Introduction Part I Mobile Radio—The First 100 Years By definition, the term “mobile-radio communications” describes any radio communication link between two terminals of which one or both are in motion or halted at unspecified locations and of which one may actually be a fixed terminal such as a base station. This definition applies to both mobile-to-mobile and mobile-to-fixed radio communica- tion links. The mobile-to-mobile link could in fact consist of a mobile- to-fixed-to-mobile radio communication link. The term “mobile” applies to land vehicles, ships at sea, aircraft, and communications satellites. In tactical situations, mobile-radio systems may include any or all of these types of mobile terminals. Mobile-radio systems are classified as radiophones, dispatching sys- tems, radio paging systems, packet radios, or radiotelephones (also known as mobile phones), including train phones. 1. Radiophones (or walkie-talkies) are two-way radios, such as CB (cit- izens band) radios, which are allocated 40 channels for anyone to use whenever the channels are free. This system affords no privacy to the user. 2. Dispatching systems use a common channel. Any vehicle driver can hear the operator’s messages to other drivers in the same fleet. The drivers can talk only to the control operator. In military applica- tions, the users can also talk to each other on an open channel. 3. Radio paging customers carry personal receivers (portable radios). Each unit reacts only to signals addressed to it by an operator. A beep sounds to alert the bearer, who then must go to a nearby tele- phone to receive the message. 1 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Source: Mobile Communications Engineering 4. Packet radio requires a form of multiple-access control that permits many scattered devices to transmit on the same radio channel with- out interfering with each other’s transmissions. Packet radios can be configured as either mobile or portable terminals. This system may become important in the future.* Each terminal is attached to a transmission control unit equipped with a radio transmitter and receiver. The data to be transmitted are formed into a “packet” within the transmission control unit. The packet contains the addresses of the receiving location and the originating terminal. A receiving device receives any packet addressed to it and transmits an acknowl- edgment if the packet appears to be free of error. The sending station waits a predetermined period for the acknowledgment. If it does not receive an acknowledgment, it transmits the packet again. For exam- ple, CDPD (cellular digital packet data) is a packet radio system. † 5. Radiotelephones include MTS (Mobile Telephone Service), IMTS (Im- proved Mobile Telephone Service), the Metroliner telephone, TACS (total access communication system), † and AMPS (Advanced Mobile Phone Service). † The Metroliner telephone is briefly discussed here, and the other types of radiotelephones are described, in greater detail, in subsequent paragraphs. The Metroliner telephone operates in the 400-MHz frequency range on the high-speed train between New York and Washington, D.C. The 225-mi railway distance is divided into nine zones. Each zone has a fixed radio transceiver located adjacent to the track right-of-way. As a train moves from one zone to another, calls that are in progress must be automatically switched from one fixed radio transceiver to the next without the customer’s being aware of any changes or interference in communication. 6. Digital Cellular † and PSC (personal communication service) † are for high capacity and data transmission. Digital Cellular in Europe is called GSM (Global System Mobile), a standard system using TDMA (time division multiple access). PCS is a cellular-like system applied at 1.8–1.9 GHz instead of 800–900 MHz for cellular. Other than that, the system protocols are the same as cellular systems. Digital cellu- lar in North America has two standards: IS-136 (TDMA) and IS-95 (CDMA). Digital cellular in Japan is called PDC (personal digital phone), a TDMA system. 7. TDD (Time Division Duplexing) systems † such as DECT (digital European cordless telephone), PHS (personal handy-phone system), 2 Introduction * S. Fralick and J. Garrett, “Technology for Packet Radio,” AFIPS Conf. Proc., vol. 44, 1975, AFIPS, Montvale, N.J.; and R. E. Kahn, S. A. Gronemeyer, J. Burchfield, and R. C. Kunzelman, “Advances in Packet Radio Technology,” Proc. IEEE, vol. 66, no. 11, Novem- ber 1978, pp. 1468–1496. † W. C. Y. Le e, Mobile Cellular Telecommunications, Analog and Digital Systems, 2d ed. McGraw Hill Co., 1995. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction and PACS (personal access communication system) use one fre- quency for both transmission and reception on a time-sharing basis. These systems are for low mobility and in-building communications. 8. Mobile Broadband Systems* will be the future public land mobile telecommunication system (FPLMTS). It will operate at a higher spectrum band (20–60 GHz), using ATM (asynchronous transfer mode) for broadband packet switching, and it will be compatible with the B-ISDN (broadband ISDN). It will be the future wireless information superhighway system. Let’s pause momentarily to review some of the historical highlights of mobile-radio communication. The first practical use of mobile-radio communication was demonstrated in 1897 by Marchese Guglielmo Marconi, who is credited with first successfully establishing radio transmission between a land-based station and a tugboat, over an 18- mi path. The following summary shows some of the important mile- stones in the history of mobile-radio communication: 1880: Hertz—Initial demonstration of practical radio communication 1897: Marconi—Radio transmission to a tugboat over an 18-mi path 1921: Detroit Police Department—Police car radio dispatch (2-MHz frequency band) 1932: New York Police Department—Police car radio dispatch (2-MHz frequency band) 1933: FCC—Authorized four channels in the 30- to 40-MHz range 1938: FCC—Ruled for regular service 1946: Bell Telephone Laboratories—152 MHz (simplex) 1956: FCC—450 MHz (simplex) 1959: Bell Telephone Laboratories—Suggested 32-MHz band for high-capacity mobile-radio communication 1964: FCC—152 MHz (full duplex) 1964: Bell Telephone Laboratories—Active research at 800 MHz 1969: FCC—450 MHz (full duplex) 1974: FCC—40-MHz bandwidth allocation in the 800- to 900-MHz range 1981: FCC—Release of cellular land mobile phone service in the 40- MHz bandwidth in the 800- to 900-MHz range for commercial oper- ation Introduction 3 *W.C.Y.Lee,Mobile Cellular Telecommunications,Analog and Digital Systems, 2d ed. McGraw Hill Co., 1995. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction 1981: AT&T and RCC (Radio Common Carrier) reach an agreement to split 40-MHz spectrum into two 20-MHz bands. Band A belongs to nonwireline operators (RCC), and Band B belongs to wireline opera- tors (telephone companies). Each market has two operators. 1982: AT&T is divested, and seven RBOCs (Regional Bell Operating Companies) are formed to manage the cellular operations. 1982: MFJ (modified final judgment) is issued by the government DOJ. All the operators were prohibited to (1) operate long-distance business, (2) provide information services, and (3) do manufacturing business. 1983: Ameritech system in operation in Chicago 1984: Most RBOC markets in operation 1986: FCC allocates 5 MHz in extended band 1987: FCC makes lottery on the small MSA and all RSA licenses 1988: TDMA voted as a digital cellular standard in North America 1992: GSM operable in Germany D2 system 1993: CDMA voted as another digital cellular standard in North America 1994: American TDMA operable in Seattle, Washington 1994: PDC operable in Tokyo, Japan 1994: Two of six broadband PCS license bands in auction 1995: CDMA operable in Hong Kong 1996: U.S. Congress passes Telecommunication Reform Act Bill. “Apparently anyone can get into anyone else’s business.” 1996: The auction money for six broadband PCS licensed bands (120 MHz) almost reaches 20 billion U.S. dollars. 1997: Broadband CDMA considered as one of the third-generation mobile communication technologies for UMTS (universal mobile telecommunication systems) during the UMTS workshop conference held in Korea. In 1970, the FCC allocated the following frequencies for domestic public mobile-radio use on land: Number of Channel Total Base transmit Mobile transmit channels spacing bandwidth Name 35.26–35.66 MHz 43.26–43.66 MHz 10 40 kHz 0.8 MHz MTS 152.51–152.81 MHz 157.77–158.07 MHz 11 30 kHz 0.6 MHz IMTS (MJ) 454.375–454.65 MHz 459.375–459.65 MHz 12 25 kHz 0.55 MHz IMTS (MK) 4 Introduction Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction Although a total of 33 channels are provided for within these fre- quency allocations, the actual number of channels used in a specified area is much smaller on account of restrictions imposed by prevailing FCC regulations, which are explained later on. In 1974, the FCC allocated a 40-MHz bandwidth in the 800- to 900- MHz frequency region for mobile telephone use. During the initial trial tests, it was utilized as follows: Number of Channel Total Base transmit Mobile transmit channels spacing bandwidth 870–890 MHz 825–845 MHz 666 30 kHz 40 MHz The Bell Telephone System used this narrow band of frequencies in trial tests of its new, high-capacity Advanced Mobile Phone Service (AMPS), which is designed for use in a cellular planned network. Because the system design is based on the reuse of allocated frequen- cies, the number of customers served is greatly increased; hence the term “high-capacity” system. Part II of this introduction describes a cellular system in greater detail. By 1976, the Bell System served approximately 40,000 mobile- telephone customers within the United States. Of this number, 22,000 were able to dial directly, whereas 18,000 required operator assistance to place a call. The various systems that serve mobile radiotelephones are classified according to their assigned frequency range. For exam- ple, the MJ system operates in the 150-MHz range, whereas the MK system operates in the 450-MHz range. Each system can provide from 1 to as many as 12 channels, with FCC regulations requiring that 12 channels of an MK system serve an area of 50 miles in diameter. To illustrate how few channels are available and how overloaded they are, in 1976 the New York Telephone Company (NYTC) operated 6 channels of the MJ system serving 318 New York City mobile- telephone subscribers, approximately 53 customers per channel, and there were 2400 applicants wait-listed for MJ mobile-telephone ser- vice. NYTC also operated six MK channels serving 225 customers, approximately 38 customers per channel, and 1300 applicants were wait-listed for MK mobile-telephone service. New York City was lim- ited to only six MK channels out of the maximum of 12 available because of the FCC regulation requiring that 12 channels serve an area of 50 miles in diameter. In 1976, there were a total of 1327 mobile-telephone systems in oper- ation across the United States. The Bell System operated 637 mobile- telephone systems within its coast-to-coast network, whereas 690 were operated by independent telephone companies. The market demand for mobile-telephone service is already much greater than the existing Introduction 5 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction available supply and is increasing very rapidly because of the great, undisputed popularity of CB radio, which is very busy and congested, with only 40 assigned operating channels (26.96 to 27.41 MHz). When the new, cellular mobile-radio systems are fully operational across the United States, high-capacity direct-dialing service at reasonable cost will entice large numbers of CB radio users to subscribe to mobile radiotelephone. The obvious advantages of mobile radiotelephone over the heavily saturated CB radio channels are: 1. Direct-dialing features equivalent to those offered to fixed-telephone subscribers 2. Absolute privacy of communication, with greatly improved quality 3. An extended range of communication utilizing the total switching resources of the commercial telephone networks 4. A theoretically unlimited number of communication channels that can be provided In this book, the theory and analyses are aimed at the mobile-to-fixed radio communication links that are designed to fit the cellular require- ments of the VHF and UHF mobile-radiotelephone systems of the 1980s and that operate in the 30-MHz to 1-GHz mobile-radio frequency ranges. For systems operating above 1 GHz, atmospheric conditions such as moisture and climatic effects must be taken into consideration. These effects are minimal at operating frequencies below 1 GHz. Below 30 MHz, path loss and signal fading are not severe; but since there are few mobile-radio frequency allocations in this region of the radio spectrum, the primary emphasis of this book is on the design of 30-MHz to 1-GHz mobile radio. Looking toward the future, the portable telephone and ultimately a pocket telephone are potential product designs that will share mobile- radiotelephone transmission facilities. Some of the major problems that must be solved before these designs are realized are the limita- tions of battery size, weight, and power capacity; radiation safety haz- ards to the user; and signal interference problems unique to the portable-telephone user’s environment. Part II Cellular Network Planning The future of mobile-radiotelephone communication is dependent upon techniques of network planning and mobile-radio equipment design that will enable efficient and economical use of the radio spectrum. One possible solution to the problem of meeting the steadily increasing cus- tomer demand for mobile-radiotelephone service, within the limitations 6 Introduction Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction of available FCC frequency allocations, is to develop a workable plan for reusing the assigned channels within each band of frequencies. To encourage the mobile-radiotelephone industry in its development of advanced high-capacity systems, the FCC in 1974 allocated a 40-MHz bandwidth in the 800- to 900-MHz frequency range for this purpose. Subsequent design research and trial tests conducted by the Bell Tele- phone Laboratories concluded that high-capacity systems based on the reuse of assigned channel frequencies in a cellular planned network are a practical solution. The system evolving from this work is known as the Advanced Mobile Phone Service (AMPS), and its functional capabilities are described in the following paragraphs. AMPS service features* In describing the service features of the AMPS, our primary area of interest is that of land mobile telephone service, which includes all of the features ascribed to normal telephone service to the extent that such services are compatible with the special characteristics of the mobile environment. This does not preclude the AMPS from providing other mobile-radio services, such as those services associated with direct dispatch, air-to-ground, and other types. Land mobile telephone service is offered as a subscriber service for privately owned vehicles, and as a public telephone service on commercial ground carriers such as buses and trains. We know from past experience that the special char- acteristics of the mobile-radio environment can have an adverse effect upon radio propagation, and consequently can affect the quality of the services provided. It is therefore essential to know the cause, extent, and methods for minimizing these effects in order to improve the qual- ity and reliability of mobile-radiotelephone communication. The effects of the mobile environment on mobile-radio performance are further examined in later chapters covering the theory of functional design. Radio enhancement techniques As previously mentioned, the FCC has allocated a 40-MHz bandwidth in the 800- to 900-MHz frequency range for high-capacity mobile radiotelephone service. On the basis of the con- cept of cellular network planning, the 40-MHz bandwidth is separated into a 20-MHz base-station transmit band in the 870- to 890-MHz range and a mobile-radio 20-MHz transmit band in the 825- to 845-MHz range. The total 40-MHz bandwidth is further subdivided into 666 two- way channels, each channel consisting of two frequencies having chan- Introduction 7 * “Advanced Mobile Phone Service,” special issue, Bell System Technical Journal, vol. 58, January 1979. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction nel bandwidths of 30 kHz each. To enable frequency separation between channels within a given area, the 666 channels are arranged for two operators in the form of a distribution matrix, as illustrated in Fig. I.1. In Fig. I.1, the Block A and Block B operators have 333 channels each. Among 333 channels, 21 channels indicated in Fig. I.1 are the setup channels. The matrix can be considered to be 21 sets of channels. To simplify distribution, the 21 sets are arranged into 3 groups of 7 and assigned suffix letters A, B, and C, respectively. The distribution of channels and channel frequencies obtained by this arrangement ensures that assignments within one geographic cell area will not inter- fere with channels assigned in adjacent cell locations. Cells that are separated by a minimum distance determined by propagation variables can simultaneously use the same channels with no risk of interference. The sample cell structure shown in Fig. I.1 illustrates the method for assigning channels among contiguous cell locations. A system operator serving a particular population center, such as a major city and its surrounding suburban communities, could provide mobile-radio coverage to large numbers of users based on cellular reuse of assigned channel frequencies. The basic cell structure is conceptu- ally hexagonal in shape and can vary in size according to the number 8 Introduction Figure I.1 Frequency-management chart. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction of channels, traffic variables, and the effectiveness of propagation- enhancement techniques. For purposes of explanation, we will tem- porarily disregard cell size. A typical area divided into cells is shown in Fig. I.2. Each block of seven cells is repeated in such a manner that cor- responding numbered cells in adjacent seven-cell blocks are located at a predetermined distance from the nearest cell having the same number. Correspondingly, the 20-MHz-bandwidth radio spectrum is divided into seven disjoint sets, with a different set allocated to each one of the seven cells in the basic block. With a total of 333 channels in 21 sets available, it is possible to assign as many as three sets to each of the seven cells constituting the basic block pattern. For blanket coverage of cell areas, each cell site is installed at the cen- ter of the cell (the dotted-line cell) and covers the whole cell, as shown in Fig. I.3. There is another way of looking at the locations of the cell sites. The three cell sites are installed, one at each alternate corner of the cell and cover the whole cell, as shown in Fig. I.3. In both cases, although the boundary of a cell is defined differently, the cell sites do not need to be moved. For convenience, the cells illustrated in Fig. I.3 are pictured as hexagonal in shape. In actual practice, the cell boundaries are defined by the minimum required signal strength at distances determined by the reception threshold limits. In the AMPS, base stations are referred to as cell sites because they perform supervision and control in addition to the transmitting and receiving functions normally associated with the con- ventional base station. Mobile-telephone subscribers within a given cell are assigned to a particular cell site serving that cell simply by the Introduction 9 Figure I.2 Basic cell block: R = radius of each cell; D = distance between two adjacent frequency-reuse cells; K = number of cells in a basic cell block. K = 7 in this illustration, and D/R = 4.6. K = (D/R) 2 ᎏ 3 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction assignment of an idle channel frequency under the control of the mobile- telephone switching office (MTSO). When a mobile unit crosses a cell boundary, as determined by the signal reception threshold limits, a new idle channel frequency is assigned by the new serving cell site.This auto- matic switching control function is referred to as a “handoff.” The problems of cochannel interference are avoided by ensuring a minimum distance between base stations using the same channel fre- quencies, and by enhancing signal level and reducing signal fading through the use of diversity schemes. These constraints limit any potential cochannel interference to levels low enough to be compatible with the transmission quality of landline networks. Two forms of diversity are used to enhance radio propagation, thus improving AMPS cell coverage. These are defined as “macroscopic” and “microscopic” diversity. Macroscopic diversity compensates for large- scale variations in the received signal resulting from obstacles and large deviations in terrain profile between the cell site and the mobile- telephone subscriber. Macroscopic diversity is obtained by installing directional antennas, one for each sector of three sectors at the cell cen- ter, or installing at the alternate corners of cells, as shown in Fig. I.3, and transferring control to the antenna providing the strongest aver- age signal from the mobile subscriber in any given time interval. For example, the three cell-site transmitters serving a particular cell area would not radiate simultaneously on an assigned channel frequency. On the basis of a computer analysis of the signals received from the mobile subscriber at each of the three sites, the one with the strongest 10 Introduction Figure I.3 Use of inward-directed antennas at alter- nate cell corners to achieve macroscopic diversity with respect to large obstructions. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Introduction [...]... features and options 10 Ruggedized construction capable of surviving in unfavorable environments 11 Attractive appearance and styling 11 Lightweight, simple to operate, and easy to remove and replace 12 Designed for theftproof installation 12 Design must use military-standard parts meeting military specifications 13 Mobile- to-base or mobile- to-base-to -mobile (for billing purposes) 13 Mobile- to-base or mobile- to -mobile. .. unlicensed bands—one for voice (10 MHz) and one for data (10 MHz)—are also shown in Fig I.7(a) There are six possible standard systems: 1 DCS (Digital Cellular System )-1 900 A GSM-version system 2 CDMA-1900 A cellular CDMA-version system 3 NA-TDMA-1900 A cellular NA-TDMA (IS-136)-version system 4 Omnipoint A hybrid system with CDMA, TDMA, and FDMA 5 B-CDMA A broadband CDMA (5-MHz or 10-MHz) system 6 PACS-1900... combined with free-space losses, collectively make up the propagation-path loss Mobile- radio signals are also affected by various types of scattering and multipath phenomena—which can cause severe signal fading— attributable to the mobile- radio communications medium Mobile- radio signal fading compounds the effects of long-term fading and short-term fading, which can be separated statistically and are described... 1990 UV-unlicensed voice UD-unlicensed date Name of Band Spectrum Bandwidth Base Rv (MHz) Base Tx (MHz) Band A Band D Band B Band E Band F Band C Unlicensed voice 30 MHz 10 30 10 10 30 10 1850–1865 1865–1870 1870–1885 1885–1890 1890–1895 1895–1910 1910–1915 1930–1945 1945–1950 1950–1965 1965–1970 1970–1975 1975–1990 1925–1930 Unlicensed data 10 1915–1925 (a) Five 50-kHz channels paired with 50-kHz channels... phenomena are not significant in air-to-ground and satellite-to-earthstation communications, because the angle of propagation precludes 21 Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website The Mobile- Radio Signal Environment 22 Chapter... Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website The Mobile- Radio Signal Environment 26 Chapter One quency distribution and geographic separation Time-delay spread and coherence bandwidth are discussed in Secs 1.5 and 1.6, respectively Under data-sampling... (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website Source: Mobile Communications Engineering Chapter 1 The Mobile- Radio Signal Environment 1.1 The Mobile- Radio Communication Medium Radio signals transmitted from a mobile- radio base station are not only subject to the same significant propagation-path losses that are encountered... velocities of ෆ V Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website The Mobile- Radio Signal Environment The Mobile- Radio Signal Environment 27 mobile- radio environment Variations in the contour and roughness of the terrain, including... Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website The Mobile- Radio Signal Environment The Mobile- Radio Signal Environment 31 (a) (b) Figure 1.3 Propagation path: (a) out of sight; (b) line of sight The propagation-path loss slope in the mobile- radio... I.5 A call originating from or terminating at a mobile unit is serviced by a cell site connected via landlines to a mobile- telephone switching office (MTSO) The MTSO provides call supervision and control, and extends call access to a commercial telephone landline network via a local central-office (CO) telephone exchange, a toll office, and any number of tandem offices required to complete the call path . installation. 12. Design must use military-standard parts meeting military specifications. 13. Mobile- to-base or mobile- to-base-to -mobile 13. Mobile- to-base or mobile- to -mobile. (for billing purposes). 14 is separated into a 20-MHz base-station transmit band in the 87 0- to 890-MHz range and a mobile- radio 20-MHz transmit band in the 82 5- to 845-MHz range. The total 40-MHz bandwidth is further subdivided. locations and of which one may actually be a fixed terminal such as a base station. This definition applies to both mobile- to -mobile and mobile- to-fixed radio communica- tion links. The mobile- to-mobile