2-MARKS(WCS-B) | Code Division Multiple Access | Cellular Network

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  EC T62 WIRELESS COMMUNICATION UNIT I: SERVICES AND TECHNICAL CHALLENGES PART-A1.Define pain!  paging systems are unidirectional wireless communications systems. They are characterized by thefollowing properties1. The user can only receive information, but cannot transmit. Consequently, a “call” message!can only be initiated by the call center, not by the user. . The information is intended for, and received by, only a single user. 2.Diffe en#ia#e PAN an$ WLAN! #hen the coverage area becomes even smaller than that of #$%&s, we spea' of  Personal Area Networks (%&s!. )uch networ's are mostly intended for simple “cable replacement” duties. *or e+ample, devices following the  Bluetooth standard allow to connect a handsfree headset to a phonewithout requiring a cable- in that case, the distance between the two devices is less than a meter. %.C&'pa e a$-(&) an$ *en*& ne#+& ,*! p to now, we have dealt with “infrastructurebased” wireless communications, where certaincomponents base stations, T/ transmitters, etc.! are intended by design to be in a fi+ed location, toe+ercise control over the networ' and interface with other networ's. The size of the networ's maydiffer from $%&s covering 0ust one apartment to cellular networ's covering whole countries!, butthe central principle of distinguishing between “infrastructure” and “user equipment” is commonto them all. There is, however, an alternative in which there is only one type of equipment, andthose devices, all of which may be mobile, organize themselves into a networ' according to their location and according to necessity. .Define '&i/i#0!   Low mobility 2 many communications devices are operated at pedestrian speeds. Cordless phones,as well as cellphones operated by wal'ing human users are typical e+amples. The effect of the lowmobility is a channel that changes rather slowly, and 3 in a system with multiple 4)s 3 handover from one cell to another is a rare event.  High mobility usually describes speed ranges from about 56 to 176 'm8h. Cellphones operated by people in moving cars are one typical e+ample.    Extremely high mobility is represented by highspeed trains and planes, which cover speeds between 566 and 1666 'm8h. These speeds pose unique challenges both for the design of the physical layer oppler shift, see Chapter 7! and for the handover between cells. .W(a# i* ene 0 )&n*'p#i&n! 9nergy consumption is a critical aspect for wireless devices. :ost wireless devices use oneway or rechargeable! batteries, as they should be free of any wires 3 both the ones used for communicationand the ones providing the power supply. .Define ISI! The runtimes for different :(Cs are different. #e have already mentioned above that this can leadto different phases of :(Cs, which lead to interference in narrowband systems. ;n a system withlarge bandwidth, and thus good resolution in the time domain,5 the ma0or consequence is signaldispersion2 in other words, the impulse response of the channel is not a single delta pulse but rather a sequence of pulses corresponding to different :(Cs!, each of which has a distinct arrival timein addition to having a different amplitude and phase . 6.W i#e a&# #(e fa$in +i#( i#* #0pe*! <apid fluctuations of the amplitude, phase as multipath delays of a radio signal over ashort period of time is called small scale fading. 3.W(a# i* fi4e$ +i e/e** a))e**! *i+ed wireless access systems can also be considered as a derivative of cordless phones or #$%&s,essentially replacing a dedicated cable connection between the user and the public landline system.The main difference from a cordless system is that i! there is no mobility of the user devicesand ii! the 4) almost always serves multiple users. *urthermore, the distances bridged by fi+edwireless access devices are much larger between 166m and several tens of 'ilometers! than those bridged by cordless telephones 5.W i#e #(e $efini#i&n f& f een)0 e*e! ;f an area is served by a single 4ase )tation, then the available spectrum can be dividedinto & frequency channels that can serve & users simultaneously. ;f more than & users are to be served, multiple 4)s are required, and frequency channels have to be reused in differentlocations. )ince spectrum is limited, the same spectrum has to be used for different wirelessconnections in different locations. This method of reusing the frequency is called as frequency reuse.  7.W i#e #(e $iffe en# #0pe* &f *e 8i)e*! i! Telephone servicesii! 4earer or =ata servicesiii! )upplementary services 19.Define '/#ipa#( p &paa#i&n! *or wireless communications, the transmission medium is the radio channel between transmitter T> and receiver <>. The signal can get from the T> to the <> via a number of different propagation paths. ;n some cases, a $ine ?f )ight $?)! connection might e+ist between T> and <>.*urthermore, the signal can get from the T> to the <> by being reflected at or diffracted by different  Interacting Objects ;?s! in the environment2 houses, mountains for outdoor environments!,windows, walls, etc. 11.W i#e *&'e /i'i#a#i&n &f *pe)# '! The spectrum available for wireless communications services is limited, and regulated by internationalagreements. *or this reason, the spectrum has to be used in a highly efficient manner. Twoapproaches are used2 regulated spectrum usage, where a single networ' operator has control over theusage of the spectrum, and unregulated spectrum, where each user can transmit without additionalcontrol, as long as s!he complies with certain restrictions on the emission power and bandwidth. ;nthe following, 12.Diffe en#ia#e TDMA an$ DMA! *=:%  the total bandwidth is divided into nonoverlapping frequency subbands.T=:% 3 divides the radio spectrum into time slots and in each slot only one user isallowed to either transmit or receive.C=:% 3 many users share the same frequency same tome with different coding. 1%.C&'pa e )e///a an$ )& $/e** #e/ep(&n0! Cellular telephony is the economically most important form of wireless communications. ;t isCharacterized by the following properties21. The information flow is bidirectional. % user can transmit and receive information at the)ame time. =ue to this reason, this boo' often draws its e+amples from cellular telephony, eventhough the general principles are applicable to other wireless systems as well. Chapters @3 A give adetailed description of the most popular cellular systems 1! #or$less %ele&hony  Cordless telephony describes a wireless lin' between a handset and a 4) that is directly connectedto the public telephone system. The main difference from a cellphone is that the cordless telephoneis associated with, and can communicate with, only a single 4) .There is thus no mobile switching center  - rather, the 4) is directly connected to the ()T&. 1.W(a# i* in#e fe en)e /i'i#e$ *0*#e'*! #ireless systems are required to provide a certain minimum transmission quality This transmission quality in turn requires a minimum 'ignal(to(Noise )atio )&<! at the receiver <>!. Consider now a situation where only a single 4) transmits, and a :obile )tation :)! receives- thus, the performance of the system is determined only by the strength of the useful! signal and the noise. %s the :) moves further away from the 4), the received signal power decreases, and at a certain distance, the )&< doesnot achieve the required threshold for reliable communications. Therefore, the range of the system is noise limited- equivalently, we can call it  signal &ower limite$ . =epending on the interpretation, it is too much noise or too little signal power that leads to bad lin' quality 1.Define # n,in a$i&! Trun'ing radio systems are an important variant of cellular phones, where there is no connection between the wireless system and the ()T&- therefore, it allows the communications of closeduser groups. ?bvious applications include police departments, fire departments, ta+is, and similar services. The closed user group allows implementation of several technical innovations that are not possible or more difficult! in normal cellular systems2 Diffe en#ia#e ;AN an$ PAN!  Bo$y Area Networks 4%&s! cover the communication between different devices attached to one body 3 e.g., from a cellphone in a hip holster to a headset attached to the ear. The range is thuson the order f 1m. 4%&s are often subsumed into (%&s.  Personal Area Networks include networ's that achieve distances of up to or about 16 m, coveringthe “personal space” of one user. 9+amples are networ's lin'ing components of computers andhome entertainment systems. =ue to the small range, the number of devices within a (%& issmall, and all are associated with a single “owner.” %lso, the number of overlapping (%&s i.e.,sharing the same space or room! is small 3 usually less than five. That ma'es cell planning andmultiple access much simpler.
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