Physical layer Taekyoung Kwon

signal physical representation of data function of time and location signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift  E.g., sinewave is expressed as s(t) = At sin(2  ft t + t)

Signal (Fourier representation) 1 1 t t ideal periodic signal real composition Digital signals need infinite frequencies for perfect transmission (UWB?) modulation with a carrier frequency for transmission (analog signal!)

signal Different representations of signals amplitude (amplitude domain) frequency spectrum (frequency domain) phase state diagram (amplitude M and phase  in polar coordinates) Q = M sin  A [V] A [V] t[s]  I= M cos   f [Hz]

Radio frequency 직진성

* Ground wave = surface wave + space wave Radio channel type * Ground wave = surface wave + space wave

Radio channel type -> Really? 802.16

Radio channel type

Why 60GHz?

Why 60GHz? Frequency reuse

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 Xmission distance detection interference

Radio propagation

Attenuation in real world Exponent “a” can be up to 6, 7

propagation reflection scattering diffraction

Signal propagation models Slow fading (shadowing) Distance between Tx-Rx Signal strength over distance fast fading Fluctuations of the signal strength Short distance Short time duration LOS vs. NLOS

Slow fading vs. fast fading Slow fading = long-term fading Fast fading = short-term fading long term fading power t short term fading

shadowing Real world Main propagation mechanism: reflections Attenuation of signal strength due to power loss along distance traveled: shadowing Distribution of power loss in dBs: Log-Normal Log-Normal shadowing model Fluctuations around a slowly varying mean

shadowing

Fast fading T-R separation distances are small Heavily populated, urban areas Main propagation mechanism: scattering Multiple copies of transmitted signal arriving at the transmitted via different paths and at different time-delays, add vector-like at the receiver: fading Distribution of signal attenuation coefficient: Rayleigh, Ricean. Short-term fading model Rapid and severe signal fluctuations around a slowly varying mean

Fast fading

Fast fading

Fast fading

The final propagation model

Real world example

Modulation and demodulation 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 UWB: no carrier -> low cost, low power

modulation Digital modulation Analog modulation digital data is translated into an analog signal (baseband) ASK, FSK, PSK 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) Frequency Division Multiplexing medium characteristics Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

Digital modulation Modulation of digital signals known as Shift Keying Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference Frequency Shift Keying (FSK): needs larger bandwidth Phase Shift Keying (PSK): more complex robust against interference 1 1 t 1 1 t 1 1 t

antenna Radiation and reception of electromagnetic waves Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna Real antennas always have directive effects (vertically and/or horizontally) z y z ideal isotropic radiator y x x

antenna Isotropic Omni-directional Directional Radiation in every direction on azimuth/horizontal plane Directional Narrower beamwidth, higher gain

Omni vs directional

Antenna (directed or sectorized) E.g. 3 sectors per BS in cellular networks 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

Switched vs. adaptive

Switched vs. adaptive

MIMO?

Why directional antenna? Wireless channel is a shared one Transmission along a single multi-hop path inhibits a lot of nodes Shorter hops help, but to a certain degree Gupta-Kumar capacity result: T = O( W / sqrt(nlogn) ) Major culprit is “omnidirectionality”

Why directional antenna? Less energy in wrong directions Higher spatial reuse Higher throughput Longer ranges Less e2e delay Better immunity to other transmission Due to “nulling” capability

Directional vs. networks One-hop wireless environments Cellular, WLAN infrastructure mode BS, AP: directional antenna Mobile: omni-directional Ad hoc, sensor networking Every node is directional

Directional antenna types Switched: can select one from a set of predefined beams/antennas Adaptive (steerable): can point in almost any direction can combine signals received at different antennas requires more signal processing

Antenna model 2 Operation Modes: Omni and Directional A node may operate in any one mode at any given time

Antenna model In Omni Mode: Nodes receive signals with gain Go While idle a node stays in omni mode In Directional Mode: Capable of beamforming in specified direction Directional Gain Gd (Gd > Go) Symmetry: Transmit gain = Receive gain

Potential benefits Increase “range”, keeping transmit power constant Reduce transmit power, keeping range comparable with omni mode Reduces interference, potentially increasing spatial reuse

neighbor Notion of a “neighbor” needs to be reconsidered Similarly, the notion of a “broadcast” must also be reconsidered

Directional neighbor When C transmits directionally Receive Beam Transmit Beam B A C When C transmits directionally Node A sufficiently close to receive in omni mode Node C and A are Directional-Omni (DO) neighbors Nodes C and B are not DO neighbors

Directional neighbor When C transmits directionally Receive Beam Transmit Beam A C B When C transmits directionally Node B receives packets from C only in directional mode C and B are Directional-Directional (DD) neighbors

Directional antenna for MAC Less energy consumption Within the boundary of omni-directional Xmission range Same energy consumption DD neighbor is possible

Directional antenna for routing same energy consumption One hop directional transmission across multi-hop omnidirectional transmission DO neighbor will be the norm

D-MAC Protocol [Ko2000Infocom]

IEEE 802.11 Reserved area F A B C D E RTS RTS CTS CTS DATA DATA ACK

Directional MAC (D-MAC) Directional antenna can limit transmission to a smaller region (e.g., 90 degrees). Basic philosophy: MAC protocol similar to IEEE 802.11, but on a per-antenna basis

D-MAC IEEE802.11: Node X is blocked if node X has received an RTS or CTS for on-going transfer between two other nodes D-MAC: Antenna T at node X is blocked if antenna T received an RTS or CTS for an on-going transmission Transfer allowed using unblocked antennas If multiple transmissions are received on different antennas, they are assumed to interfere

D-MAC Protocols Based on location information of the receiver, sender selects an appropriate directional antenna Signature table

D-MAC Scheme 1 Uses directional antenna for sending RTS, DATA and ACK in a particular direction, whereas CTS sent omni-directionally Directional RTS (DRTS) and Omni-directional CTS (OCTS)

D-MAC Scheme 1: DRTS/OCTS B C D E DRTS(B) DRTS(B) - Directional RTS including location information of node B OCTS(B,C) OCTS(B,C) DRTS(D) OCTS(D,E) DATA OCTS(B,C) – Omni-directional CTS including location information of nodes B and C DATA ACK ACK

Drawback of Scheme 1 Collision-free ACK transmission not guaranteed ? DRTS(B) OCTS(B,C) OCTS(B,C) DRTS(A) DATA DRTS(A) ACK

D-MAC Scheme 2 Scheme 2 is similar to Scheme 1, except for using two types of RTS Directional RTS (DRTS) / Omni-directional RTS (ORTS) both used If none of the sender’s directional antennas are blocked, send ORTS Otherwise, send DRTS when the desired antenna is not blocked

D-MAC Scheme 2 Probability of ACK collision lower than scheme 1 Possibilities for simultaneous transmission by neighboring nodes reduced compared to scheme 1