Code Division Multiple Access(CDMA)

Presented By:

Muhammad Umer Khan(140411002)

Saleh Ali Chaudhry(140411024)

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Presentation Outline

‡ What is Multiple Access & Its types?‡ Introduction to CDMA.‡ History of CDMA.‡ Working of CDMA.‡ CDMA Transmission & Reception.‡ CDMA Channels.‡ Systems using CDMA.‡ Characteristics & Uses of CDMA.‡ Advantages & Disadvantages of CDMA.‡ Future Trends.‡ Conclusion.‡ Thanks.‡ Q &A Session.

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Prologue….!!!

• For Wireless systems there are two simple and common resources, frequency and time.

• Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA).

• Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA).

• In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections

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Multiple Access & Its Types

•Practically the frequency spectrum or the Bandwidth is a finite resource

•Engineers are always trying to find ways to take the maximum capacity out from the available limited Bandwidth

•To effectively support a large number of users, some technique for sharing the spectrum is required

•Techniques used for this purpose are called Multiple Access Techniques e.g. FDMA, TDMA, CDMA e.t.c.

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Multiple Access & Its Types

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What is CDMA???

• CDMA is a method in which users occupy the same time and frequency allocations and are distinguished by unique assigned codes

• The signals are separated at the receiver by using a co-relator (a special demodulator) that accepts signals only from the desired channel

• Undesired signals contribute only to the noise and are ignored

• In CDMA every channel uses the full available spectrum.

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History And Development Of CDMA

• A US based Company named Qualcomm created communications chips and designs for CDMA technology and later claimed patents on the technology.

• Claude Shannon and Robert Pierce had provided CDMA framework in

1949.

• De-Rosa-Rogoff defined the direct sequence spread spectrum method in 1950.

• Cellular spread-spectrum application was suggested by Cooper and Nettleton in 1978.

• IS-95, the narrow band CDMA mobile network, has been standardized in 1993 and commercial networks were introduced in 1995.

• 3G wideband CDMA systems, such as CDMA2000 in U.S. and European WCDMA developed from 1990s and still ongoing.

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How CDMA Works?

• CDMA receiver treats all the incoming signals as the noise signals except the one actually desired by it

• The undesired noise signals are summed up and are ignored

• As its name implies, CDMA assigns unique codes to each communication to differentiate it from others in the same spectrum

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How CDMA Works?

• Each CDMA signal consists of a different pseudorandom binary sequence called PN code that modulates the carrier resulting in spreading of the spectrum

• The required signals in the receivers are separated using a co-relator that accepts only the signals from selected binary sequence and de-spreads its spectrum

• The other user’s signal whose code does not match are not de-spread and contribute only as noise

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How Desired Signal Is Detected in CDMA?

• The receiver correlates its input with the desired noise signal and enhances the SNR at the detector

• This enhancement provides adequate SNR at the detector that it overcomes the summed undesired noise signal

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Different Parameters of CDMA

• Rx: 869-894MHz Tx: 824-849MHz• 20 Channels spaced 1250kHz apart (798

users/channel)• QPSK/(Offset) OQPSK modulation

scheme• 1.2288Mbps bit rate• IS-95 standard (A/B), CDMA 2000• Operates at both 800 and 1900 MHz

frequency bands

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CDMA Coding Procedure

• CDMA is based on Coding Theory.

• Each station is assigned a code which is a sequence of numbers called CHIPS.

• In this example we have four stations each has a sequence of chips which designate as A,B,C, and D.

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Rules For Encoding

We Adopt the following Rules for encoding. • If a station needs to send a 0 bit it sends -1.

• If a station needs to send a 1 bit it sends +1.

• When a Station is idle, it sends no signal which is represented by 0.

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CDMA Spreading Rate/Chip Rate

• A CDMA signal is generated by spreading the symbols by a wideband code sequence that is produced at a rate of 1.2288 Mcps

• This much chip rates generates a CDMA signal that is spreaded over a bandwidth of 1.25 MHz

• More spreading, more data rate available and thus more capacity

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CDMA Multiplexer

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CDMA Demultiplexer

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Spread Spectrum Technique Used in CDMA

• Direct sequence spread spectrum is the technique that is used in CDMA systems

• The information signal is narrowband signal on the order of less than 10 KHz

• The energy from this narrowband signal is spread over a much larger bandwidth (1.25 MHz) by multiplying the information signal by a wideband spreading code

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CDMA Signal Generation & Detection

CDMA Transmission Side CDMA Reception Side

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Steps in Generating a CDMA Signal

1. Voice is digitized at PSTN

2. Digitized voice is Vocoded at BSC

3. Digital vocoded signal is encoded and interleaved at BTS

4. The encoded and interleaved signal is spreaded (channelized) at BTS

5. The final signal is the conversion and transmission as Radio signal at BTS.

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CDMA Channels

• There are two main channels which are used in CDMA to handle all sorts of traffic, i.e.

1. Forward link Channel.2. Reverse Link Channel.

•Both Channels comprise of Traffic & Control Channels.

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The Two Channels

• On the Reverse link there are 2 types of channels used to transmit control and voice data to the mobile

• These channels are:1. Access2. Traffic

•Pseudorandom noise (PN) codes on reverse link to channelize users

• On the Forward link there are 4 channels used to transmit control and voice data to the mobile

• These channels are:1. Pilot2. Sync3. Paging4. Traffic

•The orthogonal sequences used in forward CDMA link are Walsh codes to separate users.

Forward Link Channel Reverse Link Channel

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Diagramatical Representation

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CDMA Different Systems

• The systems which are practically implementing the CDMA technology are:1. IS-95 (Used mainly in US)2. IS-95 A/B System.3. CDMA-2000 (which is also called

WCDMA in Europe).

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Hybrid CDMA FDMA

• The CDMA system can also be a hybrid of FDMA & CDMA technique where the total system BW is divided into a set of wideband channels, each of which contains a number of CDMA signals

• In this case each operator will have more than 1 carrier.

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Characteristics of CDMA

• CDMA can effectively reject narrow band interference.

• In a CDMA system, the same frequency can be used in every cell, because channelization is done using the pseudo-random codes.

• The frequency reuse factor of CDMA system is 1 and frequency reuse efficiency 56%.

• Hand off Technique. 25

Uses of CDMA

• One of the early applications for code division multiplexing is in GPS

• The Qualcomm standard IS-95, marketed as cdma-One.

• The Qualcomm standard IS-2000, known as CDMA2000. This standard is used by several mobile phone companies, including the Global-star satellite phone network.

• CDMA has been used in the Omni-TRACS satellite system for transportation logistics

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Advantages of CDMA

• Efficient practical utilization of fixed frequency spectrum. Many users of CDMA use the same frequency, TDD or FDD may be used

• Flexible allocation of resources.

• No absolute limit on the number of users, Easy addition of more users.

• Impossible for hackers to decipher the code sent

• CDMA is compatible with other cellular technologies; this allows for nationwide roaming.

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Disadvantages of CDMA

• As the number of users increases, the overall quality of service decreases

• Near- Far- problem arises

• Path Loss

• Multipath Fading

• Shadowing

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Future Trends

• CDMA has overcome most cynicism to dominate the worldwide wireless voice market.

• Ongoing researches on CDMA are as follows.– Increase capacity by joint decoding (multi-user

detection & interference cancellation)

– Applying CDMA to other applications: optical CDMA, ad hoc networks, dense wireless LANs.

– “Multi-CDMA”: multiple antenna CDMA, multi-carrier CDMA, multi-code CDMA

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Conclusion• CDMA is probably the most interesting multiple access

method provided by spread-spectrum technology.

• Nowadays systems such as CDMA2000, its evolution versions and European WCDMA are becoming more and more popular as the networks are open commercially around the world.

• CDMA infrastructure is widespread and forms the basis for 3rd Generation networks. It is considered standard protocol for 3G Networks

• Cdma2000 and other 3G technologies bring telecommunications into the packet-switched domain, adding a host of new services and network complexities in the process

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Queries

• Now, if you’ve got any question in mind, please don’t hesitate to ask….!!!

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Comparison of 8K & 13K Vocoder

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Encoding

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With Interleaver

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Without Interleaver

• Burst Error will occur

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Illustrating Step 4 & 5

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IS-95 System• Standard was finished in 1993 and first commercially

launched in 1996

• Basic data rate is 9.6 kbps, chip rate of 1.2288 Mchip/s

• Allocated bandwidth is 1.25 MHz, fixed spreading code of length 64

• Uses pilot channel in downlink direction to provide synchronization,

• Channel tracking, and handover functions. In the uplink direction,

• Orthogonal modulation is used, which permits the more robust non-coherent demodulation to be used.

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IS 95 A/B

• IS-95 A is 2G digital cellular service capable of providing data at a speed of 14.4 kbps in addition to voice

• IS-95 B is 2.5 G digital cellular technology capable of providing data throughput of 64kbps in addition to voice

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CDMA-2000

• CDMA2000 is an improvement on IS-95

• CDMA2000 represents a family of ITU-approved, IMT-2000 (3G) standards and includes CDMA2000 1X and CDMA2000 1xEV technologies

• They deliver increased network capacity to meet growing demand for wireless services and high-speed data services

• cdma2000 optimized for integrated voice and medium data rates

• It provides a significant improvement in voice capacity and expanded data capability, and is backward-compatible with IS-95 handsets

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