OFDMA, SDMA, PDMA, CSMA-DA, CSMA-CA Difference between GSM CDMA

Orthogonal frequencyvdivision multiplexing (OFDM)

• Access technique used in Long-Term Evolution (LTE) to accommodate multiple users in a given bandwidth

• Modulation method that divides a channel into multiple narrow orthogonal bands that are spaced so they don’t interfere with one another.
• DATA divided into many lower bit streams.
• Time slot bw each subchannel are used to package the data.
• This technique is very spectrally efficient, so it provides very high data rates. It also is less
• affected by multipath propagation effects.
• To implement OFDMA, each user is assigned a group of subchannels and related time slots.

Space division multiple access SDMA

• SDMA uses physical separation methods that permit the sharing of wireless channels.
• single channel may be used simultaneously if the users are spaced far enough from one another to avoid interference. Known as frequency reuse.
• Useful in gsm

Pdma polarization division multiple access (PDMA)

• One unique variation of SDMA, polarization division multiple access (PDMA), separates signals by using different polarizations of the antennas. 
• Two different signals then can use the same frequency, one transmitting a vertically polarized signal and the other transmitting a horizontally polarized signal.

CSMA-CD

• A unique and widely used method of multiple access is carrier sense multiple access with collision detection (CSMA-CD).
• used in Ethernet local-area networks (LANs). 
• It allows multiple users of the network to access the single cable for transmission. 
• All network nodes listen continuously. When they want to send data, they listen first and then transmit if no other signals are on the line. 
• If two or more transmissions occur simultaneously, a collision occurs.
• The network interface circuitry can detect a collision, and then the nodes will wait a random time before retransmitting.

Csma ca

variation of this method is called carrier sense multiple access with collision avoidance (CSMA-CA).

This method is similar to CSMA-CD. However, a special scheduling algorithm is used to determine the appropriate time to transmit over the shared channel.

 While the CSMA-CD technique is most used in

wired networks, CSMA-CA is the preferred method in wireless networks.

Differentiate between GSM and CDMA

GSM

- GSM uses multiple frequencies.

- Uses TDMA and FDMA for accessing

signals.

- The voice rate is 9.6 KBPS.

- Uses less bandwidth.

CDMA

- CDMA uses single frequency as

carrier.

- The voice rate is 14.4 KBPS.

- CDMA power control to access

method is better as Phased Locked

Loops are used.

- Follows soft handoff, hence call

handling is more efficient.