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Multiple Access Techniques

  • When multiple communications happen simultaneously, others messages can interfere with our communication. Such interference is called noise. To avoid such noise, we use the following techniques

Code Division Multiple Access (CDMA)

  • All users use the same frequency and time, but separated by unique codes
  • Used in 2G/3G mobile systems and military communications

In this method, we keep a chip sequence. To send a '1' bit, we send this chip sequence, to send a '0' bit, we send the complement of the chip sequence. To transmit 1 bit, we need to transmit 8 bits (practically 64 or 128 bit chip sequence)

Example

Consider there are 4 systems A, B, C and D

The chip sequence of each are

A = 00011011 B = 00101110 C = 01011100 D = 01000010 

In CDMA, 
- 1 means '1' bit transmission
- -1 means '0' bit transmission
- 0 means no transmission
Therefore, these chip sequences can be rewritten as
A = -1 -1 -1 1 1 -1 1 1 B = -1 -1 1 -1 1 1 1 -1 C = -1 1 -1 1 1 1 -1 -1 D = -1 1 -1 -1 -1 -1 1 -1 At time t1, let C transmit '1' S1 = C = -1 1 -1 1 1 1 -1 -1 At time t2, let B and C transmit '1' S2 = B + C = -2 0 0 0 2 2 0 -2 At time t3, let A transmit '1' and B transmit '0' S3 = A + B' = 0 0 -2 2 0 -2 0 2 At time t4, let A and B transmit '1' and C transmit '0' S4 = A + B + C' = -1 -3 1 -1 1 -1 3 1

Consider, I am D and I am communicating with C, and all other signals are just noise to me. I just wanna know what C is speaking. For that, we take dot product of the signal I receive with C's chip sequence At time t1, S1.C = (1 + 1 + 1 + 1 + 1 + 1 + 1 + 1) / 8 = 1 => C transmitted '1' At time t2, S2.C = (2 + 0 + 0 + 0 + 2 + 2 + 0 + 2) / 8 = 1 => C transmitted '1' At time t3, S3.C = (0 + 0 + 2 + 2 + 0 - 2 + 0 - 2) / 8 = 0 => C transmitted nothing At time t4, S4.C = (1 - 3 - 1 - 1 + 1 - 1 - 3 - 1) / 8 = -1 => C transmitted '0'

In this way, we can filter out the required data from the noise

Frequency Division Multiple Access (FDMA)

  • Each station is given different frequency range.
  • Guard band prevents overlapping
  • The allowed frequency band will be equally split to all the devices
  • Devices can communicate simultaneously

Time Division Multiple Access (TDMA)

  • Same frequency spectrum is given to all the devices
  • But each device is allotted different time interval
  • A device can communicate only in its allotted time
  • Every station gets the complete bandwidth
  • Time is allotted in Round Robin fashion

Statistical Time Division Multiple Access (STDMA)

  • Similar to TDMA
  • Difference is in the fact that, the time allotted for a device is dynamically calculated based on the traffic for each station

Orthogonal Frequency Division Multiplexing (OFDM)

  • When one station is using the peak frequency, the other will be at zero
  • Used in phones

Difference between FDMA and OFDM

OFDM vs FDMA.png