For radio systems there are two resources: frequency and time. Division by frequency, so that each pair of communicators is allocated part of the spectrum for all of the time, results in Frequency Division Multiple Access (FDMA). Division by time, so that each pair of communicators is allocated all (or at least a large part) of the spectrum for part of the time results in Time Division Multiple Access (TDMA). In Code Division Multiple Access (CDMA), every communicator will be allocated the entire spectrum all of the time. CDMA uses codes to identify connections.
A digital method for simultaneously transmitting signals over a shared portion of the spectrum by coding each distinct signal with a unique code. CDMA is a wireless communications technology that uses the principle of spread spectrum communication. Advantages Multiple access capability Protection against multipath interference Privacy Interference rejection Ant jamming capability Low probability of interception
CDMA Protocols Concept behind the Protocols Averaging Systems Avoidance Systems Modulation Method DS FH TH Chirp SSHybrid
o Characteristics: o All users use same frequency and may transmit simultaneously. o Narrowband message signal multiplied by wideband spreading signal, or codeword o Each user has its own pseudo-codeword (orthogonal to others). o Receivers detect only the desired codeword. All others appearas noise. o Receivers must know transmitter’s codeword.
Signal transmission consists of the following steps: 1. A pseudo-random code is generated, different for each channel and each successive connection. 2. The Information data modulates the pseudo-random code (the Information data is “spread”). 3. The resulting signal modulates a carrier. 4. The modulated carrier is amplified and broadcast. Signal reception consists of the following steps: 1. The carrier is received and amplified. 2. The received signal is mixed with a local carrier to recover the spread digital signal. 3. A pseudo-random code is generated, matching the anticipated signal. 4. The receiver acquires the received code and phase locks its own code to it. 5. The received signal is correlated with the generated code, extracting the Information data.
Advantages: o Increased capacity o Improved voice quality o Eliminating the audible effects of multipath fading o Enhanced privacy and security o Reduced average transmitted power o Reduced interference to other electronic devices Disadvantages: o Wide bandwidth per user required o Precision code synchronization needed