Introduction to Wireless Networks

1. Introduction to Wireless Networks

2. Wireless Network • A simplified diagram • Base station provides users with radio access to the Internet that delivers messages among users QAM 64 QAM 16 QPSK Adaptive Modulation and Coding

3. Different Types of Wireless Networks • Coverage size differs WWAN UMTS / LTE / 5G WMAN IEEE802.16(WiMAX) WLAN IEEE802.11/ HyperLAN WPAN Bluetooth WAN WAN-MAN PAN MAN MAN-LAN LAN-PAN Pico-Cell Personal Operating Space ~50km ~2km 0km ~10m

4. Signal Strength vs. Link Speed?

5. Screenshots of using Raspberry Pi Signal Strength vs. Link Speed?

6. Screenshots of using G-NetTrack Lite Signal Strength vs. Link Speed? https://www.youtube.com/watch?v=8IKZL9qf18A

7. 好奇想知道 … • Quantitative relationship (if any) between the received signal strength (RSS) and link speed? • How to derive RSS? In what unit? • For RSS, how good is good? How bad is bad? • Factors affecting RSS? Which are major factors? • What toolkits can be used to measure RSS? • Over Android, iPhone, Windows, Linux • Relevant measurements • RSSI (received signal strength indicator) • RCPI (received channel power indicator) • SNR (signal-to-noise ratio) • CINR (carrier to interference-plus-noise ratio) • …

8. Why Wireless? • Communications on the move • Mobile users, fleets of trucks, taxis, buses, … • Flexibility • Portable office • For rescue workers at disaster sites • Military use • Reducing line costs (cabling saved)

9. Electromagnetic (電磁) Signal • A function of time • Can also be expressed as a function of frequency • Signal consists of components of different frequencies

10. Time-Domain Concepts • Analog signal: signal intensity varies in a smooth fashion over time • Digital signal: signal intensity maintains a constant level for some period of time and then changes to another constant level

11. Time-Domain Concepts • Peak amplitude(A): maximum value or strength of the signal over time; typically measured in volts • Frequency (f ): rate, in cycles per second, or Hertz (Hz) at which the signal repeats • Period (T): amount of time it takes for one repetition of the signal (T = 1/f ) • Phase (): measure of the relative position in time within a single period of a signal • Wavelength (): distance occupied by a single cycle of the signal • Distance between two points of corresponding phase of two consecutive cycles •  = v T (v: velocity)

12. Sine Wave Parameters • General sine wave • s(t) = A sin(2ft + ) • The next slide shows the effect of varying each of the three parameters (a) A = 1, f = 1 Hz,  = 0; thus T = 1s (b) Reduced peak amplitude; A=0.5 (c) Increased frequency; f = 2, thus T = ½ (d) Phase shift;  = /4 radians (45 degrees) • Note: 2 radians = 360° = 1 period

13. Sine Wave Parameters A=1; f=1 A=0.5 f=2  = /4

14. Frequency Decomposition An electromagnetic signal can be made up of many frequencies f = 1/T + = f = 3/T

15. Frequency-Domain Concepts • Fundamental frequency: when all of frequency components of a signal are integer multiples of some frequency f, f is called the fundamental frequency • Spectrum: range of frequencies that a signal contains • 頻譜 = 訊號頻率的變動範圍 • Absolute bandwidth: width of the spectrum of a signal • 頻譜的寬度 • Effective bandwidth (or just bandwidth): narrow band of frequencies that most of the signal’s energy is contained in

16. Fourier Series • Any reasonably behaved periodic function, g(t), with period T can be constructed by summing a (possibly infinite) number of sines and cosines: • f=1/T is the fundamental frequency • an and bn are the sine and cosine amplitudes of the n-th harmonics (terms)

17. Fourier Series: Harmonics b1 a1 1st harmonic 0 0 -a1 T T -b1 a2 b2 2nd harmonic 0 0 -a2 T -b2 T b3 a3 3rd harmonic 0 0 -a3 T T -b3

18. 0 1 1 0 0 0 1 0 1 0 T Original Binary Signal 0 1 1 0 0 0 1 0 0 1 1 0 0 0 1 0 1 1 0 0 T T With the first 4 Harmonics With the first Harmonic 0 1 1 0 0 0 1 0 0 1 1 0 0 0 1 0 1 1 0 0 T T With the first 2 Harmonics With the first 8 Harmonics A Binary Signal and Its Harmonics A data signal (e.g. character ‘b’ with 8 bits) that has a finite duration T can be handled by just imagining that it repeats the entire pattern over and over forever.

19. Data Rate vs. Bandwidth bandwidth 50ms bandwidth 25ms

20. Relationship between Data Rate and Bandwidth • The greater the bandwidth, the higher the information-carrying capacity • Summary • Any digital waveform will have infinite bandwidth • But the transmission system will limit the bandwidth that can be transmitted • For any given medium, the greater the bandwidth transmitted, the greater the cost • Digital information is generally approximated by a signal of limited bandwidth • Limiting the bandwidth creates distortions → interpreting the received signal becomes more difficult • The more limited the bandwidth, the greater the distortion and the greater the potential for error by the receiver

21. Data Communication Terms • Data: entities that convey meaning or information • Signals: electric or electromagnetic representations of data • Transmission: communication of data by the propagation and processing of signals • Analog data: Video, audio • Digital data: Text, integers

22. Analog Signals • A continuously varying electromagnetic wave that may be propagated over a variety of media, depending on frequency • Example media • Copper wire media (twisted pair and coaxial cable) • Fiber optic cable • Atmosphere or space propagation • Analog signals can propagate analog and digital data

23. Digital Signals • A sequence of voltage pulses that may be transmitted over a copper wire medium • Generally cheaper than analog signaling • Less susceptible to noise interference • Suffer more from attenuation (衰減) • Digital signals can propagate analog and digital data

24. Channel Capacity • Impairments like noise limit data rate that can be achieved • For digital data, to what extent do impairments limit data rate? • Channel capacity: maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions

25. Concepts Related to Channel Capacity • Data rate: rate at which data can be communicated (bps, bit per second) • Bandwidth: the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz) • Noise: average level of noise over the communications path • Error rate: rate at which errors occur • Error = transmit 1 and receive 0; transmit 0 and receive 1

26. Nyquist Bandwidth • For binary signals (two voltage levels) • C = 2B • With multilevel signaling • C = 2B log2M • M = number of discrete signal elements or voltage levels B: Bandwidth C: Channel Capacity

27. Signal-to-Noise Ratio • Ratio of the power in a signal to the power contained in the noise that is present at a particular point in the transmission • Typically measured at a receiver • Signal-to-noise ratio (SNR, or S/N) • A high SNR means a high-quality signal, low number of required intermediate repeaters • SNR sets upper bound on achievable data rate

28. Shannon Capacity Formula • SNR sets upper bound on achievable data rate • Represents theoretical maximum that can be achieved • In practice, only much lower rates achieved • Formula assumes white noise • Impulse noise is not considered • Attenuation distortion or delay distortion not accounted for

29. Nyquist and Shannon Formulations: Example (1/2) • Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dB • Using Shannon’s formula

30. Nyquist and Shannon Formulations: Example (2/2) • How many signaling levels are required?

31. Remarks: Signal Strength and Decibels (1/3) • Signal strength is an important parameter in any transmission system • As a signal propagates along a transmission medium, there will be a loss (attenuation) of a signal strength • Signal attenuation is often compensated by use of amplifiers • Losses and gains are expressed in terms of decibel • The decibel is a logarithmic ratio • Signal strength often falls off exponentially • Attenuation itself occurs logarithmically • This allows for easy addition and subtraction

32. Remarks: Signal Strength and Decibels (2/3) • Decibel is given by • It is a measure of relative and not absolute difference • A measure of absolute difference can be obtained through the use of dBW • A power level of 1W is used as the reference

33. Remarks: Signal Strength and Decibels (3/3) • Another common unit is dBm • The reference power level is 1mW • Example • 問：Given a system with 4mW input power, calculate the output power if the signal is transmitted overthe first element (a transmission line) with 12 dB loss, the second element (an amplifier) with 35 dB gain and the third element (a transmission line) with 10 dB loss. • 答：GdB = 35-12-10 = 13 = 10log(Pout/4mW)Pout = 4x101.3 mW = 79.8 mW

34. Read Section 2.1, pp. 90—95. 於2019/10/8(二)16:00前完成

35. Remarks: SNRand Decibels • (SNR)dB indicates the difference in decibels between the received signal and the background noise level (noise floor) • 例：If a radio (client device) receives a signal of -75 dBm and the noise floor is -90 dBm, the SNR is 15 dB • Data corruption and re-transmissions will occur if the received signal is too close to the noise floor

36. Relationship Between Data Rate, SNR, and RSS • 若以WiFi技術為例，三者之間的關係如下表 Source https://goo.gl/TNrY64