Introduction to Wireless Networks

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

Different Types of Wireless Networks Coverage size differs WWAN UMTS / LTE 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

Signal Strength vs. Link Speed?

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

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

Curious to Know … 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) …

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

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

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

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)

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

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

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

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

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)

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

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. 1 T 0 1 1 0 0 0 1 0 Original Binary Signal 1 T 0 1 1 0 0 0 1 0 With the first Harmonic 1 T 0 1 1 0 0 0 1 0 With the first 4 Harmonics 1 T 0 1 1 0 0 0 1 0 With the first 2 Harmonics 1 T 0 1 1 0 0 0 1 0 With the first 8 Harmonics

Data Rate vs. Bandwidth bandwidth 50ms bandwidth 25ms

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 http://ham.stackexchange.com/questions/3692/is-free-space-path-loss-dependent-on-frequency

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

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

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

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

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

Nyquist Bandwidth For binary signals (two voltage levels) B: Bandwidth C = 2B With multilevel signaling C = 2B log2 M M = number of discrete signal elements or voltage levels B: Bandwidth C: Channel Capacity

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

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

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

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

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

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

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 over the 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

Get prepared by September 28th. Read Section 2.1, pp. 90—95. Get prepared by September 28th.

Remarks: SNR and 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

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