Chapter#1-Mediam Access, CDMA_1

8/3/2019 Chapter#1-Mediam Access, CDMA_1

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Duplexing

Duplexing allows separation between the uplink and downlink in acellular system

FDD = Frequency Division Duplexing.Uplink and Downlink on different (usually paired) frequencies.

TDD = Time Division Duplexing.Uplink and Downlink on same frequency, at different times

(usually paired).

CDD = Code Division Duplexing.Uplink and Downlink on same frequency at same time using

orthogonal spreading codes.

1

frequencyDuplex Spacing

Uplink DownlinkGuard

Band



Duplex Schemes

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5



6



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8



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11



Introduction to Spread SpectrumCommunications

12

•The signal occupies a bandwidth much greater than that which is

necessary to send the information.

•The bandwidth is spread by means of a code which is independent of the data.

•The receiver synchronizes to the code to recover the data.




13

General Model of Spread Spectrum Communication Systems




14

Spread Spectrum

FHSS

Frequency Hopping Spread Spectrum

DSSS

Direct Sequence Spread Spectrum



15



Frequency Hopping SpreadSpectrum

(FHSS)

16

With FHSS the signal is broadcast over a seemingly random series of radio

frequencies, hopping from frequency to frequency.

The receiver hops between frequencies in synchronization with the transmitter

Both the receiver and the transmitter hop from frequency to frequencyin the same order

The transmitter has the responsibility to let the receiver

know in which order to hop

The basic approach for FHSS is to have carrier frequencies forming channels.

k

2k

2



FHSS using BFSK (Binary Frequency ShiftKeying)

17

For transmission, binary data are fed into a modulator using

some digital-to analog encoding scheme, such as BFSK.

A pseudo-noise serves as spreading code.

-Mathematically:

sd (t ) = Acos (2 (f 0 + 0.5 (b + 1)Δf ) t ); where

A = amplitudef 0 = base frequency

Δf = frequency separator (Change)

b = value of the bit (1 for binary 1; -1 for binary 0)

π




18

As

sd (t ) = Acos (2 (f 0 + 0.5 (b + 1)Δf ) t );

b = -1 sd (t ) = Acos (2 f 0t )

b=1 sd (t ) = Acos (2 (f 0 + Δf ) t )1

⇒

⇒

π

π

π




19




20



At the Receiver:

21



Direct Sequence SpreadSpectrum (DSSS)

22

Direct sequence: The digital data is directly coded at a

much higher frequency. The code is generated pseudo-

randomly, the receiver knows how to generate the same

code, and correlates the received signal with that code toextract the data.



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25

Signal Transmission

• A pseudo-random code is generated, different for each channel and

each successive connection.

•The Information data modulates the pseudo-random code (the

Information data is “spread”).

•The resulting signal modulates a carrier.

•The modulated carrier is amplified and broadcast.




26

Signal Reception

•The carrier is received and amplified.

•The received signal is mixed with a local carrier to recover the spread digitalsignal.

• A pseudo-random code is generated, matching the anticipated signal.

•The receiver acquires the received code and phase locks its own code to it.

•The received signal is correlated with the generated code, extracting the

Information data



Code Division Multiple Access (CDMA)

27

The CDMA is a DSSS System. The CDMA system works directly on 64kbit/sec digital signals. These signals can be digitized voice, ISDN channels,

modem data, etc.



Signal transmission consists of thefollowing steps:

• 1. A pseudo-random code isgenerated, different for each channeland each successive connection.

• 2. The Information data modulatesthe pseudo-random code (theInformation data is “spread”).

• 3. The resulting signal modulates acarrier.

• 4. The modulated carrier is amplified

and broadcast. 28



Signal reception consists of thefollowing steps:

• 1. The carrier is received andamplified.

• 2. The received signal is mixed with

a local carrier to recover the spreaddigital signal.

• 3. A pseudo-random code is

generated, matching the anticipatedsignal.

• 4. The receiver acquires the received

code and phase locks its own code to29



Implementing CDMA Technology

30

The system works with 64 kBits/sec data, but can accept input rates of 8,16, 32, or 64 kBits/sec. Inputs of less than 64 kBits/sec are padded with

extra bits to bring them up to 64 kBits/sec.

For inputs of 8, 16, 32, or 64 kBits/sec, the system applies Forward Error

Correction (FEC) coding, which doubles the bit rate, up to 128 kbits/sec.

http://www.bee.net/mhendry/vrml/library/cdma/201066.JPG



Generating Pseudo-Random Codes

31

For each channel the base station generates a unique code

that changes for every connection.

The base station adds together all the coded transmissions

for every subscriber.

The subscriber unit correctly generates its own matching

code and uses it to extract the appropriate signals.



Properties of Pseudo-RandomCodes

32

It must be deterministic. The subscriber station must beable to independently generate the code that matches the

base station code.

It must appear random to a listener without prior

knowledge of the code (i.e. it has the statistical properties

of sampled white noise).

The cross-correlation between any two codes must be

small

The code must have a long period (i.e. a long time before

the code repeats itself).



Pseudo-Noise Spreading

33The FEC coded Information data modulates the pseudo-random code.

Chipping Frequency (fc): the bit rate of the PN code.

Information rate (fi): the bit rate of the digital data.

Chip: One bit of the PN code.

Epoch: The length of time before the code starts repeating itself (the period of the

code). The epoch must be longer than the round trip propagation delay (The epoch

is on the order of several seconds).




Frequency Spreading

34

the bandwidth of a digital signal is twice its bit rate. The bandwidths of the information

data (fi) and the PN code are shown together. The bandwidth of the combination of the

two, for fc>fi, can be approximated by the bandwidth of the PN code.




Processing Gain

35

This is a theoretical system gain that reflects the relative

advantage that frequency spreading provides. The processinggain is equal to the ratio of the chipping frequency to the data

frequency:

There are two major benefits from high processing gain:

Interference rejection: the ability of the system to reject interference is directly

proportional to Gp.

System capacity: the capacity of the system is directly proportional to Gp.

So the higher the PN code bit rate (the wider the CDMA bandwidth), the better the

system performance.



Complex Modulator

36

Transmitting Data The resultant coded signal next modulates an RF carrier for transmission using

Quadrature Phase Shift Keying (QPSK).



Complex Modulation

37

Symbol I Q Phase shift

00 +1 +1 45°

01 +1 -1 315°

10 -1 +1 135°

11 -1 -1 225°

QPSK uses four different states to encode each symbol. The four states are phaseshifts of the carrier spaced 90_ apart. By convention, the phase shifts are 45, 135, 225,

and 315 degrees. Since there are four possible states used to encode binary

information, each state represents two bits. This two bit “word” is called a symbol.

Figure below shows in general how QPSK works.





Working with Complex Data

38

•The conversion of the Information data into complex symbols occurs before the

modulation.

• The system generates complex PN codes made up of two independent

components, PNi +jPNq.

•To spread the Information data the system performs complex multiplication

between the complex PN codes and the complex data.

• Many channels are added together and transmitted simultaneously

• This addition happens digitally at the chip rate


















Working with Complex Data

39

In CDMA, each bit time is subdivided into m short intervals called chips.

Typically there are 64 or 128 chips per bit.

Each station is assigned a a unique m-bit chip sequence.

To transmit a 1 bit, a station sends its chip sequence.

To transmit a 0 bit, it sends the one's complement of its chip sequence.

No other patterns are permitted. Thus for m = 8, if station A is assigned

the chip sequence 00011011, it sends a 1 bit by sending 00011011 and

a 0 bit by sending 11100100.



































Example

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4141

A PN generator is typically made of N cascaded flip-flop circuits and a

specially selected feedback arrangement as shown in figure below:

The flip-flop circuits when used in this way are called a shift register ,

since each clock pulse applied to the flip-flops causes the contents of each

flip-flop to be shifted to the right.

The period of the PN sequence is:

PN Sequence Generation
















4242

Example:Starting with the register in state 001

the next 7 states are :

100, 010, 101, 110, 111, 011

and then 001 again and the states

repeats

The output taken from the right-most flip-flop is 1001011 and thencontinue to repeat.

The three-stage shift register shown, the period is

















4343

The tap connections are based on primitive polynomials on the order of the

number of registrars.

The polynomial should be irreducible for the sequence to be an m-sequence

and have the desired properties.


For example, IS-95 specifies the in-phase PN generator shall be built

based on the characteristic polynomial:












4444

Properties of PN sequences

Balance Property: The number of ones in a code is always one morethan the number of zeroes in a sequence.

Correlation Property: The correlation value of two N-bit

sequences can be obtained by counting the number of similar

(Ns) and disimilarlar (Nd) bits and inserting them

into the following equation: P = (1/N) * (Ns – Nd)

1 for same sequence and very small for different sequence.















4545

IS-95 and IS-2000 use two types of m-sequences but have special names

and uses and are called:

Long codes and Short codes


Long code

The long PN code is generated by a 42-stage linear shift register.

The length of the Long code is

This code runs at the chip frequency of 1.2288 Mc/s

The time it takes to recycle this length of code at this speed is 41.2 days


















4646

It is used to both spread the signal and to encrypt it.

A cyclically shifted version of the long code is generated by the cell phone

during call setup.

The shift is called the Long Code Mask and is unique to each phone call


Long Codes

















47

Short code

The short code used in CDMA system is based on a m-sequence created

from a LFSR of 15 registers.

The code Length is L =


The short code repeats every 26.666 milliseconds. The sequences

repeat exactly 75 times in every 2 seconds.

These codes are used for synchronization in the forward and reverselinks and for cell/base station identification in the forward link.



















4848

During call setup, the mobile looks for a short code and needs to be ablefind it fairly quickly as two seconds is the maximum time that a mobile will

need to find a base station.

If one is present because in 2 seconds the mobile has checked each of

the allowed base stations in its database against the network signal it isreceiving.

Each base-station is assigned one of these codes.

Since short code is only one sequence, each station gets the same

sequence but cyclically shifted.


PN Off t d PN R ll


















4949

PN Offset and PN Roll

Different cells and cell sectors all use the same short code, but use

different phases or shifts, which is how the mobile differentiates one base

station from another.

The phase shift is known as the PN Offset

For short code there can be 32,768 PN offsets.

The moment when the Short code wraps around and begins again is

called a PN Roll

There are actually two short codes per base station. One for each I and Q

channels to be used in the quadrature spreading and despreading of CDMA signals.

PN Off t d PN R ll























5050

From properties of the m-sequences, the shifted version of a m-sequenceshas a very small cross correlation and so each shifted code is an

independent code.

if two adjacent offsets are used, a multi-path of the leading sequence

(delayed by exactly one chip) would look identical to the lagging sequence.

In IS-95, a 64 chip separation is recommended for each adjacent station.

This gives 512 different short PN offsets used for different cells and cell

sectors, that is how the mobile differentiates one base station from another.

PN Offset and PN Roll
















51

Walsh codes

51

Addition to Long and short codes there is a special kind of codes used in

IS-95 called Walsh Codes

IS-95 uses 64 Walsh codes, which allow creation of 64 channels from the

base station.

A base station can talk up to maximum of 55 as the remaining channelsare used for pilot and synch.

Walsh codes have just one outstanding quality i.e., all codes are

orthogonal to each other and are used to create channelization within the

1.25 MHz band.
















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Creation of Walsh codes

52

Walsh codes are created out of Hadamard matrices and Transform

starting with H0 = [0]. The Hadamard matrix is built by:

For example, here are the Walsh-Hadamard codes of length 2, 4 and 8respectively

From the corresponding matrix, the Walsh-Hadamard code-words are given by

the rows. 0's are mapped to 1's and 1's are mapped to -1.

Walsh-Hadamard codes


























53

Walsh Hadamard codesOrthogonality

53

Walsh-Hadamard codes form the basis for orthogonal codes with different

spreading factors.

H1.2 => (1 -1 | 1 -1) and H2.3 => (1 1 -1 -1)

Computing the orthogonality, we get: (multiplying elements by elements)(1 x 1) + (-1 x 1) + (1 x -1) + (-1 x -1) = 1 - 1 - 1 + 1 = 0

Hence, H1.2

and H2.3

are orthogonal.

For example, let's see if the second codeword of H1 which we will denote H1.2

and the third codeword of H2, H2.3, are orthogonal. Since they are of different

length, we repeat H1.2 to match the length of H2.3. Hence we get the followingtwo code-words, in polar form:

W l h C d













































































54

Walsh Codes

54

The main purpose of Walsh codes in CDMA is to provide orthogonalityamong all the users in a cell.

Each user traffic channel is assigned a different Walsh code by the

base

station. IS-95 has capability to use 64 codes, whereas CDMA 2000 canuse up to 256 such codes.

Walsh code 0 is reserved for pilot channels

Walsh 1 to 7 for synch and paging channels and rest for traffic

channels.

They are also used to create an orthogonal modulation on the forward

link and are used for modulation and spreading on the reverse channel.

O S C































5555

Orthogonal Variable Spreading Factor Codes

Tree-Structuretree structure allows better visualization of the relation between different

code length and orthogonality between them.

C l i b PN S













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Correlation between PN Sequences

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Orthogonality between PN







57

Orthogonality between PNSequences

5757

Orthogonality between PN









585858

Orthogonality between PNSequences








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59

C d d i CDMA






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Codes used in CDMA

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Simplified model for code usage in CDMA

IS 95 Channels








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IS-95 Channels

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Channel waveform properties







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Channel waveform properties

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The communications between the mobile and the base station takes

place using specific channels.

The forward link (from base station to mobile) is made up of the

following channels:

Pilot channel (always uses Walsh code W0) (Beacon Signals)

Paging channel(s) (use Walsh codes W1-W7)

Sync channel (always uses Walsh code W32)

Traffic channels (use Walsh codes W8-W31 and W33-W63)

The reverse link: (from mobile to base station) is made up of the

following channels:

Access channel

Traffic channel

Forward Channel (Base to Mobile)






























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63

A CDMA mobile has to lock onto a system gradually. At first, everything looks like

noise.

The MS searches for the strongest pilot channel, which is un-modulated by data but

spread by a known sequence.

Then a Sync channel can be acquired to obtain basic system parameters, then the

mobile can move on to a paging channel to wait for a command from the user or the

network before finally reaching a traffic channel to transmit voice or data. This processcan be illustrated as:

IS 95 CDMA Forward Channel
















64

IS-95 CDMA Forward Channel

The forward link uses the same frequency spectrum as AMPS (824-849

MHz)Each carrier 1.25MHz

4 types of logical channel: A pilot, a synchronization, 7 paging, and 55

traffic channels

Channels are separated using different spreading codes

QPSK is the modulation scheme

Orthogonal Walsh codes are used (64 total)

After orthogonal codes, they are further spread by short PN spreading

codes


















65


65

Basic Spreading Procedure








66

p gforward Channel

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Channels Description








67


67

The pilot channel

Provide a reference signal for all MSs

provides the phase reference for Demodulation

4-6 dB stronger than all other channels.

Obtained using all zero Walsh code; i.e. contains no information except

the RF carrier

Spread using the PN spreading code to identify the BS

(512 different BS*64 offsets)

No power control in the pilot channel

















68


68

Used to acquire initial time synchronization

Synch message includes system ID (SID), network ID (NID), the

offset of the PN short code, the state of the PN-long code, and the

paging channel data rate (4.8/9.6 Kbps)

Uses W32 for spreading

Sync channel














69


69

Paging channels

Used to page the MS in case of an incoming call, or to carry the control

messages for call set upUses W1-W7

There is no power control

Additionally scrambled by PN long code, which is generated by LFSR of

length 42

The rate 4.8 Kbps or 9.6Kbps
















70

C a e s esc p o

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The traffic channels

Carry user information

Two possible date rates

RS1={9.6, 4.8, 2.4, 1.2 Kbps} & RS2={14.4, 7.2, 3.6, 1.8 Kbps}

RS1 is mandatory for IS-95, but support for RS2 is optional

Also carry power control bits for the reverse channel

Forward Channel Description













71

Forward Channel Description

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A base station can communicate on up to 64 channels

It has one pilot signal, one synch channel and 7 paging channels.

The remaining are used for traffic with individual mobiles.

Reverse Channels Description



















72


72

In IS-95, there are just two channels on which the mobile transmits,and even that never simultaneously.

It is either on the Access channel or Traffic channel.

The channel structure is similar but simpler to the forward channel,

with the addition of 64-ary modulation.

















73


73

64-array Modulation







74

y

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This block takes a group of 6 incoming bits (which makes = 64

different bit sequences of 6 bits) and assigns a particular Walsh code

to each.

We know that each Walsh code sequence is orthogonal to all the

others so in this way, a form of spreading has been forced on the

arbitrarily created symbols of 6 bits.

And this spreading also forces the symbols to be orthogonal. It is not

really a modulation but is more of a spreading function because we still

have not up converted this signal to the carrier frequency.

64-array Modulation
















75

64 array Modulation

75

Next comes multiplication with the long code starting at a particular private

start point.

Then comes serial to parallel conversion, and application of base-band

filtering which can be a Gaussian or a root cosine shaping.

Then the Q channel (or I, it makes no difference) is delayed by half a

symbol.

The reason this is done is to turn this into an offset QPSK modulated

signal.

.

64-array Modulation
















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64 array Modulation

76

Each I and Q channel is multiplied by the RF carrier, (a sine and acosine of frequency fc) and off the signal goes to the base station.

On the demodulation side, the most notable item is the Rake receiver.

Due to the presence of multipath, Rake receivers which allow maximal

combining of delayed and attenuated signal, make the whole thing workwithin reasonable power requirements. Without Rake receivers, cell

phone would not be as small as it is.













Thank You


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Chapter#1-Mediam Access, CDMA_1