NOTE (1): ActiveXperts SMS Messaging Server is an SMS messaging framework to allow sending, receiving and processing SMS messages. It is designed to implement any project that requires SMS messaging. Read more »
NOTE (2): ActiveXperts SMS Component provides developers with a fast and reliable SMPP API. Simply connect to the SMPP provider, bind using your credentials and call SubmitSms to send out the messages. Read more »
CDMA is a form of Direct Sequence Spread Spectrum communications. In general, Spread Spectrum communications is distinguished by three key elements:
In order to protect the signal, the code used is pseudo-random. It appears random, but is actually deterministic, so that the receiver can reconstruct the code for synchronous detection. This pseudo-random code is also called pseudo-noise (PN).
There are three ways to spread the bandwidth of the signal:
CDMA is a Direct Sequence Spread Spectrum system. The CDMA system works directly on 64 kbit/sec digital signals. These signals can be digitized voice, ISDN channels, modem data, etc.
Signal transmission consists of the following steps:
Signal reception consists of the following steps:
CDMA works on Information data from several possible sources, such as digitized voice or ISDN channels. Data rates can vary, here are some examples:
|Data Source||Data Rate|
|Voice||Pulse Code Modulation (PCM)||64 kBits/sec|
|Adaptive Differential Pulse Code Modulation (ADPCM)||32 kBits/sec|
|Low Delay Code Excited Linear Prediction (LD-CELP)||16 kBits/sec|
|ISDN||Bearer Channel (B-Channel)||64 kBits/sec|
|Data Channel (D-Channel)||64 kBits/sec|
The system works with 64 kBits/sec data, but can accept input rates of 8, 16, 32, or 64 kBits/sec. Inputs of less than 64 kBits/sec are padded with extra bits to bring them up to 64 kBits/sec.
For inputs of 8, 16, 32, or 64 kBits/sec, the system applies Forward Error Correction (FEC) coding, which doubles the bit rate, up to 128 kbits/sec. The Complex Modulation scheme (which we'll discuss in more detail later), transmits two bits at a time, in two bit symbols. For inputs of less than 64 kbits/sec, each symbol is repeated to bring the transmission rate up to 64 kilosymbols/sec. Each component of the complex signal carries one bit of the two bit symbol, at 64 kBits/sec, as shown below.
For each channel the base station generates a unique code that changes for every connection. The base station adds together all the coded transmissions for every subscriber. The subscriber unit correctly generates its own matching code and uses it to extract the appropriate signals. Note that each subscriber uses several independant channels.
In order for all this to occur, the pseudo-random code must have the following properties:
In this context, correlation has a specific mathematical meaning. In general the correlation function has these properties:
Intermediate values indicate how much the codes have in common. The more they have in common, the harder it is for the receiver to extract the appropriate signal. There are two correlation functions:
The receiver uses cross-correlation to separate the appropriate signal from signals meant for other receivers, and auto-correlation to reject multi-path interference.
The FEC coded Information data modulates the pseudo-random code, as shown in Figure 2a. Some terminology related to the pseudo-random code:
An important concept relating to the bandwidth is the processing gain (Gp). This is a theoretical system gain that reflects the relative advantage that frequency spreading provides. The processing gain is equal to the ratio of the chipping frequency to the data frequency.