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Multiuser Systems (Ch. 14) EEE-534 Wireless Communications Spring 2011 Bilkent University Department of Electrical and Electronics Engineering. Multiuser Systems. In multiuser systems , system resources should be divided among multiple users.
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Multiuser Systems (Ch. 14) EEE-534 Wireless Communications Spring 2011 Bilkent University Department of Electrical and Electronics Engineering
Multiuser Systems • In multiuser systems, system resources should be divided among multiple users. • Multiple-access:Allocation of signal space dimensions to multiple users. • Two approaches: • Fixed-assignment multiple-access methods: FDMA, TDMA, CDMA, … [Section 14.2] • Random access (packet radio) methods: ALOHA, CSMA, … [Section 14.3] • Consider two types of multiuser channels: • Downlink channels • Uplink channels
Multiuser Channels: The Downlink • Downlink channel: Also called a broadcast channel or forward channel • One transmitter sending to multiple receivers • s(t) = s1(t) + … + sK(t) • Examples include • Radio and TV broadcast • Transmission link from base station to mobile terminals in a cellular system • From a satellite to multiple ground stations
Multiuser Channels: The Uplink • Uplink channel: Also called a multiple-access channel or reverse channel • Multiple transmitters sending to a receiver • Examples include • Laptop wireless LAN cards transmitting to a wireless LAN access point • Transmissions from mobile terminals to a base station in a cellular system • From ground stations to a satellite
Duplexing • Most communication systems are bi-directional (duplex); that is, communication occurs in both directions* [Each device Transceiver] • Frequency division duplexing (FDD): Assigns separate frequency bands for transmission and reception • Time division duplexing (TDD): Assigns non-overlapping (“orthogonal”) timeslots for transmission and reception • An advantage ofTDD is that bi-directional channels are typically symmetrical in their channel gains, so channel measurementsmade in one direction can be used to estimate the channel in the other direction.This is not necessarily the casefor FDD in frequency-selective fading. * Otherwise, a simplex system, in which one device can only transmit and the other one can only receive.
Multiple-Access • When dedicated channels are allocated to different users (fixed-assignment), it is often called multiple access (such techniques are used for both uplinks and downlinks). • Divide up the total signaling dimensions into channels and then assign these channelsto different users. • Various techniques in different dimensions: • Time-division multiple access (TDMA) • Frequency-division multiple access (FDMA) • Code-division multiple access (CDMA) • Space-divisionmultiple access (SDMA) • Some combinations of those (hybrid) • For a given signal space of dimension 2BT,Northogonal channels of dimension 2BT/N can be obtained regardless of the channelization method.
SDMA • There are two ways in which space provides multiple access capability: • Physical separation • Directional antennas • Physical separation: Two transmitters can use the same part of the radio spectrum if both are not within radio range of the same receiver. • Television Broadcast: Channels are re-used in different cities. (TV signals won’t travel much beyond line of sight.) • Cellular Telephones: Transmitters in different cells can use the same frequencies. (Low-power transmissions won’t propagate much beyond the cell.)
SDMA • Directional Antennas: Antennas can be designed to transmit and/or receive in specific angular directions. • Sectorization of a cell: Cells are divided into a number of sectors by using directional antennas
FDMA • In FDMA, the available radio spectrum is divided into channels of fixed bandwidth, which are then assigned to different users. • While a user is assigned a given channel, no one else is allowed to transmit in that channel. • Commonly, guard bands are used between the channels to prevent interference. f, frequency C2 C1 C3 C1 = channel 1 C2 = channel 2 etc. Total available bandwidth
FDMA Applications • FM Radio: In a given market, the frequency range 88-108 MHz is divided into 200kHz-wide channels: 88.1, 88.3, ……… 107.9 [AM radio, TV broadcast, are similar.] • GSM (Global System of Mobile Telecommunication): Uses FDMA in combination with TDMA. • Advanced Mobile Phone Service (AMPS) - U.S. Analog Cellular: 50 MHz of total bandwidth is available • 869 - 894 MHz for the “forward” (base to mobile) link • 824 - 849 MHz for the “reverse” (mobile to base) link • These are divided into 30kHz-wide (FM voice) channels.
FDMA Example • Consider a bandwidth of 25 MHz allocated to a analog cellular operator (first-generation (1G) system). Suppose each user needs 24 kHz bandwidth for analog voice communications (using FM), and 3 kHz guard bands are needed on each side to prevent interference between users. In addition, assume that 10 kHz guard bands are needed on both sides of the 25 MHz band to prevent interference to other systems. Calculate the total number of users that can be supported by this operator. • Solution: • Each user needs 24 KHz + 2(3 kHz) = 30 kHz • ( 25 MHz – 2(10 kHz) ) / 30 kHz = 832.6 832 users can be supported.
t S2 S2 S1 S3 …. .. S1 S3 …. .. …….. Interval 1 Interval 2 S1 = slot 1 S2 = slot 2 etc. TDMA • In TDMA, time is divided into intervals of regular length, and then each interval is subdivided into slots. • Each user is assigned a slot number, and can transmit over the entire bandwidth during its slot within each interval. • Commonly, guard intervals are used between different time slots.
TDMA Applications • T1 Line: • A T1 is a high-speed digital telephone “trunk” line. • It uses TDMA with a time period of 125 microseconds (µs) during which it multiplexes twenty-four 64-kbps digital phones channels. • So the time slots are each approximately 5.2 µs long, during which 8 bits from a single source are sent. • The bulk data rate is 1.544 Mbps. • GSM:Uses TDMA in combination with FDMA. • U.S. Digital Cellular (USDC) (also called IS-54/IS-136) • 30 kHz AMPS channels are subdivided using TDMA • 6 subchannels (for 4 kbps digital voices) • Time intervals are about 1/4 millisecond • Time slots are about 1/24 ms • Also called Digital AMP (D-AMPS)
TDMA Example • The original GSM design uses 25 MHz of bandwidth for the uplink and for the downlink (same as AMPs). This bandwidth is divided into 125 TDMA channels of 200 kHz each. Each TDMA channel consists of 8 user timeslots; the 8 timeslots along with a preamble and trailing bits form a frame, which is cyclically repeated in time. a) Find the total number of users that can be supported in the GSM system and the channel bandwidth of each user. b) If the RMS delay spread of the channel is 10 μs, will inter-symbol interference (ISI) mitigation be needed in this system? • Solution: a) 8 users per channel and 125 channels Total number of users that can be supported in this system 125 × 8 = 1000 users. The bandwidth of each TDMA channel is 25 × 106/125 = 200 kHz. b) Channel coherence bandwidth Bc ≈ 1/10μs = 100 kHz, which is less than the TDMA channel bandwidth of 200 KHz. Thus, ISI mitigation is needed.
CDMA • In FDMA, users are divided into distinct frequency channels, which they can exclusively use while connected to the network. • In TDMA, users are divided into distinct time slots, again for their exclusive use while connected. • In CDMA, all users are allowed all the available bandwidth all of the time while connected. • The manner in which these resources is controlled is by a code or pattern, unique to each user. • The receiver knows the pattern of time/frequency use of the various users, and can separate them accordingly. • Two basic types of CDMA: • Frequency hopping • Direct sequence
Frequency Hopping (FH) • In frequency hopping an ordinary source (say voice) is modulated into a carrier as usual. • But, instead of having a single carrier frequency, the carrier frequency is “hopped”, seemingly at random, throughout the entire range of available frequencies. • The hopping pattern is not really random but is merely very complex so as to appear random (this is called pseudorandom pattern) • The receiver knows the hopping pattern, and can demodulate simply by hopping the demodulator’s frequency accordingly.
Spread Spectrum (SS) • Because the transmitted signal with frequency hopping occupies a bandwidth much larger than that of the source, this is an example of spread spectrum modulation. • Spread spectrum was originally developed for military communications because of two advantages: • It is hard to jam • It is hard to intercept • It also has the advantage that it is less susceptible to some physical channel impairments (e.g., frequency-selective fading).
Frequency Hopping CDMA (FH/CDMA) • Frequency hopping can be used as a multiple access technique by assigning each user a distinct hopping pattern. • Although sometimes two users may hop to the same frequency, this can be fixed through error-control coding. • An advantage is that FH users can randomly access the channel without need for a reserved channel or time slot. • FH/CDMA is used very commonly in tactical communications, and in some wireless LANs. Also GSM uses some elements of FH to reduce inter-cell interference. • Example: Wireless LAN's (IEEE 802.11 standard) • frequency band 2.4-2.4835 GHz (ISM Band) • source: data at 1 - 2 Mbps • modulation: FSK • the carrier hops 2.5 times per second through 79, 1-MHz sub-bands.
0 T 2T 3T 4T 5T 6T 7T 8T Direct Sequence Spread Spectrum (DSSS) Consider a binary baseband data signal: +1 m(t) - 1 It has a spectrum:
DSSS - Spreading Suppose we multiply the baseband data signal by another binary baseband data signal, with a much higher symbol rate. c(t) ... time Tc The resulting time signal p(t) = c(t) m(t) is also a high-rate baseband data signal.
DSSS – Baseband Spectrum And the resulting baseband spectrum looks like: time
DSSS - Despreading • Now suppose p(t)=c(t)m(t) is modulated onto a carrier and then demodulated at a receiver. • If the receiver knows the higher-rate signal c(t), then it can form • c(t)p(t) = c2(t)m(t) = m(t) • (since c(t) = +1 or -1 and so c2(t) = 1 ) • This process called despreading recovers the baseband data signal.
DSSS - Spectra Baseband data spectrum Baseband spread data spectrum f Transmitted spectrum f
DSSS – Block Diagram m(t) p(t) Modulator Channel c(t) Demodulator y(t) c(t) c2(t) = 1 m(t)
Comments on DSSS • The transmitted bandwidth is 2/Tc, which is much larger than the 2/T bandwidth required by OOK or PSK, and so this is another form of spread spectrum. • It's called direct sequence because the "sequence" c(t) is modulated directly onto the baseband data signal (instead of via the carrier, as in FH).
Chips & Pseudo-Noise Signals • Like the hopping pattern in FH, the sequence of symbols used to create c(t) is chosen pseudo-randomly; this sequence is called the spreading code. • The symbols are called chips (to distinguish them from the bits of the actual data source.) • The signal c(t) is called the pseudo-noise (PN) signal; it is usually chosen to be periodic and to have other structure to make it easy to generate.
Spreading Ratio • The spreading ratio is a key parameter in spread-spectrum systems; it refers to the factor by which the bandwidth of the source signal is spread. • For DSSS, • spreading ratio = T/Tc = the no. of chips per bit. • 1/Tc is called the chip rate.
DS/CDMA • Like frequency hopping, direct-sequence can be used as a multiple-access technique. • Different users are assigned different spreading codes. • The receiver can pick out a given user by despreading with its code. • DS/CDMA has a number of advantages: • robustness to physical impairments of mobile radio channels (frequency-selective fading). • allows greater privacy / security • allows greater flexibility in assignment of users (“graceful degradation”) • in cellular systems allows re-use of frequencies • in adjacent cells (greater capacity) • can take advantage of bursty traffic and amplitude fading of interferers. • can be overlaid on existing services (good for use in ISM bands).
DS/CDMA Applications • US CDMA Cellular (IS-95): • frequency band same as AMPS • source: digital voice at 9.6 kbps • modulation DQPSK (downlink) • spreading gain 128 chips/bit • chip rate is 1.2288 Mchips/second (Mcps) • 3rd Generation (3G) Cellular: Wideband CDMA (W-CDMA) • source: digital voice or multimedia (rates range from 9.6kbps to 2Mbps) • variable spreading gain • chip rates up to 5Mcps
Hybrid Multiple-Access Techniques • Many systems are hybrids of these techniques. • Examples: • The TDMA channels of USDC are themselves within the FDMA channels of AMPS. • The CDMA channels of IS-95 within 1.25 MHz subchannels of the cellular band. • The TDMA channels of GSM are frequency hopped to reduce co-channel interference.
Fixed Channel Assignment • FDMA, TDMA and CDMA are called fixed-assignment channel-access methods because each user is given a share of the channel resources (e.g., a frequency band, a time-slot, or a code) through which to transmit. • These methods make relatively efficient use of radio resources when there is a steady flow of information from the source — e.g., voice, a data file, a fax. • However, for sources generating short messages at random times, this is inefficient; and random-access methods— also called packet radio — are of interest.
Data Packets • In random-access systems, a data sequence from a digital source is broken down into smaller pieces which are organized into data packets. • A data packet is a series of digital symbols with a structure something like the following
Data Packets Contents • Details may vary from system to system, best illustrated by example: • HEADER: a sequence that contains information about the source and destination of the packet. • ID: identifies the packet as an element of a group of packets, and specifies its place in the order of these packets (e.g., “packet 5 of 62 packets”). • DATA PAYLOAD: a piece of the source data sequence to be transmitted.
Data Packets Contents – Cont’d • ERROR CONTROL: a sequence of symbols used to determine whether there are errors in the packet -“cyclic redundancy check” (CRC). • TAIL: a sequence indicating that the packet is ending. [not always needed] • Note: to transmit over radio, a data packet may be • embedded in a radio packet, which also contains further symbols aiding in its demodulation [synch bits, “training symbols”, etc.]
Random Access Protocols • Packets are transmitted to a destination through a shared radio network without explicit channel assignment. [They can also be switched through a backbone network.] • When they all arrive safely at the destination, the payloads are reassembled into the original data sequence from the information source. • Since the channel is shared, protocols must be observed to assure the fair and orderly transfer of data. • We'll talk about two basic protocols: • ALOHA • Carrier-sense Multiple Access (CSMA)
Packet Radio Basics • Subscribers attempt to access a single radio channel by transmitting packets to a common receiver — say, a base station — in a minimally coordinated fashion. • If the packet is correctly received (as assessed by the CRC), an ACK (acknowledgement) identifying the received packet is broadcast back to the subscribers. • If the receiver detects a collision of two packets or otherwise erroneous reception, it broadcasts a NACK (negative acknowledgement). The transmitter then must re-send the packet.
Contention Protocols • Protocols establish the manner in which packets can be sent originally, and how they should be re-sent if a NACK is received. • Such schemes are called contention techniques. • They key parameters are • - Throughput: the average number of packets successfully transmitted per unit time • - Delay: the average delay experienced by a • typical packet
ALOHA • ALOHA: developed at the Univ. of Hawaii for bursty low-data-rate transmission over satellite systems. • Pure ALOHA: • a user transmits as soon as a packet is ready to go • if a collision occurs (NACK received) the transmitter waits a random period of time and then retransmits • simple, but low throughput • Other forms improve throughput, but reduce flexibility. • Slotted ALOHA: transmission can occur only at the beginning of specific time slots (doubles throughput). • Reservation ALOHA: a transmitter with a long file can reserve slots.
Carrier-Sense Multiple Access (CSMA) • The transmitter "listens" to see if the channel is idle (i.e., no carrier is detected). • If the channel is idle, the user transmits according to a fixed protocol. • Collisions still occur because of simultaneous transmission, and also because of transmission delay.
CSMA Varieties • Different types: • 1-persistent CSMA: • packet is transmitted as soon as the channel • is idle. • non-persistent CSMA: • NACK'ed packets are retransmitted only after • a random amount of time. • CSMA with collision detection (CSMA/CD): • The transmitter listens while transmitting to see if anyone else is also transmitting — ("listen while talk"). If so, transmission is aborted immediately.
CSMA Applications • Ethernet • —uses CSMA/CD • Wireless (IEEE 802.11) • —uses CSMA/CA (“collision avoidance”) • Cellular Digital Packet Data (CDPD) • —packet service over idle AMPS channels • —uses a form of CSMA/CD called “digital sense multiple access” (DSMA)
References • H. V. Poor, Lecture Slides for ELE538 - Wireless Communications: Signal Processing Principles, Princeton University, 2001. • Multiple-access techniques: • Chap. 8 of Rappaport, Wireless Communications. • Radio protocols: • Chap. 9 of Rappaport, Wireless Communications. • A. Goldsmith, Wireless Communications.