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- Spectrum Shortage - The major problem faced by CDMA technology is lack of available spectrum and this is hindering their subscriber growth. In India CDMA operators are having problem in allocation of spectrum in 1900 MHz band which is common in other regions.
According to B.B. Anand (President for Regulatory Affairs at Reliance Infocomm) "It is of the utmost importance that the 1900 MHz spectrum be allocated to CDMA operators in India to ensure that the region remains part of the international community of roaming wireless subscribers worldwide,"
In India the band was allocated to Defense Services for their mobile communication usage. However upon the launch of mobile communication services for public, coordination was sought from the Defense department to make the spectrum available for mobile services. The Defense is releasing some spectrum through auction but it is opposed by the GSM operators on the commercial ground for preventing growth of CDMA.
- Low performance in hilly regions- The towers in CDMA interfere with each other and normally they are installed on much shorter towers and because of this CDMA (IS-95 standard) may not perform well in hilly terrains and India has plenty of such region in north region.
- Near Far effect
In Near Far effect the code transmitted from a transmitter (A) to receiver is interfered by another transmitter (B) which is nearer to the receiver as compared to sender transmitter and this make difficult for identification of the original signals by the receiver .This effect became a major obstacle for successful implementation of CDMA technology.
It is very expensive to solve the near far problem in a CDMA system. The precision power control algorithm are used to solve near far problem which consists of open loop power control and closed loop power control and this make a CDMA transreciever very complicated in both hardware and software implementation.
- Breathing of Base Stations, where coverage area shrinks under load. In CDMA as number of subscribers using a particular sit increases the range of that site goes down. In other words unlike GSM where number of users are finite but in CDMA users are not finite and in more load the voice quality goes down.
Design And Implementation of CDMA
The CDMA technology works on basic principle of communicating through specific code on available frequency instead of assigning a specific frequency. Since users are specified by code they are able to use same carrier frequency and this eliminates the frequency reuse problem encountered in other technologies.
Steps in generation of CDMA signals
- Analog to Digital Conversion of Voice: CDMA uses Pulse code Modulation (PCM) for converting analog voice or audio to digital signal. This process is also known as digitalization.
- Voice Compression: CDMA uses a special device VOCODER to do voice compression. They are located at the BSC and in the phone. While talking we give space or pause between words and CDMA takes advantage of these pauses in speech activity by using a variable rate vocoder. There are four data rates at which vocoder compresses the voice signal and they are ½, ¼, 1/8 and Full. Vocoder uses its full rate when a person is talking very fast. It uses the 1/8 rate when the person is nearly silent.
- Encoding and Interleaving: Encoders and interleavers are built into the BTS and the phones. They build redundancy into the signal so that information lost in the transmission can be recovered. CDMA uses convolutional encoding to encode audio signals provided by VOCODERS.
Interleaving is used to reduce the effects of burst errors and recovering lost bits.
- Channelization: The encoded data after encoding and interleaving is again encoded to separate it from other encoded data. After that encoded signals are spreaded over the entire channel.
CDMA uses WLASH code to channelize user on the forward or upward link i.e. from BTS to mobile and Pseudorandom noises to channelize user on reverse link or downward link i.e. from mobile to BTS.
- Converting digital signal to a Radio Frequency (RF) signal. The channelized data from all calls are combined into one signal and then it is converted into radio frequency signal for transmission.
A more relevant and self explanatory example is show below of transmission through base station. As explained above each conversation is compressed with a vocoder and the output is doubled by a convolutional encoder that adds redundancy for error checking. The encoded bit are then channelized and all calls are combined and modulated onto a carrier frequency.
At receiving site the steps followed are just opposite of sender site
- Conversion of RF signal to digital signal
- Despreading the signal
- Deinterleaving and decoding
- Voice decompression
- Digital to analog voice recovery
RF signal are converted into digital signal by quantization into bits ("chips") by the analog-to-digital converter (ADC). The output is deschannellized and decoded through a decoder know as Viterbi decoder which corrects the errors using the convolutional code. The output goes to the vocoder for decompression and then to digital-to-analog converter (DAC), which decompresses the bits and turns them back into waveforms (sound).
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