Chapter 1 Introduction

Transcription

1 Chapter 1 Introduction 1.1 Definition of mobile radio communications and examples Definition: Mobile communication means that the sender and/or receiver are not at a fixed location. The obvious means to enable mobile communications is wireless (radio) communications. Radio communications allow the highest possible degree of mobility. Examples: Sound and television broadcasting wireless LAN wireless local loop trunked mobile radio system cordless telephone cellular mobile radio system Fundamentals of mobile radio communications

2 1.2 Basic architectures and requirements Basic architectures and requirements Direct and indirect transmission m(t) S E ^m(t) direkte Übertragung m(t) S Zwischenmedium E ^m(t) indirekte Übertragung m(t) S Zwischenmedium E ^m(t) indirekte Übertragung S(ender) transmitter E(mpfänger) direkte Übertragung indirekte Übertragung Zwischenmedium receiver direct transmission indirect transmission intermediate medium Fig Direct and indirect transmission

3 1.2 Basic architectures and requirements Simplex and duplex transmission m(t) S E ^m(t) Simplexübertragung 1. Endgerät 2. Endgerät 1 m 1 (t) S 1 E 2 2 E 1 S 2 ^m 1 (t) Duplexübertragung (FDD, TDD) ^m 2 (t) 1. Endgerät 2. Endgerät m 2 (t) Endgerät FDD TDD terminal frequency division duplex time division duplex Fig Simplex and duplex transmission

4 1.2 Basic architectures and requirements Addressing E 1 ^m(t) m(t) S E 2 ^m(t) keine selektive Adressierung (Rundfunk) ^m(t) E 3 keine selektive Adressierung Rundfunk no selective addressing broadcast transmission Fig Non-selective addressing

5 1.2 Basic architectures and requirements 5 E 1 ^m 1 (t) Mobilstation m 1 (t) m 2 (t) m 3 (t) S Basisstation E 2 Mobilstation ^m 2 (t) selektive Adressierung (Downlink, Mobilfunk) E 3 ^m 3 (t) Mobilstation Mobilfunk mobile radio Fig Selective addressing

6 1.2 Basic architectures and requirements Multiple access m 1 (t) S 1 E 3 ^m 3 (t) m 2 (t) S 2 E 2 ^m 2 (t) (Mobilfunk) m 3 (t) S 3 E 1 ^m 1 (t) Vielfachzugriff multiple access Fig Multiple access

7 1.3 Cellular concept Cellular concept Frequency band reuse B: total available bandwidth for mobile communications system B u : bandwidth required by each user Number of supportable users, if frequency bands are not reused: [ ] B K = B (1.1) B u [x] w := largest whole number contained in x, see Fig spektrale Leistungsdichte B u w B 0 B u K f 0 B 2 f 0 + B 2 Teilnehmerfrequenzband f spektrale Leistungsdichte Teilnehmerfrequenzband Einteilung Gesamtübertragungsbandbreite Bandbreite spectral power density user frequency band subdivision total transmission bandwidth bandwidth Fig Subdivision of total transmission bandwidth B into K user frequency bands of width B u

8 1.3 Cellular concept 8 The number of supportable users can be increased, if the frequency band B is reused. To this purpose the whole service area is divided into cells, which ideally are hexagonal (hexagonal cell net). Fig. 1.7 shows such a cell with a base station (BS) in the center and four mobile stations (MS) distributed over the cell area. The MSs cannot directly communicate with each other and with MSs in other cells. Therefore, the transmission is indirect. MS 4 MS 1 MS 3 BS MS 2 MS BS sechseckig Einzelzelle mobile station base station hexagonal single cell Fig Hexagonal single cell Fig. 1.8 shows a hexagonal cell net. With the cell radius ρ 0 the cell area becomes A = ρ2 0 =2.598ρ 2 0. (1.2)

9 1.3 Cellular concept 9 ρ 0 Cell with the cell radius ρ 0 Fig Section of a cell net Number of supportable users per unit area: If in each cell the total frequency band B is utilized, the number of supportable users per unit area becomes K A = B B u A = For a given value B, K A can be increased by B B u ρ (1.3) 1. small ρ 0, i.e. small cells. For this reason there is a trend to make the cells smaller and smaller (from macro cells to micro cells and pico cells). However, there is a trade off between investment costs for new base stations and an increase of K A. 2. small required B u. This can be achieved by modern source encoding algorithms which reduce the required data rate without a perceptible impact on transmission quality (e.g. speech quality).

10 1.3 Cellular concept Clustering If the total frequency band B is reused in each cell, connections which use the same frequencies are geographically rather close together and the mutual disturbances of connections using the same frequency band are rather high. This problem can be solved by using only a part of the total frequency band B in each cell. Clustering: the total available frequency band of width B is subdivided into r K partial bands of width B u = B r K, (1.4) the r K partial bands are arranged in r subgroups with K partial bands each, the r different subgroups are evenly distributed over the cell net. The bandwidth available in each cell is B/r and is termed cell bandwidth. Fig. 1.9 shows an example with r K = 6 partial frequency bands, r = 3 subgroups. The r subgroups form a cluster. Therefore, r is termed the cluster size. Fundamentals of mobile radio communications

11 1.3 Cellular concept 11 spectral power density B B u f r = ; ; ; 6 5; ; ; 4 3; ; 2 1; ; ; ; ; 4 3; 4 3; 4 Cluster with r = 3 cells Usage of the ::: ; ::: user frequency bands 1 and 2 3; 4 ::: user frequency bands 3 and ; ::: user frequency bands 5 and 6 Fig Reuse-pattern for r equal 3 Fundamentals of mobile radio communications

12 1.3 Cellular concept 12 The subgroups are reused in every r-th cell, see Fig A uniform or even distribution of the r clusters over the cell net is only possible for certain values of r which are given by the rhombic numbers r = i 2 + j 2 + ij, i, j N 0,i+ j 0. (1.5) In such a uniform distribution, each of the connections is disturbed to the same degree by connections in other cells using the same frequency bands. Usual cluster sizes are 1, 3, 4, 7, 9, 12,.... If clustering is applied when performing frequeny reuse, the number of supportable users per unit area becomes K A = B B u A r = B B u ρ 2 0 r (1.6) In order to obtain a large K A, r should be chosen as small as possible. The lower limit of r is set by mutual disturbances of connections using the same frequency band.

13 1.3 Cellular concept Cell sizes and formats Cell sizes: cell type cell radius ρ 0 /km hyper cell sparsely populated, rural > 20 macro cell densely populated, sub-urban micro cell cities pico cell office, home, pedestrian < 0.1 Formats: Real world cells deviate from the ideal hexagonal form depending on the topography and morphology of the service area. Overlay cells: Hot spots within large cells can be serviced by local small cells. large cell BS l small cell in hot spot area fast moving users in hot spot area are supported by BS l slowly moving users are supported by BS s BS s Fig Overlay cells Handover: As shown in Section 1.3.2, the number of supportable users can be increased by choosing smaller cells. However, in the case of smaller cells the mobile user more frequently changes the BS to which he is assigned. Such a changing procedure is termed handover. It causes additional signaling traffic and thus reduces the capacity. Necessary balance: Going to smaller cells increases capacity, but also the number of required handovers. Fundamentals of mobile radio communications

14 1.4 Generations of cellular systems Generations of cellular systems First generation Technology: Analog. Examples: C-Netz (Germany), 450 MHz, NMT (Nordic Mobile Telephone; 900 MHz Scandinavia, Switzerland), 450 MHz and TACS (Total Access Communications System, UK), 900 MHz, AMPS (Advanced Mobile Phone System, USA), 900 MHz, NTTM-H (MCS-L2, Mobile Communications-Land 2 nd 900 MHz. Generation, Japan), Second generation Technology: Digital. Applications: Mainly voice, low-rate data ( 10 kbit/s). Introduction: Started in the early 1990ies. Examples: GSM(Global System for Mobile Communications; European origin; 150 countries, 150 Mio customers; Germany: D1 and D2 (900 MHz), e-plus (1800 MHz), PDC (Personal Digital Cellular, Japan, 900 MHz and 1500 MHz), IS-95 (Qualcomm; USA, Korea, China); Uplink 925 MHz 960 MHz, Downlink 880 MHz 915 MHz. Fundamentals of mobile radio communications

15 1.5 Operational issues Third generation Applications: Voice and data up to 2 Mbit/s; new types of services (multi-media); worldwide roaming. Introduction: From 2001 on. Examples: Europe: UMTS (Universal Mobile Telecommunications System) standardized by ETSI (European Telecommunications Standards Institute). The air interface of UMTS has the acronym UTRA (UMTS Terrestrial Radio Access). It consists of the two modes TD-CDMA for the TDD-bands and W-CDMA for the FDD-bands. World: IMT-2000 (International Mobile Telecommunications) to be standardized by ITU (International Telecommunications Union). 1.5 Operational issues Noise limited and interference limited systems The performance of mobile radio systems is hampered by noise (thermal, atmospheric, man made...), interference, i.e. signals from other users of the same mobile radio system. Usually, cellular mobile radio systems are interference limited due to sufficiently large transmit powers. However, for MSs operating at the cell border, that is far from their BS, noise limitation may be the case. Fundamentals of mobile radio communications

16 1.5 Operational issues 16 system level interference link level system load QoS Fig Relation between system load, interference and QoS System load versus Quality of Service Each user expects a certain Quality of Service (QoS). QoS is defined by the probability that a user gets the desired connection, and that the speech quality or error probability (data transmission) fulfil given requirements. Interference increases with increasing system load, i.e. number of supported users. QoS decreases with increasing interference, see Fig Therefore, QoS decreases with increasing system load as shown in Fig. 1.12: QoS system load Fig QoS versus system load Fundamentals of mobile radio communications

17 1.5 Operational issues 17 The operator of the system has to find an adequate compromise between system load and QoS. There are many different options with soft transitions between them. Wired systems versus radio systems: In wired systems blocking occurs, when all resources are consumed. Such systems are hardware limited Spectrum efficiency and spectrum capacity Spectrum efficiency: K: NumberofuserspercellwhichcanbesupportedwithagivenQoS, R: Data rate per user, B: available total bandwidth, B u : bandwidth assigned to each user, r: cluster size. η = K R B = K R B u K r = R (spectrum efficiency), (1.7) B u r bit [η] = s Hz cell. (1.8) Spectrum capacity: η c = K B = K B u r K = 1 B u r, (1.9) no. of users per cell [η c ]=. (1.10) Hz η c depends on the type of service. It also depends on the way in which the mobile radio network is organized. Coming and going of users is a dynamic process. For instance, η c is high, if a resource released by a certain user can be immediately re-occupied by another user.