Subject: Wireless Technology

Topic: Fundamentals of Wireless Communication

Difficulty: Medium

Where channel access methods are used in point-to-point networks (such as cellular networks) for dividing forward and reverse communication channels on the same physical communications medium, they are known as duplexing methods, such as time-division duplexing and frequency-division duplexing,

Time-division duplexing:

• Time-division duplexing (TDD) is the application of time-division multiplexing to separate outward and return signals. It emulates full duplex communication over a half-duplex communication link.
• Time-division duplexing has a strong advantage in the case where there is asymmetry of the up link and down link data rates. As the amount of uplink data increases, more communication capacity can be dynamically allocated, and as the traffic load becomes lighter, capacity can be taken away. The same applies in the downlink direction.
• For radio systems that aren't moving quickly, another advantage is that the uplink and downlink radio paths are likely to be very similar. This means that techniques such as beam forming work well with TDD systems

• Examples of time-division duplexing systems are:

  a) UMTS 3G supplementary air interfaces TD-CDMA for indoor mobile telecommunications.

  b) The chinese TD-LTE 4-G, TD-SCDMA 3-G mobile communications air interface.

  c) IEEE 802.16 WiMAX

  d) PACTOR

Frequency-division duplexing:

• Frequency-division duplexing (FDD) means that the transmitter and receiver operate at different carrier frequencies. The term is frequently used in ham radio operation, where an operator is attempting to contact a repeater station. The station must be able to send and receive a transmission at the same time, and does so by slightly altering the frequency at which it sends and receives. This mode of operation is referred to as duplex mode or offset mode

• Uplink and downlink sub-bands are said to be separated by the frequency offset. Frequency-division can be efficient in the case of symmetric traffic. In this case time-division duplexing tends to waste bandwidth during the switch-over from transmitting to receiving has greater inherent latency, and may require more complex circuitry.

• Another advantage of frequency- division duplexing is that it makes radio planning easier and more efficient, since base stations do not "hear" each other (as they transmit and receive in different sub-bands) and therefore will normally not interfere with each other.

Examples of frequency- division duplexing systems are:

• ADSL and VDSL
• Most cellular systems, including the UMTS/WCDMA use frequency- division duplexing mode and the cdma2000 system.
• IEEE 802.16 WiMax also uses frequency-division duplexing