Unit V. Multi-User Radio Communication

Transcription

1 Unit V Multi-User Radio Communication ADVANCED MOBILE PONE SERVICE (AMPS) 1906: 1 st radio transmission of Human voice. What s the medium? Used an RC circuit to modulate a carrier frequency that radiated up and down an antenna. Receiver had a matched RC circuit with an antenna 1910: Lars Ericsson in Sweden invents the first car phone. However, Morse Code continues to be the primary method of sending information without a wire. 1934: Federal Communications Commission is founded by Roosevelt s New Deal. Charged to allocate the radio spectrum with the public interest in mind. The FCC was corrupt until the mid 60 s, it propped up AM radio for years to keep out newer FM stations. The FCC gave priority in terms of broadcast channels to emergency and government units. WW2: Many innovations, including RADAR. Also the first mobile FM Transmitter/Receiver. Weighed over 30 lbs. More like a Walkie Talkie. Developed by Motorola. 1946: In St. Louis, AT&T and Southwestern Bell introduced the first mobile telephone service. There were 6 channels in the 150 MHz band with 60 KHz allocated to each channel. A very powerful antennae sat atop a centrally located building. All calls were routed through here. Not full duplex; it was like a walkie/talkie. Operators routed all calls 1954/1958: Silicon transistor and integrated circuits are invented at Texas Instruments. Walkie/Talkies were now the size of a large shoe. 1960: Bell employees informally outline a cellular plan and request 75 MHz of bandwidth around the 800 MHz band. Everything that was needed to have mobile communications was invented at this point except the microprocessor (1971 by Intel). CS Analog & Digital Communication,Unit 5 Page 1

2 1973: Martin Cooper from Motorola files for a patent on the first handheld mobile phone. He didn t invent cellular phones, however, Bell had a working system on trains 4 years earlier, but it wasn t handheld. They were both cellular with frequency reuse. 1974: The FCC releases all the requested bandwidth 1975: Bell receives permission to start a commercial cellular network in Chicago. They order 135 phones at a cost of over $500, : Lucent makes the first DSP on one chip. October 12, 1983: Bell rolls out the first full-scale cellular network in Chicago. Covers 2100 square miles with 12 cellular sites. Uses a system called AMPS Operated in the 800 MHz band that had been allocated by the FCC. Telephones were expensive suit case type phones 1983 are cell phone. AMPS: Advanced Mobile Phone System Known as First Generation Wireless Analog channels of 30 KHz. Uses Frequency Division Multiple Access Uses frequency reuse people in other cells can use your frequency without interference. Very susceptible to static. Very easy to ease drop. Introduced in Must be supported by every wireless carrier until February, OnStar still uses AMPS. In 2005, 15% of Alltel s customers were still using AMPS. Replaced by TDMA and now CDMA (all digital) technologies. First generation of cell phones 3 basic devices mobile base transceiver mobile telephone switching office (MTSO) Voice and Control Channels Outgoing from mobile input phone number and press send CS Analog & Digital Communication,Unit 5 Page 2

3 mobile links to base xcvr via control channel base to MTSO to POTS MTSO routes connection back to mobile via voice channel mobile shifts from control to voice Incoming to mobile call goes from POTS to MTSO on control channel, MTSO searches for mobile by PAGING every active mobile if found, MTSO rings it and establishes voice channel connection uses xcvr with strongest signal from mobile The only system available in the United States until about 1997 The first system used for cellular telephony- is analog Uses the 800 MHz frequency band of the spectrum Is still being used widely - The number of subscribers began to decrease in 1999 due to migration to digital Utilizes FDMA (Frequency division multiple access) to separate users In FDMA, users are separated in frequency. i.e. mobile phones communicate at different frequencies than the others within each cell. The radio spectrum is shared among users GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS (GSM) A version of time division multiple access (TDMA) technology, because it divides frequency bands into channels and assigns signals time slots within each channel. Makes more efficient use of limited bandwidth than the IS-136 TDMA standard common in the United States. Makes use of silences in a phone call to increase its signal compression, leaving more open time slots in the channel. More than 800 million end users in 190 countries and representing over 70% of today's digital wireless market. Israel source: GSM Association Orange uses GSM Pelephone and Cellcom are about to use GSM GSM Services GSM was designed to do 3 things: 1. Bearer data services: Faxes, text messages, web pages. Basic GSM had a basic data rate that is limited to 9.6 kbps CS Analog & Digital Communication,Unit 5 Page 3

4 Extended by GMRS and EDGE to around 384 Kbps 2. Tele Services: Voice traffic But, at a lower quality than analog. 3. Supplementary services Call forwarding, caller id, etc Meaning, we need to connect to the SS7 network GSM functional architecture System architecture As with all systems in the telecommunication area, GSM comes with a hierarchical, complex system architecture comprising many entities, interfaces, and acronyms. A GSM system consists of three subsystems, the radio sub system (RSS), the network and switching subsystem (NSS), and the operation subsystem (OSS). Each subsystem will be discussed in more detail in the following sections. Generally, a GSM customer only notices a very small fraction of the whole network - the mobile stations (MS) and some antenna masts of the base transceiver stations (BTS) Radio subsystem Base station subsystem (BSS): A GSM network comprises many BSSs, each controlled by a base station controller (BSC). The BSS performs all functions necessary to maintain radio connections to an MS, coding/decoding of voice, and rate adaptation to/from the wireless network part. Besides a BSC, the BSS contains several BTSs. Base transceiver station (BTS): A BTS comprises all radio equipment, i.e., antennas, signal processing, amplifiers necessary for radio transmission. A BTS can form a radio CS Analog & Digital Communication,Unit 5 Page 4

5 cell or, using sectorized antennas, several cells, and is connected to MS via the Um interface (ISDN U interface for mobile use), and to the BSC via the Abis interface. The Um interface contains all the mechanisms necessary for wireless transmission (TDMA, FDMA etc.) and will be discussed in more detail below. The Abis interface consists of 16 or 64 kbit/s connections. A GSM cell can measure between some 100 m and 35 km depending on the environment (buildings, open space, mountains etc.) but also expected traffic. Base station controller (BSC): The BSC basically manages the BTSs. It reserves radio frequencies, handles the handover from one BTS to another within the BSS, and performs paging of the MS. The BSC also multiplexes the radio channels onto the fixed network connections at the A interface. Mobile station (MS): The MS comprises all user equipment and software needed for communication with a GSM network. An MS consists of user independent hard- and software and of the subscriber identity module (SIM), which stores all user-specific data that is relevant to GSM.3 While an MS can be identified via the international mobile equipment identity (IMEI), a user can personalize any MS using his or her SIM, i.e., userspecific mechanisms like charging and authentication are based on the SIM, not on the device itself. Device-specific mechanisms, e.g., theft protection, use the device specific IMEI. Without the SIM, only emergency calls are possible. The SIM card contains many identifiers and identity number (PIN), a PIN unblocking key (PUK), an authentication key Ki, and the inter-national mobile subscriber identity tables, such as card-type, serial number, a list of subscribed services, a personal (IMSI) The PIN is used to unlock the MS. Using the wrong PIN three times will lock the SIM. In such cases, the PUK is needed to unlock the SIM. The MS stores dynamic information while logged onto the GSM system, such as, e.g., the cipher key Kc and he location information consisting of a temporary mobile subscriber identity (TMSI) and the location area identification (LAI). Typical MSs for GSM900 have a transmit power of up to 2 W, whereas for GSM W is enough due to the smaller cell size. Apart from the telephone interface, an MS can also offer other types of interfaces to users with display, loudspeaker, microphone, and programmable soft keys. Further interfaces comprise computer modems, IrDA, or Bluetooth. Typical MSs, e.g., mobile phones, comprise many more vendor-specific functions and components, such as cameras, fingerprint sensors, calendars, address books, games, and Internet browsers. Personal digital assistants (PDA) with mobile phone functions are also available. The reader should be aware that an MS could also be integrated into a car or be used for location tracking of a container. Network and switching subsystem The heart of the GSM system is formed by the network and switching subsystem (NSS). The NSS connects the wireless network with standard public networks, performs handovers between different BSSs, comprises functions for worldwide localization of users and supports charging, accounting, and roaming of users between different CS Analog & Digital Communication,Unit 5 Page 5

6 providers in different countries. The NSS consists of the following switches and databases: Mobile services switching center (MSC): MSCs are high-performance digital ISDN switches. They set up connections to other MSCs and to the BSCs via the A interface, and form the fixed backbone network of a GSM system. Typically, an MSC manages several BSCs in a geographical region. A gateway MSC (GMSC) has additional connections to other fixed networks, such as PSTN and ISDN. Using additional interworking functions (IWF), an MSC can also connect to public data networks (PDN) such as X.25. An MSC handles all signaling needed for connection setup, connection release and handover of connections to other MSCs. The standard signaling system No. 7 (SS7) is used for this purpose. SS7 covers all aspects of control signaling for digital networks (reliable routing and delivery of control messages, establishing and monitoring of calls). Features of SS7 are number portability, free phone/toll/collect/credit calls, call forwarding, three-way calling etc. An MSC also performs all functions needed for supplementary services such as call forwarding, multi-party calls, reverse charging etc Home location register (HLR): The HLR is the most important database in a GSM system as it stores all user-relevant information. This comprises static information, such as the mobile subscriber ISDN number (MSISDN), sub-scribed services (e.g., call forwarding, roaming restrictions, GPRS), international mobile subscriber identity (IMSI). Dynamic information is also needed, e.g., the current location area (LA) of the MS, the mobile subscriber roaming number (MSRN), the current VLR and MSC. As soon as an MS leaves its current LA, the information in the HLR is updated. This information is necessary to localize a user in the worldwide GSM network. All these user-specific information elements only exist once for each user in a single HLR, which also supports charging and accounting. HLRs can manage data for several million customers and contain highly specialized data bases which must fulfill certain real-time requirements to answer requests within certain time-bounds. Visitor location register (VLR): The VLR associated to each MSC is a dynamic database which stores all important information needed for the MS users currently in the LA that is associated to the MSC (e.g., IMSI, MSISDN, HLR address). If a new MS comes into an LA the VLR is responsible for, it copies all relevant information for this user from the HLR. This hierarchy of VLR and HLR avoids frequent HLR updates and long-distance signaling of user information. Some VLRs in existence, are capable of managing up to one million customers. Operation subsystem The third part of a GSM system, the operation subsystem (OSS), contains the necessary functions for network operation and maintenance. The OSS possesses network entities of its own and accesses other entities via SS7 signaling The following entities have been defined: CS Analog & Digital Communication,Unit 5 Page 6

7 Operation and maintenance center (OMC): The OMC monitors and controls all other network entities via the O interface (SS7 with X.25). Typical OMC management functions are traffic monitoring, status reports of network entities, subscriber and security management, or accounting and billing. OMCs use the concept of telecommunication management network (TMN) as standardized by the ITU-T. Authentication centre (AuC): As the radio interface and mobile stations are particularly vulnerable, a separate AuC has been defined to protect user identity and data transmission. The AuC contains the algorithms for authentication as well as the keys for encryption and generates the values needed for user authentication in the HLR. The AuC may, in fact, be situated in a special protected part of the HLR. Equipment identity register (EIR): The EIR is a database for all IMEIs, i.e., it stores all device identifications registered for this network. As MSs are mobile, they can be easily stolen. With a valid SIM, anyone could use the stolen MS. The EIR has a blacklist of stolen (or locked) devices. In theory an MS is useless as soon as the owner has reported a theft. Unfortunately, the blacklists of different providers are not usually synchronized and the illegal use of a device in another operator s network is possible (the reader may speculate as to why this is the case). The EIR also contains a list of valid IMEIs (white list), and a list of malfunctioning devices (gray list). CDMA Third generation system Code Division Multiple Access (CDMA) Separates users by assigning them digital codes within a broad range of the radio frequency First technology to use soft-handoff Employs spread spectrum technique Advantages Improved capacity, coverage, voice quality, and immunity from interference Each voice signal is digitized and assigned a unique code, and then small components of the signal are issued over multiple frequencies using the spread spectrum technique. CS Analog & Digital Communication,Unit 5 Page 7

8 Cells In a city, there might be one MTSO There will be many xcvrs, each in its own area or CELL Cell is hexagonal, with dia. of 6 miles or less Channels Every mobile in a cell who is talking must have its own channel Otherwise, there will be interference More channels = more users Rule of thumb - one channel can support 20 users Cell Sectorization Cells can be divided into sectors to provide a smaller coverage area, and therefore, more frequency reuse. Cell Area Cellular areas aren t really circular as the area depends on the terrain and the interference that s present. Why Hexagons? CS Analog & Digital Communication,Unit 5 Page 8

9 Using hexagons, as opposed to circles or boxes, allows for a better visualization of the coverage areas. Also, a system of hexagons helps offset cells from linear road boundaries (where cell phones were envisioned to be used). Making a Call When a mobile is idle, i.e., it is not experiencing the process of a call, then it searches all the FCCs to determine the one with the highest signal strength. The mobile then monitors this particular FCC. However, when the signal strength falls below a particular threshold that is insufficient for a call to take place, the mobile again searches all the FCCs for the one with the highest signal strength. For a particular country or continent, the control channels will be the same. So all mobiles in that country or continent will search among the same set of control channels. However, when mobile moves to a different country or continent, then the control channels for that particular location will be different and hence the mobile will not work. Each mobile has a mobile identification number (MIN). When a user wants to make a call, he sends a call request to the MSC on the reverse control channel. He also sends the MIN of the person to whom the call has to be made. The MSC then sends this MIN to all the base stations. The base station transmits this MIN and all the mobiles within the coverage area of that base station receive the MIN and match it with their own. If the MIN matches with a particular MS, that mobile sends an acknowledgment to the BS. The BS then informs the MSC that the mobile is within its coverage area. The MSC then instructs the base station to access specific unused voice channel pair. The base station then sends a message to the mobile to move to the particular channels and it also sends a signal to the mobile for ringing. In order to maintain the quality of the call, the MSC adjusts the transmitted power of the mobile which is usually expressed in db or dbm. When a mobile moves from the coverage area of one base station to the coverage area of another base station i.e., from one cell to another cell, then the signal strength of the initial base station may not be sufficient to continue the call in progress. So the call has to be transferred to the other base station. This is called handoff. In such cases, in order to maintain the call, the MSC transfers the CS Analog & Digital Communication,Unit 5 Page 9

10 call to one of the unused voice channels of the new base station or it transfers the control of the current voice channels to the new base station. Spectral Allocation In U.S., we use two 25mHz bands l one band from mobile to xcvr l one band from xcvr to mobile Each channel uses 30 khz So - about 832 channels for a city But - channels split between two companies 21 for control, 395 for voice So - can support about 7900 users Frequency Reuse Also called Spatial Allocation Cells are small, so signals can be low power Can use same freq. in a cell that is far away, but not one nearby l at least 7 cells must intervene So - each cell can use about 57 channels So - each cell can support 1140 users Hand-offs Mobile phones move from cell to cell As signal fades, mobile asks MTSO for a new base xcvr MTSO polls all base xcvrs to see which has strongest signal with mobile Mobile shifts to new channel coming from new base Roaming Mobile has moved to a different city or subscriber area MTSO will check with your provider, via land-line Then will allow you access Keeps track of billing data Multiple Access First generation system Based on FDMA (Frequency Division Multiple Access), where frequency band is divided into a number of channels. Each channel carries only one voice conversation at a time. AMPS operates on 800 MHz or 1800 MHz CS Analog & Digital Communication,Unit 5 Page 10

11 FDMA Advantages: Widest coverage Limitations: Inadequate to satisfy the increasing demand Poor security Not optimized for data TDMA Second generation system Enables users to access the whole channel bandwidth for a fraction of the time, called slot, on a periodic basis Has applications in satellite communications Advantages Improved capacity CS Analog & Digital Communication,Unit 5 Page 11

12 CDMA Third generation system Separates users by assigning them digital codes within a broad range of the radio frequency First technology to use soft-handoff Employs spread spectrum technique Advantages Improved capacity, coverage, voice quality, and immunity from interference CS Analog & Digital Communication,Unit 5 Page 12

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