processes:
Cyclic Redundancy Check (CRC)
Forward Error Correction (FEC)
Interleaving
CYCLIC REDUNDANCY CHECK (CRC)
Cyclic Redundancy Check (CRC) is used to detect if there are any uncorrected errors left after error correction.
Blocks of data are passed through a CRC generator, which will perform a mathematical division on the data producing a remainder or checksum. This is added to the block of data and transmitted.
The same division is performed on the data block in the receiver. If a different checksum is produced the receiver will know that there is an error in the block of data (alternatively there is an error in the received checksum). Based on this knowledge the receiver has two choices:
1) Discard the block if it is a voice transmission or
2) Ask for a retransmission in the case of packet data.
The longer the checksum, the greater is the accuracy of the process. It could be imagined that various combinations of errors on the data and the checksum would produce the same checksum. The longer the checksum the less likely it is for this to happen.
WCDMA specifications specify a range of checksum lengths ranging from 0 to 24 bits. PKzip, used to compress files in the computer industry uses a 32-bit checksum for greater accuracy.
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FORWARD ERROR CORRECTION (FEC)
The next block in the transmitter is Forward Error Correction. This is sometimes called ‘channel coding’. The function of this block is to help the receiver correct bit errors caused by the air interface.
One method for correcting these errors would be to send the information a number of times. Provided this is more than twice, the receiver could select which message is most correct by a “best out of three” decision. The more times the data is transmitted the better is the error protection. However the bandwidth is also increased proportionally
What is required is a system that provides forward error correction with minimal increase in bandwidth.
There are two basic types of FEC available, Block or Continuous codes.
Block Codes work by processing the data into unique code words. This would be similar to transmitting “New York City” to represent ‘NYC’. These redundant bits provide the error correction. As this type of system works on blocks of data it is not suitable for conversational transmissions.
Continuous codes, such as Convolutional Codes and Turbo Codes, on the other hand, are continuously produced as the data is fed to the FEC device. The result will contain redundant bits that help to correct errors.
IS-95, CDMA2000 and WCDMA utilize Convolutional Coding, for low data rates where a low latency and real time processing are required. For high data rate services where latency and processing power is not a problem Turbo Coding is used. This type of coding gives a much better error correction performance than traditional methods.
When Convolutional Coding is applied at the transmitter, a Viterbi decoder is used to recover the original data at the receiver.
The data rate is increased by a factor of two in the case of Convolutional Coding rate ½ and by a factor of 3 when Turbo or Convolutional Coding rate 1/3 is employed.
There is also a small additional overhead added by CRC and tail or trellis termination bits required before FEC.
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INTERLEAVING
A radio channel produces bursty errors. Because Convolutional and Turbo coding are most effective against random errors, interleaving is used to randomize these bursty errors. The interleaving length is determined by the delay requirements of the service. Speech service, for example, requires a shorter delay than data services.
The interleaving lengths supported in the WCDMA RAN are 10, 20, 40 and 80msec.
Figure 1-2 below, shows some examples of error protection used in the WCDMA RAN.
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• CRC 12
• Convolutional Coding (Rate1/2 and 1/3)
• 20 msec InterleavingSpeech
• CRC 16
• Turbo Coding (Rate 1/3)
• 20 msec Interleaving
CS 64
• CRC 16
• Turbo Coding (Rate 1/3)
• 10 msec Interleaving
Interactive
Error Protection
CHn
Data rate is increased by a factor of 2 (Rate
1/2) or 3 (Rate (1/3)
plus
overhead added by CRC and Convloutional
or Turbo coding
Figure 1-2 Error Protection Examples
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SERIAL TO PARALLEL CONVERSION
Since I/Q modulation is used, the data is split into an ‘I’ and ‘Q’ stream. The mapping is such that even and odd numbered bits are mapped to the I and Q branch respectively. This process has the effect of halving the rate of the data on each branch. The data ‘bits’ on the I and Q branches are referred to as ‘symbols’ and measured in ksymbols per second (ksps). This reduction in data rate is illustrated in Figure 1-3 below.
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Serial to Parallel Conversion
Serial
to
Parallel
I
Q
kbits per second (kbps)
Eg. 60 kbps
ksymbols per second (ksps)
Eg. 30 ksps
Figure 1-3 Serial to Parallel Conversion
In the example in Figure 1-3 above the result of the process is that two 30 ksps streams are produced from the input 60 kbps.
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SPREADING
The symbols from the Serial to Parallel Convertor are spread to the chip rate (3.84 Mcps) by a Channelization Code (Cch,SF,m). This is illustrated in Figure 1-4 below.
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Spreading
Cch,SF,m
I + jQ
+Symbol rate (xx ksps)
Chip rate (3.84 Mcps)
Figure 1-4 Spreading
The chip rate of the channelization is fixed at 3.84 Mcps but the symbol rate will vary depending in what channel is being spread. An example of a 960 ksps channel being spread is illustrated in Figure 1-5 below.
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Spreading Example
Symbols are spread to the chip rate by Channelization Code
Spreading Factor (SF) = Chip Rate/Symbol rate
1 -1 1 1 -1Symbols
@960 ksps
1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1chips
SF=4
1 -1 1 -1 1 -1 1 -1 -1 1 -1 1-1
1x-1=-1
1
1x 1= 1
-1
1x-1=-1
-1 1 -1 1
-1x 1= -1
-1x-1= 1
-1x 1= -1
-1x-1= 1
Result 1
1x 1= 1
X
Figure 1-5 Spreading Example
In this example there are four chips representing one bit of data. In other words the Spreading Factor (SF) is 4.
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The formula below can be used to calculate the SF when the symbol rate is known:
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Spreading Example
Symbols are spread to the chip rate by Channelization Code
Spreading Factor (SF) = Chip Rate/Symbol rate
1 -1 1 1 -1Symbols
@960 ksps
1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1chips
SF=4
1 -1 1 -1 1 -1 1 -1 -1 1 -1 1-1
1x-1=-1
1
1x 1= 1
-1
1x-1=-1
-1 1 -1 1
-1x 1= -1
-1x-1= 1
-1x 1= -1
-1x-1= 1
Result 1
1x 1= 1
X
ORTHOGONAL CODES (OC)
Channelization codes are said to be orthogonal. This means that when represented as vectors the will always be at right angles to each other. These are ideally suited for providing channel separation of spread channels, as it is not possible for any channel to corrupt all the chips of another when added together, regardless of power imbalance. This is illustrated in the example of addition of a SF 128 channel (Ch1) and SF 8 channel (Ch2), illustrated in Figure 1-6 below.
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Channelization codes are ‘Orthogonal’
CH1
(SF 128)
(1,1)
CH2
(SF 8)
(1,-1)
(1,-1)
Ch1 + CH 2(1,-1)
CH1
(SF 128)
CH2
(SF 8)
Lost one chip= 1/128 symbols
Since codes are Orthogonal one channel cannot currupt both chips, regardless of
the power imbalance
Figure 1-6 Channelization codes are ‘Orthogonal’
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Figure 1-7, below shows the orthogonal code tree. Two codes are said to be orthogonal when their inner product is zero. The inner product is the sum of all the terms we get by multiplying two codes element by element.
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Orthogonal Code Tree
Chip Rate = 3.840 Mcps
480 ksp/s 480 ksp/s 480 ksp/s 480 ksp/s 480 ksp/s 480 ksp/s 480 ksp/s 480 ksp/s
1
1-1 11
1-11-1 1-1-11 11-1-1 1111
1-11-11-11-11-11-1-11-11 1-1-111-1-111-1-11-111-1 11-1-111-1-111-1-1-1-111 1111-1-1-1-1 11111111
Figure 1-7 Orthogonal Code Tree
For example, (1, 1, 1, 1) and (1, 1, -1, -1) are orthogonal since
(1 * 1) + (1 * 1) + (1 * -1) + (1 * -1) = 0
The code tree corresponds to different discrete Spreading Factor (SF) levels, SF=1, 2, 4, 8…(2n). Different spreading factor levels mean different code lengths, and they are therefore normally referred to as Orthogonal Variable Spreading Factors (OSVF). The idea is to be able to combine different messages with different spreading factors and keep the orthogonality between them. We therefore need codes of different lengths that are still orthogonal. Of course, the chip rate remains the same for all codes, so short ones will be transmitted at a higher information rate than longer ones. The longer the code is the lower will the data rate. The spreading factor corresponds to the length of the code and the number of channels sending at a certain bit rate.
SF: 4-512 is allowed in the WCDMA Downlink.
SF: 4-256 is allowed in the WCDMA Uplink.
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The Channelization Code naming scheme from the 3GPP Technical Specification no. 25.201 is illustrated in Figure 1-8 below.
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Channelization Code naming scheme
Designation: cch, SF , code number
1
1 -1
1 1
1 1 1 1
1 1 -1 -1
1 -1 1 -1
1 -1 -1 1
C1,0
C2,0
C2,1
C4,0
C4,1
C4,2
C4,3
SF = 1 SF = 2 SF = 4
3GPP TS 25.2013GPP TS 25.201
Downlink = SF 4 ----------------> SF 512
Uplink= SF 4 -----------> 256
Figure 1-8 Channelization Code naming scheme
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SCRAMBLING
The spread data is scrambled with a cell specific Scrambling Code (Sdl,n) to separate it from other cells in the area. This is illustrated in Figure 1-9 below.
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Scrambling
Sdl,n
I + jQSpread symbols Scrambled symbols
Cell specific Scrambling code
Figure 1-9 Scrambling
SCRAMBLING CODE GENERATION
Scrambling Codes are created from Pseudo Noise (PN) sequences generated using a Shift Register as illustrated in Figure1-10 below.
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Scrambling Code Generation
Scrambling Codes are created from Pseudo Noise (PN) sequences which are generated using a Shift Register
D D
clock
D D
1 2 3 N
1010010010001110101...
Figure 1-10 Scrambling Code Generation
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Shift register sequences are not orthogonal, but they do have a narrow autocorrelation peak. By using a single shift register, maximum length (M) sequences can be obtained. Such sequences can be created by applying a single shift register with a number of specially selected feedback taps. If the shift register size is (n) then the length of the code is equal to 2n-1. The number of possible codes is dependent on the number of possible sets of feedback taps that produce an M sequence.
The mathematics of these generators is equivalent to the operation of ordinary algebra applied to abstract polynomials over an indeterminate X, with binary valued coefficients. Each sequence is based on a generator polynomial
G(X)= bnXn + bn-1Xn-1 + bn-2Xn-2 +……+ b1X1 + 1
Figure 1-11 below, illustrates auto- and cross correlation plots for a 2000 chip length PN code.
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PN Code Correlation Plots
Autocorrelation of 2000-bit PN sequenceTime offset = 0
-800 -600 -400 -200 0 200 400 600 800 1000-100
0
100
200
300
400
500
600
Single, centered correlation peak indicates that two signalsare identical, with zero time offset
Cross-correlation of two different PN sequences
-800 -600 -400 -200 0 200 400 600 800 1000-100
0
100
200
300
400
500
600
Figure 1-11 PN Code Correlation Plots
As can bee seen from the auto-correlation plot, a peak is only produced when both codes are perfectly aligned, any slippage produces noise. When two different PN codes are correlated as is the case in the cross-correlation plot, it can bee seen that only noise is produced regardless of the time alignment of the codes.
Because of this property, it is possible to distinguish transmitters in a WCDMA network regardless of their phases.
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For this reason SCs are used to distinguish transmitters, that is cells in the downlink and UEs in the uplink, as illustrated in Figure 1-12 below.
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Use of Scrambling codes in the network
SCa SCb
SCc SCd
SC1 SC1
SC2 SC2
Uplink: 16,777,216 Scrambling codes used to distinguish each UE Downlink: 512 Scrambling codes used to distinguish each cell
Figure 1-12 Use of Scrambling codes in the network
In the WCDMA RAN there are 16,777,216 SCs available for distinguishing UEs in the uplink and 512 SCs used to distinguish cells in the downlink.
An example of the error protection scheme, scrambling and channelization codes taken from the TIW L3 Presentation window is illustrated in Figure 1-13 below.
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Example from TIW L3 Presntation Window
Error protection scheme
Uplink Scrambling and Channelization codes
Downlink Channelization code
Figure 1-13 Example from TIW L3 Presentation Window
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SPREADING AND SCRAMBLING EXAMPLE
An example of how a channel at 960ksps could be spread and scrambled by a transmitter is illustrated in Figure 1-14 below.
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1 -1 1 1 -1Data
1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1C4,2
1 1 -1 1 1 1 -1 -1 11 -1 11 -1 -1 11 -1 -1 1 1 -1 -1SC
SF=4
1 -1 1 -1 1 -1 1 -1 -1 1 -1 1-1
1x-1=-1
1
1x 1= 1
-1
1x-1=-1
-1 1 -1 1-1x 1= -1
-1x-1= 1
-1x 1= -1
-1x-1= 1
Result 1
1x 1= 1
X
X
11 -1 -1 -1 -1 1 1 -1 1 1 1 1 -1 -1 -1 11 -1 1 1 -1
1x 1= 1
1x-1=-1
-1x 1= -1
Coded signal
Transmitter
Figure 1-14 Spreading and Scrambling Example
This data would be recovered in the receiver as illustrated in Figure 1-15 below.
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Data
1 -1 1 -1 1 -1 1 -1 -1 1 -1 1-1 1 -1 11
-1x-1= 1
-1
-1x 1= -1
4 4 -4
Result 1
1x 1= 1X
1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1C4,2
1 1 -1 1 1 1 -1 -1 11 -1 11 -1 -1 11 -1 -1 1 1 -1 -1SC
11 -1 -1 -1 -1 1 1 -1 1 1 1 1 -1 -1 -1 11 -1 1 1 -1Coded signal
-1x1 = -1
-1
Data
X
1 x 1 = 1
1
-1x-1= 1
1
1 x 1 = 1
1
-1x-1= 1
1
1x-1=-1
-1
-1x1 = -1
-1
1x-1= -1
-1
-1x1 =-1
-1
Receiver
Figure 1-15 De-Scrambling and de-Spreading Example
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CHANNEL ADDITION
Each complex-valued spread and scrambled channel is separately weighted by a weight factor Gn. All downlink physical channels are then combined using complex addition. This is illustrated in Figure1-16 below.
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Channel Addition
ΣG1
Gn Resulting symbol
CH1
CHn
Figure 1-16 Channel Addition
The resulting baseband symbols will still be at the chip rate and are applied to the RF modulator.
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MODULATION
The type of modulation used in the WCDMA downlink is Quadrature Phase Shift Keying (QPSK). The baseband symbols are split into real and imaginary streams, filtered and applied to the I and Q inputs of the modulator. The I and Q branches modulate the RF carrier represented by Cos (ωt) and - Sin (ωt) respectively as illustrated in Figure 1-17 below.
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Quadrature Phase Shift Keying (QPSK)
Pulse shaping
Pulse shaping
+
Cos (ωt)
- Sin (ωt)
Real
&
Imag
split
+ Cos (ωt)Or
- Cos (ωt)
+ Sin (ωt)Or
- Sin (ωt)
+ 1Or- 1
+ 1Or- 1
RF output (2Ghz)
Baseband Input
(3.84Mcps)
I
Q
Figure 1-17 Quadrature Phase Shift Keying (QPSK)
After modulation the I branch will swing between Cos (ωt) and - Cos (ωt) and the Q branch between Sin (ωt) and - Sin (ωt). These are combined to produce a waveform that swings between four phases.
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The scenario where I and Q are both positive is illustrated in Figure 1-18 below.
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Modulation Example ( I =+1, Q=+1)
Q Branch
I Branch
Sum
Output
45o
Figure 1-18 Modulation Example ( I =+1, Q=+1)
The four possible combinations are illustrated in the constellation diagram shown in Figure 1-19 below.
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QPSK Constellation Diagram
1 Modulation Symbol represents 2 data bits
Modulation efficiency = 2 bits/ symbol
I
Q
( I = 1, Q = 1 )
( I = -1, Q = -1 )
( I = -1, Q = 1 )
( I = 1, Q = -1 )
RF Carrier amplitude
RF Carrier phase angle
Figure 1-19 QPSK Constellation Diagram
This modulation efficiency of QPSK is two bits per modulation symbol.
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