Wireless WANS and MANS. Chapter 3

Transcription

1 Wireless WANS and MANS Chapter 3

2 Cellular Network Concept Use multiple low-power transmitters (100 W or less) Areas divided into cells Each served by its own antenna Served by base station consisting of transmitter, receiver, and control unit Band of frequencies allocated Cells set up such that antennas of all neighbors are equidistant (hexagonal pattern) 2009/10/19 J. P. Sheu 2

3 2009/10/19 J. P. Sheu 3

4 Frequency Reuse Adjacent cells assigned different frequencies to avoid interference or crosstalk Objective is to reuse frequency in nearby cells 10 to 50 frequencies assigned to each cell Transmission power controlled to limit power at that frequency escaping to adjacent cells The issue is to determine how many cells must intervene between two cells using the same frequency N: reuse factor N= I 2 + J 2 + (I x J) 2009/10/19 J. P. Sheu 4

5 Frequency Reuse 2009/10/19 J. P. Sheu 5

6 2009/10/19 J. P. Sheu 6

7 Capacity Enhancement Adding new channels Frequency borrowing frequencies are taken from adjacent cells by congested cells Cell splitting cells in areas of high usage can be split into smaller cells Cell sectoring cells are divided into a number of wedge-shaped sectors, each with their own set of channels Microcells antennas move to buildings, hills, and lamp posts 2009/10/19 J. P. Sheu 7

8 Micro-cell Macro-cell Pico-cell Figure 3.2. Cell-splitting 2009/10/19 J. P. Sheu 8

9 Sectorization Space Division Multiple Access (SDMA) 2009/10/19 J. P. Sheu 9

10 Channel Allocation Algorithms Fixed channel allocation Allowed to borrow some channels from neighbors Dynamic channel allocation Require a centralized arbitrator to allocate channels Hybrid channel allocation A set of local channels and another set of borrowable channels 2009/10/19 J. P. Sheu 10

11 Cellular System Overview 2009/10/19 J. P. Sheu 11

12 Cellular Systems Terms Base Station (BS) includes an antenna, a controller, and a number of transceivers Mobile telecommunications switching office (MTSO) connects calls between mobile units Two types of channels available between mobile unit and BS Control channels used to exchange information having to do with setting up and maintaining calls Traffic channels carry voice or data connection between users 2009/10/19 J. P. Sheu 12

13 Steps in an MTSO Controlled Call Between Mobile Users Mobile unit initialization MH scans and select the strongest setup control channel Mobile-originated call MH sending the called unit on the pre-selected setup channel Paging The MTSO sends a paging message to certain BSs Call accepted Ongoing call Handoff 2009/10/19 J. P. Sheu 13

14 2009/10/19 J. P. Sheu 14

15 Handoff Performance Metrics Handoff delay delay time in the transfer an ongoing call from the current cell to the new cell Duration of interruption the duration of time during a handoff which is not connected to either BS Handoff success probability probability that a handoff is successful Probability of unnecessary handoff probability of the pin-pong effect 2009/10/19 J. P. Sheu 15

16 Improved Handoff Strategies Prioritization: a certain number of channels are reserved for handoff Relative signal Strength: a minimum time for which an MT must be in a cell before it can request a handoff Soft handoffs: a period of time when more than one BS handles a call can be allowed Predictive handoffs: predict the mobility pattern of mobile users Adaptive handoffs: users may have to be shifted across different layers, from micro- to macro- cellular 2009/10/19 J. P. Sheu -16

17 Handoff Strategies Used to Determine Instant of Handoff Relative signal strength (at L 1 ) Relative signal strength with threshold (at L 2 for Th 2 ) Relative signal strength with hysteresis (at L 3 ) Relative signal strength with hysteresis and threshold (at L 3 for Th 1 or Th 2, at L 4 for Th 3 ) Prediction techniques: based on the expected future value of the received signal strength 2009/10/19 J. P. Sheu 17

18 2009/10/19 J. P. Sheu 18

19 Power Control Design issues making it desirable to include dynamic power control in a cellular system Received power must be sufficiently above the background noise for effective communication Desirable to minimize power in the transmitted signal from the mobile Reduce co-channel interference, alleviate health concerns, save battery power In SS systems using CDMA, it s desirable to equalize the received power level from all mobile units at the BS 2009/10/19 J. P. Sheu 19

20 Types of Power Control Open-loop power control Depends solely on mobile unit No feedback from BS Not as accurate as closed-loop, but can react quicker to fluctuations in signal strength Closed-loop power control Adjusts signal strength in reverse channel based on metric of performance BS makes power adjustment decision and communicates to mobile on control channel 2009/10/19 J. P. Sheu 20

21 2009/10/19 J. P. Sheu 21

22 First-Generation Analog Advanced Mobile Phone Service (AMPS) In North America, two 25-MHz bands allocated to AMPS One for transmission from base to mobile unit One for transmission from mobile unit to base Each band split in two to encourage competition Frequency reuse exploited 2009/10/19 J. P. Sheu 22

23 Differences Between First and Second Generation Systems Digital traffic channels first-generation systems are almost purely analog; second-generation systems are digital Encryption all second generation systems provide encryption to prevent eavesdropping Error detection and correction second-generation digital traffic allows for detection and correction, giving clear voice reception Channel access second-generation systems allow channels to be dynamically shared by a number of users 2009/10/19 J. P. Sheu 23

24 Mobile Wireless TDMA Design Considerations Global System for Mobile Communications (GSM) Number of logical channels (number of time slots in TDMA frame): 8 Maximum cell radius (R): 35 km Frequency: region around 900 MHz,1800 MHz, or 1900 MHz Maximum vehicle speed (V m ):250 km/hr Maximum coding delay: approx. 20 ms Maximum delay spread ( m ): 10 µs (mutipath delay) Bandwidth: Not to exceed 200 khz (25 khz per channel) Maximum data rate for each channel is 34Kbps 2009/10/19 J. P. Sheu 24

25 Control Channels Broadcast Control Channel (BCCH) A downlink channel that contains the BS s idendity and channel status. All MTs monitor the BCCH to detect if the have moved to a new cell. Dedicated Control Channel (DCCH) Used for call-setup, locations updates, and all callmanagement related information exchange. Every call has its own allotted DCCH Common Control Channels (BCCH) Consists of the downlink paging channel to page any MT MT to BS for call-initiation, and the access grant channel 2009/10/19 J. P. Sheu 25

26 2009/10/19 J. P. Sheu 26

27 Speech Coding Traditional speech coding use pulse code modulation (PCM) The data rate of PCM: 64 kbps This rate is undesirably high for use in cellular radio With current technology, 12kbps is enough Due to the maximum coding delay is 20 ms it is reasonable to form the encoded speech into blocks of 20 ms duration, or speech samples of 240 bits 2009/10/19 J. P. Sheu 27

28 ITU s View of Third-Generation Capabilities Voice quality comparable to the public switched telephone network 144 kbps data rate available to users in highspeed motor vehicles over large areas 384 kbps available to pedestrians standing or moving slowly over small areas Support for Mbps for office use Symmetrical / asymmetrical data transmission rates Support for both packet switched and circuit switched data services 2009/10/19 J. P. Sheu 28

29 ITU s View of Third-Generation Capabilities (IMT 2000) An adaptive interface to the Internet to reflect efficiently the common asymmetry between inbound and outbound traffic More efficient use of the available spectrum in general Support for a wide variety of mobile equipment Flexibility to allow the introduction of new services and technologies 2009/10/19 J. P. Sheu 29

30 Table 3.1. Evolution plan to 3G standards Country Existing 2G Standard 3G Standard Europe GSM W-CDMA (UMTS) Japan PDC W-CDMA (DoCoMo) USA IS-95 / cdma one Cdma2000 USA IS-136 UWC /10/19 J. P. Sheu 30

31 Table 3.2. IMT-2000 Service Types Service Upstream Downstrea m Interactive Multimedia High Multimedia Medium Multimedia Switched Data Simple Messaging Example 256 Kbps 256 Kbps Video conference Switching Circuit 20 Kbps 2 Mbps TV Packet 19.2 Kbps 768 Kbps Web surfing Packet 43.2 Kbps 43.2 Kbps Fax Circuit 28.8 Kbps 28.8 Kbps Packet Speech 28.8 Kbps 28.8 Kbps Telephony Circuit 2009/10/19 J. P. Sheu 31

32 The Problems with 3G Systems Difficult to find a common slice of spectrum to enable global roaming Disappointing performance of CDMA in practice It is hard to find suitable applications 2009/10/19 J. P. Sheu 32

33 Table 3.3 CDMA The debate Claims Capacity of 20 times that of AMPS No more dropped calls No problem of interference Quality if speech promised at 8 Kbps Reality Only 3-4 times that of AMPS 40 percent dropped calls when loaded Interference from existing AMPS Had to change to 13 Kbps 2009/10/19 J. P. Sheu 33

34 Wireless in Local Loop Local loop: the last hop connectivity between the subscriber and PSTN Advantages: Easy deployment, high scalability Low investment cost, cost effective 2009/10/19 J. P. Sheu 34

35 FSU FSU Telephone Telephone FSU BTS Telephone Figure 3.6. WLL architecture 2009/10/19 J. P. Sheu 35

36 IEEE Standard Metropolitan area networks (MANs) span several km and cover large parts of cities MANs much larger in size than LANs and their functionalities differ from those LANs IEEE is called air interface for fixed broadband wireless access systems Based on OSI model, specifies air interface including data link layer and physical layer 2009/10/19 J. P. Sheu 36

37 Differences between and IEEE was designed for broadband data such as digital video and telephony The number of users and BW usage per user is much higher than IEEE IEEE is completely connection-oriented and QoS guarantees 2009/10/19 J. P. Sheu 37

38 Physical Layer Uses traditional narrow-band radio (10-66 GHz) with conventional modulation scheme Two new protocols attempt to close to IEEE a operates in the 2 11 GHz and b operates in 5 GHz ISM band Since the signal strength falls off sharply with distance from the BS, the following three modulation scheme are used QAM-64 used by subscribers located near the BS QAM-16 used by subscribers located at intermediate distance from the BS QPSK used by subscribers located far away from the BS 2009/10/19 J. P. Sheu 38

39 Data Link Layer DLL of IEEE can be subdivided into three sublayers Security sublayer: only the payloads are encrypted MAC sublayer: deals with channel management and slot allocation to stations Services specific convergence sublayer: interface to the network layer 2009/10/19 J. P. Sheu 39

40 MAC Sublayer On the downlink, data to the SS is TDM and on the uplink, the medium is shared by the SSs using TDMA Each uplink connection has four classes of services: Constant bit rate service (voice) Real-time variable bit rate service (multimedia) Non-real-time variable bit rate service (file transfer) Best effort service (others) 2009/10/19 J. P. Sheu 40

41 Table 3.4. A brief comparison among IEEE b WLANs, IEEE WMANs, and GSM WWANs Feature IEEE B WLANs IEEE WMANs GSM WWANs Range Few hundred meters Several Km Few tens of Km Frequency 2.4 GHz ISM band GHz 900 or 1800 MHz Physical Layer CCK, BPSK, QPSK QAM-64, QAM16, QPSK GMSK Maximum Data Rates 11 Mbps Mbps 9.6 Kbps/user Medium Access CSMA/CA TDM/TDMA FDD/TDMA QoS Support DCF-No Yes Yes PCF-Yes Connectivity DCF-connectionless Connection oriented Connection oriented PCF-connection oriented Typical Applications Web browsing, Multimedia, digital TV broadcasting Voice 2009/10/19 J. P. Sheu 41

42 Homework 3, 4, /10/19 J. P. Sheu 42