CDMA 120 Section 3-9User Input Channelization Using Orthogonal Spreading By spreading, each symbol is XORed with all the chips in the orthogonal sequence Walsh sequence assigned to the u
Trang 1CDMA 120 Section 3-9
User Input
Channelization Using Orthogonal Spreading
By spreading, each symbol is XORed with all the chips in the orthogonal sequence (Walsh
sequence) assigned to the user The resulting sequence is processed and is then transmitted over thePhysical Channel along with other spread symbols
In this figure, a 4-digit code is used The product of the user symbols and the spreading code is asequence of digits that must be transmitted at 4 times the rate of the original encoded binary signal
Trang 2CDMA 120 Section 3-10
cdma university Channelization Example (continued)
Recovery of Spread Symbols
The receiver despreads the chips by using the same Walsh code used at the transmitter Notice thatunder no-noise conditions, the symbols or digits are completely recovered without any error Inreality, the channel is not noise free, but cdmaOne systems employ Forward Error Correction
(FEC) techniques to combat the effects of noise and enhance the performance of the system
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cdma university Channelization Example (continued)
Recovery of Spread Symbols using Wrong Function
When the wrong Walsh sequence is used for despreading, the resulting correlation yields an
average of zero This clearly demonstrates the advantage of the orthogonality property of the
Walsh codes
Whether the wrong code is mistakenly used by the target user or by other users attempting to
decode the received signal, the resulting correlation is always zero because of the orthogonalityproperty of the Walsh sequences
Trang 4CDMA 120 Section 3-12
Spread Waveform Representation of User B's signal
Spread Waveform Representation of User C's signal
Analog signal formed by the summation of the three spread signals
An Example of Spreading with Three Subscribers
In this example, three users A, B, and C are assigned three orthogonal codes for spreading
purposes:
z User A signal = 00, Spreading code = 0101
z User B signal = 10, Spreading code = 0011
z User C signal = 11, Spreading code = 0000
The analog signal shown on the bottom of the page is the composite signal when all of the spreadsymbols are summed together
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+1 -1
t
Walsh Code for User A: "0101"
Received Composite Signal
At the receiver of user A, the composite analog signal is multiplied by the Walsh code
corresponding to user A and the result is then averaged over the symbol time This process is calledcorrelation Note that the average voltage value over one symbol time is equal to 1 Therefore, theoriginal bit transmitted by A was “0.”
You may try to decode the symbols for users B or C in the same manner This process occurs in theCDMA mobile unit for recovering the signals
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• Two Short Codes (215 = 32,768)
– Termed “I” and “ Q” codes (different taps)– Used for Quadrature Spreading
– Unique offsets serve as identifiers for a Cell or a Sector– Repeat every 26.67 msec (at a clock rate of 1.2288Mcps)
• One Long Code (242= 4400 Billion)
– Used for spreading and scrambling– Repeats every 41 days (at a clock rate of 1.2288Mcps)
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Pseudorandom Noise (PN) Codes
PN codes are deterministic codes that mimic randomness properties The state of the code
resembles the outcome of tossing a two-sided coin with “1” and “0,” rather than “heads” or “tails.”However, if the current state and the generating function of the PN code are known, the future state
of the code can be predicted
The two short codes and one long code used in cdmaOne systems are time-synchronized to
midnight, January 6, 1980 (GPS time) In cdmaOne systems, all Base Stations and all mobiles usethe same three PN sequences
Note: For CDMA2000 Spreading Rate 3, the short code length is 3 times the short code length
given above or 3 x 215in length Spreading Rate 3 is discussed in the CDMA2000 Overview
section of this course
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(# of agreements - # of disagreements)/N
0 1
-1/N
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Autocorrelation of a Pseudorandom Noise Code
PN sequences have an important property: time-shifted versions of the same PN sequence havevery little correlation with each other
Autocorrelation is the measure of correlation between a PN code and a time-shifted version of the
same code The figure shows the autocorrelation1 function, and it is clear that it is a two-valuedfunction As long as the time shift is greater than the chip time, correlation is very small
The channelization of users in the Reverse link is accomplished by assigning them different shifted versions of the long code, thus making them uncorrelated with each other This property isthen exploited to separate subscriber’s signals in the BTS receivers
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Out
• Seed Register with 001
• Output will be a 7-digit sequence that repeats continually: 1001011
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PN Code Generation
PN codes are generated from prime polynomials using modulo 2 arithmetic The state machinesgenerating these codes are very simple and consist of shift registers and XOR gates
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cdma university PN Code Generation (continued)
PN codes are maximum length In general, if there are N shift registers (N = number of shift
registers), the length of the PN code is equal to 2N-1
In this example, the number of distinct states in the shift registers is 23-1=7
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cdma university PN Code Generation (continued)
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Masking will cause the generator to producethe same sequence, but offset in time
0
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PN Offset (Masking)
Masking provides the shift in time for PN codes Different masks correspond to different time
shifts In cdmaOne systems, Electronic Serial Numbers (ESN) are used as masks for users on theTraffic Channels
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Sequence Produced by a Masked Generator
This example illustrates how a mask produces the same original sequence shifted in time
The content of the 3-digit mask determines the offset of the sequence Masking is used to produceoffsets in both the short codes and the long code The offsets of the short PN codes are used touniquely identify the Forward Channels of individual sectors or cells The offsets of the Long PNcode are used to separate code channels in the reverse direction
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cdma university
SECTION REVIEW
105AC_00
Section Review
• Orthogonal and PN Codes
• Properties of Orthogonal and PN Code Sequences
• Generation of Orthogonal and PN Code Sequences
• Masking the PN Code Sequences
• Spreading and Despreading using these Two Codes
• Properties of PN Short and Long Code Sequences
Section Review
z Orthogonal and PN code sequences
z Properties of Orthogonal and PN code sequences
z Generation of Orthogonal and PN code sequences
z Masking the PN code sequences
z Spreading and despreading using these two codes
z Properties of PN short and long code sequences
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= -169 dB/Hz
PN SOURCE
DATA
DIGITAL FILTER
DEINTERLEAVE
& DECODE
1.25 MHz BPF
PN SOURCE
CORRELATOR
1.2288 Mcps 1.2288
Mcps
BACKGROUND NOISE
EXTERNAL INTERFERENCE
OTHER CELL INTERFERENCE (IOC)
1.25 MHz
f0OTHER USER NOISE (ISC)
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SECTION INTRODUCTION
106AC_00.emf
• CDMA Modulation Overview
• Modulation of the Forward CDMA Channel
• Modulation of the Reverse CDMA Channel
Section Learning Objectives
After completing this section, you will be familiar with the cdmaOne physical layer and will beable to:
z Describe the generation of the Forward and Reverse links
z Describe the function of the Control Channels in the Forward direction:
– Pilot Channel
– Sync Channel
– Paging Channel
z Describe the function of the Access Channel in the Reverse direction
z Define the terms “Fundamental Channel” and “Supplemental Code Channel.”
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= -169 dB/Hz
PN SOURCE
DATA
DIGITAL FILTER
DEINTERLEAVE
& DECODE
1.25 MHz BPF
PN SOURCE
CORRELATOR
1.2288 Mcps 1.2288
Mcps
BACKGROUND NOISE
EXTERNAL INTERFERENCE
OTHER CELL INTERFERENCE (IOC)
1.25 MHz
f 0
OTHER USER NOISE (ISC)
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CDMA Channel Generation Block Diagram
A CDMA signal is generated by spreading the code symbols by a wideband code sequence that isproduced at a rate of 1.2288 Mcps (Megachips per second)
This spreading of the information signal provides a substantial gain that is referred to as Processing
Gain The Processing Gain is defined as the ratio of the bandwidth to the information rate (W/R).
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Pilot
Forward Traffic ChannelSync
PagingForward Traffic Channel
Forward Traffic Channel
Forward CDMA Channel
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Forward Link Channels
The Forward link consists of:
z One Pilot Channel
z One to seven Paging Channels
z Traffic Channels assigned as needed
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cdma university Forward Link Channels (continued)
CHANNELS
Pilot
FORWARD CDMA LINK
Forward Link Channels
In the Forward link, channels are divided by assigning each channel a unique Walsh code:
z Walsh code 0 — reserved for Pilot Channel
z Walsh codes 1 through 7 — reserved for Paging Channel Unused Walsh codes may be
assigned to Traffic Channels
z Walsh code 32 — reserved for Sync Channel
z Remaining Walsh codes — assigned to subscriber Traffic Channels as required
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Filter Offset I PN
The Pilot Channel
Walsh code 0 is reserved for the Pilot Channel Every cell or sector must transmit a Pilot Channelfor each frequency that is supported Pilot signals act as beacons to notify potential users of theexistence of a CDMA Base Station
Pilot signals contain no messages; however, they are used by mobiles for Traffic Channel
demodulation Finally, mobiles use the Pilots for power strength comparison, which is essential forthe process of handoff
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Filter Offset I PN
Offset Q PN
Out
W32
1.2288 Mcps
Filter
Block Interleaver
Convolutional Encoder & Repetition
Sync
Channel
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The Sync Channel
The Sync Channel is used by mobiles to obtain timing and cell specific information
Mobiles must acquire the Sync Channel and decode its message in order to synchronize with thesystem The Sync message is spread with Walsh code 32 and is broadcast continuously
The Sync message includes the following information:
z Pilot PN offset
z System time
z State of the Long PN code
z Common Air Interface Revision Level
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Filter Offset I PN
Offset Q PN
Out
Wp
1.2288 Mcps
Filter
Block Interleaver
Convolutional Encoder & Repetition
Paging
Channel
Messages
Long Code PN Generator
Paging Channel Address Mask
Analog
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Paging Channels
A Paging Channel is used by the CDMA system to transmit overhead messages and
mobile-directed messages Walsh codes 1 through 7 may be used for Paging Channels A system operatormay choose to support less than seven Paging Channels In this case, unused codes may be
assigned to Traffic Channels
A Paging Channel transmits the configuration messages:
z System Parameters message
z Neighbor List message
z Access Parameters message
z Global Service Redirection message