System Design in Wireless Communication. Ali Khawaja

Transcription

1 System Design in Wireless Communication Ali Khawaja University of Texas at Dallas December 6,

2 Abstract This paper deals with the micro and macro aspects of a wireless system design. With the growing popularity of wireless devices, there is a greater need to ensure not only the reliability of the system, but also the quality of service given to the customers. The purpose of this project is to describe the main objectives involved in a cellular system design and the techniques currently used to entertain the ever growing community of wireless users with extreme emphasis on speech quality, system reliability, quality of service and cost of service. 2

3 Description The purpose of this project is to design a wireless communication system. The emphasis of the project is on the technological aspects and the techniques involved in a system design to ensure reliability, quality of service (QOS), efficiency and economy. Young 1 cited the basic objectives for any radio or cellular system as follows: Growth capability Large subscriber capacity Efficient use of spectrum "Telephone" quality of service The DCS 1900 (Digital Cellular System 1900) standard is used in this design. DCS 1900 is a GSM (Global System for Mobile); a European digital standard that was adapted as PCS (Personal Communication System), standard in the U.S in 1994 for commercial implementations. DCS 1900 uses more efficient and advance TDMA access standard and accommodates for 8 users as an access scheme per radio channel that achieves huge bandwidth efficiency. This advantage becomes more significant as the demand for mobile communication is increasing almost exponentially. On the same notion, incorporation of digital cellular system with the more general multimedia standard of Integrated Service Digital Network System (ISDN) is another advance futuristic aspect of the PCS system. Specification Following specification will be used to design the cellular system: PCS Band: 30 MHz Coverage Area: Dallas (~2500 a) 3

4 Reverse Link: MHz Forward Link: MHz Users per 200 khz radio channel: 8 Radio Channels / Base Stations: 64 The main task in a wireless system design is to find the minimum, but adequate number of cells required, reason being that it eventually comes down to the cell level to insure service reliability, efficient frequency reuse for un-congested network and QOS. Before going in to the design process, two most important cellular concepts, Frequency Reuse and Cell Splitting, are described blow: Frequency Reuse: Frequency reuse refers to the use of radio channels on the same carrier frequency to cover different areas that are separated from one another by sufficient distances so that cochannel interference is not B objectionable. Instead of covering A F an entire local area from one land C H transmitter site with high power at a high elevation, the service provider D E G I J can distribute transmitters of moderate power throughout the coverage area. Each site then cover a nearby sub area, referred to, as cell. Base stations in adjacent cells are assigned channel groups that contain completely different channels than neighboring cells. By limiting the coverage area to within the boundaries of a cell, the same group of channels may be used to cover different cells that are separated from on another by distances large enough to keep interference levels within tolerable limits 2. 4

5 Cells labeled with different letters in the above figure must be served by distinct sets of channel frequencies to avoid interference problems. Cells far apart, as A and H in the above figure, may use same set of channels. Through frequency reuse, a cellular system in one coverage area can handle simultaneous calls greatly exceeding the total number of channel frequencies. Co-channel reuse ratio (D/R): Co-channel reuse ratio can be set large or small depending on the design requirements. Increasing the radius of the cell decreases D/R and is beneficial if low cost and large capacity is to be achieved. On the other hand, making D/R as small as possible increases the transmission quality. Cell Splitting: If the cellular system contains N cells with total allocation of C channels, then each cell will contain S = C/N channels. To grow further from the capacity S, the cell boundaries has to be revised so that each cell now contains several cells. This process is called cell splitting. In the figure below, after reaching its capacity, 4 smaller cells were superimposed on cell C. Therefore cell splitting is not necessarily done for all cells B in the system. Further cell splitting increases the number of voice paths, or the number of A D C2 C1 C4 C3 F G H J simultaneous conversations E I possible within the same region. Cell splitting decreases the cell area, and thus allows the system to meet the growing cellular demand. 5

6 Design Process: Calculations & Results Maximum Number of radio channels: Based on the spectrum allocated (30 MHz), and the bandwidth of the radio channel (200 khz), maximum number of radio channels, a, that can be used in the system, can be calculated as follows: RCT 6 3*10 200*10 = 3 = 150 Radio Channels Maximum User Support per Base Station: With 64 users per base station, and 8 users per radio channel, the maximum number of users supported per base station are: U = 64 *8 = 512 Users/Base Station BS Path Loss Model for Urban Areas: The purpose of a path loss model is to study the propagation characteristics for evaluation of the quality of service of the cellular or any radio system. Hata model was derived from Okumura s report to use his propagation predictions effectively in a wireless system. The propagation loss is given by A+B log 10 R where A and B are antenna height function and R is the distance 3. The formula presented was standard for the urban area signal propagation and corrections equations were supplied for application to other situations. The standard formula for median path loss in urban area is given by L = log10 fc 13.82*log10 hb a( hm) * + ( * log 10hb ) log P 10 * Correction factor for a(h m ), vehicular station antenna height: small to medium sized city a(h m ) = (1.1 log(a) - 0.7)a - (1.56 log(a) -0.8) db large city a(h m ) = 8.29 ( log a ) db a300 MHz R 6

7 a(h m ) = 3.20 ( log a ) db a300 MHz f c = Frequency in MHz from 150 to 1500 h b = Base station effective antenna height ( m) h m = Vehicular station antenna height (1 to 10 m) R = Distance (km) Number of Cells required for coverage of 2500 km 2 area: Cell Requirement: A 90 dbm signal level is assumed to be required for 90% coverage of each cell. Mobile has a 3 dbi gain antenna whereas the base station uses 10 db gain omni-directional antenna. σ is assumed to be 8 db and path loss exponent n equals 4. n indicates the rate at which the path loss increases with distance d. Total Number of Cells: N C = * R 2 Where 2500 km 2 is the total coverage area and area of the cell (hexagon) is given by *R 2. The radius R of the cell can be found using the following expression: 1 1 γ [ P + + t ( PL( d0 ) 10n log( R / d0) 10n log( r / R))] Pr[Pr( r) > γ ] = erf 2 2 σ 2 Where Pr[Pr{ r ) > γ ] = 0. 9, P t = 20W = 13 db, γ = -90 dbm = -69 db, r = R. The value obtained for R using the above expression is approximately km. Therefore, total number of cells sites for 2500 km 2 area is: N C = 2500 = 320 cells * R 7

8 Based on the radius, the co-channel interference becomes minimal with the cochannel separation distance of about 3.4 times the radius of the cell that is calculated from the expression D R = 3N with a 4-cell cluster, i.e., N=4. Percentage of Coverage Area With the given path loss specifications, its very beneficial to compute the percentage of area with signal level greater than the threshold level, given a known likelihood of coverage at the cell boundary 4. The expression used for this purpose is: 1 U γ ) = 1 erf 2 1 2ab ( 2 1 ab ( a) + exp 1 erf b b Where γ P t + PL( d0) + 10n log( R / d0 ) a = and σ 2 10nlog e b = σ 2 Following set of curves show the percentage of useful coverage area, U(γ), where γ is a threshold level of the received signal plotted against σ/n. If σ/n = 4, then with 75% boundary coverage provides 93% area coverage; with σ/n = 6, area coverage is 82% with 65% boundary coverage, etc. 8

9 Grade of Service (GOS) In calculating the total number of user for 5% blocking probability, the number of user per cell is found then multiplied by the total number of cells in the system. This is a valid extrapolation because the graph of Erlang B 4 becomes very much linear with respect to channel number for values over 100 channels. The total traffic intensity A = 32.6 for 38 radio channels per base stations was found from the Erlang B table. Also the average and general traffic intensity per user A u of 0.1 Erlang is assumed. The total user for the system with the GOS of 0.05 is calculated to be 104,300. 9

10 Bibliography 1. Young, W. R., "AMPS: Introduction, Background, and Objectives." B.S.T.J., The Bell Systems Technical Journal, Vol. 58, No. 1, January MacDonald, V.H,. "The Cellular Concept," The Bell Systems Technical Journal, Vol. 58, No. 1, pp , January Hata, Masaharu, "Empirical Formulation for Propagation Loss in Land Mobile Radio Service," IEEE Transactions on Vehicular Technology," Vol. VT-29, No. 3, pp , August Rappaport, Theodore, S., Wireless Communication: Principles and Practice, Prentice Hall, ISGN , July,