Chapter 2 Overview Part 1 (2 weeks ago) Digital Transmission System Frequencies, Spectrum Allocation Radio Propagation and Radio Channels Part 2 (last week) Modulation, Coding, Error Correction Part 3 (today) Interference, noise, capacity limits Media Access Protocols Duplexing, Multiple Access - 1 -
Interference Signals by other senders which are transmitting in the same frequency band in cellular networks inherently existing, because neighboring cells using the same frequency The strength of the interference depends on pathloss between sender and receiver By equal transmitter power of all subscribers, the interference is only depending on the geometrical constellation - 2 -
Transmission over noisy channels (1) interference or noise source channel sink sender - 3 -
Transmission over noisy channels (2) nt p(x) ~ signal power S TP x 0 1 ~ noise power N Probability of detecting 0 although 1 was sent decision threshold x The greater S and the smaller N, the smaller is the error probability. - 4 -
Carrier-to-Interference Ratio (CIR) (Uplink Situation) Ratio of Carrierto Interference-power at the receiver C/I = C / (Σ I + N) I I C Typical in GSM: C/I=15dB (Factor 32) I C/I often also termed S/N - 5 -
Channel Capacity (Shannon) Bandwidth and S/N are reciprocal to each other Means: With low bandwidth very high data rate is possible provided S/N is high enough Example: higher order modulation schemes With high noise (low S/N) data communication is possible if bandwidth is high enough Example: spread spectrum Long time capacity as theoretical limit since turbo coding (1993) practical systems with 0.5 db to Shannon channel bandwidth - 6 -
Shannon Theorem The error probability is reverse proportional to the signal-to-noise ratio S/N. The transmission rate is proportional to the frequency bandwidth (b). The maximum achievable throughput over a noisy channel is: C = b log 2 (1+S/N) [bit/s] In many cases S/N 1, therefore simplified: C 1/3 b S/N db - 7 -
Interference-limited systems Mobile stations are in the coverage zone I C at the receiver is sufficient, but too much interference is received at the receiver C C/I is too low - 8 -
Range-limited systems Mobile stations are at the border or beyond the coverage zone C at the receiver is too low, because the path loss between sender and receiver is too high C I always exists, at least due to channel noise C/I is too low - 9 -
Capacity-limited systems Mobile stations are in the illuminated zone I C at the receiver is sufficient, I is small enough C/I is sufficient C No more resources (channels) available - 10 -
Examples GSM GSM 1992 Range-limited system, because no no wide-area coverage is is available, few few users, little little interference GSM GSM 2000 (European 900 900 MHz MHz networks) Interference-limited system, because many subscribers cause interferences. Interference-limiting countermeasures like like Power Control or or Frequency Hopping are are applied GSM GSM 2000 (European 1800 MHz MHz networks) Capacity-limited systems, because enough spectrum for for large clusters (little interference) is is available - 11 -
Media Access Duplexing Multiplexing/Multiple Access (PHY layer): TDMA, FDMA, FTDMA, CDMA Media Access Protocols (MAC layer) - 12 -
Time Division Duplex (TDD) Duplex Schemes Up-/downlink on the same channel, separated by time time Frequency Divison Duplex: FDD up/downlink separated by frequency, permanent transmission possible More RF bandwidth needed (worst case: double) time - 13 -
Basic Multiplex Schemes Divide the spectrum into portions in order to implement different channels In case of assigning these channels to different users, also called multiple access schemes Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) Space Division Multiple Access (SDMA) - 14 -
Frequency Multiplex/FDMA 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 Example: radio stations Disadvantages: waste of bandwidth if the traffic is distributed unevenly inflexible guard spaces t c k 1 k 2 k 3 k 4 k 5 k 6 f - 15 -
Time Multiplex/TDMA A channel gets the whole spectrum for a certain amount of time Advantages: only one carrier in the medium at any time throughput high even for many users c k 1 k 2 k 3 k 4 k 5 k 6 Disadvantages: f precise synchronization necessary t - 16 -
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 t coordination required c k 1 k 2 k 3 k 4 k 5 k 6 f - 17 -
Each channel has a unique code Code Multiplex/CDMA 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 k 1 k 2 k 3 k 4 k 5 k 6 t c f - 18 -
Space Multiplex/SDMA Each user can be separated from other users by means of directional antennas All users use the same spectrum at the same time Advantages: bandwidth efficient no coordination and synchronization necessary Disadvantages: Mutual interference cannot be fully avoided lower user data rates more complex signal regeneration Example: Beamforming k 1 k 2 k 3 k 4 k 5 k 6-19 -
Comparison SDMA/TDMA/FDMA/CDMA Approach SDMA TDMA FDMA CDMA Idea Terminals Signal separation Advantages Disadvantages Comment segment space into cells/sectors only one terminal can be active in one cell/one sector cell structure, directed antennas very simple, increases capacity per km² inflexible, antennas typically fixed only in combination with TDMA, FDMA or CDMA useful segment sending time into disjoint time-slots, demand driven or fixed patterns all terminals are active for short periods of time on the same frequency synchronization in the time domain established, fully digital, flexible guard space needed (multipath propagation), synchronization difficult standard in fixed networks, together with FDMA/SDMA used in many mobile networks segment the frequency band into disjoint sub-bands every terminal has its own frequency, uninterrupted filtering in the frequency domain simple, established, robust inflexible, frequencies are a scarce resource typically combined with TDMA (frequency hopping patterns) and SDMA (frequency reuse) spread the spectrum using orthogonal codes all terminals can be active at the same place at the same moment, uninterrupted code plus special receivers flexible, less frequency planning needed, soft handover complex receivers, needs more complicated power control for senders still faces some problems, higher complexity, lowered expectations; will be integrated with TDMA/FDMA - 20 -
FDD/FDMA - General Scheme, Example GSM 960 MHz f 124 935.2 MHz 915 MHz 1 124 20 MHz 200 khz 890.2 MHz 1 t - 21 -
TDD/TDMA - general scheme, example DECT 417 µs 1 2 3 11 12 1 2 3 11 12 downlink uplink t - 22 -
Motivation: Media Access for Wireless Can media access methods from fixed networks be applied? Example CSMA/CD Carrier Sense Multiple Access with Collision Detection Checks if medium is free (CS), if ok, sends data, continues to listen if collision occurs (CD), if yes stops transmission, sends jam signal (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 and 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 - 23 -
Motivation - hidden and 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 for C Exposed terminals A B C 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 - 24 -
Motivation - Near and Far Terminals Terminals A and B send, C receives signal strength decreases proportional to the square of the distance the signal of terminal B therefore drowns out A s signal C cannot receive A A B C If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer Also severe problem for CDMA networks - precise power control needed! - 25 -
Contention based Wireless Access Protocols Decentralised, random access Assignment based Base station polls mobile stations Reservation based Allocate fixed resources (time slots) to mobile stations Centralised and decentralised possible In decentralised case, contention based access used for reservation - 26 -
Aloha/Slotted Aloha Mechanism random, distributed (no central arbiter), time multiplex Slotted Aloha additionally uses time slots, sending must always start at slot boundaries Aloha collision sender A sender B sender C Slotted Aloha collision t sender A sender B sender C - 27 - t
Carrier Sense Multiple Access (CSMA) Use classical ALOHA, but listen to channel before transmitting Drastic decrease of collision probability 1-persistent: each station checks continously channel and starts as soon as free non persistent: checks channel in stochastic intervals only and starts transmitting directly after channel detected free p-persistent: uses slots, if detects slot free, transmits with probability p, with (1-p) waits for next slot, then same procedure as for previous slot CSMA/CA: load dependent waiting time, before listening is allowed after collision; used e.g. in IEEE 802.11 Tanenbaum, Computernetzwerke - 28 -
Transmission Efficiency S (throughput per packet time slot) pure partitioned G (tries per packet time slot) Tanenbaum, Computernetzwerke - 29 -
DAMA Demand Assigned Multiple Access Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution for packet arrival and 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 for satellite links Examples for reservation algorithms: Explicit Reservation according to Roberts (Reservation- ALOHA) Implicit Reservation (PRMA) Reservation-TDMA - 30 -
DAMA: Explicit Reservation DAMA Demand Assigned Multiple Access Explicit Reservation (Reservation Aloha): two modes: ALOHA mode for reservation: competition for small reservation slots, collisions possible reserved mode for data transmission within successful reserved slots (no collisions possible) it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time collision Aloha reserved Aloha reserved Aloha reserved Aloha t - 31 -
MACA collision avoidance MACA (Multiple Access with Collision Avoidance): No base station, no hidden station problem, flexibility of ALOHA and dynamic reservation MACA 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 size Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC) - 32 -
MACA 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 MACA 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 MACA examples RTS CTS A B C RTS CTS CTS RTS A B C - 33 -
PRMA Implicit reservation (PRMA - Packet Reservation MA): a certain number of slots form a frame, frames are repeated stations compete for 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 for this slots starts again as soon as the slot was empty in the last frame reservation ACDABA-F ACDABA-F AC-ABAF- A---BAFD ACEEBAFD frame 1 frame 2 frame 3 frame 4 frame 5 1 2 3 4 5 6 7 8 time slot 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 collision at reservation attempts - 34 -
Reservation TDMA Reservation Time Division Multiple Access every frame consists of N mini slots and x data-slots every station has its own mini slot and 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-effort traffic) N mini slots N k data slots e.g. N=6, k=2 reservations for data slots other stations can use free data-slots based on a round-robin scheme - 35 -
End Chapter 2 Mobile Communications - 36 -