EPL 657 Wireless Environment and Mobility Issues. Andreas Pitsillides, Dept. of Computer Science, UCY

Transcription

1 EPL 657 Wireless Environment and Mobility Issues Andreas Pitsillides, Dept. of Computer Science, UCY 1

2 Overview Why study? Frequency bands The wireless environment Signal distortion wireless channels 2

3 Why study? 3

4 Why study? In a wireless environment (open space) carrying data using radio signals, over given frequency bands: Many additional complexities in comparison to fixed media transmission, (as e.g. electrical signals in copper, or optical in fibre), which can seriously degrade the performance of wireless networking systems 4

5 Wireless networks compared to fixed networks Higher loss-rates due to interference, plus signal attenuation RF emissions of, e.g., engines, lightning Restrictive regulations of frequencies frequencies have to be coordinated, useful frequencies are almost all occupied Low transmission rates (this is changing fast Gbit/sec speeds discussed) local some Mbit/s, regional currently, e.g., 9.6kbit/s with GSM Higher delays, higher jitter (again there is much work being done here; 100s of ms for LTE) connection setup time with GSM in the second range, several hundred milliseconds for other wireless systems Lower security, simpler active attacking radio interface accessible for everyone, base station can be simulated, thus attracting calls from mobile phones Always shared medium secure access mechanisms important

6 Effects of mobility Channel characteristics change over time and location signal paths change different delay variations of different signal parts different phases of signal parts quick changes in the power received (short term fading) Additional changes in distance to sender obstacles further away slow changes in the average power received (long term fading)

7 Mobile communication Two (wishful?) aspects of mobility: user mobility: users communicate (wireless) anytime, anywhere, with anyone device portability: devices can be connected anytime, anywhere to the network Wireless vs. mobile Examples stationary computer notebook in a hotel with fixed access wireless LANs in historic buildings Personal Digital Assistant (PDA) The demand for mobile communication creates the need for integration of wireless networks into existing fixed networks: local area networks: standardization of IEEE Internet: Mobile IP extension of the internet protocol IP wide area networks: e.g., internetworking of 3G/4G and PSTN IoTs, M2M

8 Challenges for wireless / mobile networks 2 grand challenges (beyond those for traditional fixed networks) Wireless link Capacity of link affected by many factors, e.g. (dynamic) spectrum allocation Quality of link connection is subjected to many (environmental) factors and can vary substantially Mobility Wireless link quality is adversely affected by device location 1 (distance) from transmitting / receiving source ( where a d varies between about 2 to 4) Device / node portability

9 Effects of device portability Power consumption limited computing power, low quality displays, small disks due to limited battery capacity CPU: power consumption ~ CV 2 f Loss of data C: internal capacity, reduced by integration V: supply voltage, can be reduced to a certain limit f: clock frequency, can be reduced temporally higher probability, has to be included in advance into the design (e.g., defects, theft) Limited user interfaces compromise between size of fingers and portability integration of character/voice recognition, abstract symbols Limited memory limited value of mass memories with moving parts flash-memory or? as alternative

10 Challenges in wireless / mobile communication Wireless Communication transmission quality (bandwidth, error rate, delay) modulation, coding, interference media access, regulations... Mobility location dependent services location transparency quality of service support (delay, jitter, security)... Portability power consumption limited computing power, sizes of display,... usability... Addressability (especially for Internet connected devices) and security Internet addresses are linked to the Network Point of Attachment (NPA) which has physical meaning In sensor networks a different meaning of addressing

11 Simple reference model used here; not always applicable Application Application Transport Transport Network Network Network Network Data Link Data Link Data Link Data Link Physical Physical Physical Physical Medium Radio Trend toward all-ip networks cross layering?

12 Influence of mobile communication to the layer model Application layer Transport layer Network layer Data link layer Physical layer service location new applications, multimedia adaptive applications congestion and flow control quality of service addressing, routing, device location hand-over authentication media access multiplexing media access control encryption modulation interference attenuation frequency

13 The wireless environment 13

14 Frequencies for communication λ f VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency Frequency and wave length: λ = c/f wave length λ, frequency f speed of light c 3x10 8 m/s, UHF = Ultra High Frequency SHF = Super High Frequency EHF = Extra High Frequency UV = Ultraviolet Light Some frequencies are strictly controlled (pre-assigned by regulating bodies), others are open to use (even by different applications), subject to some given constraints, e.g. Max. Transmit Power 14

15 Frequencies for mobile communication optimum antenna size can be related to λ (λ/2 or similar for optimum power transfer) VHF-/UHF-ranges for mobile radio simple, small antenna for cars deterministic propagation characteristics, reliable connections SHF and higher for directed radio links, satellite communication small antenna, focusing large bandwidth available Wireless LANs use frequencies in UHF to SHF spectrum smaller antenna some systems planned up to EHF limitations due to absorption by water and oxygen molecules (resonance frequencies) 15

16 Recall: Signals physical representation of data function of time and location signal parameters: parameters representing the value of data classification continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values Signal parameters of periodic signals: period T, frequency f=1/t, amplitude A, phase shift ϕ sine wave as special periodic signal for a carrier: s(t) = At sin(2 π ft t + ϕt) 16

17 Transmitted signal <> received signal! Wireless transmission distorts any transmitted signal Received <> transmitted signal; results in uncertainty at receiver about which bit sequence originally caused the transmitted signal Abstraction: Wireless channel describes these distortion effects Sources of distortion Attenuation energy is distributed to larger areas with increasing distance Reflection/refraction bounce of a surface; enter material Absorption energy is absorbed without any reflection Diffraction start new wave from a sharp edge Scattering multiple reflections at rough surfaces Doppler fading shift in frequencies (loss of center) 17

18 Example wireless signal strength in a multi-path environment Brighter color = stronger signal Obviously, simple (quadratic) free space attenuation formula is not sufficient to capture these effects Source / access point Jochen Schiller, FU Berlin 18

19 Distortion effects: Non-line-of-sight paths Because of reflection, scattering,, radio communication is not limited to direct line of sight communication (good or bad?) Effects depend strongly on frequency, thus different behavior at higher frequencies Line-ofsight path Non-line-of-sight path Different paths have different lengths = propagation time Results in delay spread of the wireless channel Closely related to frequency-selective fading properties of the channel With movement: fast fading LOS pulses multipath pulses signal at receiver Jochen Schiller, FU Berlin 19

20 Gain, Attenuation and path loss 20

21 Attenuation results in path loss Effect of attenuation: received signal strength is a function of the distance d between sender and transmitter Captured by Friis free-space equation Describes signal strength at distance d relative to some reference distance d 0 < d for which strength is known d 0 is far-field distance, depends on antenna technology Power received is inversely proportional to distance (free space) 21

22 Suitability of different frequencies Attenuation Attenuation depends on the used frequency Can result in a frequencyselective channel If bandwidth spans frequency ranges with different attenuation properties

23 Generalizing the attenuation formula To take into account stronger attenuation than only caused by distance (e.g., walls, ), use a larger exponent > 2 is the path-loss exponent Rewrite in logarithmic form (in db): Take obstacles into account by a random variation Add a Gaussian random variable with 0 mean, variance 2 to db representation Equivalent to multiplying with a lognormal distributed r.v. in metric units! lognormal fading 23

24 Range and coverage range maximum distance at which two radios can operate and maintain a connection. can use simple geometry to determine the coverage area of an Access Point using the formula to determine the area of a circle (π)r2 where the radius (r) is the range of the Wi-Fi signal. The coverage area of an Access Point is often referred to as a cell and these terms are usually used interchangeably. See tutorial 24

25 Link formulas 25

26 Range Basics Function of data rate (tradeoff) the higher the data rate, the shorter the range. determining the range of an Access Point, a few terms need to be defined and a basic understanding of the mathematics that goes into determining the distance by which a radio signal will travel needs to be provided. In an open environment, or what is referred to as Free Space, Power varies inversely with the square of the distance between two points (the receiver and the transmitter). The stronger the Transmit Power, the higher the signal strength or Amplitude. Antenna Gain also increases Amplitude and will be further discussed. While Gain and Power increase the distance a wireless signal can travel, the expected signal loss (Path Loss) between the transmitter and a receiver reduces it. 26

27 Path Loss and RSSI Path Loss is the reduction in signal strength that a signal experiences as it travels through the air or through objects between the transmitter and receiver. relative strength of that signal at the receiver is measured as the Received Signal Strength Indicator (RSSI). RSSI is normally expressed in dbm or as a numerical percentage. For clarification purposes, a db (Decibel) is a measure of the ratio between two quantities (10Log10 x/y) while dbm is a Decibel with respect to milliwatts of power. An overall Link Budget can be defined by taking into account all the gains and losses of a signal as it moves from a transmitter to a receiver. dbm (sometimes dbmw) is an abbreviation for the power ratio in decibels (db) of the measured power referenced to one milliwatt (mw) note 0dBm is equivalent to 1 milliwatt. It is used in radio, microwave and fiber optic networks as a convenient measure of absolute power because of its capability to express both very large and very small values in a short form. By comparison, the decibel (db) is a dimensionless unit, used for quantifying the ratio between two values, such as signal-tonoise ratio. 27

28 dbm Zero dbm equals one milliwatt. A 3 db increase represents roughly doubling the power, which means that 3 dbm equals roughly 2 mw. For a 3 db decrease, the power is reduced by about one half, making 3 dbm equal to about 0.5 milliwatt. To express an arbitrary power P as x dbm, or go in the other direction, the following equations may be used: or, where P is the power in W and x is the power ratio in dbm. 28

29 Below is a table summarizing useful cases: dbm level Power Notes 80 dbm 100 kw Typical transmission power of FM radio station with 50 km range 60 dbm 1 kw = 1000 W 50 dbm 100 W 40 dbm 10 W 37 dbm 5 W 36 dbm 4 W 33 dbm 2 W 30 dbm 1 W = 1000 mw 27 dbm 500 mw Typical combined radiated RF power of microwave oven elements Maximum allowed output RF power from a ham radio transceiver (rig) without special permissions Typical thermal radiation emitted by a human body Typical maximum output RF power from a ham radio transceiver (rig) Typical PLC (Power Line Carrier) Transmit Power Typical maximum output RF power from a hand held ham radio transceiver (rig) Typical maximum output power for a Citizens' band radio station (27 MHz) in many countries Maximum output from a UMTS/3G mobile phone (Power class 1 mobiles) Maximum output from a GSM850/900 mobile phone Typical RF leakage from a microwave oven - Maximum output power for DCS 1800 MHz mobile phone Maximum output from a GSM1800/1900 mobile phone Typical cellular phone transmission power Maximum output from a UMTS/3G mobile phone (Power class 2 mobiles) 26 dbm 400 mw Access point for Wireless networking 29

30 24 dbm 250 mw Maximum output from a UMTS/3G mobile phone (Power class 3 mobiles) 23 dbm 200 mw Maximum output in interior environment from a WiFi 2.4Ghz antenna (802.11b/g/n). 22 dbm 160 mw 21 dbm 125 mw Maximum output from a UMTS/3G mobile phone (Power class 4 mobiles) 20 dbm 100 mw Bluetooth Class 1 radio, 100 m range Maximum output power from unlicensed AM transmitter per U.S. Federal Communications Commission (FCC) rules [1]. Typical wireless router transmission power. 15 dbm, 10 dbm, 6 dbm, 5 dbm, 4 dbm 32 mw, 10 mw, 4.0 mw, 3.2 mw, 2.5 mw Typical WiFi transmission power in laptops. 3 dbm 2.0 mw Bluetooth Class 2 radio, 10 m range More precisely (to 8 decimal places) mw 30

31 0 dbm 1.0 mw = 1000 µw Bluetooth standard (Class 3) radio, 1 m range 1 dbm 794 µw 3 dbm 501 µw 5 dbm 316 µw 10 dbm 100 µw Typical maximum received signal power ( 10 to 30 dbm) of wireless network 20 dbm 10 µw 30 dbm 40 dbm 50 dbm 60 dbm 1.0 µw = 1000 nw 100 nw 10 nw 1.0 nw = 1000 pw The Earth receives one nanowatt per square metre from a magnitude +3.5 star [2] 70 dbm 80 dbm 100 dbm 111 dbm 100 pw 10 pw 0.1 pw pw = 8 fw Typical range ( 60 to 80 dbm) of wireless received signal power over a network ( variants) Thermal noise floor for commercial GPS single channel signal bandwidth (2 MHz) dbm fw = 178 aw Typical received signal power from a GPS satellite 174 dbm aw = 4 zw Thermal noise floor for 1 Hz bandwidth at room temperature (20 C) dbm zw = 56 yw dbm 0 W Thermal noise floor for 1 Hz bandwidth in outer space (4 kelvins) Zero power is not well-expressed in dbm (value is negative infinity) 31

32 Antennas: isotropic radiator How do we get signals through space? E.M radiation. 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 e.m. radiation around an antenna See tutorial 32

33 Antennas: directed and sectorized Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley) 33

34 Antennas: directed and sectorized Cell sizes 34

35 Antenna gain Antenna Gain (also known as Amplification) improves range of an antenna extends range of a Wi-Fi network. Gain refers to an increase of the Amplitude or Signal Strength One of the advantages of a directional antenna (e.g. a dipole) is greater antenna Gain; this is a result of the RF energy pattern being focused vs. an isotropic design. Other types of antennas are more directional in design taking their radiated energy and squeezing it into a very narrow pattern. good analogy: think of the isotropic antenna like a light bulb radiating energy equally in all directions, and the directional antenna like a flash light with the light focused in one direction the energy of the directional antenna is concentrated in a particular direction, enabling the beam to travel much farther than an isotropic antenna. Antenna Gain is bi-directional so it will amplify the signal as it is being transmitted and as it is received. So if a directional antenna is providing 6db Gain on transmit, it will also increase received sensitivity an equal amount, so the antenna design of the Wi-Fi Access Point plays a critical role in the amount of range (coverage) delivered. 35

36 Antenna gain basics dbi dbd db(isotropic) the forward gain of an antenna compared with the hypothetical isotropic antenna, which uniformly distributes energy in all directions. Linear polarization of the EM field is assumed unless noted otherwise. db(dipole) the forward gain of an antenna compared with a half-wave dipole antenna. 0 dbd = 2.15 dbi 36

37 Attenuation RF signal strength is reduced as it passes through various materials. This effect is referred to as Attenuation. As more Attenuation is applied to a signal, its effective range will be reduced. The amount of Attenuation will vary greatly based on the composition of the material the RF signal is passing through. Note: A change in power ratio by a factor of two is approximately a 3 db change 20dB is a factor of

38 EIRP EIRP - Effective Isotropic Radiated Power EIRP = Power out (dbm) + antenna gain (dbi) cable loss (db) EIRP Regulations 38

39 Simplistic Range Calculations The Model For indoor environment the signal power at the receiver SRx is related to the transmit power TRx as shown below (this model can be used as the reference analysis model) Where C=speed of light, f=center frequency, N: path loss coefficient. ITU recommends N=3.1 for 5-GHz and N=3 for 2.4-GHz

40 Simplistic Range Calculations IEEE b (with N=3) With EIRP of 30dBm max range=154m With EIRP of 19dBm max range=66.4m With EIRP of 15dBm max range=48.4m IEEE a (with N=3.1) With EIRP of 18dBm range=14m with 54Mbits /s With EIRP of 23dBm range=30m with 54Mbits/s

41 Receiver Sensitivity For IEEE b receiver should be able to detect - 76dBm with BER of min 10e-5 in the absence of Adjacent Chanel Interference (ACI). If ACI is present the receiver must be able to detect -70dBm For IEEE a as follows

42 Link Budget Example: Consider a WLAN access point (AP) transmitting to an AP 1.5 km away Transmistting antenna gain = 13.5 dbi transmitting power = 100 mw Distance to receiver AP = 1500 metres Receiving AP antenna gain =13.5 dbi Rx sensitivity = -82 dbi. The free space path loss = db. The Rx Power Level = = The Loss Budget equals -(-82) (safety margin) = 14.7 Because 14.7 is greater than 0, the link will work. 42

43 Signals in noise and interference 43

44 Signal-to-Noise Ratio (SNR) The range of an Access Point is a function of data rate. notion that higher data rates do not appear to travel as far as the lower data rates is a function of the Signal to Noise Ratio (SNR) and not because the Access Point and the client can t necessarily hear each other. SNR is the ratio of the desired signal to that of all other noise and interference as seen by a receiver. SNR is important as it determines which data rates can be correctly decoded in a wireless link. It is expressed in db as a ratio. The received signal and the noise level, determine the SNR. As data rates increase from 6 Mbps to 54 Mbps, more complex modulation and encoding methods are used that require a higher SNR to properly decode the signal. E.g. a 54 Mbps per second signal requires 25 db of SNR: signal will not be properly decoded at greater distances because as the signal moves further from the source, a greater amount of path loss occurs (the signal is attenuated). Lower data rate transmissions, can be more easily decoded and as a result appear to travel farther. E.g. in an outdoor environment with just free space loss, a 6 Mbps signal can actually be decoded 7 times further away than a 54 Mbps. 44

45 SNR for different modulation schemes The more complex (and higher efficiency) modulation schemes require higher SNR to decode signal 45

46 Noise and interference So far: only a single transmitter assumed Only disturbance: self-interference of a signal with multi-path copies of itself In reality, two further disturbances Noise due to effects in receiver electronics, depends on temperature Typical model: an additive Gaussian variable, mean 0, no correlation in time Interference from third parties Co-channel interference: another sender uses the same spectrum Adjacent-channel interference: another sender uses some other part of the radio spectrum, but receiver filters not good enough to fully suppress it Effect: Received signal is distorted by channel, corrupted by noise and interference What is the result on the received bits? 46

47 Symbols and bit errors Extracting symbols out of a distorted/corrupted wave form is fraught with errors Depends essentially on strength of the received signal compared to the corruption Captured by signal to noise and interference ratio (SINR) SINR allows to compute bit error rate (BER) for a given modulation Also depends on data rate R (# bits/symbol) of modulation E.g., for simple DPSK, data rate corresponding to bandwidth: 47

48 Examples for SINR! BER mappings BER Coherently Detected BPSK Coherently Detected BFSK e-05 1e-06 1e SINR 48

49 Signal Important quantities Important quantities to measure the strength of the signal to the receiver, noise, interference e.g. SNR. Signal to Noise Ratio in db SIR = Signal to Interference Ratio; received power of reference user in dbm/received power of all interferers in dbm C/I. Carrier over Interference in db Carrier Power (dbm) / received power of all interferers in dbm 49

50 Signal Important quantities - Examples SNR Signal to Noise Ratio Assumptions to simplify things: - All the users are equally distributed in the coverage area so that they have equal distances to the TRX Antenna - The power level they use is the same thus the interference they cause is on the same level. - All the UEs use the same Baseband rate e.g. 60 kbits/sec for Streaming Video. If assumed that there are X users under the same TRX Coverage (in the same Cell) and the above assumptions are applied, it means that there are X 1 users causing interference to one (1) user. This indicates the Signal to Noise Ratio and when expressed in mathematical format the outcome is the following equation: SNR P P ( X 1) Where P is the power required for information transfer in one channel and is a multiple of the energy used per bit (E b ) and the Baseband rate ( P = E b x Baseband rate) 50

51 Bit Error Rate IEEE b for BER better than 10e-5 then min S/N IEEE a for BER better than 10e-5 then min S/N

52 Signal Important quantities - Examples SIR Signal to Interference Ratio The Signal to Interference Ratio (SIR) at the receiver is considered as a quality parameter and is determined by the ratio of the desired signal power to the total interference power from all the other users. For e.g. The capacity of CDMA is limited by the amount of interference that can be tolerated from other users. System Capacity is maximized if the transmitted power of each terminal is controlled so that its signal arrives at the Base Station with the minimum required SIR. If a terminal's signal arrives at the Base Station with a too low received power value then the required QoS of the radio Connection can not be met. If the received power value is too high, the performance of this terminal is good, however, interference to all other terminal transmitters sharing the channel is increased and may result is unacceptable performance for other users, unless their number is reduced. MORE LATER WHEN DISCUSSING RRM TECHNIQUES FOR 3G 52

53 Signal Important quantities - Examples C/I Carrier to Interference Ratio The Wideband Signal to Interference (SIR) Ratio is also called as Carrier to Interference Ratio (C/I). The Carrier to Interference (C/I) Ratio is very important in Cellular systems in order to determine the maximum allowed interference level for which the system will work. Eb/No: The Required Eb/No (measured in db) for a service denotes the value that the signal energy per bit (Eb) divided by the interference and noise power density (No) should have for achieving a certain BER (Bit Error Rate) so as to satisfy the required QoS of a service. Eb/No is the measure of signal to noise ratio for a digital communication system. It is measured at the input to the receiver and is used as the basic measure of how strong the signal is. it is the fundamental prediction tool for determining a digital link's performance. Another, more easily measured predictor of performance is the carrier-to-noise or C/N ratio See f b -bit rate, Bw receiver noise bandwidth 53

54 Signal Important quantities Eb/No. Signal Energy per bit to noise Power Density per hertz. These curves show the best performance that can be achieved across a digital link with a given amount of RF power. -Eb/No = Signal energy (per bit ) dbm / noise Power dbm. Measures how strong the signal is. -Different forms of modulation BPSK, QPSK, QAM, etc. have different curves of theoretical bit error rates versus Eb/No. Eb/No e.g. For DBPSK/DQPSK 8dB required Eb/No is adequate to achieve a desired BER of 10E-3 db 54

55 Example calculation Consider a 12.2 kbps speech service spread over a 5 MHz Carrier and that an Eb/No of 5.0 db is required to achieve a 0.01 BER performance. After the dispreading in the receiver, the signal power needs to be typically a few decibels (db) above the interference and noise power. Since an Eb/No of 5.0 db is enough for efficiently detecting the signal, the required wideband Signal to Interference Ratio (SIR) will be 5.0 db minus the Processing Gain of 25 db that can be achieved for the corresponding service (10 x log (WCDMA Chip Rate/Bit Rate)). The chip rate is equal with 3.84 Mcps. Thus, the signal power can be 20 db under the interference and thermal noise power, and the WCDMA receiver can still efficiently detect and interpret the signal correctly. 55

56 56

57 The big picture... 57

58 Effects of Mobility on channel 58

59 Effects of mobility on channel Channel characteristics change over time and location signal paths change different delay variations of different signal parts different phases of signal parts quick changes in the power received (short term fading) Additional changes in distance to sender obstacles further away slow changes in the average power received (long term fading) See mobility models papers for modelling Mobility paper 1, paper 2 59

60 Supplementary slides 60

61 Signal propagation ranges Transmission range communication possible low error rate Detection range detection of the signal possible no communication possible Interference range signal may not be detected signal adds to the background noise 61

62 Signal propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d n (d = distance between sender and receiver, n depends on medium, usually 2, but can be higher, e.g. 4, see later) Receiving power additionally influenced by fading (frequency dependent) shadowing reflection at large obstacles refraction depending on the density of a medium scattering at small obstacles diffraction at edges 62

63 Real world example signal coverage 63

64 Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction Time dispersion: signal is dispersed over time interference with neighbor symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted distorted signal depending on the phases of the different parts 64

65 Typical large-scale path loss 65

66 Measured large-scale path loss 66

67 Partition losses 67

68 Measured indoor path loss 68

69 Measured indoor path loss 69

70 Measured received power levels over a 605 m 38 GHz fixed wireless link in clear sky, rain, and hail [from [Xu00], IEEE]. 70

71 Measured received power during rain storm at 38 GHz [from [Xu00], IEEE]. 71