Mobile Computing Unit 1 WIRELESS COMMUNICATION FUNDAMENTALS

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1 WIRELESS COMMUNICATION FUNDAMENTALS Objective Unit I present some basics about wireless transmission technology. The topics covered include: frequencies used for communication, signal characteristics, antennas, signal propagation, and several fundamental multiplexing and modulation schemes. This unit does not require profound knowledge of electrical engineering nor does it explore all details about the underlying physics of wireless communication systems. Its aim is rather to help the reader understand the many design decisions in the higher layers of mobile communication systems. Also, it presents a broad range of media access technologies. It explains why media access technologies from fixed networks often cannot be applied to wireless networks, and shows the special problems for wireless terminals accessing space as the common medium. Different multiplexing schemes are also discussed. Introduction Computers for the next decades? Computers are integrated o small, cheap, portable, replaceable -no more separate devices Technology is in the background o computer are aware of their environment and adapt ( location awareness ) o computer recognize the location of the user and react appropriately (e.g., call forwarding, fax forwarding, context awareness )) Advances in technology o more computing power in smaller devices o flat, lightweight displays with low power consumption o new user interfaces due to small dimensions o more bandwidth per cubic meter o multiple wireless interfaces: wireless LANs, wireless WANs, regional wireless telecommunication networks etc. ( overlay networks ) Mobile communication Two aspects of mobility: o user mobility: users communicate (wireless) anytime, anywhere, with anyone o device portability: devices can be connected anytime, anywhere to the network Wireless vs. mobile Examples x x stationary computer x notebook in a hotel x wireless LANs in historic buildings 1

Personal Digital Assistant (PDA) The demand for mobile communication creates the need for integration of wireless networks into existing fixed networks: o local area networks: standardization of IEEE

2 Personal Digital Assistant (PDA) The demand for mobile communication creates the need for integration of wireless networks into existing fixed networks: o local area networks: standardization of IEEE o o Applications Internet: Mobile IP extension of the internet protocol IP wide area networks: e.g., internetworking of GSM and ISDN, VoIP over WLAN and POTS Vehicles o transmission of news, road condition, weather, music via DAB/DVB-T o personal communication using GSM/UMTS o position via GPS o local ad-hoc network with vehicles close-by to prevent accidents, guidance system, redundancy o vehicle data (e.g., from busses, high-speed trains) can be transmitted in advance for maintenance Emergencies o early transmission of patient data to the hospital, current status, first diagnosis o replacement of a fixed infrastructure in case of earthquakes, hurricanes, fire etc. o crisis, war,... Typical Application Mobile and wireless services Always Best Connected 2

Applications II Traveling salesmen o direct access to customer files stored in a

Replacement of fixed networks o remote sensors, e.g.

3 Applications II Traveling salesmen o direct access to customer files stored in a central location o consistent databases for all agents o mobile office Replacement of fixed networks o remote sensors, e.g., weather, earth activities o flexibility for trade shows o LANs in historic buildings Entertainment, education,... o outdoor Internet access o intelligent travel guide with up-to-date location dependent information o ad-hoc networks for multi user games Location dependent services 3

4 Location aware services o what services, e.g., printer, fax, phone, server etc. exist in the local environment Follow-on services o automatic call-forwarding, transmission of the actual workspace to the current location Information services o push : e.g., current special offers in the supermarket o pull : e.g., where is the Black Forrest Cheese Cake? Support services o caches, intermediate results, state information etc. follow the mobile device through the fixed network Privacy who should gain knowledge about the location Mobile devices Effects of device portability Power consumption o limited computing power, low quality displays, small disks due to limited battery capacity o CPU: power consumption ~ CV2f C: internal capacity, reduced by integration V: supply voltage, can be reduced to a certain limit f: clock frequency, can be reduced temporally Loss of data o higher probability, has to be included in advance into the design (e.g., defects, theft) 4

5 Limited user interfaces o compromise between size of fingers and portability o integration of character/voice recognition, abstract symbols Limited memory o limited usage of mass memories with moving parts o flash-memory or? as alternative Wireless networks in comparison to fixed networks Higher loss-rates due to interference o emissions of, e.g., engines, lightning Restrictive regulations of frequencies o frequencies have to be coordinated, useful frequencies are almost all occupied Low transmission rates o local some Mbit/s, regional currently, e.g., 53kbit/s with GSM/GPRS or about 150 kbit/s using EDGE Higher delays, higher jitter o connection setup time with GSM in the second range, several hundred milliseconds for other wireless systems Lower security, simpler active attacking o radio interface accessible for everyone, base station can be simulated, thus attracting calls from mobile phones Always shared medium o secure access mechanisms important Wireless Transmission o Frequencies o Signals, antennas, signal propagation o Multiplexing o Spread spectrum, modulation Cellular systems Frequencies for communication 5

6 Frequencies for mobile communication VHF-/UHF-ranges for mobile radio o simple, small antenna for cars o deterministic propagation characteristics, reliable connections SHF and higher for directed radio links, satellite communication o small antenna, beam forming o large bandwidth available Wireless LANs use frequencies in UHF to SHF range o some systems planned up to EHF o limitations due to absorption by water and oxygen molecules (resonance frequencies) weather dependent fading, signal loss caused by heavy rainfall etc. Frequencies and regulations ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) 6

7 Signals physical representation of data function of time and location signal parameters: parameters representing the value of data classification o continuous time/discrete time o continuous values/discrete values o analog signal = continuous time and continuous values o digital signal = discrete time and discrete values signal parameters of periodic signals: period T, frequency f=1/t, amplitude A, phase shift ϕφ o sine wave as special periodic signal for a carrier: s(t) = Atsin(2 πft t + ϕt) Fourier representation of periodic signals 7

8 Different representations of signals o amplitude (amplitude domain) o frequency spectrum (frequency domain) o phase state diagram (amplitude M and phase ϕ in polar coordinates) o o Composed signals transferred into frequency domain using Fourier transformation Digital signals need infinite frequencies for perfect transmission modulation with a carrier frequency for transmission (analog signal! Antennas: isotropic radiator Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission Isotropic radiator: equal radiation in all directions (three dimensional) -only a theoretical reference antenna Real antennas always have directive effects (vertically and/or horizontally) Radiation pattern: measurement of radiation around an antenna 8

Signal propagation ranges Transmission range o communication possible o low error rate Detection range o detection of the signal possible o no communication possible Interference range o signal may

9 Signal propagation ranges Transmission range o communication possible o low error rate Detection range o detection of the signal possible o no communication possible Interference range o signal may not be detected o signal adds to the background noise Multiplexing Multiplexing in 4 dimensions o space (si) o time (t) o frequency (f) o code (c) Goal: multiple use of a shared medium Important: guard spaces needed! Modulation 9

Digital modulation o digital data is translated into an analog signal (baseband) o ASK, FSK, PSK -main focus in this chapter o differences in spectral efficiency, power efficiency, robustness Analog

10 Digital modulation o digital data is translated into an analog signal (baseband) o ASK, FSK, PSK -main focus in this chapter o differences in spectral efficiency, power efficiency, robustness Analog modulation o shifts center frequency of baseband signal up to the radio carrier Motivation o smaller antennas (e.g., λ/4) o Frequency Division Multiplexing o medium characteristics Basic schemes o Amplitude Modulation (AM) o Frequency Modulation (FM) o Phase Modulation (PM) Spread spectrum technology Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference Solution: spread the narrow band signal into a broad band signal using a special code protection against narrow band interference 0 Side effects: coexistence of several signals without dynamic coordination tap-proof Alternatives: Direct Sequence, Frequency Hopping 10

11 MEDIUM ACCESS CONTROL Can we apply media access methods from fixed networks? Example of CSMA/CD Carrier Sense Multiple Access with Collision Detection send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) Problems in wireless networks a radio can usually not transmit and receive at the same time signal strength decreases proportionally to the square of the distance or even more the sender would apply CS and CD, but the collisions happen at the receiver it might be the case that a sender cannot hear the collision, i.e., CD does not work furthermore, CS might not work if, e.g., a terminal is hidden Hidden and exposed terminals Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a free medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is hidden for C 11

12 A B C Exposed terminals B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is exposed to B Motivation - near and far terminals Terminals A and B send, C receives signal strength decreases proportional to the square of the distance the signal of terminal B therefore drowns out A s signal C cannot receive A A B C If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer Also severe problem for CDMA-networks - precise power control needed! Access methods SDMA/TDMA/FDMA/CDMA 12

13 SDMA (Space Division Multiple Access) segment space into sectors, use directed antennas cell structure TDMA (Time Division Multiple Access) assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time FDMA (Frequency Division Multiple Access) assign a certain frequency to a transmission channel between a sender and a receiver permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) CDMA (Code Division Multiple Access) assign an appropriate code to each transmission channel (DSSS, Direct Sequency Spread Spectrum) frequency hopping over separate channels (FHSS, Frequency Hopping Spread Spectrum) Some medium access control mechanisms for wireless 13

14 SDMA TDMA FDMA CDMA Fixed Used in GSM FHSS DSSS Used in Bluetooth Used in UMTS Fixed Aloha CSMA Reservations DAMA Pure Slotted Non-persistent p-persistent CSMA/CA Used in (mandatory) Multiple Access with Collision Avoidance Copes with hidden and exposed terminal RTS/CTS Used in (optional) Polling MACAW MACA-BI FAMA CARMA TDMA/TDD example: DECT 417 µs downlin k uplin k 1 2 t DECT: Digital Enhanced Cordless Telecommunications TDD: Time Division Duplex 14

15 FDMA/FDD example: GSM 960 MHz f downlin k MHz khz 915 MHz MHz MHz 1 uplin k t Aloha/slotted aloha Mechanism random, distributed (no central arbiter), time-multiplex Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries Aloha collision sender A sender B sender C Slotted Aloha 15

16 collision sender A sender B sender C Carrier Sense Multiple Access (CSMA) t Goal: reduce the wastage of bandwidth due to packet collisions Principle: sensing the channel before transmitting (never transmit when the channel is busy) Many variants: Collision detection (CSMA/CD) or collision avoidance(csma/ca) Persistency (in sensing and transmitting) 1-Persistent CSMA Stations having a packet to send sense the channel continuously, waiting until the channel becomes idle. As soon as the channel is sensed idle, they transmit their packet. If more than one station is waiting, a collision occurs. Stations involved in a collision perform a the backoff algorithm to schedule a future time for resensing the channel Optional backoff algorithm may be used in addition for fairness Non-Persistent CSMA Attempts to reduce the incidence of collisions Stations with a packet to transmit sense the channel If the channel is busy, the station immediately runs the back-off algorithm and reschedules a future sensing time If the channel is idle, then the station transmits Demand Assigned Multiple Accesses (DAMA): 16

17 Channel efficiency only 18% for Aloha, 36% for Slotted Aloha Reservation can increase efficiency to 80% a sender reserves a future time-slot sending within this reserved time-slot is possible without collision reservation also causes higher delays typical scheme for satellite links Examples for reservation algorithms: Explicit Reservation (Reservation-ALOHA) Implicit Reservation (PRMA) Reservation-TDMA DAMA / Explicit Reservation Explicit Reservation (Reservation Aloha): two modes: ALOHA mode for reservation: competition for small reservation slots, collisions possible reserved mode for data transmission within successful reserved slots (no collisions possible) it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time collision Aloha reserved Aloha reserved Aloha reserved Aloha t DAMA / Packet reservation (PRMA) Implicit reservation based on slotted Aloha a certain number of slots form a frame, frames are repeated stations compete for empty slots according to the slotted aloha principle 17

18 once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send competition for a slot starts again as soon as the slot was empty in the last frame reservation ACDABA-F ACDABA-F AC-ABAF- A---BAFD ACEEBAFD frame 1 frame 2 frame 3 frame 4 frame time-lot A C D A B A F A C A B A A B A F A B A F D A C E E B A F D DAMA / Reservation-TDMA Reservation Time Division Multiple Access every frame consists of N mini-slots and x data-slots every station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k). other stations can send data in unused data-slots according to a round-robin N minislots sending scheme (best-effort traffic) N * k dataslots e.g. N=6, k=2 reservations for dataslots Polling mechanisms If one terminal can be heard by all others, this central terminal (e.g., base station) can poll all other terminals according to a certain scheme all schemes known from fixed networks can be used (typical mainframe - terminal scenario) Example: Randomly Addressed Polling other stations can use free dataslots based on a round-robin scheme 18

19 base station signals readiness to all mobile terminals terminals ready to send can now transmit a random number without collision with the help of CDMA or FDMA (the random number can be seen as a dynamic address) the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address) the base station acknowledges correct packets and continues polling the next terminal this cycle starts again after polling all terminals of the list Inhibit Sense Multiple Access (ISMA) Current state of the medium is signaled via a busy tone the base station signals on the downlink (base station to terminals) if the medium is free or not terminals must not send if the medium is busy terminals can access the medium as soon as the busy tone stops the base station signals collisions and successful transmissions via the busy tone and acknowledgements, respectively (media access is not coordinated within this approach) mechanism used, e.g., for CDPD (Cellular Digital Packet Data) Similar approach was proposed for Packet Radio Networks (Kleinrock + Tobagi, 1975) Code Division Multiple Access Principles 19

20 all terminals send on the same frequency and can use the whole bandwidth of the transmission channel each sender has a unique code The sender XORs the signal with this code the receiver can tune into this signal if it knows the code of the sender tuning is done via a correlation function Disadvantages: higher complexity of the receiver (receiver cannot just listen into the medium and start receiving if there is a signal) all signals should have approximately the same strength at the receiver Advantages: all terminals can use the same frequency, no planning needed huge code space (e.g., 232) compared to frequency space more robust to eavesdropping and jamming (military applications ) forward error correction and encryption can be easily integrated Principle (very simplified) Spreading Despreading A k A k A d X A s X C+ D A d A s + B s B k B k B d Example: X B s X C+ D B d 20

21 Sender A sends Ad = 1, key Ak = (assign: 0 = -1, 1 = +1) sending signal As = Ad * Ak = (-1, +1, -1, -1, +1, +1) Sender B sends Bd = 0, key Bk = (assign: 0 = -1, 1 = +1) sending signal Bs = Bd * Bk = (-1, -1, +1, -1, +1, -1) Both signals superimpose in space interference neglected (noise etc.) As + Bs = (-2, 0, 0, -2, +2, 0) Receiver wants to receive signal from sender A apply key Ak bitwise (inner product) Ae = (-2, 0, 0, -2, +2, 0) Ak = = 6 result greater than 0, therefore, original bit was 1 receiving B Be = (-2, 0, 0, -2, +2, 0) Bk = = -6, i.e. 0 SAMA (Spread Aloha Multiple Access) Aloha has only a very low efficiency, CDMA needs complex receivers to be able to eceive different senders with individual codes at the same time. Idea: use spread spectrum with only one single code (chipping sequence) for spreading for all senders accessing according to aloha sender Asender B spread the signal e.g. using the chipping sequence ( CDMA without CD ) narro w band send for a shorter period with higher power Problem: find a chipping sequence with good characteristics t Comparison SDMA/TDMA/FDMA/CDMA 21

Summary This unit introduced the basics of wireless communication. As we have only one medium for wireless transmission, several multiplexing schemes can be applied to raise the overall capacity.

22 Summary This unit introduced the basics of wireless communication. As we have only one medium for wireless transmission, several multiplexing schemes can be applied to raise the overall capacity. The standard schemes are SDM, FDM, TDM and CDM. To achieve FDM, data has to be translated into a signal with a certain carrier frequency. Therefore, tow modulation steps can be applied. Digital modulation encodes data into a base band signal, whereas analog modulation encodes data into a base band signal, whereas analog modulation then shifts the centre frequency of the signal up to the radio carrier. Some advanced schemes have been presented that can code many bits into a single phase shift, raising the efficiency. Keywords SAMA (Spread Aloha Multiple Access) CDMA(Code Division Multiple Access ) CSMA(Carrier Sense Multiple Access ) FDMA(Frequency Division Multiple Access) TDMA(Time Division Multiple Access) SDM Space Division Multiplexing FDM- Frequency division multiplexing TDM- Time Division Multiplexing 22

23 CDM- Code Division Multiplexing Multiple choice questions 1. CDMA with only a single code, is called a)sama b) CDMA c)fdma d)tdma systems use exactly these codes to separate different users in code space and to enable access to a shared medium without interference. a)sama b) CDMA c)fdma d)tdma 3. In a sender senses the medium (a wire or coaxial cable) to see if it is free. If the medium is busy, the sender waits until it is free. If the medium is free, the sender starts transmitting data and continues to listen into the medium. a)cdma b)csma c)fdma d)tdma comprises all algorithms allocating frequencies to transmission channels according to the frequency division multiplexing (FDM) scheme. a)cdma b)csma c)fdma d)tdma comprises all technologies that allocate certain time slots for communication. a)cdma b)csma c)fdma d)tdma was to provide a mobile phone system that allows users to roam throughout Europe and provides voice services compatible to ISDN and other PSTN systems (a)gps (b)gsm (c)cdma (d)tetra 7. Separation of whole spectrum into smaller frequency bands is (a)sdm (b)fdm (c)tdm (d)cdm 8.Precise Synchornization is necessary in (a)sdm (b)fdm (c)tdm (d)cdm 9. Each Channel has unique code and all the channels use the same spectrum at the same time is (a)sdm (b)fdm (c)tdm (d)cdm 10. Which are the following multiplexing are used for secured wireless transmission? (a)sdm (b)fdm (c)tdm (d)cdm 23

24 Part-A (2 Marks) 24

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27 Part B 1. Explain about Mobile services (16) 2. Explain System architecture (16) 3. Explain briefly about TETRA (16) 4. Explain about UTRAN (16) Review Questions and Exercises 27

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References Introduction.pdf Wireless_Transmission.pdf http://cst.mi.fu-berlin.de/resources/mobkom/material/english/pdf-handout/c01- http://cst.mi.fu-berlin.de/resources/mobkom/material/english/pdf-handout/c02- http://cst.

29 References Introduction.pdf Wireless_Transmission.pdf Media_Access.pdf 29

Mobile Computing. Chapter 3: Medium Access Control

Mobile Computing. Chapter 3: Medium Access Control Mobile Computing Chapter 3: Medium Access Control Prof. Sang-Jo Yoo Contents Motivation Access methods SDMA/FDMA/TDMA Aloha Other access methods Access method CDMA 2 1. Motivation Can we apply media access