What is CDMA technology?

What is CDMA technology?

Code division multiple access(CDMA) is achannel access method utilized by various radio communication technologies. It should not be confused with the mobile phone standardscalledcdmaOneandCDMA2000(which are often referred to as simply "CDMA"), which use CDMA as an underlyingchannel access method.

One of the basic concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share abandwidthof different frequencies. This concept is calledmultiplexing. CDMA employsspread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast,time division multiple access(TDMA) divides access bytime, whilefrequency-division multiple access(FDMA) divides it byfrequency. CDMA is a form of"spread-spectrum"signaling, since the modulated coded signal has a much higherdata bandwidththan the data being communicated.

  • 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 ofmultiplexing, which allows numerous signals to occupy a single transmissionchannel, optimizing the use of availablebandwidth.
  • The technology is used in ultra-high-frequency (UHF)cellular telephonesystems in the 800-mhzand 1.9-ghzbands.

Now a day's importance of CDMA technology

  • One of the important wireless technologies widely in use today is CDMA which has been adopted by many service providers.
  • For anybody aspiring to start their career in the wireless communication industry, the knowledge of the CDMA technology is a must.
  • This course provides the basic knowledge on the working of the CDMA technology.

Modulation technique

Asynchronous CDMA's main advantage over CDM (SynchronousCDMA), TDMA and FDMA is that it can use the spectrum more efficiently in mobile telephony applications. (In theory, CDMA, TDMA and FDMA have exactly the same spectral efficiency but practically, each has its own challenges - power control in the case of CDMA, timing in the case of TDMA, and frequency generation/filtering in the case of FDMA.) TDMA systems must carefully synchronize the transmission times of all the users to ensure that they are received in the correct timeslot and do not cause interference.

Most modulation schemes try to minimize the bandwidth of this signal since bandwidth is a limited resource. However, spread spectrum techniques use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. One of the initial reasons for doing this was military applications including guidance and communication systems. These systems were designed using spread spectrum because of its security and resistance to jamming.


  • CDMA is a spread spectrum multiple access technique. A spread spectrum technique is one which spreads the bandwidth of the data uniformly for the same transmitted power.
  • Spreading code is a pseudo-random code which has a narrowAmbiguity functionunlike other narrow pulse codes.
  • In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. Data for transmission is simply logicallyXOR(exclusive OR) added with the faster code.
  • The figure shows how spread spectrum signal is generated. The data signal with pulse duration ofTbis XOR added with the code signal with pulse duration ofTc. (Note:bandwidthis proportional to1 /TwhereT= bit time) Therefore, the bandwidth of the data signal is1 /Tband the bandwidth of the spread spectrum signal is1 /Tc. SinceTcis much smaller thanTb, the bandwidth of the spread spectrum signal is much larger than the bandwidth of the original signal.

Code Division Multiplexing (SynchronousCDMA)

  • Synchronous CDMA exploits mathematical properties oforthogonallybetweenvectors representing the data strings.
  • For example, binary string "1011" is represented by the vector (1, 0, 1, 1). Vectors can be multiplied by taking theirdot product, by summing the products of their respective components.
  • If the dot product is zero, the two vectors are said to beorthogonalto each other (note: if u=(a,b) and v=(c,d), the dot product uv = a*c + b*d). Some properties of the dot product aid understanding of howW-CDMAworks.

Asynchronous CDMA

  • The previous example of orthogonal Walsh sequences describes how 2 users can be multiplexed together in a synchronous system, a technique that is commonly referred to as Code Division Multiplexing (CDM).
  • The set of 4 Walsh sequences shown in the figure will afford up to 4 users, and in general, an NxN Walsh matrix can be used to multiplex N users.
  • Multiplexing requires all of the users to be coordinated so that each transmits their assigned sequencev(or the complement, -v) so that they arrive at the receiver at exactly the same time.
  • Thus, this technique finds use in base-to-mobile links, where all of the transmissions originate from the same transmitter and can be perfectly coordinated.

What is benefit of CDMA Technology?

  • Outstanding Voice and Call Quality
  • CDMA filters out background noise, cross-talk, and interference so you can enjoy crystal-clear voice quality, greater privacy, and enhanced call quality. QUALCOMM's CDMA variable rate vocoder translates voice into digital transmissions, zeroes and ones, at the highest translation rates possible (8kbps or 13kbps).

  • Greatest Coverage for Lower Cost
  • CDMA's spread spectrum signal provides the greatest coverage in the wireless industry, allowing networks to be built with far fewer cell sites than is possible with other wireless technologies.

  • Longer Talk Time, Longer Battery Life and Smaller Phones
  • You can leave your phone on with CDMA. CDMA uses power control to monitor the amount of power your system and handset need at any time.

Greater Capacity

CDMA allows the largest number of subscribers to share the same radio frequencies, helping service provider's increase their profitability. You can leave your phone on with CDMA. CDMA uses power control to monitor the amount of power your system and handset need at any time.

  • Reduced Background Noise and Interference
  • CDMA combines multiple signals and improves signal strength. This leads to the near elimination of interference and fading.

  • Improved Security and Privacy
  • CDMA's digitally encoded, spread spectrum transmissions resist eaves dropping. Designed with about 4.4 trillion codes, CDMA virtually eliminates cloning and other types of fraud.

Difference between CDMA Technology and GSM Technology

1. CDMA stands for code division multiple access it use cdma Techniques as access mechanism.

GSM is stands for global system for mobile communication. It use tdma & fdma techniques as access mechanism.

2. CDMA phone instruments are dedicated to the service Provider.

GSM phone instruments are portable across service Providers through SIM cards.

3. in CDMA the entire frequency band is available to the End-user. So, the Frequency Re-use factor is 1 in CDMA.

In GSM, the entire frequency band is not available to the end-user.

4. in cdma we allocate a Unique code for every user separately and allocate bandwidth to user.

In gsm we divide bandwidth in to time slots for better utilization of bandwidth.

5. Here the handset is network locked. You have to buy a complete package from the service provider that includes both, the handset and the connection (prepaid / post-paid).

If you opt for GSM technology, you have the freedom to choose handset separately from the wide variety of big handset companies like Nokia, Motorola, Sony Ericsson, etc who retail their products in the market.

6. You cannot change the service provider and still keep the same handset.

You even enjoy the freedom to switch the service provider and still continue with the same handset.

7. Here you need to check for International roaming tie ups if you travel abroad frequently. You do not enjoy the freedom like GSM mobiles.

To save yourself from roaming charges from your service provider, you can purchase a local SIM card with call value and a local number in the country you are visiting.

CDMA Advantages:

  • The first and the most important advantage in CDMA is the cost of calls is lower than GSM. Although this difference is becoming transparent day by day still roaming costs will remain low.
  • CMDA services in India are backed up India's most wealthy companies Tata and Reliance.
  • Right now the call quality is better than GSM. But as soon as the subscriber base increases the call quality will start going low because of the way in which CDMA system works.


  • although many handsets are available in CDMA service u don't find as much variety as u would in GSM handsets.
  • The biggest disadvantage for CDMA handsets is they aren't compatible with GSM handsets. U cannot use a CDMA handset with a SIM card. So u gotta stick around with one handset for a long time.
  • Another disadvantage is that in CDMA only 2 service providers are there Reliance and Tata. While Tata is still improving so u gotta stick to Reliance u has no option? Offcourse Reliance is too increasing day by day.
  • The web based services like messenger, downloading ringtones etc from websites are not yet available in CDMA services yet.

Security issues

Parameter Negotiation

After the call has been set up using the protocols H.225 and Q.931, the H.245 protocol is now used to negotiate the parameters of the call. It uses the H.245 control channel, which is always open.

  • each side starts out by announcing its capabilities: eg whether it can support video or conference calls, which codecs it supports, bit rate etc.
  • once each side knows what the other one can handle, two unidirectional data channels are set up and a codec and other parameters assigned to each one
  • after all negotiations are complete, data flow can begin using RTP, which is managed by RTCP, which handles congestion control and audio/video synchronisation
  • when either party hangs up, the Q.931 call signalling channel is used to tear down the connection.

SIP - Session Initiation Protocol

This application-layer protocol describes how to set up Internet telephone calls, video conferences, and other multimedia connections. SIP is a single module, designed to interwork well with existing Internet applications.


A VPN consists of a set of computers that interconnect over an insecure network such as the Internet and make use of encryption and other protocols to provide security. At each private site LANs link workstations, servers and databases under the control of an IT department.

Virtual LANs

Users can be assigned to LANS based on their department and not on their physical location. Advantages of Virtual LANs (VLANs) include:

  • When hosts change from one department to another it is not required to physically move the computer
  • Security is enhanced as data is kept within a department
  • Network loading is equitable - eg a department may use high-bandwidth applications - other departments on different VLANS will not be affected.

ADSL - ASYMMETRIC Digital Subscriber Line

Network Interface Device (NID) is installed on customer premises. The NID marks the end of telephone company property and start of customer property. Close to NID or combined with it is a splitter, an analogue filter that separates the 0-4000Hz band used by POTS from the data stream.

POTS signal is routed to the existing telephone or fax machine and the data signal is routed to the ADSL modem.

ADSL modem is a Digital Signal Processor that has been set up to act as 250 QAM modems operating in parallel.

Since most current ADSL modems are external, a high-speed interface for the computer is required.

Usually this is an Ethernet connection but the USB port could be used instead. Internal ADSL modems will probably become available as happened for dial-up access.


Data traffic has increased exponentially, unlike voice traffic, which shows only a linear increase. As a consequence of this, many packet-switching network operators started showing interest in carrying voice over their data networks. The amount of additional bandwidth required for voice is negligible since the packet networks are already built for the data traffications are being invented regularly.

Implementation of CDMA transmitter for a multi-standard SDR base band platform

It is clear now that it is virtually impossible to adopt or impose on a single standard for all wireless networks. Hence, software defined radio (SDR) technology emerged which addresses this issue. This paper describes the design of CDMA digital transmitter for a multi-standard SDR base band platform. The platform consists of reconfigurable and reprogrammable hardware platform which provide different standards with a common platform. This paper focus also on the methods of generate VHDL model of CDMA transmitter. This model can be implemented with Field Programmable Gate Array (FPGA).











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