Mobile Computing Seminar Technologies and standards in wireless communication Summary

Recently there has been a growing interest in wireless communication in small devices Although Infrared is already widely implemented in small low cost devices there is rapid movement towards wireless communication in the Giga Hertz spectrum which allows for new concepts such as hidden computing The presentation rst describes some of the techniques used in transmitting information wirelessly It then presents various standards that are going to play an important role in wireless communication distinguishing them between infrastructure networks and ad hoc networks Finally an attempt is made at evaluating how well these standards are suited for the implementa tion in small devices Radio Transmission Techniques We rst have a look at techniques for transmitting information reliably over radio waves in the GHz spectrum Three techniques in radio transmission are presented Frequency Hopping Spread Spectrum Direct Se quence Spread Spectrum and Orthogonal Frequency Division Multiplexing They all try to achieve resis tance against narrow band interference caused by other radio sources Frequency Hopping Spread Spectrum FHSS In Frequency Hopping the transmission band is divided into di erent frequency channels typically and a logical channel i e a channel on which two or more devices communicate hops periodically from one frequency channel to another with a pseudo random hopping sequence Direct Sequence Spread Spectrum DSSS Also in Direct Sequence the frequency band is divided into fewer but larger channels but no hopping is used To compensate for noise on a channel a technique called chipping is implemented each bit is converted into a redundant bit pattern called chip sequence whereby an n chip code spreads the signal by a factor of n If some interference destroys part of the chip sequence the original bit may still be recovered from the remaining chips Orthogonal Frequency Division Multiplexing OFDM The general idea of OFDM is to split a high rate data stream into lower rate streams which are trans mitted simultaneously over a number of subcarriers These subcarriers are orthogonal to each other in the sense that when listening to one subcarrier the others don t interfere hence the name Orthog onal Frequency Division Multiplexing By using this technique multipath delay spread and inter symbol interference are considerably decreased because only low bit rate streams are employed Infrastructure Networks In this section two wireless network standards are presented IEEE and ETSI HiperLAN They are usually intended for use as Wireless LANs WLAN Table summarises the characteristics of these two standards Table and HiperLAN