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  1. CDMA
  2. What is CDMA? The term “CDMA” (Code Division Multiple Access) refers to both a spread spectrum technique and a cellular standards. Initially it was restricted to the armed forces, this technology was commercially launched in 1995. Multiple access basically means that information by several transmitters can be sent simultaneously onto a single communication channel.
  3. There are multiple users which are provided or assigned variant CDMA codes and thus the users can access the entire band of frequencies or the whole bandwidth. This method does not limit the frequency range of the user. Hence, with the help of CDMA, multiple users can share a band of frequencies without any kind of undue interference between them. CDMA makes the use of spectrum technology along with analog to digital conversion(ADC). It is thus used by various radio communication technologies. Mainly, it is used for mobile communication.
  4. History of CDMA • The CDMA technology was originated during World War II. • It was developed by English allies for protection of their wireless transmissions from jamming from enemies. • This technology was patented by Qualcomm post war and was made commercially available for use. • The first CDMA system was launched in 1995 in Hongkong by Hutchison Telephone Company.
  5. How it Works? • Code: It refers to the string of binary sequence that the transmitter and the receiver share. This code encodes the information into a low frequency signal before it is transmitted over a channel. This same code is used by the receiver to decode the information. The receiver gets the code with the help of the nearest base station. • Division: In CDMA a single channel is divided into numerous slots which can be used by multiple users. This is possible because of the use of unique code. • Multiple Accesses: Due to code based communication, multiple users can communicate and access the same channel simultaneously without any undesirable interference and loses.
  6. Features of CDMA Some of the key features of CDMA are: • Used in Global Positioning System (GPS) • Every channel uses full available spectrum • Better capacity for voice and data communications • CDMA does not require synchronization • Allocation of code word to each user helps to reduce interference • Used by several mobile companies (Qualcomm standard IS-2000 known as CDMA2000) • Emits less radiation • Faster data transfers • CDMA supports high speed push to talk and push to email services • UTMS 3G mobile phone standard uses W-CDMA • OmniTRACS satellite system uses CDMA for transportation
  7. Encoder for DS-SS CDMA
  8. • The input provided to the CDMA encoder can be in the form of Pulse code modulation (PCM) encoded voice band signal or can be a digital signal from computer. • It is multiplied with N bit, which is a unique chip code. • The output of balanced modulator is the product code. • In IF carrier it is used as an PSK modulation. • The balanced modulator is sometimes referred to as multiplier. • Further the modulated signal is then converted to RF band and is used for transmission purpose. • The high-power amplifier basically raises the level of power to a very high level and then the antennas transmit this signal. • The encoder is also called as multiplexer.
  9. Decoder for DS-SS CDMA
  10. • The decoder helps in reconverting the RF signal to IF. • A coherent PSK carrier is been obtained from IF. • The chip code is been used by the receiver and it helps in synchronizing the receiver station’s code generator. • The recovered chip is then multiplied with recovered PSK carrier to generate PSK modulated signal which contains PSK carrier and the chip code. • The IF signal which is received, contains chip code, PSK carrier and data. In correlator it is compared with the received IF signal. • The correlator helps in comparing this two signals and helps in recovering the original data. • The decoder is also called as demultiplexer.
  11. Limitations of CDMA • Some of the Limitations of CDMA are: • A large code length could induce delay or interference • Time synchronization is required • Reduction in capacity due to gradual transfers • Constant tight power control is required resulting in multiple handovers (Causing dropped calls)
  12. Types of CDMA Technologies There are two types of CDMA technologies: • Synchronous CDMA (orthogonal codes) • Asynchronous CDMA (pseudorandom codes)
  13. Difference between Synchronous CDMA (orthogonal codes) & Asynchronous CDMA (pseudorandom codes) CDMA Technology Definition Advantages Disadvantages Best suited for Synchronous CDMA It exploits the mathematical properties orthogonally between vectors representing the data strings. •Can accommodate large number of subscribers •Gives better performance •Orthogonal codes are used •Digital modulation is analogues •Issue of dropped calls •Quality degrades with increased user base Mainly used by mobile telephone operators. Asynchronous CDMA Makes use of code space . Pseudo-random or pseudo- noise sequences are used. •Pseudo-random code (Pseudo-random Noise) or Gold code is used •Flexible allocation of resources •Slow speeds •Uses random or irregular time intervals Ideally suited for mobile networks having large number of transmitters are producing small amount of traffic at irregular intervals.
  14. Advantages • Increased user capacity is an advantage of the CDMA as it supports a lot more users in comparison to TDMA or FDMA. • CDMA is more secure as the information transmitted is below the noise floor making the intrusion of the spectrum difficult. • CDMA systems have comparatively fewer dropouts than GSM. Thus, it can also be used in rural areas. • The cost of the calls in CDMA is lower in comparison to the cost in GSM. • CDMA provides a high quality of voice with almost no noise during the calls. • Using CDMA problems like multipath and fading do not occur. • CDMA has a very low power requirement.
  15. Disadvantages • CDMA lacks the facility of international roaming which is provided by GSM. • Since there is no limit to the number of users the system performance degrades with an increase in the number of users. • Self-jamming problem occurs in CDMA systems because of loss of orthogonality. • The problem of channel pollution occurs in CDMA systems which thus degrades the quality of audio. • Since most of the mobile companies use GSM thus there is a lack of handsets for CDMA technology.