Contents
CODE DIVISION MULTIPLE ACCESS
CDMA it is a method in which users occupy the same time and frequency
allocations and are channelized by unique assigned codes. The signals are
separated at the receiver by using a correlator that accepts only signal energy
from the desired channel. Undesired signals contribute only to noise.
1. Basic Units Of Digital Wireless System:
A digital wireless system has 4 basis components:1.1 Subscriber Units
This is the generic term used for all types of telephone: mobiles, portable, or Wireless Local Loop (WLL).
1.2.Base Station Transceiver Subsystem (BTS)
The BTS is a radio link between the subscriber and the system. A BTS is a system element that provide that CDMA signal generation and transmits the signal at the appropriate frequency.
1.3 Base Station Controller (BSC)
The Base controls several BTS elements. The BSC perform the switching operation provide other services such as billing.
1.4 Public Switched Telephone Network (PSTN)
The PSTN is the link that connects the wireless system to all international and domestic landline traffic.
2.Steps in Generating a CDMA signal:
1. Voice is digitized (PSTN)2. Digital voice is vocoded (BSC)
3. Digital signal is encoded and interleaved (BTS)
4. Encoded signal are spread (channelized) (BTS)
5. The digital signal converted and transmitted as radio signal (BTS)
2.1.Analog to digital conversion
The voice is converted to a digital signal. It consists of:
1. Low pass filtering - It limits the frequency of the analog signal.
2. Sampling - The amplitude of a voice signal is measured at regular time intervals.
3. Quantization - A signal is sampled into each 2n levels, where n is the number bits needed per sample to achieve the accuracy desired.
4. Binary coding - The resulting levels are coded into a base band digital signal.
5. PCM is the most common method used to convert a voice signal into a digital signal.
2.1.1 Variable rate vocoder
The vocoder (Voice Coder) is used to compress the digital signal. The vocoder used in CDMA system compresses the voice signal into various data rates. CDMA takes advantage of the fact that we pause between syllables and words. The data rate is dynamically determined by the users’ speech activity. The vocoders are located in the phone and the BSC.
2.1.2 Encoding
During transmission part of the transmitted signal maybe lost. To compensate for this the Encoder introduces redundancy in the signal. The Encoder takes steps to ensure that data in the signal can be retrieved using a backup even if part of the transmitted signal is not decipherable. For example if the signal ABCD is to be propagated, the Encoder codesthem in a format as AAABBBCCCDDD.
2.1.3 Interleaving
This is a method of data retrieval used by the Encoder. Here, the Encoder repeats transmitted bits at certain fixed intervals in a fixed order instead of sending one continuous stream of bits. Such a signal will be deinterleaved at the receiving station before being converted to Analog form by the Decoder. For example, if the encoded signal is AAABBBCCCDDD then after interleaving the actual signal that is sent is of the format ABCDABCDABCD. This is Deinterleaved and then Decoded as ABCD at the receiving station.
2.1.4 Coding & Spreading
The encoded voice data is encoded using a code that will separate it from other encoded voice data. This process, known as channelization, will spread the encoded symbols over the entire bandwidth of the CDMA channel.
2.1.5 Transmission
The channelized data from all calls will be combined into a one signal that is transmitted is converted to an analog RF signal which is transmitted over the air.
2.1.6 Receiver Decoding / Despreading
The code used to channelize and spread the encoded voice data is known to the receiver.The receiver will use the code to despread / decode the signal and recover the encoded voice data.
2.1.7 Codes used in CDMA
Orthogonal (Walsh) codes are used on the forward link to channelize users. Pseudorandom noise (PN) code is used on the reverse link to channelize users.
2.2 Digital to Analog Conversion:
2.2.1 Deinterleaving and Decoding
A fter the signal is despread the receiver will deinterleave and decode the signal into the original vocoded voice. The deinterleaving and decoding is done at the BTS and in the phone.
2.2.2 Voice Decompression
The decompression of the voice signal is done at the BSC and then sent to the PSTN.
2.2.3 Voice Recovery
The receiver converts the vocoded voice into an analog voice signal. The D/A conversion is done at the PSTN.
