Wireless Transmission Basics: A Comprehensive Guide for Mobile Computing, Lecture notes of Mobile Computing

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Mobile Computing

Wireless Transmission

Wireless Transmission Basics

• Data transmission occurs between transmitter and

receiver over transmission medium.

• Transmission media may be classified as guided or

unguided.

• In both cases, communication is in the form of electromagnetic waves.
• With guided media, the waves are guided along some

physical path. Ex. Twisted pair, coaxial cable, fiber

• Unguided media provides a means for transmitting

electromagnetic waves but do not guide them.

• Ex. Propagation through air, vacuum, and sea water

>This form of transmission is usually referred to as wireless transmission

Wireless Transmission Signal Propagation

• Wireless communication networks have

senders and receivers of signals

• However, in connection with signal

propagation, wired and wireless networks

exhibit considerable differences.

• In wireless, the signal has no wire to

determine the direction of propagation,

where as signals in wired networks only travel

along the wire.

Wireless Transmission Signal Propagation

transmission

sender

detection

interference Distance

Wireless Transmission Multiplexing

• A fundamental mechanism in communication systems and networks
• Multiplexing describes how multiple users share a medium with minimum or no interference
• The task of multiplexing is to assign space, time, frequency, and code to each communication channel with a minimum of interference and a maximum of medium utilization
• In wireless communication,

multiplexing can be carried

out in four dimensions: space, time, frequency, and code

Multiplexing Space Division Multiplexing (SDM)

• In SDM, channels are assigned on the basis of “space” (but operate on same frequency)
• The assignment makes sure that the transmissions do not interfere with each (with a guard space in between). - The space between the interference ranges is sometimes called guard space.
• In wireless transmission, SDM implies a separate sender for each communication channel with a wide enough distance between senders
• This multiplexing scheme is used, for example, at FM radio stations where the transmission range is limited to a certain region – many radio stations around the world can use the same frequency without interference

Multiplexing Frequency Division Multiplexing (FDM)

• FDM describes schemes to subdivide the frequency

dimension into several non-overlapping frequency bands.

• For channels Kn, each channel K i is allotted its own

frequency band. Senders using a certain frequency band can use this band continuously.

• Again, guard spaces are needed to avoid frequency

band overlapping [adjacent channel interference]

• This scheme is used for radio stations within the same

region, where each radio station has its own frequency

FDM…

Each channel Ki has different frequency f i but at same space

TDM…

Each channel Ki uses same frequency f But at different times

Multiplexing Code Division Multiplexing (CDM)

• While SDM and FDM are well known from the early days of radio transmission and TDM is used in connection with many applications, CDM is a relatively new scheme in commercial communication systems - used in military applications due to its inherent security features
• A channel Ki uses the same frequency at the same time for transmission. Separation is now achieved by assigning each channel its own “code”, guard spaces are realized by using codes with the necessary distance in code space.
• The main advantage of CDM for wireless transmission is that it gives good protection against interference and tapping. Different codes have to be assigned but code space is huge compared with frequency space.

Reading Assignment I

Modulation

Medium Access Control (MAC)

• MAC schemes from wired networks
  • Aloha
  • Token ring/Bus
  • CSMA/CD – carrier sense multiple access with collision detection
  • CSMA/CA - carrier sense multiple access with collision avoidance
• CSMA/CD
  • a sender senses the medium (a wire or coaxial cable) to see if it is free. If the medium is busy, the sender waits until it is free. If the medium is free, the sender starts transmitting data and continues to listen into the medium. If the sender detects a collision while sending, it stops at once and sends a jamming signal

MAC…

• Why does CSMA/CD fail in Wireless?
  • CSMA/CD is not really interested in collisions at the sender but rather in those at the receiver. The signal should reach the receiver without collisions
  • The signal strength is same in wired networks at almost all points. So it doesn’t matter whether it’s being detected by the sender or the receiver.
  • In the case of wireless networks, the signal strength decreases proportionally with the square of the distance to the sender. - Hidden and exposed terminals problem