Mobile Communications Chapter 3 : Media Access Motivation SDMA, FDMA, TDMA Aloha

Mobile Communications Chapter 3 : Media Access Motivation SDMA, FDMA, TDMA Aloha

Mobile Communications Chapter 3 : Media Access Motivation SDMA, FDMA, TDMA Aloha

Stevens, Shannon, Morning Drive-Time Personality has reference to this Academic Journal, PHwiki organized this Journal Mobile Communications Chapter 3 : Media Access Motivation SDMA, FDMA, TDMA Aloha Reservation schemes Collision avoidance, MACA Polling CDMA SAMA Comparison Motivation Can we apply media access methods from fixed networks Example CSMA/CD Carrier Sense Multiple Access with Collision Detection send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) Problems in wireless networks signal strength decreases proportional to the square of the distance the sender would apply CS in addition to CD, but the collisions happen at the receiver 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” Motivation – hidden in addition to exposed terminals Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is “hidden” as long as C 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, there as long as e waiting is not necessary C is “exposed” to B B A C

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Motivation – near in addition to far terminals Terminals A in addition to B send, C receives signal strength decreases proportional to the square of the distance the signal of terminal B there as long as e drowns out A’s signal C cannot receive A If C as long as example was an arbiter as long as sending rights, terminal B would drown out terminal A already on the physical layer Also severe problem as long as CDMA-networks – precise power control needed! A B C Access methods SDMA/FDMA/TDMA SDMA (Space Division Multiple Access) segment space into sectors, use directed antennas cell structure FDMA (Frequency Division Multiple Access) assign a certain frequency to a transmission channel between a sender in addition to a receiver permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) TDMA (Time Division Multiple Access) assign the fixed sending frequency to a transmission channel between a sender in addition to a receiver as long as a certain amount of time The multiplexing schemes presented in chapter 2 are now used to control medium access! FDD/FDMA – general scheme, example GSM f t 124 1 124 1 20 MHz 200 kHz 890.2 MHz 935.2 MHz 915 MHz 960 MHz

TDD/TDMA – general scheme, example DECT 1 2 3 11 12 1 2 3 11 12 t downlink uplink 417 µs Aloha/slotted aloha Mechanism r in addition to om, distributed (no central arbiter), time-multiplex Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries Aloha Slotted Aloha sender A sender B sender C collision sender A sender B sender C collision t t DAMA – Dem in addition to Assigned Multiple Access Channel efficiency only 18% as long as Aloha, 36% as long as Slotted Aloha (assuming Poisson distribution as long as packet arrival in addition to packet length) Reservation can increase efficiency to 80% a sender reserves a future time-slot sending within this reserved time-slot is possible without collision reservation also causes higher delays typical scheme as long as satellite links Examples as long as reservation algorithms: Explicit Reservation according to Roberts (Reservation-ALOHA) Implicit Reservation (PRMA) Reservation-TDMA

Access method DAMA: Explicit Reservation Explicit Reservation (Reservation Aloha): two modes: ALOHA mode as long as reservation: competition as long as small reservation slots, collisions possible reserved mode as long as data transmission within successful reserved slots (no collisions possible) it is important as long as all stations to keep the reservation list consistent at any point in time in addition to , there as long as e, all stations have to synchronize from time to time Aloha reserved Aloha reserved Aloha reserved Aloha collision t Access method DAMA: PRMA Implicit reservation (PRMA – Packet Reservation MA): a certain number of slots as long as m a frame, frames are repeated stations compete as long as empty slots according to the slotted aloha principle once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send competition as long as this slots starts again as soon as the slot was empty in the last frame frame1 frame2 frame3 frame4 frame5 1 2 3 4 5 6 7 8 time-slot collision at reservation attempts A C D A B A F A C A B A A B A F A B A F D A C E E B A F D t ACDABA-F ACDABA-F AC-ABAF- A–BAFD ACEEBAFD reservation Access method DAMA: Reservation-TDMA Reservation Time Division Multiple Access every frame consists of N mini-slots in addition to x data-slots every station has its own mini-slot in addition to can reserve up to k data-slots using this mini-slot (i.e. x = N k). other stations can send data in unused data-slots according to a round-robin sending scheme (best-ef as long as t traffic) N mini-slots N k data-slots reservations as long as data-slots other stations can use free data-slots based on a round-robin scheme e.g. N=6, k=2

MACA – collision avoidance MACA (Multiple Access with Collision Avoidance) uses short signaling packets as long as collision avoidance RTS (request to send): a sender request the right to send from a receiver with a short RTS packet be as long as e 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 size Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC) MACA examples MACA avoids the problem of hidden terminals A in addition to C want to send to B A sends RTS first C waits after receiving CTS from B MACA avoids the problem of exposed terminals B wants to send to A, C to another terminal now C does not have to wait as long as it cannot receive CTS from A B RTS CTS CTS B RTS CTS RTS MACA variant: DFWMAC in IEEE802.11 idle wait as long as the right to send wait as long as ACK sender receiver packet ready to send; RTS time-out; RTS CTS; data ACK RxBusy idle wait as long as data RTS; RxBusy RTS; CTS data; ACK time-out data; NAK ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy time-out NAK; RTS

Polling mechanisms If one terminal can be heard by all others, this “central” terminal (a.k.a. base station) can poll all other terminals according to a certain scheme now all schemes known from fixed networks can be used (typical mainframe – terminal scenario) Example: R in addition to omly Addressed Polling base station signals readiness to all mobile terminals terminals ready to send can now transmit a r in addition to om number without collision with the help of CDMA or FDMA (the r in addition to om number can be seen as dynamic address) the base station now chooses one address as long as polling from the list of all r in addition to om numbers (collision if two terminals choose the same address) the base station acknowledges correct packets in addition to continues polling the next terminal this cycle starts again after polling all terminals of the list ISMA (Inhibit Sense Multiple Access) Current state of the medium is signaled via a “busy tone” the base station signals on the downlink (base station to terminals) if the medium is free or not terminals must not send if the medium is busy terminals can access the medium as soon as the busy tone stops the base station signals collisions in addition to successful transmissions via the busy tone in addition to acknowledgements, respectively (media access is not coordinated within this approach) mechanism used, e.g., as long as CDPD (USA, integrated into AMPS) Access method CDMA CDMA (Code Division Multiple Access) all terminals send on the same frequency probably at the same time in addition to can use the whole b in addition to width of the transmission channel each sender has a unique r in addition to om number, the sender XORs the signal with this r in addition to om number the receiver can “tune” into this signal if it knows the pseudo r in addition to om number, tuning is done via a correlation function Disadvantages: higher complexity of a receiver (receiver cannot just listen into the medium in addition to start receiving if there is a signal) all signals should have the same strength at a receiver Advantages: all terminals can use the same frequency, no planning needed huge code space (e.g. 232) compared to frequency space interferences (e.g. white noise) is not coded as long as ward error correction in addition to encryption can be easily integrated

CDMA in theory Sender A sends Ad = 1, key Ak = 010011 (assign: „0“= -1, „1“= +1) sending signal As = Ad Ak = (-1, +1, -1, -1, +1, +1) Sender B sends Bd = 0, key Bk = 110101 (assign: „0“= -1, „1“= +1) sending signal Bs = Bd Bk = (-1, -1, +1, -1, +1, -1) Both signals superimpose in space interference neglected (noise etc.) As + Bs = (-2, 0, 0, -2, +2, 0) Receiver wants to receive signal from sender A apply key Ak bitwise (inner product) Ae = (-2, 0, 0, -2, +2, 0) Ak = 2 + 0 + 0 + 2 + 2 + 0 = 6 result greater than 0, there as long as e, original bit was „1“ receiving B Be = (-2, 0, 0, -2, +2, 0) Bk = -2 + 0 + 0 – 2 – 2 + 0 = -6, i.e. „0“ CDMA on signal level I data A key A signal A data key key sequence A Ad Ak As Real systems use much longer keys resulting in a larger distance between single code words in code space. CDMA on signal level II signal A data B key B key sequence B signal B As + Bs data key 1 0 0 0 0 0 1 1 0 1 0 1 0 0 0 0 1 0 1 1 1 1 1 1 0 0 1 1 0 1 0 0 0 0 1 0 1 1 1 Bd Bk Bs As

CDMA on signal level III Ak (As + Bs) Ak integrator output comparator output As + Bs 0 1 0 1 0 1 data A Ad CDMA on signal level IV integrator output comparator output Bk (As + Bs) Bk As + Bs data B Bd CDMA on signal level V comparator output wrong key K integrator output (As + Bs) K As + Bs

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SAMA – Spread Aloha Multiple Access Aloha has only a very low efficiency, CDMA needs complex receivers to be able to receive different senders with individual codes at the same time Idea: use spread spectrum with only one single code (chipping sequence) as long as spreading as long as all senders accessing according to aloha 1 sender A 0 sender B 0 1 t narrow b in addition to send as long as a shorter period with higher power spread the signal e.g. using the chipping sequence 110101 („CDMA without CD“) Problem: find a chipping sequence with good characteristics 1 1 collision Comparison SDMA/TDMA/FDMA/CDMA

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