EEE 309 Communication Theory Semester: January 2016 Dr. Md. Farhad Hossain Associate Professor Department of EEE, BUET Email: mfarhadhossain@eee.buet.ac.bd Office: ECE 331, ECE Building
Part 08 Multiplexing l i and Multiple Access Techniques 2
Multiplexing and Multiple Access Multiple cars are sharing the same road Requires proper traffic management policies Improper traffic management: accidents, traffic jam, delay, inefficient utilization of resources 3
Multiplexing and Multiple Access (MA) Multiple users shares the same channel Multiple users under a single base station Multiplexing: Multiplexing technique combine signals from several sources Thus allows one channel to be used by multiple sources to send multiple messages without interfering each other It works on the physical layer (L1) of OSI model Multiple Access (MA): Decides on - Who will transmit? Whom to transmit? When to transmit? How to transmit? MA techniques are channel access methods based on some principles including multiplexing Allocates channels to different users and also handles the situation ti when there are more message sources than available channels It works on the data link layer (L2) of OSI model 4
Multiplexing Techniques Multiplexing techniques allow sharing a channel by keeping the transmitted signals from various sources separate so that they do not interfere with one another This separation is accomplished by making the signals orthogonal to one another in the dimensions of frequency, time, code, space, etc. Various types: Time division multiplexing (TDM) Frequency division multiplexing (FDM) Wavelength division multiplexing (WDM) Code division multiplexing (CDM) Space division multiplexing (SDM) Orthogonal frequency division multiplexing (OFDM): a variety of FDM Polarization division multiplexing (PDM) 5
FDM P t 3D view User 1 User 2 User N Sub-channel 1 Sub-channel 2 Sub-channel N f Channel 2D view Available bandwidth of the common channel is divided into bands Signals are orthogonal (separated) in frequency domain Requires guard bands to avoid adjacent-channel interference Requires filtering to minimize adjacent channel interference: costly 6
FDM (2) Block Diagram of an FDM System 7
TDM (1) A digital transmission technology Transmission time is divided into time-slots and unique time slot(s) are allocated to each user Different users can transmit or receive messages, one after the next in the same bandwidth but in different time slots: Orthogonal in time-domain 8
TDM (2) Block Diagram of a TDM System Increases the transmission efficiency (i.e., better resource utilization) Permits the utilization of all the advantages of digital it techniques: digital it speech interpolation, source coding, channel coding, error correction, bit interleaving, etc. Suitable for asymmetric (i.e., unequal uplink and downlink data rate) data rate Equipment is becoming increasingly i cheaper Requires a significant amount of signal processing for synchronization as the transmission i of all users must be exactly synchronized Requires guard times between time slots to compensate clock instabilities and transmission time delay 9
TDM Frame: Four Signals 4 4 4 1 1 1 2 2 2 3 3 3 Time TDM Frame TDM Frame TDM Frame 10
TDM Frame One TDMA Frame Preamble Information Message Trail Bits Slot 1 Slot 2 Slot 3 Slot N Trail Bits Sync. Bits Information Data Guard Bits Typical TDMA frame formation 11
Example 1 Following figure shows synchronous TDM with a data stream for each input and one data stream for the output. The unit of data is 1 bit. Find (a) the input bit duration, (b) the output bit duration, (c) the output bit rate, and (d) the output frame rate. a. The input bit duration is the inverse of the bit rate: 1/1 Mbps = 1 μs b. The output bit duration is one-fourth of the input bit duration, or 1/4 μs c. The output bit rate is the inverse of the output bit duration, i.e., 4 Mbps d. The frame rate is always the same as any input rate. So the frame rate is 1,000,000 frames per second 12 School of Electrical and Information Engineering
Example 2 We have four sources, each creating 250 8-bit characters per second. If the interleaved unit is a character and 1 synchronizing bit is added to each frame, find (a) the data rate of each source (b) the duration of each character in each source (c) the frame rate (d) the duration of each frame (e) the number of bits in each frame (f) the data rate of the link Solution a. The data rate of each source is 250 8 = 2000 bps = 2 kbps 13
Example 2 b. Each source sends 250 characters per second. Therefore, the duration of a character is 1/250 s, or 4 ms. c. Each frame has one character from each source, which means the link needs to send 250 frames per second to keep the transmission i rate of each source. d. The duration of each frame is 1/250 s, or 4 ms. Note that the duration of each frame is the same as the duration of each character coming from each source. e. Each frame carries 4 characters and 1 extra synchronizing bit. This means that each frame is 4 8 + 1 = 33 bits f. 33 bits are transmitted in 4 ms. Hence the data rate = 33 x 1000 /4 = 8250 bps 14
Digital Carrier Systems using TDM Two main systems for voice communications: 1. T-carrier Developer: Bell Labs, USA Used in North America, Japan and South Korea US system based on DS-1 signaling format ITU-T use a similar (but different) system Formats: T-1, T-2, T-3, T-4 2. E-Carrier Developer: European Conference of Postal and Telecommunications Administrations (CEPT) With some revisions, ITU-T has accepted it Used throughout Europe and most of the rest of the world * DS = Digital Signal, ** ITU T = ITU Telecommunication Standardization Sector 15
T-Carrier (1) 24 channels per frame 1 bit per frame (The first bit of a frame) is framing bit used for synchronization 8 khz sampling rate and 8 bits/sample = 64 kbps per channel Uses μ-law with μ = 255 T-1 Lines for Multiplexing Telephone Lines 16
T-Carrier (2) T-1 Frame Structure (Frame duration: 125 µs) (Bit duration: 0.6477 µs) 17
T-Carrier (3) Can also interleave DS-1 channels: For example, DS-2 is four DS-1 giving 6.312 Mbps 18
E-Carrier (1) E-Carrier system multiplexes 32 DS-0 channels (time slots each carrying 8 bits) together to form an E-1 circuit Time slot 0 is devoted d to transmission i management and dtime slot 16 for signaling The rest slots are assigned for voice/data transport Data rate: 32*8*8 kbps = 2.048 Mbps Uses A-law ** DS = Digital Signal 19
E-Carrier (2) 20
E-Carrier (3) 21
Joint TDM and FDM For certain applications, such as synchronous optical network (SONET) or synchronous digital hierarchy (SDH), both TDM and FDM can be employed simultaneously 22
WDM Block Diagram of an WDM System Conceptually same as FDM, except that multiplexing and demultiplexing involves light signals transmitted through fibre-optic channels Combines different frequency signals (same as FDM). However, the frequencies are very high. WDM is designed to utilize the high data rate capability of fibre-optic cable 23
Multiple Access (MA) Techniques Decides on - who will transmit? whom to transmit? when to transmit? How to transmit? Random access (contention methods): No station is superior to another station and none is assigned the control over another. No station permits, or does not permit, another station to send. Controlled access: The stations consult one another to find which station has the right to send. A station cannot send unless it has been authorized by other stations. Channelization techniques: The available bandwidth of a link is shared in time, frequency, or through code, between different stations. Usually, it is controlled by a system administrator. 24
Multiple Access (MA) Techniques Various forms of channelization techniques: Frequency division multiple access (FDMA): e.g., 1G cellular system Time division multiple access (TDMA) : e.g., 2G GSM system Wavelength division multiple access (WDMA) Code division multiple access (CDMA): e.g., 2G CDMA, 3G UMTS system Orthogonal frequency division multiple access (OFDMA): e.g., LTE, WiMAX Space division multiple access (SDMA) These techniques can be used in combination 25
Case Study: GSM TDMA Frames Hyperframe Superframe Multiframe Frame Bursts 26
CDMA A spread spectrum (SS) multiple access technique, which allows multiple signals occupying the same bandwidth to be transmitted simultaneously without interfering with one another In a CDMA system, each user is assigned a particular code, named as pseudo-noise (PN) code, which are ideally supposed to be unique for each user This unique code enables the desired message to be extracted at the receiver The transmissions from other users looks like interference What is a spread spectrum (SS) system? Spreads a narrowband communication signal over a wide range of frequencies Signal spreading is done before transmission i by using a spreading sequence De-spreads it into the original data bandwidth at the receiver Same sequence is used at the receiver to retrieve the signal Power Frequency Narrowband (High Peak Power) Spread Spectrum (Low Peak Power) 27
FDMA, TDMA and CDMA 28
Two types: Direct sequence CDMA (DS-CDMA) Frequency hoping CDMA (FH-CDMA) CDMA: Principle (1) DS-CDMA System: Symbol Bit Duration duration T T b S Data d(t) 1 1 b(t) 0 PN PN sequence Sequence c(t) a(t) Power Narrowband b(t) () b(t)a(t) d(t)c(t) () () Spread Spectrum b(t)a(t) Chip Duration T C Time Frequency Processing gain, G = No. of chips per bit = T b /T c 29
CDMA System Spreaded signal for user 1, b S1 Data of user 1, b 1 PN 1 Modulator Transmitted signal of user 1, TX1 Data of user 2, b 2 PN code of User 1 PN 2 b S2 Modulator Transmitted signal of user 2, TX2 Input Signal of Receiver 1 before Despreading, b S1' Despreading Demodulator PN 1 Output of Receiver 1, b 1 ' PN code of User 2 PN code of User 1 b SK Receiver Data of user 2, b K PN N Modulator Transmitted signal of user K, TXK PN code of User K Transmitter 30
CDMA: Principle (2) 31
CDMA: Principle (3) PN code 32
CDMA: Principle (4) 33
CDMA: Principle (5) Detection by receiver (station) 2: 34
CDMA System with Multi-User (1) Spreaded signal for user 1, b S1 Data of user 1, b 1 PN 1 Modulator Transmitted signal of user 1, TX1 Data of user 2, b 2 PN code of User 1 PN 2 PN code of User 2 Input Signal of Receiver 1 before Despreading, b b S1 ' S2 Despreading Modulator Transmitted signal of user 2, TX2 Demodulator PN 1 PN code of User 1 Output of Receiver 1, b 1 ' b SK Receiver Data of user 2, b K PN N Modulator Transmitted signal of user K, TXK PN code of User K Transmitter 35
CDMA System with Multi-User (2) Data of User 1, b 1 Spreading Spreaded Signal for User 1, b S1-2f - f s 0 f s s 2f - f C 0 f f s f C Input signal of receiver 1 before despreading, b S1 ' User K User 3 Total Output of Receiver 1, b 1 ' User 1 User 3 User 2 User 1 Despreading User 2 User K - f C 0 f C f - f C - f s 0 f s f C f 36
CDMA with Narrowband Interference Spreading TX b Input Data, b t (t) PN t PN Code Channel Narrowband / Wideband Interference Despreading RX b Output Data, b PN r (t) r PN Code Data Signal B t (f) Spreading RX b (f) DS-CDMA Signal (spread) Narrowband Interference - 2f - f s 0 f s s 2f s - f f C 0 f C f Whitened Interference B r (f) DS-CDMA Signal (despread) Despreading - f c - f 0 f f s s c f 37
CDMA with Wideband Interference Data Signal of User 1 B t (f) Spreading DS-CDMA Signal User 1 (spread) RX b (f) Wideband Interference of User 2-2f - f s 0 f s s 2f s - f f C 0 f C f Wideband Interference of User 2 B r (f) DS-CDMA signal of User 1 (despread) Despreading -f c - f 0 f f s s c f 38
PN Sequence Generation M-sequence Gold sequence Walsh code Kasami sequence Gold sequence generator 39
FH-CDMA (1) 40
FH-CDMA (2) Transmitter 41
FH Spread Spectrum: Invention Invention (1941): For controlling radio-controlled torpedoes US patent: Secret Communication System, August 1942 First implementation (modified form): For the sake of national defense, government did not allow publication of its details. First implemented by US Defense during Cuban Missile Crisis in 1962. Award: Pioneer Awards, Electronic Frontier Foundation, 1997 Actress and inventor (1914 2000) George Antheil (1900 1959) Composer, pianist, author, and inventor 42
Some of the advantages: CDMA: Advantages Hard to intercept: secure communications Difficult to jam Improved interference rejection and suppression No guard-band like FDMA or guard-time like TDMA Easy addition of more users Can accommodate more users than TDMA and FDMA Improved multi-path effect mitigation Graceful degradation of performance as the number of simultaneous users increases Less susceptible to effects induced from a changing environment 43
Requires high bandwidth CDMA: Drawbacks Self-jamming problem due to spreading sequences not being exactly orthogonal Power control necessary for mitigating near-far problem Inappropriate p for ultra high rate wireless access because Tremendous width of BW necessary Hardware complexity Synchronization problem 44
Duplexing Duplexing refers to the technique of separating the transmitting and receiving channels Communication Systems: Simplex, Half duplex, Full duplex FDD Frequency division duplexing (FDD): Transmitter and receiver operate at different carrier frequencies TDD Time division duplexing g( (FDD): Transmitter and receiver operate at same carrier frequencies, but through different time slots 45
MA and Duplexing Schemes in Use (Don t need to memorize the followings for exam) System Multiple Access Advanced Mobile Phone System (AMPS) FDMA/FDD 2G Global System for Mobile (GSM) TDMA/FDD US Digital Cellular (USDC) TDMA/FDD Digital European Cordless Telephone (DECT) FDMA/TDD US Narrowband Spread Spectrum (IS-95) CDMA/FDD Satellite Communication TDMA, FDMA, CDMA 3G WCDMA/FDD LTE OFDMA/FDD or TDD WiMax OFDMA/FDD or TDD 46
OFDM OFDM dates back to the 1960 s, whereas its implementation in commercial silicon only became possible in the late 1990 s A variant of FDM: Sub carriers are orthogonal to each other requiring no guard band Applications of OFDM/OFDMA: LTE WiMAX DSL ADSL PLC Digital it TV WLAN 47
OFDM System 48