CDMA(CODE DIVISION

MULTIPLE ACCESS)

By : Nur Azlin Binti Razali

Nur Fadzilah Binti Johar

Nur Fattahiah Binti Haslauhuddin

Siti Norhidayah Binti Kamarudin

Introduction to CDMA

CDMA (Code-Division Multiple Access) refers to any of several protocols used in so-called second-generation (2G) and third-generation (3G) wireless communications.

As the term implies, CDMA is a form of multiplexing, which allows numerous signals to occupy a single transmission channel, optimizing the use of available bandwidth. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands.

All users can use same carrier frequency and may transmit simultaneously. Each user in a CDMA system uses a different code to modulate their signal.

The best performance will occur when there is good separation between the signal of a desired user and the signals of other users.

The separation of the signals is made by correlating the received signal with the locally generated code of the desired user. If the signal matches the desired user's code then the correlation function will be high and the system can extract that signal.

How CDMA works? The words "code" and "division" are important parts of

how CDMA works. CDMA uses codes to convert between analog voice

signals and digital signals and also uses codes to separate (or divide) voice and control data into data streams called "channels."

CDMA converts speech into digital information, which is transmitted as a radio signal over wireless network.

The unique about CDMA is that it uses a unique code to distinguish each different call, thus enabling many people to share the airwaves simultaneously; it thereby eliminates cross talk and interference.

CDMA is also used in data technology apart from voice technology.

There are five steps in generating a CDMA signal :

i) analog to digital conversion ii) vocodingiii) encoding and interleavingiv) channelizing the signalsv) conversion of the digital signals to a Radio Frequency

(RF) signals

Analog to digital conversion The first step of CDMA signal generation

is analog to digital conversion, sometimes called A/D conversion. CDMA uses a technique called Pulse Code Modulation (PCM) to accomplish A/D conversion.

Vocoding The second step of CDMA signal generation is voice compression Vocoder : to accomplish voice compression. The term "vocoder" is a contraction of the words "voice" and "code. Vocoders are located at the BSC and in the phone. People pause between

syllables and words when they talk. CDMA takes advantage of these pauses in speech activity by using a variable rate vocoder. A CDMA vocoder varies compression of the voice signal into one of four data rates based on the rate of the user's speech activity.

Encoding and interleaving

encoding and interleaving that is built into the BTS and the phones

The purpose of the encoding and interleaving is to build redundancy into the signal so that information lost in transmission can be recovered

The type of encoding done at this stage is called "convolutional encoding.

A digital message consists of four bits (A, B, C, D) of vocoded data. Each bit is repeated three times. These encoded bits are called symbols.

The decoder at the receiver uses a majority logic rule. Thus, if an error occurs, the redundancy can help recover the lost information.

A burst error is a type of error in received digital telephone signals. Burst errors occur in clumps of adjacent symbols. These errors are caused by fading and interference. Encoding and interleaving reduce the effects of burst errors.

Interleaving is a simple but powerful method of reducing the effects of burst errors and recovering lost bits. In the example shown here the symbols from each group are interleaved (or scrambled) in a pattern that the receiver knows. De-interleaving at the receiver unscrambles the bits, spreading any burst errors that occur during transmission.

The encoded voice data is further encoded to separate it from other encoded voice data. The encoded symbols are then spread over the entire bandwidth of the CDMA channel. This process is called channelization. The receiver knows the code and uses it to recover the voice data.

Channelizing

CDMA uses two important types of codes to channelize users.

Walsh codes channelize users on the forward link (BTS to mobile).

Pseudorandom Noise (PN) codes channelize users on the reverse link (mobile to BTS)

 Conversion of the digital signals to a Radio Frequency (RF) signals

The BTS combines channelized data from all calls into one signal. It then converts the digital signal to a Radio Frequency (RF) signal for transmission.

Architecture of CDMA

A CDMA network consists of the following components :

Mobile Station (MS ) Base Station ( BS ) Base Station Controller ( BSC ) Mobile Switching Center ( MSC ) Home Location Register ( HLR ) Visitor Location Register ( VLR ) Authentication Center (AC) Operation and Administration (OAM)

 

 

cCOMPONENT FUNCTIONMobile station The CDMA mobile station (or mobile phone) communicates with other parts of the system through

the base station system.

Base station (BS) The base station (BS) handles the radio interface to the mobile station. The base station is the radio equipment (transceivers and antennas).

Base station Controller (BSC) The BSC provides the control functions and physical links between the MSC and BTS. It provides functions such as hand cover, cell configuration data and control of RF power levels in as transceiver stations.

Mobile switching center (MSC)

The MSC performs the telephony switching functions of the system. Its also performs such functions as toll ticketing, network interfacing ,common channel signaling, and others.

Home location register (HLR) The HLR database is used for storage and management of subscriptions. The home location register stores permanent data about subscribers, including a subscriber’s service profile, location information, and activity status.

Visitor location register (VLR) The VLR database contains temporary information about subscribers that is needed by the mobile services switching center (MSC) in order to service visiting subscribers. When a mobile station roams into a new mobile services switching center (MSC)Area, the visitor location register (VLR) connected to that MSC will request data about the mobile station from the HLR, reducing the need for interrogation of the home location register (HLR).

Authentication (AC) The AC provides authentication and encryption parameters that verify the user’s identity and ensure the confidentiality of each call. The authentication center (AUC) also protects network operators from fraud.

Operation and administration (OAM)

The OAM is the functional entity from which the network operator monitors and control the system. The purpose of operation and support system is to offer support for centralized, regional and local operational and maintenance activities that are required for a CDMA network.

Architecture of CDMAExample CDMA 2nd Generation:

Architecture of CDMAExample CDMA 3rd Generation:

CDMA Applications : One of the early applications for code division multiplexing is

in GPS. This predates and is distinct from its use in mobile phones.

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.

The UMTS 3G mobile phone standard, which uses W-CDMA.

CDMA has been used in the Omni TRACS satellite system for transportation logistic

 

Advantages of CDMa Disadvantages of CDMA

more advanced digital interface technology

the transmission power is extremely small, so conversation is not only less radiation,

also longer battery life mobile phones.

better call quality, almost no noise when making a call,

there is no health adverse effects because its radiation is negligible

faster download speeds best privacy (spread spectrum), hardest to crack (the AIR

INTERFACE) cost of calls is cheaper than in

GSM.

one major problem in CDMA technology is channel polution,where signals from too many cell sites are present in the subscriber’s phone but none of them is dominant.(the quality of the audio degrades)

limited to North America and a few other places, GSM is worldwide

limited variety of the handset because at present the major mobile companies use GSM technologies.

CDMA VS GSM

ADVANTAGES DISADVANTAGES

CDMA •Increased cellular communications security.•Simultaneous conversations•Increased efficiency•Smaller phones•Low power requirements and little cell-to-cell coordination needed by operators.•Extended reach

•Due to its proprietary nature, all of cdma’s flaws are not known•CDMA is relatively new, and all the network is not as mature as GSM.•Cannot offer international roaming

GSM •Already used worldwide•Permits international roaming•More stable network•Smart cards that provide security data encryption

•Lack of access to burgeoning American market.

Frequency hopping CDMAThere are two kinds of frequency hopping ; Slow Frequency Hopping (SFH)

In this case one or more data bits are transmitted within one hop. An advantage is that coherent data detection is possible. Often, systems using slow hopping also employ (burst) error control coding to restore loss of (multiple) bits in one hop.

Fast Frequency Hopping (FFH)One data bit is divided over multiple hops. In fast hopping, coherent signal detection is difficult, and seldom used. Mostly, FSK or MFSK modulation is used.

Direct sequence CDMA In Direct Sequence spread spectrum transmission, the

user data signal is multiplied by a code sequence. Mostly, binary sequences are used. The duration of an element in the code is called the "chip time". The ratio between the user symbol time and the chip time is called the spread factor. The transmit signal occupies a bandwidth that equals the spread factor times the bandwidth of the user data.

As despreading is the same operation as spreading a possible narrowband jammer signal in the radio channel is spread before the detector. Thus, the jam signal is attenuated by the spread factor ("spreading gain").