1. CDMA
BASIC PRINCIPLE

2. Objectives
Upon completion of this lesson, the student will be able to:
Describe the differences between CDMA, TDMA, FDMA
What is spread spectrum modulation
Identify we use DSSS in CDMA
Walsh codes
Short PN and long PN
The purpose of Vocoding

3. Multiple Access Types of Media -- Examples: Twisted pair - copper
Each pair of users enjoys a dedicated, private circuit through the transmission medium, unaware that the other users exist.
Since the beginning of telephony and radio, system operators have tried to squeeze the maximum amount of traffic over each circuit.
Multiple Access: Simultaneous private use of a transmission medium by multiple, independent users.
Transmission
Medium
Types of Media -- Examples:
Twisted pair - copper
Coaxial cable
Fiber optic cable
Air interface (radio signals)
Advantages of Multiple Access
Increased capacity: serve more users
Reduced capital requirements since fewer media can carry the traffic
Decreased per-user expense
Easier to manage and administer

4. Channels FDMA TDMA CDMA FDMA Frequency Division Multiple Access
Time
Power
FDMA
TDMA
CDMA
Channel: An individually-assigned, dedicated pathway through a transmission medium for one user’s information.
The transmission medium is a resource that can be subdivided into individual channels according to the technology used.
FDMA Frequency Division Multiple Access
Each user on a different frequency
A channel is a frequency
TDMA Time Division Multiple Access
Each user on a different window period in time (“time slot”)
A channel is a specific time slot on a specific frequency
CDMA Code Division Multiple Access
A channel is a unique code pattern
Each user uses the same frequency all the time, but mixed with different distinguishing code patterns

5. Defining Our Terms CDMA Channel or CDMA Carrier or CDMA Frequency
Duplex channel made of two 1.25 MHz-wide bands of electromagnetic spectrum, one for Base Station to Mobile Station communication (called the FORWARD LINK or the DOWNLINK) and another for Mobile Station to Base Station communication (called the REVERSE LINK or the UPLINK)
In 800 Cellular these two simplex 1.25 MHz bands are 45 MHz apart
In 1900 MHz PCS they are 80 MHz apart
CDMA Forward Channel
1.25 MHz Forward Link
CDMA Reverse Channel
1.25 MHz Reverse Link
CDMA Code Channel
Each individual stream of 0’s and 1’s contained in either the CDMA Forward Channel or in the CDMA Reverse Channel
Code Channels are characterized (made unique) by mathematical codes
Code channels in the forward link: Pilot, Sync, Paging and Forward Traffic channels
Code channels in the reverse link: Access and Reverse Traffic channels
45 or 80 MHz
CDMA CHANNEL
CDMA
Reverse
Channel 1.25 MHz
Forward

6. CDMA Is a Spread-Spectrum System
Spread Spectrum Payoff: Processing Gain
Spread Spectrum
TRADITIONAL COMMUNICATIONS SYSTEM
Slow
Information
Sent TX
Recovered RX
Narrowband
Signal
SPREAD-SPECTRUM SYSTEM
Fast
Spreading
Sequence
Wideband Signal
Traditional technologies try to squeeze the signal into the minimum required bandwidth
Direct-Sequence Spread spectrum systems mix their input data with a fast spreading sequence and transmit a wideband signal
The spreading sequence is independently regenerated at the receiver and mixed with the incoming wideband signal to recover the original data

7. Spread Spectrum Principles
SHANON Formula
C = Blog2(1+S/N)
Where,
C is capacity of channel, b/s
B is signal bandwidth, Hz
S is average power for signal, W
N is average power for noise, W
It is the basic principle and theory for spread spectrum communications.

8. How DSSS Spectrum Change
User 1
Code 1
Composite
Time
Frequency + =
Direct Sequence CDMA

9. Spread Spectrum Principles
1.25 MHz
30 KHz
Power is “Spread” Over a Larger Bandwidth
MATH
HAMMER
MATH
HAMMER

10. Spread Spectrum Principles
Many code channels are individually
“spread” and then added together to
create a “composite signal”

11. Spread Spectrum Principles
UNWANTED POWER
FROM OTHER SOURCES
Using the “right” mathematical
sequences any Code Channel
can be extracted from the received
composite signal
Eb/No PG

12. CDMA Forward Signal in Spectrucm Analyzer (Fixed Overhead Power)
Pilot, Paging
and Sync Combined
Amplitude
(Fixed Overhead Power)
1.25 MHz
Downlink

13. Spectrum Usage and Capacity:
Each wireless technology (AMPS, NAMPS, D-AMPS, GSM, CDMA) uses a specific modulation type with its own unique signal characteristics
The total traffic capacity of a wireless system is determined largely by radio signal characteristics and RF design
RF signal vulnerability to Interference dictates how much interference can be tolerated, and therefore how far apart same-frequency cells must be spaced
For a specific S/N level, the Signal Bandwidth determines how many RF signals will “fit” in the operator’s licensed spectrum
AMPS, D-AMPS, N-AMPS
CDMA 30
10 kHz
200 kHz
1250 kHz 1 3
1 Users
8 Users
20 Users 2 4 5 6 7
Typical Frequency Reuse N=7
Typical Frequency Reuse N=4
Typical Frequency Reuse N=1
Vulnerability:
17 dB dB dB
GSM
17 dB = 50
14 dB = 25
12 dB = 16

14. Relationship Between Eb/N0 and S/N
Signal Power
Bit Rate S R
E / t
B / t
Noise Power
Bandwidth N W
Eb =
N0 = = = =
Signal to Noise S R N W Eb N0 = S R W N X S N W R X = =
Processing
Gain W
1,250,000
8 Kb vocoder
(Full Rate) 10
2.11 = =
130 = =
21.1 dB R
9,600 W
1,250,000
13 Kb vocoder
(Full Rate) 10
1.94 = = 87 = =
19.4 dB R
14,400

15. Anything We Can Do, We Can Undo
ORIGINATING SITE
DESTINATION
Spreading
Sequence
Input
Data
(Base Band)
Recovered
Spread Data Stream
(Base Band + Spreading Sequence)
Any data bit stream can be combined with a spreading sequence
The resulting signal can be de-spread and the data stream recovered if the original spreading sequence is available and properly synchronized
After de-spreading, the original data stream is recovered intact

16. CDMA Spreading Principle Using Multiple Codes
Sequence A B C
Input
Data X
Recovered
X+A
X+A+B
X+A+B+C
Spread-Spectrum Chip Streams
ORIGINATING SITE
DESTINATION
Multiple spreading sequences can be applied in succession and then reapplied in opposite order to recover the original data stream.
The spreading sequences can have different desired properties.
All spreading sequences originally used must be available in proper synchronization at the recovering destination.

17. “Shipping and Receiving” via CDMA
FedEx
Data
Mailer
Shipping
Receiving
Whether in shipping and receiving or in CDMA, packaging is extremely important!
Cargo is placed inside “nested” containers for protection and to allow addressing.
The shipper packs in a certain order, and the receiver unpacks in the reverse order.
CDMA “containers” are spreading codes.

18. Discriminating Among Forward Code Channels
Sync
Pilot
FW Traffic
(for user #1)
Paging
(for user #2)
(for user #3)
A Mobile Station receives a Forward Channel from a sector in a Base Station.
The Forward Channel carries a composite signal of up to 64 forward code channels.
Some code channels are traffic channels and others are overhead channels.
A set of 64 mathematical codes is needed to differentiate the 64 possible forward code channels.
The codes in this set are called “Walsh Codes”

19. Discriminating Among Base Station
Up to 64
Code Channels
A mobile Station is surrounded by Base Stations, all of them transmitting on the same CDMA Frequency.
Each Sector in each Base Station is transmitting a Forward Traffic Channel containing up to 64 forward code channels.
A Mobile Station must be able to discriminate between different Sectors of different Base Stations.
Two binary digit sequences called the I and Q Short PN Sequences (or Short PN Codes) are defined for the purpose of identifying sectors of different base stations.
These Short PN Sequences can be used in 512 different ways in a CDMA system. Each one of them constitutes a mathematical code which can be used to identify a particular sector.

20. Discriminating Among Reverse Code Channels
RV Traffic
from M.S. # # #
System Access
Attempt by M.S. #
(on access channel #1)
The CDMA system must be able to identify each Mobile Station that may attempt to communicate with a Base Station.
A very large number of Mobile Stations will be in the market.
One binary digit sequence called the Long PN Sequence (or Long PN Code) is defined for the purpose of uniquely identifying each possible reverse code channel.
This sequence is extremely long and can be used in trillions of different ways. Each one of them constitutes a mathematical code which can be used to identify a particular user (and is then called a User Long Code) or a particular “user Reverse Traffic channel”.

21. Correlation of Walsh Code #23 with Walsh Code #59
Walsh Codes
64 Sequences, each 64 chips long
A chip is a binary digit (0 or 1)
Each Walsh Code is Orthogonal to all other Walsh Codes
This means that it is possible to recognize and therefore extract a particular Walsh code from a mixture of other Walsh codes which are “filtered out” in the process
Two same-length binary strings are orthogonal if the result of XORing them has the same number of 0s as 1s
WALSH CODES
# Chip Sequence
EXAMPLE:
Correlation of Walsh Code #23 with Walsh Code #59 # #
XOR
Correlation Results: 32 1’s, 32 0’s: Orthogonal!!

22. Correlation and Orthogonality
Code #
–(Code #23)
Code #
PARALLEL
XOR: all 0s
Correlation: 100%
(100% match)
ORTHOGONAL
XOR: half 0s, half 1s
Correlation: 0%
(50% match, 50% no-match)
ANTI-PARALLEL
XOR: all 1s
Correlation: –100%
(100% no-match)
#23
–(#23)
#59
Correlation is a measure of the similarity between two binary strings

23. Properties of the Walsh Codes
When a Walsh code is XORed chip by chip with itself, the result is all 0’s (100% correlation)
When a Walsh code is XORed chip by chip with its logical negation, the result is all 1’s (–100% correlation)
When a Walsh code is XORed chip by chip with any other code or its logical negation, the result is half 0’s and half 1’s (0% correlation)

24. Walsh Code Table
1 1 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63

25. Short PN Sequences
The two Short PN Sequences, I and Q, are 32,768 chips long I Q
32,768 chips long
26 2/3 ms.
(75 repetitions in 2 sec.)
100% Correlation: All bits = 0
Short PN Sequence vs. 0 Offset
Orthogonal: 16,384 1’s + 16,384 0’s
Short PN Sequence vs. Any Offset
Unique Properties:
Together, they can be considered a two-dimensional binary “vector” with distinct I and Q component sequences, each 32,768 chips long
Each Short PN Sequence (and, as a matter of fact, any sequence) correlates with itself perfectly if compared at a timing offset of 0 chips
Each Short PN Sequence is special: Orthogonal to a copy of itself that has been offset by any number of chips (other than 0)

26. Short PN: 4-bits register example1 p1 p2 p3 p4 p5
p5 = p1 + p4
The PN sequences are deterministic and periodic.
The length of the generated string is 2n-1, where “n” is the number of elements in the register
The number of zeroes in the sequence is equal to the number of ones minus 1

27. Long Code Register (@ 1.2288 MCPS)
The Long PN Sequence
Long Code Register MCPS)
Public Long Code Mask
(STATIC)
User Long Code
Sequence
MCPS) 1 P E R M U T D S N
AND =
Modulo-2 Addition
Each mobile station uses a unique User Long Code Sequence generated by applying a mask, based on its 32-bit ESN, to the 42-bit Long Code Generator which was synchronized with the CDMA system during the mobile station initialization.
Generated at Mcps, this sequence requires 41 days, 10 hours, 12 minutes and 19.4 seconds to complete.
Portions of the User Long Codes generated by different mobile stations for the duration of a call are not exactly orthogonal but are sufficiently different to permit reliable decoding on the reverse link.

28. Long PN:4-bits shift register example1
Original PN
sequence
XOR
mask
AND
AND
AND
AND
XOR)
New PN
sequence
Attention:different mask lead to different offset!

29. Functions of the CDMA Forward Channels
Pilot Walsh 0
Walsh 19
Paging Walsh 1
Walsh 6
Walsh 11
Walsh 20
Sync Walsh 32
Walsh 42
Walsh 37
Walsh 41
Walsh 56
Walsh 60
Walsh 55
PILOT: WALSH CODE 0
The Pilot is a beacon which does not contain a character stream. It is a timing source used in system acquisition and for measurement device during handoff.
SYNC: WALSH CODE 32
This carries system identification and parameter information used by mobiles during system acquisition.
PAGING: WALSH CODE 1
One paging channel at either full rate (9.6Kbps) or half rate (4.8Kbps) as determined by capacity needs. Carries pages, system parameters information, and call setup orders.
TRAFFIC: any remaining WALSH codes
The traffic channels are assigned to users to carry call traffic. All remaining Walsh codes are available, subject to overall capacity limited by noise.

30. Coding Process on CDMA Forward Channels
WALSH 19
BTS
Pilot Walsh 0
Walsh 19
Paging Walsh 1
Walsh 6
Walsh 11
Walsh 20
Sync Walsh 32
Walsh 42
Walsh 37
Walsh 41
Walsh 56
Walsh 60
Walsh 55
PN OFFSET 116
PN OFFSET 226
PN OFFSET 510 S PN
372 x
PN OFFSET
ANALOG
SUM/MUX
PN OFFSET 372
Each user is assigned one of the 64 Walsh Codes and their traffic is spread by the Walsh code to establish a code channel.
All user’s code signals are then analog-summed with the pilot, sync and paging signals to produce one composite waveform.
The composite waveform is then combined with the I and Q Short PN Code sequences using a specific offset to identify the sector.

31. Functions of the CDMA Reverse Channels
BTS
REG
1-800
242
4444
There are two types of CDMA Reverse Channels:
TRAFFIC CHANNELS are used by individual users during their actual calls to transmit traffic to the BTS.
ACCESS CHANNELS are used by mobile stations not yet in a call to transmit registration requests, call setup requests, page responses, order responses, and other signaling information.

32. Coding Process on CDMA Reverse Channels
Each mobile is uniquely identified by the User Long Code, which it generates internally.
All mobile stations transmit simultaneously on the same 1.25 MHz wide frequency band.
Any nearby BTS can dedicate a channel element to the mobile station and successfully extract its signal.
Mobile stations also use the other CDMA spreading sequences, but not for channel-identifying purposes.
Short PN Sequences are used to achieve phase modulation.
Walsh Codes are used as Orthogonal Modulation to give ultra-reliable communications recovery at the BTS.
User Long Code
BTS
BSC
MSC

33. Summary of Characteristics & Functions
Walsh Codes
Short PN Sequences
Long PN
Sequences
Type of
Sequence
Mutually Orthogonal
Orthogonal with itself at any time shift value except 0
near-orthogonal if shifted
Special
Properties 64 2 1
How
Many
64 chips
1/19,200 sec.
32,768 chips
26-2/3 ms
75x in 2 sec.
242 chips
~41 days
Length
Orthogonal Modulation
(information carrier)
Quadrature Spreading (Zero offset)
Distinguish users
Reverse Link Function
User identity
within cell’s signal
Distinguish Cells & Sectors
Data Scrambling to avoid strings of 1’s or 0’s
Forward Link Function I Q
32,768 chips long
26-2/3 ms.
(75 repetitions in 2 sec.)
64 codes
64 chips long
AND = S U M
Modulo-2 Addition
Cell
Each CDMA spreading sequence is used for a specific purpose on the forward link and a different purpose on the reverse link.
The sequences are used to form “code channels” for users in both directions.

34. Variable Rate Vocoder
A-to-D C O N V E R T
64 Kbps D
“Codebook” Instruction
8Kbps
MSC
Speech coding algorithms (digital compression) are necessary to increase cellular system capacity.
Coding must also ensure reasonable fidelity, that is, a maximum level of quality as perceived by the user.
Coding can be performed in a variety of ways (for example, waveform, time or frequency domain).
Vocoders transmit parameters which control reproduction of voice instead of the explicit, point-by-point waveform description.

35. Variable Rate Vocoding
CDMA uses a superior Variable Rate Vocoder
Full rate during speech
Low rates in speech pauses
Increased capacity
More natural sound
Voice, signaling, and user secondary data may be mixed in CDMA frames
DSP QCELP VOCODER
Codebook
Pitch
Filter
Formant
Coded Result
Feed-
back
20ms Sample

36. Variable Rate Vocoding
Rate Set 2 Frame Sizes
bits
Full Rate Frame
1/2 Rate Frame
1/4 Rt.
1/8 36 72
144
288
Rate Set 1 Frame Sizes 24 48 96
192
The output is 20 ms frames at fixed rates: Full Rate, 1/2 Rate , 1/4 Rate , 1/8 Rate, & Blank
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.
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.
Current vocoder rates are 8kbps, 13kbps, and 8kbps EVRC (Enhanced Variable Rate Coder)

37. Variable Rate Voice Bit and PCM
Where is vocoder?
BTS
BSC
MSC
Analog voice
Variable Rate
PCM

38. The End!