COMPUCOM INSTITUTE OF TECHNOLOGY & MANAGEMENT, JAIPUR (DEPARTMENT OF ELECTRONICS & COMMUNICATION) Notes Wieless Communication (Subject Code: 7EC3) Pepaed By: LOKESH KUMAR ARYA Class: B. Tech. IV Yea, VII Semeste
Syllabus UNIT : LINE OF SIGHT MICOWAVE COMMUNICATION- Link Engineeing, Fequency planning, Fee space loss, Fesnel zone cleaance bending of adio beam, Effective eath adius, Building blocks of Tansmitte & Receive. Beyond the Syllabus Fequency Reuse Leaning Objectives This unit gives the detailed knowledge about diffeent paametes of data links in wieless communication. Pepaed By: LOKESH KUMAR ARYA Page
Unit.1 Link Engineeing: A link in a communication system can be defined as connecting two points sepaated at a distance, whee one point may act as tansmitte and anothe point act as a eceive. Implementation of data link is an integal pat of communication engineeing design and pefomance of data links significantly effect the oveall communication system pefomance. Boadly data links ae divided in fou types: i. Wie line link ii. Radio wave link iii. Micowave link Radio link systems opeate in the MHz to GHz ange (micowaves). A micowave system consists of a numbe of gound base stations. Tansmitting and eceiving antennas must be in diect line of sight of each othe. Radio link systems wee intoduced as an altenative to coaxial cable on long haul outes. They ae also used fo links to islands and difficult ual situations. Advantages of adio link systems include: high bandwidth low level of signal attenuation can be used ove ough teain which would be unsuitable fo cabled media Disadvantages of adio link systems include: expensive ove shot distances thee can be no obstacles between the tansmitting and eceiving antennas can suffe fom intefeence due to climatic conditions and othe micowave souces Digital Point-to-Point Micowave Link: The tem digital communications coves a boad aea of communications techniques, including digital tansmission and digital adio. Digital tansmission is the tansmittal of digital pulses between two points in a communications system. Digital adio is the tansmittal of digitally modulated analog caies between two points in a communications system. Digital tansmission systems equie physical facility between the tansmitte and eceive, such as a metallic wie pai, a coaxial cable o a fibe optic cable. In digital adio systems, the tansmission medium is fee space o the eath's atmosphee. Figue.1 shows simplified block diagams of digital tansmission point-to-point micowave system. In a digital tansmission system, the oiginal souce infomation may be in digital Pepaed By: LOKESH KUMAR ARYA Page 3
o analog fom. If it is in analog fom, it must be conveted to digital pulses pio to tansmission and conveted back to analog fomat the eceive end. In a digital adio system, the modulating input signal and the demodulated output signal ae digital pulses. The digital pulses could oiginate fom a digital tansmission system, fom a digital souce such as a mainfame compute o fom the binay encoding of an analog signal. Tansmission Lines A tansmission line is a device that tansfes enegy (infomation) fom one point to anothe with minimum amount of loss. Infomation can take the fom of voice, video and data signals. In othe wods, the tansmission line must be efficient. Efficiency is the eal key to a tansmission. Tansmission media can be classified as eithe: Cabled twisted pai coaxial cable fibe optic cable Non-cabled cellula adio systems adio link systems satellite system. Fequency planning: Pepaed By: LOKESH KUMAR ARYA Page 4
Electomagnetic Spectum fo Telecommunications Electomagnetic Spectum Fo wieless communication, antenna is needed. In tansmission, antenna adiates electomagnetic enegy in space and in eception, antenna picks up EM waves fom suounding medium. In geneal, thee ae thee majo anges of fequencies which ae used fo wieless communication: i) Micowaves ii) Radio Waves iii) Infaed waves When electons move, they ceate electomagnetic waves that can popagate though fee space even in a vacuum. By attaching an antenna of the appopiate size to an electical cicuit, the electomagnetic waves can be boadcast efficiently and eceived by a eceive some distance away. All wieless communication is based on this pinciple. The electomagnetic spectum is shown in the following figue. The adio, micowave, infaed and visible light potion of the spectum can all be used fo tansmitting infomation by modulating the amplitude, fequency, o phase of the wave. Ultaviolet light, X- Ray and gamma ays would be even bette, due to thei highe fequencies but they ae had to poduced and modulate, do not popagate well though buildings and ae dangeous to living things. The bands listed below at the bottom of electomagnetic spectum ae the official ITU names and based on the wave lengths. So the LF band goes fom 1km to 10 km (appoximately 30 KHz to 300 KHz). The tems LF, MF Pepaed By: LOKESH KUMAR ARYA Page 5
and HF efe to low, medium and high fequency espectively. The amount of infomation that an electomagnetic wave can cay is elated to its bandwidth. Fequency band fo communication Table: Fequency bands: S. No. Fequency Band Fequency Range 1 Extemely Low Fequency (ELF) 0 to 3 KHz Vey Low Fequency (VLF) 3 KHz to 30 KHz 3 Low Fequency (LF) 30 KHz to 300 KHz 4 Medium Fequency (MF) 300 KHz to 3000 KHz 5 High Fequency (HF) 3 MHz to 30 MHz 6 Vey High Fequency (VHF) 30 MHz to 300 MHz 7 Ulta-High Fequency (UHF) 300 MHz to 3000 MHz 8 Supe high Fequencies (SHF) 3 GHz to 30.0 GHz (Micowave) 9 C-band 3600 MHz to 705 MHz 10 X-band: 7.5 GHz to 8.4 GHz 11 Ku-band 10.7 GHz to 14.5 GHz 1 Ka-band 17.3 GHz to 31.0 GHz 13 Extemely High Fequencies (EHF) 30.0 GHz to 300 GHz (Millimete Wave Signals) 14 Infaed Radiation 300 GHz to 430 THz 15 Visible Light 430 THz to 750 THz 16 Ultaviolet Radiation 1.6 PHz to 30 PHz 17 X-Rays 30 PHz to 30 EHz 18 Gamma Rays 30 EHz to 3000 EHz.3 Fee space loss: In telecommunication, fee-space path loss (FSPL) is the loss in signal stength of an electomagnetic wave that would esult fom a line-of-sight path though fee space (usually ai), with no obstacles neaby to cause eflection o diffaction. It does not include factos such as the gain of the antennas used at the tansmitte and eceive, no any loss associated with hadwae impefections. A discussion of these losses may be found in the aticle on link budget. Pepaed By: LOKESH KUMAR ARYA Page 6
Fee-space path loss fomula Fee-space path loss is popotional to the squae of the distance between the tansmitte and eceive, and also popotional to the squae of the fequency of the adio signal. Fo any type of wieless communication the signal dispeses with distance. Theefoe, an antenna with a fixed aea will eceive less signal powe the fathe it is fom the tansmitting antenna. Fo satellitee communication this is the pimay mode of signal loss. Even if no othe souces of attenuation o impaiment ae assumed, a tansmitted signal attenuates ove distance because the signal is being spead ove a lage and lage aea. This fom of attenuation is known as fee space loss, which can be expess in tems of the atio of the adiatedd powe to the powe eceived by the antenna o, in decibels, by taking 10 times the log of that atio. Fo the ideal isotopic antenna, fee space loss is The equation fo FSPL is whee: P P t = ) ( ) (44 pd 4pfd = l c is the signal wavelength (in metes), is the signal fequency (in hetz), is the distance fom the tansmitte (in metes), is the speed of light in a vacuum,.9979458 10 8 metes pe second. This equation is only accuate in the fa field whee spheical speading can be assumed; it does not hold close to the tansmitte. Pepaed By: LOKESH KUMAR ARYA Page 7
Fee-space path loss in decibels A convenient way to expess FSPL is in tems of db: P L db = 10log P æ = 0logç è t 4 pd l ö ø Fo othe antennas, we must take into account the gain of the antenna, which yields the following fee space loss equation: ( ) ( d) ( d) ( cd) Pt 4 = p P G Gl G t = gain of tansmitting antenna G = gain of eceiving antenna A t = effective aea of tansmitting antenna A = effective aea of eceiving antenna t = l A A t = f A A t The thid faction is deived fom the second faction using the elationship between antenna gain and effective aea defined in Equation. We can ecast the losss equation as L db =0 log(λ)+ 0 log(d)-10 log(a t A ) =-0 log(f)+ 0 log(d)-10 log(a t A )+169.54 db Thus, fo the same antenna dimensions and sepaation, the longe the caie wavelength (lowe the caie fequency), the highe is the fee space path loss. It is inteesting to compae Equations. Equation indicates that as the fequency inceases, the fee space loss also inceases, which would suggest that at highe fequencies, losses become moe budensome. Howeve, Equation shows that we can easily compensate fo this inceased loss with antenna gains. In fact, thee is a net gain at highe fequencies, othe factos emaining constant. Equation shows that at a fixed distance an incease in fequency esults in an inceased loss measued by 0log(f). Howeve, if we take into account antenna gain, and fix antenna aea, then the change in loss is measued by -0log(f) that is, thee is actually a decease in loss at highe fequencies. Pepaed By: LOKESH KUMAR ARYA Page 8
.4 Fesnel Zone: If unobstucted, adio waves will tavel in a staight line fom the tansmitte to the eceive. But if thee ae obstacles nea the path, the adio waves eflecting off those objects may aive out of phase with the signals that tavel diectly and educe the powe of the eceived signal. On the othe hand, the eflection can enhance the powe of the eceived signal if the eflection and the diect signals aive in phase. Sometimes this esults in the counteintuitive finding that educing the height of an antenna inceases the S+N/N atio. Fesnel povided a means to calculate whee the zones ae whee obstacles will cause mostly in phase and mostly out of phase eflections between the tansmitte and the eceive. Obstacles in the fist Fesnel will ceate signals that will be 0 to 90 degees out of phase, in the second zone they will be 90 to 70 degees out of phase, in thid zone, they will be 70 to 450 degees out of phase and so on. Odd numbeed zones ae constuctive and even numbeed zones ae destuctive. The concept of Fesnel zone cleaance may be used to analyze intefeence by obstacles nea the path of a adio beam. The fist zone must be kept lagely fee fom obstuctions to avoid intefeing with the adio eception. Howeve, some obstuction of the Fesnel zones can often be toleated, as a ule of thumb the maximum obstuction allowable is 40%, but the ecommended obstuction is 0% o less. Fo establishing Fesnel zones, fist detemine the RF Line of Sight (RF LoS), which in simple tems is a staight line between the tansmitting and eceiving antennas. Now the zone suounding the RF LoS is said to be the Fesnel zone. The geneal equation fo calculating the Fesnel zone adius at any point P in the middle of the link is the following: whee, F n = The nth Fesnel Zone adius in metes d 1 = The distance of P fom one end in metes d = The distance of P fom the othe end in metes λ = The wavelength of the tansmitted signal in metes Pepaed By: LOKESH KUMAR ARYA Page 9
Fequency Reuse Fequency euse is a technique of eusing fequencies and channels within a communications system to impove capacity and spectal efficiency. Fequency euse is one of the fundamental concepts on which commecial wieless systems ae based that involves the patitioning of an RF adiating aea (cell) into segments of a cell. One segment of the cell uses a fequency that is fa enough away fom the fequency in the bodeing segment that it does not povide intefeence poblems. Fequency e-use in mobile cellula systems means that each cell has a fequency that is fa enough away fom the fequency in the bodeing cell that it does not povide intefeence poblems. The same fequency is used at least two cells apat fom each othe. This pactice enables cellula povides to have many times moe customes fo a given site license. The key chaacteistic of a cellula netwok is the ability to e-use fequencies to incease both coveage and capacity. As descibed above, adjacent cells must use diffeent fequencies, howeve thee is no poblem with two cells sufficiently fa apat opeating on the same fequency. The elements that detemine fequency euse ae the euse distance and the euse facto. The euse distance, D is calculated as whee R is the cell adius and N is the numbe of cells pe cluste. Cells may vay in adius in the anges (1 km to 30 km). The boundaies of the cells can also ovelap between adjacent cells and lage cells can be divided into smalle cells. The fequency euse facto is the ate at which the same fequency can be used in the netwok. It is 1/K (o K accoding to some books) whee K is the numbe of cells which cannot use the same fequencies fo tansmission. Pepaed By: LOKESH KUMAR ARYA Page 10
Common values fo the fequency euse facto ae 1/3, 1/4, 1/7, 1/9 and 1/1 (o 3, 4, 7, 9 and 1 depending on notation). In case of N secto antennas on the same base station site, each with diffeent diection, the base station site can seve N diffeent sectos. N is typically 3. A euse patten of N/K denotes a futhe division in fequency among N secto antennas pe site. Some cuent and histoical euse pattens ae 3/7 (Noth Ameican AMPS), 6/4 (Motoola NAMPS), and 3/4 (GSM). If the total available bandwidth is B, each cell can only use a numbe of fequency channels coesponding to a bandwidth of B/K, and each secto can use a bandwidth of B/NK. Code division multiple access-based systems use a wide fequency band to achieve the same ate of tansmission as FDMA, but this is compensated fo by the ability to use a fequency euse facto of 1, fo example using a euse patten of 1/1. In othe wods, adjacent base station sites use the same fequencies, and the diffeent base stations and uses ae sepaated by codes athe than fequencies. While N is shown as 1 in this example, that does not mean the CDMA cell has only one secto, but athe that the entie cell bandwidth is also available to each secto individually. Depending on the size of the city, a taxi system may not have any fequency-euse in its own city, but cetainly in othe neaby cities, the same fequency can be used. In a big city, on the othe hand, fequency-euse could cetainly be in use. Recently also othogonal fequency-division multiple access based systems such as LTE ae being deployed with a fequency euse of 1. Since such systems do not spead the signal acoss the fequency band, inte-cell adio esouce management is impotant to coodinate esouce allocation between diffeent cell sites and to limit the inte-cell intefeence. Thee ae vaious means of Inte-cell Intefeence Coodination (ICIC) aleady defined in the standad. Coodinated scheduling, multi-site MIMO o multi-site beam foming is othe examples fo inte-cell adio esouce management that might be standadized in the futue. Pepaed By: LOKESH KUMAR ARYA Page 11