COSC 393: Lecture 2 Radio Fundamentals

Radio signals Spectrum Transmitter Signal propagation Modulation Radio Communication Radio signals Spectrum Transmitter Signal propagation Modulation

Radio Wave s(t) = At sin(2  ft t + t)

Frequency and Wave length Relationship:  = c/f wave length , speed of light c  3x108m/s, frequency f

Radio Spectrum

VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency twisted pair coax cable optical transmission 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz VLF LF MF HF VHF UHF SHF EHF infrared UV visible light VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency UHF = Ultra High Frequency SHF = Super High Frequency EHF = Extra High Frequency UV = Ultraviolet Light

Antennas Isotropic radiator: Equal radiation in all directions (3D) - theoretical antenna Real antennas always have directive effects (vertically and/or horizontally) Different antennas have different radiation pattern.

Example: Radiation pattern of a simple Hertzian dipole Dipoles with lengths /4 or Hertzian dipole with length /2 (length proportional to wavelength) Example: Radiation pattern of a simple Hertzian dipole Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power) /2 /4 y y z simple dipole x z x side view (xy-plane) side view (yz-plane) top view (xz-plane)

Often used for base stations in a cellular system (e. g Often used for base stations in a cellular system (e.g., covering a valley) y y z directed antenna x z x side view (xy-plane) side view (yz-plane) top view (xz-plane) z z sectorized antenna x x top view, 3 sector top view, 6 sector

Effect of a transmission 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 sender transmission distance detection interference No effect

Signal propagation property Radio signal behaves like light in free space (straight line) Receiving power proportional to 1/d² (d = distance between sender and receiver) So ideally, the transmitter and a receiver must see each other! Really?

Three means of propagation Ground wave Tropospheric wave Ionospheric or sky wave

Ground Wave travels in contact with earth’s surface reflection, refraction and scattering by objects on the ground transmitter and receiver need NOT see each other affects all frequencies at VHF or higher, provides more reliable propagation means signal dies off rapidly as distance increases

Tropospheric Wave bending(refraction) of wave in the lower atmosphere VHF communication possible over a long distance bending increases with frequency – so higher frequency more chance of propagation More of an annoyance for VHF or UHF (cellular)

Ionospheric or Sky Wave Reflected back to earth by ionospheric layer of the earth atmosphere By repeated reflection, communication can be established over 1000s of miles Mainly at frequencies below 30MHz More effective at times of high sunspot activity

4 possible events Radio wave Radio wave Radio wave shadowing scattering Radio wave reflection diffraction

Multipath Characteristics A signal may arrive at a receiver - many different times - many different directions - due to vector addition . Reinforce . Cancel - signal strength differs from place to place

Mobile System Usually Base Station is not mobile Receiver could be moving (65mph!) Whenever relative motion exists - Doppler shift - Fading Even the motion of scatterers cause fading

Free Space Propagation Suppose we have unobstructed line-of-sight Pr(d) = (Pt Gt Gr l^2)/(4p)^2 d^2 L) -Pt transmitted power -Gt, Gr Antenna gain -l wavelength in meters - d distance in meters - L (>= 1) system loss factor (not related to propagation.

Propagation Losses Two major components - Long term fading m(t) - Short term fading r(t) Received signal s(t) s(t) = m(t) r(t)

dB - decibel Decibel, a logarithmic unit of intensity used to indicated power lost or gained between two signals.  Named after Alexander Graham Bell. 10 log (P1/P2)

Radio Signal Fading Short term fading Signal strength (dB) Long term fading T Time

Short term fading Also known as fast fading – caused by local multi paths. Observed over distance = ½ wave length 30mph will experience several fast fades in a sec. Given by Rayleigh Distribution This is nothing but the square root of sum of the square of two Gaussian functions. r = square root ( Ac * Ac + As * As) Ac and As are two amplitude components of the field intensity of the signal

Long term fading Long term variation in mean signal level is also known as slow fading Caused by movement over large distances. The probability density function is given by a log-normal distribution - normal distribution on a log scale P(m) = (1/m s(m) 2p) e^[-(log m – E(m))^2/(2 s(m)^2)]

Delay Spread Signal follows different paths to reach same destination. So same signal may arrive many times at different time intervals. t

Delay Spread In digital system, delay spread causes intersymbol interference. Therefore, there is a limit on the maximum symbol rate of a digital multipath channel. Obviously, delay spreads are different in different environment. (roughly between 0.2 to 3 microseconds)

Capacity of Channel What is the maximum transmission rate so that the channel has very high reliability? - error free capacity of a channel C.E. Shannon’s work suggest that signaling scheme exists for error-free transmission if the rate of transmission is lower than the channel capacity.

Shannon’s work C - channel capacity (bits/s) B – transmission bandwidth (Hz) E – energy per bit of received signal (Joule) R – information rate (bits/s) S = E R – signal power N – single-sided noise power spectral density (W/Hz) (C/B) = log [1+(S/(NB))] = log [1+(E/N)(R/B)] Suppose R = C we have (C/B) = log [1+(E/N)(C/B)]

Shannon’s work - continued Solving for (E/N) (aka. signal to noise ratio) (E/N) = (2^a –1)/a where a = (C/B). So given C= 19.2kb/s and bandwidth = 30kHz What is E/N required for error-free transmission? R/B = 19.2/30 = 0.64 Substituting we get E/N = 0.8724 = -0.593dB So control transmission power to obtain this E/N.

Propagation models in built-up areas Propagation is strongly influenced by the environment - building characteristics - vegetation density - terrain variation Perfect conductors reflect the wave where as nonconductors absorb some energy!

Empirical models to predict propagation losses Okumura’s model - based on free space path loss + correction factors for suburban and rural areas, irregular terrain, street orientations Sakagmi and Kuboi model - extend Okumura’s model using regression analysis of data. Hata’s model - empirical formula to describe Okumura’s data

More models Ibrahim and Parsons model - equations developed to best fit data observed at London. (freq. 168-900 MHz) Lee’s model Use at 900MHZ 3 parameters (median trasmission loss, slope of the path loss curve and adjustment factor)

Freq. for mobile communication VHF-/UHF-ranges for mobile radio simple, small antenna SHF and higher for directed radio links, satellite communication small antenna, focusing large bandwidth available Wireless LANs use frequencies in UHF to SHF spectrum limitations due to absorption by water and oxygen weather dependent fading, signal loss due to by heavy rainfall etc. 2.2.1

Modulation Digital modulation Analog modulation Motivation digital data is translated into an analog signal ASK, FSK, PSK (… Shift Keying) differences in spectral efficiency, power efficiency, robustness Analog modulation shifts center frequency of baseband signal up to the radio carrier Motivation smaller antennas (e.g., /4)

Types of Modulation Amplitude modulation Frequency modulation Phase modulation Combination modulation

analog baseband signal digital data digital modulation analog modulation radio transmitter 101101001 radio carrier analog baseband signal digital data analog demodulation synchronization decision radio receiver 101101001 radio carrier

Amplitude Modulation

Frequency Modulation

Phase Modulation

Digital modulation Amplitude Shift Keying (ASK): 1 1 Amplitude Shift Keying (ASK): simple low bandwidth susceptible to interference Frequency Shift Keying (FSK): somewhat larger bandwidth Phase Shift Keying (PSK): more complex (both ends) robust against interference t 1 1 t 1 1 t