Ασύρματα Δίκτυα και Κινητές Επικοινωνίες Ενότητα # 6: Κυτταρική Δομή Κινητών Επικοινωνιών και Συστήματα Κινητής Τηλεφωνίας 1ης & 2ης Γενιάς Διδάσκων: Βασίλειος Σύρης Τμήμα: Πληροφορικής

Κυτταρική Δομή Κινητών Επικοινωνιών l Mobile Communications Principles l Early Mobile Telephone System Architecture l Co-channel Interference and Frequency Reuse l The Cellular Concept l Cellular System Architecture l Frequency Reuse l Cell Splitting l Handoff l Handoff Initiation l Cellular System Components l Digital Cellular l Time Division Multiple Access (TDMA)

Mobile Communications Principles l each mobile (station) u uses a separate, temporary radio channel to talk to the cell site l the cell site (base station) u talks to many mobiles at once, using one channel per mobile l a pair of frequencies are used for communication u one (the forward link) for transmitting from the cell site u another frequency (the reverse link) for the cell site to receive calls from the users l mobiles must stay near the base station u to maintain communications u radio energy dissipates over distance l mobile (voice) networks include u mobile radio service n operates in a closed network n no access to the telephone system u mobile telephone service n interconnection to the telephone network

                               Early Mobile Telephone System Architecture l Traditional mobile service u structured in a fashion similar to TV broadcasting u one powerful transmitter in a (e.g.,  metropolitan) area n could broadcast in a radius of up to 50  km l The cellular concept u different! u many low-power transmitters placed throughout an area

Co-channel Interference and Frequency Reuse l co-channel interference u caused by mobile units using the same channel in adjacent areas u all channels cannot be (re-)used in every cell u areas have to be skipped before the same channel is reused l frequency reuse is still a key technique for mobile communications systems l interference u is not proportional to the distance between areas, but to the ratio of the distance between areas to the transmitter power (radius) of the areas u reducing the radius of an area by 50%, increases the number of potential customers in an area 4x n systems with a 1 Km radius can have 100 times more channels than systems with areas 10 Km in radius

The Cellular Concept l variable low-power transmission levels u allow cells to be sized according to n subscriber density n traffic demands l as the population or traffic grows u cells can be added to accommodate that growth l frequencies used in one cell cluster can be re-used in other clusters l conversations can be handed-off from cell to cell u to maintain continuous service as the user moves between cells l the base station can communicate with mobiles as long as they are within range                               

Cellular System Architecture l engineering plan u clusters u frequency reuse u handovers l cells u basic geographic unit of a cellular system u base stations transmit over small geographic areas u often represented as hexagons u true shape of cells is not a perfect hexagon n because of constraints imposed by m natural terrain m man-made structures u cell size varies depending on the landscape l clusters u a group of cells u no channels are reused within a cluster

Frequency Reuse l no channels are reused within a cluster l cells with same number have same group of frequencies u they are far enough so that there is no interference l number of available frequency groups is 7 u frequency reuse factor=7 u each cell is using 1/7 of available channels l Hexagon cell pattern, values of N=1,3,7,9,12,13,16,19,21

Reuse patterns A F E D B G C A F E D B G C N = 7 A B C B B C C C B B A C A A A N = 3

Cellular architecture l Cellular backhaul or Radio Access Network: interconnect base stations to backbone l Can be wired or wireless (point-to-point)

Increasing cellular capacity l Cell splitting u smaller cells in high demand areas u smaller cells => more base stations & more frequent handoffs u smaller cells (micro cells) => reduced transmission power l Frequency borrowing u congested cells borrow frequencies from less congested cells u dynamic allocation of frequencies l Sectorization u cell divided into sectors (typically 2-6) u use directional (sector) antennas 3-sector cell

Cell Splitting l creating full systems with many small areas impractical l cell splitting u as a service area becomes full of users u split a single area into smaller ones l urban centers u can be split into as many areas as necessary u to provide acceptable service levels in heavy-traffic regions l rural regions u larger, less expensive cells

Handoff l mobile subscriber travels from one cell to another during a call u adjacent cells do not use the same radio channels u call must either n be dropped or n transferred from one radio channel to another when a user crosses cells u dropping the call is unacceptable l hand-off (US, hand-over in UK) u mobile telephone network automatically transfers a call u from radio channel to radio channel, as a mobile crosses adjacent cells

Handoff Initiation l when MS moves out of the coverage area of a given cell site u the reception becomes weak u the cell site (in use) or the MS requests a handoff u system switches the call to n a new site (and channel) with n a stronger signal u without interrupting the call or alerting the user u call continues as long as the user is talking, and the user does not notice the handoff at all

Cellular System Components l public switched telephone network (PSTN) l mobile telephone switching office (MTSO) l cell site with antenna system l mobile subscriber unit (MSU)

Cellular System Component Definitions l PSTN ( public switched telephone network) u made up of: n local networks n the exchange area networks, and n the long-haul network (on a worldwide basis) l Mobile Telephone Switching Office (MTSO) u the central office for mobile switching u houses the mobile switching center (MSC), field monitoring, and relay stations for switching calls from cell sites to wireline central offices (PSTN) u in analog cellular networks, the MSC controls the system operation: controls calls, tracks billing information, and locates cellular subscribers l the Cell Site u refers to the physical location of radio equipment (providing coverage within a cell) u includes: n power sources n interface equipment n radio frequency transmitters and receivers, and n antenna systems

Συστήματα Κινητής Τηλεφωνίας 1ης & 2ης Γενιάς l 1st generation u AMPS l 2nd generation u GSM u DAPMS, IS-54, IS-136 (TDMA) n CDPD (Packet Data extension) u IS-95 (CDMA)

North American Analog Cellular Systems l originally devised in the late 1970s to early 1980s l operate in the 800-MHz range l system development takes into consideration many different, and often opposing, requirements u often a compromise between conflicting requirements results l cellular development involves the following basic topics:  frequency and channel assignments  type of radio modulation and modulation parameters  maximum power levels  messaging protocols  call-processing sequences

AMPS: Advanced Mobile Phone Service l U.S. (’80s) l Analog l cell size: 1km 2 … 100km 2 l indoor coverage: problematic                               

Advanced Mobile Phone Service (AMPS) l released in 1983 l a fully automated mobile telephone service l the first standardized cellular service in the world l used throughout the world n United States n South America n China n Australia l designed for use in cities, later expanded to rural areas l maximized the cellular concept of frequency reuse u by reducing radio power output l AMPS telephones (or handsets) u have the familiar telephone-style user interface u are compatible with any AMPS base station l mobility between service providers (roaming) possible

AMPS (cont.) l using the 800-MHz to 900-MHz frequency band l 30-kHz bandwidth for each channel l frequency modulation (FM) for radio transmission l in the US, Frequency Division Duplex (FDD): u separate frequencies for n transmissions from mobile to base station n transmissions from the base station to the mobile subscriber u (alternative to FDD is Time Division Duplex--TDD)

AMPS Limitations l low calling capacity l limited spectrum l no room for spectrum growth l poor data communications l minimal privacy l inadequate fraud protection

Narrowband Analog Mobile Phone Service (NAMPS) l introduced as an interim solution to AMPS capacity problems l operational in 35 U.S. and overseas markets l combines u existing voice processing u with digital signaling l tripling the capacity of today's AMPS systems l 3 channels from a single AMPS 30-kHz channel u FDM u provides 3 users in an AMPS channel l increased probability of interference u thus lower voice quality u because channel bandwidth is reduced for a call (compared to AMPS)

Συστήματα Κινητής Τηλεφωνίας 2ης Γενιάς l GSM (discussed extensively later) l TDMA (IS-54, then IS-136) l CDMA (IS-95) u “marketing name” PCS (IS-95 based) n smaller cells, higher speeds

AMPS/DAMPS Comparison

CDPD: Cellular Digital Packet Data l CDPD: data (IP) on AMPS (extended) l AMPS: U.S. (’80s) u Analog u cell size: 1km 2 … 100km 2 u modems for data (error control) n s set-up n up to 4.8 Kb/s l CDPD u exploiting 30+% idle time on AMPS circuit-switching channels u 40 ms delay for voice to recapture channel u channel hopping u up to 19.2 Kb/s u IP

GSM - Global System Mobile l European, introduced in 1992 l digital cellular, TDMA based l extended for data traffic at 9.6 Kb/s l short messages (160 Bytes) l frequencies u 900 Mhz, 1800 Mhz (DCS), in Europe u 1900 Mhz in the U.S.                               

Τέλος Ενότητας # 6 Μάθημα: Ασύρματα Δίκτυα και Κινητές Επικοινωνίες Ενότητα # 6: Συστήματα Κινητής Τηλεφωνίας1ης & 2ης Γενιάς Διδάσκων: Βασίλειος Σύρης Τμήμα: Πληροφορικής