26. Radio Transmission: Duplexing and Multiple Access

 

26. Radio Transmission: Duplexing and Multiple Access



17.12.2018

Radio Transmission: Duplexing and Multiple Access – Video: Register for Course


Selected Issues:

Radio Transmission Duplexing

  • Now – so far we discussed how the single transmission comes –we have a receiver and transmitter and there is one way transmission, bt in real systems we have AP and STA, UE and eNB etc.
  • So how do we get them to talk to each other – this is refered to as duplexing
  • One possibility is that each one talks to eahc other at different Times but using the same carrier frequency and BW – this is TDD, when STA transmits, the AP is listening and vice versa
  • Another option is FDD< when both parties are transmistting and receiving at the same time, but using different carrier frequencies (e.g. 2490 and 2450MHz)
  • The third option is a mixo of the other tow – half FDD – they also use different frequencies but when one transmits, the other listens (this is useful for simpler receivers – that do not have to simultaneously transmit and receiver)

Multiple Access Schemes Overview

  • Ok but this is only when we have a tx and Rx and a single link
  • What if we have multiple devices contacting the same receiver (or the same transmitter transmits to different receivers) e.g. is WIFi and multiple STAs, or cellular networks and multiple devices – this is refered to as multiple access
  • The first method is called FDMA – the transmitter transmits simultaneously to all the receivers but each receiver is assigned different carrier frequency (this was used in 1G system)
  • Other option is called TDMA – in this case all of them are using the same carrier frequency, but in different timeslots (this was used in GSM)
  • Next one is CDMA – in this case each of the receiver is assigned a different spreading code and by multiplexing at the refeiver with the same PRS, it gets all the data for itself and others are seen as noise (example here is DSSS), (this was used inUMTS)
  • Another one is called OFDMA – used in LTE and 5G. In this case different subcarriers are used for different users (the same principle as in OFDM< but in OFDM we had all subcarriers for the same user)

Carrier Sense Multiple Access

  • The scheme that is used by WiFi is called CSMA
  • It uses DSSS (in earlier versions of wifi) or OFDM (in the latest)
  • In this scheme, we use the same carrier frequency for all users – while only one can transmit at a time, and all of them has to listen before they can transmit (this is called LBT)
  • The listening is via a receiver module that has to detect if the channel is busy (i.e. if the signal power at the given frequency is above certain threshold/value) if not the STA can transmit and if busy should wait
  • After the transmission is ended,all the transmitters calculate a random number that is called backoff, that tells them how long they should wait additionally before thy can transit – this is in order to minimize the number of collisions – i.e. so that the one that calculates the lower number starts to transmit before others will, (this is CA)
  • Lets look at the example –
  • The first transmits, others see that channel is busy and waits
  • When it transmitted all the data, allo fot them backs off with ifferent period of time and the one who hwas the lowest number ransimits, and the next one s are waiting, and so on and so forth

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