Wireless Environment

Frequencies for communication 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 Frequency and wave length: λ = c/f wave length λ, speed of light c ≅ 3x108m/s, frequency f

Frequencies for mobile communication VHF-/UHF-ranges for mobile radio simple, small antenna for cars deterministic propagation characteristics, reliable connections SHF and higher for directed radio links, satellite communication small antenna, focusing large bandwidth available Wireless LANs use frequencies in UHF to SHF spectrum some systems planned up to EHF limitations due to absorption by water and oxygen molecules (resonance frequencies)

Antennas: isotropic radiator Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission Isotropic radiator: equal radiation in all directions (three dimensional) - only a theoretical reference antenna Real antennas always have directive effects (vertically and/or horizontally) Radiation pattern: measurement of radiation around an antenna

Antennas: directed and sectorized Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley)

Antennas: directed and sectorized Cell Sizes Fig. 2.11

Signals physical representation of data function of time and location signal parameters: parameters representing the value of data classification continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values Signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift ϕ sine wave as special periodic signal for a carrier: s(t) = At sin(2 π ft t + ϕt)

Signal Important quantities Important quantities to measure the strength of the signal to the receiver , noise , interference e.g. SNR . Signal to Interference Ratio in dB SIR =received power of reference user in dBm/received power of all interferers in dBm C/I . Carrier over Interference in dB Carrier Power (dBm) / received power of all interferers in dBm

Signal Important quantities Eb/No . Signal Energy per bit to noise Power Density per hertz. -Eb/No = Signal energy (per bit ) dBm / noise Power dBm .Measures how strong the signal is . -Different forms of modulation BPSK, QPSK, QAM, etc. have different curves of theoretical bit error rates versus Eb/No. Eb/No

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

Signal propagation Propagation in free space always like light (straight line) Receiving power proportional to 1/d²(d = distance between sender and receiver) Receiving power additionally influenced by fading (frequency dependent) shadowing reflection at large obstacles refraction depending on the density of a medium scattering at small obstacles diffraction at edges

Real world example

Multipath propagation Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction Time dispersion: signal is dispersed over time interference with “neighbor” symbols, Inter Symbol Interference (ISI) The signal reaches a receiver directly and phase shifted distorted signal depending on the phases of the different parts

Typical large-scale path loss Fig. 2.15

Measured large-scale path loss Fig. 2.16

Partition losses Fig. 2.16

Measured indoor path loss Fig. 2.16

Measured indoor path loss Fig. 2.16

Fig. 2.8 Measured received power levels over a 605 m 38 GHz fixed wireless link in clear sky, rain, and hail [from [Xu00], ©IEEE].

Fig. 2.9 Measured received power during rain storm at 38 GHz [from [Xu00], ©IEEE].

Effects of mobility Channel characteristics change over time and location signal paths change different delay variations of different signal parts different phases of signal parts quick changes in the power received (short term fading) Additional changes in distance to sender obstacles further away slow changes in the average power received (long term fading)

Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes: interference only one node can send successfully at a time multiple access protocol distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit communication about channel sharing must use channel itself! what to look for in multiple access protocols:

Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R. 2. When M nodes want to transmit, each can send at average rate R/M 3. Fully decentralized: no special node to coordinate transmissions no synchronization of clocks, slots 4. Simple

MAC Protocols: a taxonomy Three broad classes: Channel Partitioning divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use Random Access channel not divided, allow collisions “recover” from collisions “Taking turns” tightly coordinate shared access to avoid collisions

Multiplexing Multiplexing in 4 dimensions space (si) time (t) frequency (f) code (c) Goal: multiple use of a shared medium Important: guard spaces needed!

Time multiplex A channel gets the whole spectrum for a certain amount of time Advantages: only one carrier in themedium at any time throughput high even for many users Disadvantages: Precise synchronization necessary

Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load. FDM (Frequency Division Multiplexing): frequency subdivided.

Frequency multiplex Separation of the whole spectrum into smaller frequency bands A channel gets a certain band of the spectrum for the whole time Advantages: no dynamic coordination necessary works also for analog signals Disadvantages: waste of bandwidth if the traffic is distributed unevenly inflexible guard spaces

Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle time frequency bands

Time and frequency multiplex Combination of both methods A channel gets a certain frequency band for a certain amount of time Example: GSM Advantages: better protection against tapping protection against frequency selective interference higher data rates compared to code multiplex but: precise coordination required

Code multiplex Each channel has a unique code All channels use the same spectrum at the same time Advantages: bandwidth efficient no coordination and synchronization necessary good protection against interference and tapping Disadvantages: lower user data rates more complex signal regeneration Implemented using spread spectrum technology

Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) unique “code” assigned to each user; i.e., code set partitioning used mostly in wireless broadcast channels (cellular, satellite, etc) all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

CDMA Encode/Decode

CDMA: two-sender interference

space division multiplex Cell structure Implements space division multiplex: base station covers a certain transmission area (cell) Mobile stations communicate only via the base station Advantages of cell structures: higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally Problems: fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

Medium access control Motivation for a specialized MAC in wireless – Consider carrier sense –CS- multiple access with collision detection-CD- (CSMA/CD) – wired nets: A sender senses the medium to see if it is free. If the medium is busy, the sender waits until it is free. If the medium is free, the sender starts transmitting data and continues to listen the medium. If the sender detects a collision while sending, it stops at once.

CSMA collisions collisions can still occur: collision: note: spatial layout of nodes collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted note: role of distance & propagation delay in determining collision probability

CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: receiver shut off while transmitting

CSMA/CD collision detection

Ethernet uses CSMA/CD No slots adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection Before attempting a retransmission, adapter waits a random time, that is, random access

Wireless MAC CSMA makes sense: Problems in wireless networks get all the bandwidth if you’re the only one transmitting shouldn’t cause a collision if you sense another transmission Problems in wireless networks signal strength decreases proportional to the square of the distance the sender would apply CS and CD, but the collisions happen at the receiver due to a second sender it might be the case that a sender cannot “hear” the collision, i.e., CD does not work furthermore, CS might not work if, e.g., a terminal is “hidden”

Wireless MAC (a) The hidden station problem. (b) The exposed station problem.

Wireless MAC Hidden terminal problem A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) C also stars sending causing a collision at B A cannot receive the collision (CD fails) and continues with its transmission A is “hidden” for C

Medium access control Exposed terminals B sends to A C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use but A is outside the radio range of C, therefore waiting is not necessary C is “exposed” to B

Medium access control Solution: CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) sensing the carrier is combined with a back-off scheme in case of a busy medium to achieve some fairness among the competing stations uses short signaling packets for collision avoidance RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive Signaling packets contain sender address receiver address packet length (the length of the future transmission)

IEEE 802.11 MAC Protocol: CSMA/CA 802.11 CSMA: sender - if sense channel idle for DISF sec. then transmit entire frame (no collision detection) -if sense channel busy then binary backoff 802.11 CSMA receiver - if received OK return ACK after SIFS (ACK is needed due to hidden terminal problem)

Collision Avoidance: RTS-CTS exchange sender transmits short RTS (request to send) packet: indicates duration of transmission receiver replies with short CTS (clear to send) packet notifying (possibly hidden) nodes hidden nodes will not transmit for specified duration: NAV

Collision Avoidance: RTS-CTS exchange RTS and CTS short: collisions less likely, of shorter duration end result similar to collision detection IEEE 802.11 allows: CSMA CSMA/CA: reservations polling from AP

IEEE 802.11 MAC Protocol: CSMA/CA CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) avoids the problem of hidden terminals – A and C want to send to B – A sends RTS first – C waits after receiving CTS from B

IEEE 802.11 MAC Protocol: CSMA/CA CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) avoids the problem of exposed terminals – B wants to send to A – C to another terminal – now C does not have to wait for it cannot receive CTS from A

Modulation Digital modulation Analog modulation Motivation digital data is translated into an analog signal (base band) ASK, FSK, PSK - main focus in this chapter differences in spectral efficiency, power efficiency, robustness Analog modulation shifts center frequency of base band signal up to the radio carrier Motivation smaller antennas (e.g., λ/4) Frequency Division Multiplexing medium characteristics Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

Modulation and demodulation

Channel allocation methods and systems for a common channel. Summary Channel allocation methods and systems for a common channel.

IEEE 802.11 MAC Protocol Fig. 2.10 Overview of the IEEE 802.11 Wireless LAN standard.

IEEE 802.11 MAC Protocol Fig. 2.12 Channelization scheme for IEEE 802.11b throughout the world.