April 21, 2023

Mobile Computing COE 446

IS-95Tarek Sheltami

KFUPMCCSECOE

http://faculty.kfupm.edu.sa/coe/tarek/COE446.htm

Principles of Wireless Networks

K. Pahlavan and P. Krishnamurth

CDMA CDMA – Code Division Multiple Access

Fully digital wireless data transmission system Not designed for voice at all

Uses special random numbers to encode bits of information

Allows multiple access by assigning different users different random numbers on the same channel.

Users have control of a very wide channel bandwidth 1.25 to 5 MHz

The only limit to the system is the computing ability of the base station and it’s ability to separate noise from actual data

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IS-95 CDMA is an access method. IS-95 was the first ‘operating system’ to

use CDMA Invented by Qualcomm Began production in 1995. At this point, this is still called 2G wireless.

Known as a narrowband system. Being supplanted by CDMA2000 (WCDMA)

and UMTS, fully 3G systems. They both use CDMA. Known as wideband systems.April 21, 2023

How IS-95 Works When a phone is turned on, it scans one of

the forward channels to find a base station identifier Camps on the strongest signal

The phone sends out an encrypted pass key and gains access to the network

It can then send and receive calls. It is assigned a 1.25 MHz wide frequency to

operate on It listens for pages on the forward channel

to let it know it has a call incoming This is all very similar to how GSM operates

so farApril 21, 2023

IS-95 Vocoders

IS-95 uses extremely advanced vocoders that use variable encoding rates just like GSM

They operate at variable rates, up to a maximum of 9600 bps

At a minimum, it encodes 1200 bps, so that the phone doesn’t seem dead

The quality, though less than AMPS, is much higher than GSM, on average

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How IS-95 Works

The access method is what makes IS-95 different The access method is called CDMA

CDMA is a transmission technique to pass information from the mobile to the base station and from the base station back to the mobileApril 21, 2023

Mobile Wireless CDMA Design Considerations

RAKE receiver – when multiple versions of a signal arrive more than one chip interval apart, RAKE receiver attempts to recover signals from multiple paths and combine them This method achieves better performance

than simply recovering dominant signal and treating remaining signals as noise

Soft Handoff – mobile station temporarily connected to more than one base station simultaneously

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CDMA..

used in several wireless broadcast channels (cellular, satellite, etc) standards

unique “code” assigned to each user; i.e., code set partitioning

all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data

encoded signal = (original data) X (chipping sequence)

decoding: inner-product of encoded signal and chipping sequence

allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

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CDMA.. Let the data rate of the ith user by Ri(t) Let the code assigned to the ith user by Ci(t)

i.e. all the codes are orthogonal Each user uses its code to spread its signal – the signal

transmitted by the ith user is Si(t) = Ri(t)Ci(t) The signal received BS is the sum of all transmitted

signals (ignore multi-path copies for the time being),

∑ Si(t) = ∑ Ri(t)Ci(t)

i j

0, i jC (t) X C (t)

1, i j

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CDMA..

Demodulation (De-spreading): Receiver dedicates a path structure per user – multiplies the received signal with the kth user code

Ck(t) X ∑ Si(t) = Ck(t) X ∑ Ri(t)Ci(t)

= Rk(t)

i.e. only the kth signal is retrieved from the kth receiver path

CDMA Analogy 10 people in a room.

5 speak English, 2 speak Spanish, 2 speak Chinese, and 1 speaks Russian

Everyone is talking at relatively the same time over the same medium – the air

Who can listen to whom and why? Who can’t you understand? Who can’t speak to anyone else?

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CDMA Spread Spectrum.

A signal takes up 6 – 10 times the bandwidth that it needs at a minimum.

This seems deliberately inefficient. The military used spread spectrum

communications because the signal is: Difficult to block. Difficult to listen in on. Difficult to even identify from noise.

Much more difficult to tune into a certain frequency.

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CDMA In CDMA, all users share the same 1.25 MHz bandwidth

They all transmit a signal that’s the exact same size, 1.25 MHz There’s actually .02 MHz of a guard band, meaning that the actual

bandwidth is 1.23 MHz However, with CDMA, unique digital codes are used to

separate each of the mobile phones Essentially, this makes each mobile phone speak a different

language Also, it’s language is very unpredictable, it starts at a random

language and changes in random fashion with a given seed. Also, the base station can speak every language as long as it is

synchronized Also, the languages are special in that they will be able to

mathematically never interfere with each other Each bit of the conversation is encoded with this special code

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CDMA Codes In IS-95, the mobile and base station must be

synchronized to a nearly perfect time clock CDMA actually uses GPS satellites to obtain a

very accurate, system wide clock This clock is obtained by every cell phone

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CDMA Codes.. The base station and mobile phone

have an algorithm for generating pseudo random numbers Uses something called Walsh Vectors

This mathematical function has a way to generate 128 bit random numbers that are orthogonal to every other random number that is has generated

This random number generator has a very large period

When they both start at the same seed (the time), both the mobile and base station should generate the same random numbers

The random number is actually only 32 bits

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CDMA Codes.. This random number is convoluted with

the data Also, a time stamp is added And error codes are added

The result is 128 bits that represent only 1 bit of data This is a very computationally intense process.

But, modern cell phones have fast processors

So, the original 9600 bps of conversation has been multiplied to 1.23 Mbps.April 21, 2023

CDMA Codes Example These codes are designed to never interfere

with any other codes to a very high probability The base station, using the mobile’s known

code, can convolute this code with everything that it received This convolution results in only what the mobile

sent. The base station does have to be smart

enough to recognize between voice traffic and noise

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CDMA Encode/Decode

slot 1 slot 0

d1 = -1

1 1 1 1

1- 1- 1- 1-

Zi,m= di.cmd0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutput

channel output Zi,m

sendercode

databits

slot 1 slot 0

d1 = -1d0 = 1

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 1 1

1- 1- 1- 1-

1 1 11

1-1- 1- 1-

slot 0channeloutput

slot 1channeloutputreceiver

code

receivedinput

Di = Zi,m.cmm=1

M

M

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CDMA: two-sender interference

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IS-95 The data message handler (DMH) collects

billing information The interworking, function (IWF) that allows

an MSC to connect to other networks The auxiliary (AUX) equipment that can

connect to an MT The authentication center (AuC) in GSM is

shown as the AC and the operation and maintenance center

(OMC) is shown as the operation system (OS)

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IS-95.. CDMA occupies the same frequency

spectrum as AMPS and IS-136 Each carrier occupies 1.25 MHz,

whereas in AMPS and IS-136 was 30 KHz IS-95 forward channel consists of 4

types of logical channels Pilot Ch Synchronization Ch Paging Ch Traffic Ch

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IS-95..Forward ChannelForward Channel Each carrier contains pilot, synchronization, up to seven

paging and number of traffic channels Channels are spread from one another using different

spreading codes Modulation scheme is QPSK Any information contained in the symbol form is

modulated by Walsh codes Each Walsh code identifies one of the 64 forward

channels After the channel symbol spread using orthogonal

codes, they are scrambled in the in-phase and quadrature phase line by Pseudorandom Noise (PN)-spreading codes

PN-codes are not orthogonal

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Why we have two spreading codes?

The orthogonal codes are used to differentiate between the transmissions within a cell

The PN spreading codes are used to isolate different cells (BSs) that are using the same frequencies

The same PN sequence is used in all BSs The offset for each BS is different. Of course, this

requires synchronization Synchronization is achieved by GPS

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The pilot channel Provide a reference signal for all MSs Spread using the PN spreading code to identify

the BS No power control in the pilot channel

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The Sync channel 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 Operates at 1200 bps

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The Paging channels Used to page the MS in case of an incoming call,

or to carry the control messages for call set up Uses W1-W7 There is no power control Additionally scrambled by PN long code, which is

generated by the Linear feedback shift register (LFSR) of length 42

The rate 4.8 Kbps or 9.6Kbps

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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

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PN-codes PN-codes are not random, they are

deterministic, periodic sequences with the following properties: PN are generated by combining the

outputs of feedback shift registers The sequence are repeated every 2n-1 The initial contents of the stages and

the feedback determine the successive contents of the stages

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PN-codes

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PN-Codes..

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Walsh Codes

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Walsh Codes..

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IS-95.. The orthogonal codes are used to

isolate the transmissions between different channels within a cell

The PN spreading codes are used to separate the transmissions between different cells

The PN sequences are used to differentiate between several BS

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DS Spread Spectrum

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IS-95 reverse channel Fundamentally different from the forward

channels Uses QPSK for power efficiency QPSK demodulation is easy 869-894 MHz range No spreading of the data using orthogonal codes Same orthogonal codes are used for WAVEFORM

encoding Two types of logical channels: The access

channels and the reverse traffic channels

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