Network Kernel Architectures and Implementation (01204423) Physical Layer Chaiporn Jaikaeo chaiporn.j@ku.ac.th Department of Computer Engineering Kasetsart University Materials taken from lecture slides by Karl and Willig

Overview Frequency bands Modulation Signal distortion – wireless channels From waves to bits

© Jochen Schiller, FU Berlin RF Spectrum VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency 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 visible light VLF LF MF HF VHF UHF SHF EHF infrared UV optical transmission coax cable twisted pair © Jochen Schiller, FU Berlin

Frequency Allocation Some frequencies are allocated to specific uses E.g., Cellular phones, analog radio broadcasting ISM bands Industrial, Scientific, Medical License-free operation Some typical ISM bands Frequency Comment 13.553 - 13.567 MHz 26.957 – 27.283 MHz 40.66 – 40.70 MHz 433 – 464 MHz Europe 900 – 928 MHz Americas 2.4 – 2.5 GHz WLAN/WPAN 5.725 – 5.875 GHz WLAN 24 – 24.25 GHz

Example: US Freq Allocation

Overview Frequency bands Modulation Signal distortion – wireless channels From waves to bits

Signals General form of signal Parameters A(t) – amplitude f(t) – frequency (t) – phase When the above parameters are all constant, the signal becomes a basic sine wave

Modulations Process of encoding data into signal By changing the three parameters Different ways of setting parameters Set the parameter to an arbitrary value  analog modulation Choose values from a finite set of legal values  digital modulation (keying) Resulting signal requires a certain bandwidth to be transmitted Centered around frequency of the basic sine wave

Modulation/Keying Examples Amplitude Shift Keying (ASK) Frequency Shift Keying (FSK) Phase Shift Keying (PSK) © Tanenbaum, Computer Networks

© Forouzan, Data communications and Networking QAM QAM – Quadrature Amplitude Modulation A combination of ASK and PSK © Forouzan, Data communications and Networking

Receiver: Demodulation Receiver matches signal with corresponding data bits Problems Carrier synchronization: frequency may vary Bit (symbol) boundary Frame boundary Received signal is not the transmitted signal!

Dealing with Synchronization E.g., Mica motes

Overview Frequency bands Modulation Signal distortion – wireless channels From waves to bits

Signal Distortion Wireless transmission distorts transmitted signal Wireless channel  abstract model describes these distortion effects Sources of distortion Attenuation Reflection/refraction Diffraction Scattering Doppler fading

Suitable Frequency Attenuation depends on the used frequency Can result in a frequency-selective channel with large enough bandwidth span © http://www.itnu.de/radargrundlagen/grundlagen/gl24-de.html © http://141.84.50.121/iggf/Multimedia/Klimatologie/physik_arbeit.htm

Path Loss Friis free-space equation: Other than free-space Ptx – Transmission Power Gt – Tx antenna gain Gr – Rx antenna gain L  1 – Loss in circuitry 2    6 – Path-loss exponent

Non-Line-Of-Sight (NLOS) Paths Different paths have different propagation times Results in delay spread of the wireless channel Closely related to frequency-selective fading properties of the channel With movement: fast fading LOS path NLOS path

Signal Strength in Multi-Path Environment Brighter color = stronger signal Simple free-space attenuation formula is not sufficient © Jochen Schiller, FU Berlin

General Path-Loss Formula  > 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

Overview Frequency bands Modulation Signal distortion – wireless channels From waves to bits

Noise and interference Due to effects in receiver electronics Depends on temperature Interference from third parties Co-channel interference Adjacent-channel interference

Symbols and Bit Errors Error ratio depends on signal strength compared to noise Captured by signal to noise and interference ratio (SINR) SINR allows to compute bit error rate (BER) Depends on modulation and symbol rate E.g., formula for BPSK

SINR vs. BER Example

SINR vs. BER Example

Summary Wireless radio communication introduces many uncertainties and vagaries into a communication system Handling the unavoidable errors will be a major challenge for the communication protocols