Wireless Communications

Transcription

1 Wireess Communications Mutipe Access Hamid Bahrami Eectrica & Computer Engineering

2 Communication System Bock Diagram

3 Dupexing Dupexing: transmit and receive at the same time Exampe: teephone, how about wakie-takie Termina A Simpex Termina B Termina A Haf-dupex Termina B Termina A Fu-dupex Termina B

4 Dupexing Frequency division dupexing (FDD) Mutipexes the Tx and Rx in one time sot in which transmission and reception is on 2 different frequencies It provides simutaneous transmission channes for mobie/base station (forward and reverse channes) At the base station, separate transmit and receive antennas are used to accommodate the two separate channes At the mobie unit, a singe antenna (with dupexer) is used to enabe transmission and reception To faciitate FDD, sufficient frequency isoation of the transmit and receive frequencies is necessary

5 Dupexing Time division dupexing (TDD) Mutipexes the Tx & Rx in one frequency at different time sots Ampitude Time Division Dupexing T R T R Time Ex: in a simpe 2-way radio where a button is pressed to tak and reeased to isten Ony possibe for digita transmission

6 Mutipexing Mutipexing (channeization) is the process of simutaneousy transmitting severa information signas using a singe communication channe

7 Frequency Division Mutipexing The avaiabe bandwidth is divided into non-overapping frequency sots and each message is assigned a frequency sot within the avaiabe band Signas are transmitted by different frequency bands and then added together to form a baseband signa The signas are narrowband and frequency imited Frequency f N-1 f N-2 f 3 f 2 f 1 f 0 Frequency Band N Frequency Band 2 Frequency Band 1 Time

8 Time Division Mutipexing Digita signas from severa sources are mutipexed in time and transmitted over a singe communication channe The communication channe is divided into frames; each frame is further segmented into sots; each user is assigned a sot (or channe) within each time frame Ony for digita communication... Sot N Sot 1 Sot 2 s 1 s 2... s k... Sot N s 1 s 2... Sync word Information or data word FRAME

9 Code Division Mutipexing (CDM) Mutipe signas are transmitted simutaneousy on the same time and same frequency Each signa is assigned a distinct code sequence The code sequences (spreading codes) are orthogona (or amost orthogona)

10 Exampe of orthogona codes Code 1 = {1, 1, 1, 1} Code 2 = {1, 1, -1, -1} Code 3 = {1, -1, -1, 1} Code 4 = {1, -1, 1, -1}

11 FDM/TDM/CDM

12 Spatia Mutipexing Mutipexing in space using mutipe antenna at the Tx and Rx

13 Waveength Division Mutipexing Optica communication

14 Mutipe Access Mutipe access: used to aow many mobie users to share simutaneousy a finite amount of radio spectrum Frequency division mutipe access (FDMA) Time division mutipe access (TDMA) Code division mutipe access (CDMA) Space Division Mutipe Access (SDMA) Demand Access Mutipe Access (DAMA) Random Access Mutipe Access (RAMA) Hybrid Mutipe Accesses

15 Mutipe Access To transmit mutipe signas in a point-to-point communication system, we use mutipexing. In a muti-user system with mutipe receivers (a point-to-mutipoint or broadcast system), a system with mutipe transmitters (a mutipoint-to-point or muti-access system), or a system composed of many users communicating directy with each other (a mutipoint-to-mutipoint system), we can use the same concept to transmit these mutipe signas. This is caed mutipe access.

16 Mutipe Access Mutipe access: used to aow many users to share simutaneousy a finite amount of radio spectrum Frequency division mutipe access (FDMA) Time division mutipe access (TDMA) Code division mutipe access (CDMA) Space Division Mutipe Access (SDMA) These are sometimes caed channeization protocos. These are contentioness protocos. Good for circuit switch. Random Access Mutipe Access (RAMA): Less coordinated and controed. Contention based protocos. Good for packet switch. Hybrid Mutipe Accesses

17 Mutipe Access Mutipe Access Protoco Contentioness (Scheduing Access) Contention (Random Access) CDMA Fixed Assigned Demand Assigned Repeated Random Access Random Access w/reservation FDMA TDMA Poing Token Passing ALOHA Sotted ALOHA Impicit Expicit

18 Frequency Division Mutipe Access Individua frequencies are assigned to individua users on demand for the duration of cas Frequency distances are far enough à no interference When the ca is finished à the channe is reeased and avaiabe for a new ca If the transmission path deterioratesà switches the system to another channe Guard band (at the edges & between) to minimize crosstak 1 2 n B! FRAME

19 Frequency Division Mutipe Access The FDMA channe carries ony one phone circuit at a time If a channe is not in use, then it sits ide and cannot be used by other users à the waste of bandwidth After the assignment of a voice channe, the base station and the mobie transmit simutaneousy and continuousy The bandwidths of FDMA channes are reativey narrow à narrowband systems The symbo time of a signa is arge as compared to the average deay spread à itte or no equaization

20 Frequency Division Mutipe Access Low compexity à but it is changing Fewer bits are needed for synchronization as frame bits Higher ce sit system cost ß the need of bandpass fiters and singe channe per carrier design Dupexers ß both tx and rx operate at the same time Require tight RF fiter to minimize the adjacent channe interference Orthogona FDMA (OFDMA) is the mutipe access scheme for 4G systems (LTE and IEEE /WiMAX)

21 Frequency Division Mutipe Access Guard band B guard : aocated at the edge of the aocated spectrum band Channe bandwidth: B c The tota spectrum aocation: B t The number of channes: N N = B t - 2B B C guard Channe 1 Channe 2... Channe N s B g B c B s MHz

22 Practice Design a Frequency Division Mutipex (FDM) signa set consisting five voice channes, each in the frequency range 300 to 3400 Hz. The mutipexed composite is to occupy the spectra region from 30k to 50 khz. Decide the aocating frequency bands for each channe by using the maximum guard band frequency. The voice channe bandwidth is 3100 Hz. The maximum guard bandwidth is ( Guard) Þ Guard 900Hz Channe 1: ~ Hz and Guard band: ~ Hz Channe 2: ~ Hz and Guard band: ~ Hz Channe 3: ~ Hz and Guard band: ~ Hz Channe 4: ~ Hz and Guard band: ~ Hz Channe 5: ~ Hz and Guard band: ~ Hz

23 Frequency Division Mutipe Access Exampe: AMPS (Advanced Mobie Phone System) Deveoped by Be Labs and introduced in G, Anaog, 800MHz FM band Each AMPS channe has a one way bandwidth of 30KHz (60KHz each dupex channe) 416 channes in MHz band (upink) and 416 channes in MHz band (downink)

24 Exampe 9.2, page 452 If a US AMPS (advanced mobie phone system) ceuar operator is aocated 12.5 MHz for each simpex band, and if B guard is 10 KHz, and B c is 30 khz, find the number of channes avaiabe in an FDMA system N = = 416

25 Time Division Mutipe Access Divide the radio spectrum into time sots In each sot, ony one user is aowed to either transmit or receive Buffer-and-burst method à non-continuous transmission (digita communication) One TDMA Frame Contro Bits Information Data Trai Bits Sot 1 Sot 2 Sot 3 Sot N Trai Bits Sync. Bits Information Data Guard Bits

26 Time Division Mutipe Access Shares a signa carrier frequency with severa users, where each user makes use of non-overapping time sots Data transmission is not continuous, but occurs in bursts à ow battery consumption Handoff process is simpe ß discontinuous transmission Dupex is not required Adaptive equaization is necessary ß high data rate The guard time shoud be minimized High synchronization

27 Time Division Mutipe Access IS-54 The origina TDMA format for AMPS By dividing a 30-kHz channe into 3 time sots, enabing 3 different users to occupy it at same time Provides a 3-fod increase in traffic capacity reative to AMPS, given the same bandwidth aocation A second phase of the IS-54 standard provides for 6 (instead of 3) TDMA user channes in each 30 khz radio channe IS-136 (aso caed caed D-AMPS) Enhanced TDMA with specia contro channes to aow short message service, battery ife extension, other features, 6 timesots, three users occupy in rotation

28 Time Division Mutipe Access GSM (Goba System for Mobie) Pan-European digita ceuar standard to repace six incompatibe anaog ceuar systems then in use in different geographic areas in eary 80 s Empoying TDMA and each radio channe carries 8 timesots (8 speech channes) and the radio channe bandwidth (for a 8 channes) is amost 270Kbps. Freq. band 850MHz or 1900MHz (in many countries 900 and 1800MHz) AT&T network is mainy a GSM based network.

29 Time Division Mutipe Access Efficiency of TDMA A measure of the percentage of transmitted data that contains information as opposed to providing overhead for the access scheme æ b ö h = OH ç 1- b 100% è T ø b b OH T = = T N f r R b r + N b t p + N b t g + N r b g b OH =overhead bits per frame N r = # of reference burst per frame N t = # of trai (preambe) burst per frame b r = # of overhead bits per reference burst b p = # of overhead bits per preambe b g = # of equivaent bits in each guard time interva T f : frame duration R: System bit rate

30 Time Division Mutipe Access Number of channes in TDMA system N = ( - 2B ) m B t B C guard m: the maximum number of TDMA users supported on each radio channe Exampe 9.4: If GSM uses a frame structure where each frame consists of eight time sots, and each time sot contains bits, and data is transmitted at kpbs in the channe. Find (1) the time duration of a bit (2) the time duration of a sot, (3) the time duration of a frame, and (4) how ong must a user occupying a singe time sot wait between two successive transmissions.

31 Exampe 9.4 The time duration of The time duration of The time duration of a frame : T Waiting time : 4.615ms 1 a bit : Tb = = 3.692µ s a sot : T = T = 0.577ms sot s = 8 T sot b = 4.615ms

32 Code Division Mutipe Access It is cassified as a spread spectrum mutipe access. Another possibiity is FHMA. The narrowband message signa is mutipied by a very arge bandwidth signa caed the spreading signa The spreading signas a pseudonoise code sequence that has a chip rate which is greater than the data rate Chinese Principes OF Engish Arabic CDMA Engish Major Hindu

33 Code Division Mutipe Access Many users of a CDMA system share the same frequency, either TDD or FDD Soft capacity imit à increasing the number of users raises the noise foor in a inear manner Mutipe fading might be substantiay reduced ß the signa is spread over a arge spectrum Channe data rates are high à the symbo duration is short and ess then the channe deay spread RAKE receiver can be used Sef-jamming is a probem ß not perfecty orthogona Near-far probem

34 Code Division Mutipe Access Reverse Link (from mobie unit to base station) Near-far probem The power of each user do not appear equa at the base station Many mobie users share the same channe Power contro: To maximize the tota user capacity To minimize power consumption of portabe units Forward Link (from base station to mobie unit) Link does not suffer much from near-far probem since a ce signas can be received at the mobie with equa power When at excessive interce interference, the power contro can be appied by increasing the power to the mobie

35 Code Division Mutipe Access Advantages Voice Activities Cyces (35% taking, 65% istening) Improved ca quaity No Equaizer Needed (ony correator needed) No hard handoff No guard time Less fading No frequency management needed Capacity advantage Enhanced privacy Coexistence Simpe system panning

36 Code Division Mutipe Access IS-95 (Interim Standard 95) aso caed cdmaone Quacomm, a San Diego-based company IS-95 was used as a 2G standard Each channe bandwidth was MHz (much arger than narrowband) Offer greater traffic capacity than GSM CDMA 2000 (3G) Supports data rate of up to 153Kbps Channe bandwidth is 1.25MHz Verizon and Sprint Nexte networks

37 CDMA Transmitter Data signa d k (t) x a k (t)d k (t) Baseband BPF Moduator Transmitted Signa x k (t) a k (t) PN Code Generator Acos ~ ( w t) c The data symbos d k (t) are spread into a k (t)d k (t) Chip Cock Then spread signa is moduated s () t = 2Pa t d t cos 2p f t+ f ( ) ( ) ( ) k k k k c k

38 (CDMA) Channe s k (t) h k (t) y k (t) h t k L j k () bd e y = å = 1 k t -t [ ] k ( ) ( t -t ) y () t = ò h t s dt k - k L kå kò- k k k k c k k = 1 ( t-t ) ( t-t ) ( ) [ t-t ] jy k = 2P b a d cos w t+ f d e dt L ( t-t ) ( t-t ) ( ) = 2P å b a d cos w t+ q k k k k k k c k = 1

39 CDMA Receiver ( ) r t Demoduator ( ) y t ò T 0 b ( ) dt Decision Device ŝk 2P cos ( w t+ q ) k c k a ( t-t ) k d Detection accompished by de-moduating & de-spreading This invoves the correation of the received signa with the deayed version of the spreading signa (despreading operation) In other words, the received signa is mutipied again by a synchronized version of the PN code

40 Performance Anaysis for Mutipe users ò K r () t = å y ( t) + n() t k k = 1 K L k k= 1= 1 K r () t = å y ( t) + n() t k k = 1 K L k k= 1= 1 ( t-t ) ( t-t ) ( ) = åå 2P b a d cos 2 p f t+ q + n( t) k k k k k k c k ( t-t ) ( t-t ) ( ) = åå 2P b a d cos 2 p f t+ q + n( t) k k k k k k c k

41 Performance Anaysis Assuming the first user is desired ( i + 1) T b it c b t t p z1 = ò r a1 cos 2 f t dt T 1 = ò b 1 ( ) ( ) ( ) z r cos 2 0 t a t p fct dt ( ) ( ) ( ) K L T 1 2 b i = åå kbkò0 k k 1 t k k cos wc cos wc + qk k= 1= 1 T b 0 ( ) ( ) cos( w ) 1 ( t-t ) ( ) ( t-t ) ( ) ( ) z P a a d t t + ò n t a t t dt c

42 Space Division Mutipe Access (SDMA) Separating the signas of mutipe users using beamforming

43 Hybrid FDMA/CDMA (FCDMA) Avaiabe spectrum is divided into subbands Each subband is then considered as a CDMA system It is the principe of muti-carrier CDMA (MC-CDMA)

44 Hybrid Direct Sequence/Frequency Hopping Mutipe Access (DS/FHSS) Signa of each user is a DS signa whose center frequency hops periodicay in a pseudo-random fashion. Avoids near-far probem, soft hand-off is not possibe

45 Other Hybrid Techniques Time Division CDMA (TCDMA) Different spreading codes are assigned to different ces Within each ce, ony one user is aocated a particuar time sot such that ony one user is transmitting in each ce at one sot Time Division Frequency Hopping (TDFH) At the start of a new TDMA frame, the user hops to a new channe This avoids severe fades or erasure in any particuar channe The user is aowed to hop according to a predefined sequence TXs are made to transmit on different frequencies at different times

46 Orthogona Frequency Division Mutipexing (OFDM) Simiar to FDM by using three principes Mutirate, mutisymbo and muticarrier Distribute the data over a arge number of carrier aows smaer transmission rate, e.g., bigger symbo duration Reduce the effect of ISI No/Less frequency seective fading Possibe guard interva between symbos Orthogona reationship between the subcarrier signas aow subcarriers to overap each other without interference No cross tak between carriers No need for inter-carrier guard band

47 OFDM Case Study Tx Wireess Loca Area Network Data à Seria to Parae à Moduation à IFFT à Parae to Seria à Guard à Mod

48 OFDM Case Study Rx Wireess Loca Area Network Demod à Guard remova à Seria to Parae à FFT à DeMod à Parae to Seria à Data

49 Orthogona Frequency Division Mutipe Access (OFDMA)

50 Muticarrier DS-CDMA Statement of probems Variety of services, incuding voice, data, image and video DS-CDMA suffers ISI and Muti-user Interference (MUI) OFDM has the potentia benefit Muticarrier DS-CDMA Marries the best of the OFDM and DS-CDMA 4G

51 Random Access In packet radio (PR), many users attempt to access a singe channe in an uncoordinated (or minimay coordinated manner). Coision can happen and is detected by the basestation (access point or AP). The AP broadcasts an ACK or a NACK signa for successfu and unsuccessfu reception of user s data. The users use a contention technique to transmit on a common channe. PR is easy to impement but has ow spectra efficiency and may cause deay. It is ony used for transmission of data (ess sensitive to deay) and not in voice systems.

52 Random Access Protocos Pure ALOHA: a user accesses a channe as soon as a packet is ready to be transmitted. After transmission the user wait for ACK. (Deveoped in University of Hawaii in eary 70 s) In case of coision (when NACK is received), the termina waits for a random period of time and then retransmits the same packet. There is amost no timing in the system. Very easy to impement but arge deay especiay when the number of users is arge.

53 Random Access Protocos Pure ALOHA Sending a packet Waiting for ACK ACK received? Yes Continue the next packet No Randomy seected a deay

54 Random Access Protocos Sotted ALOHA: Time is divided into equa timesots. A users have synchronized cocks. If a user has a packet to transmit, its transmission is done ony at the beginning of each time-sot. The rest of the protoco is simiar to ALOHA. This prevents partia coision and improves the throughput (reduces deay). Sti for arge number of users, the deay can be high.

55 Random Access Protocos Sotted ALOHA

56 Random Access Protocos In pure and sotted ALOHA protocos the users don t isten to the channe. By istening to the channe before transmission, greater efficiency is achieved. Carrier Sense Mutipe Access (CSMA): A users with a packet to transmit, constanty monitor the channe. If the channe is ide, the user transmits. CSMA with Coision Detection (CD): The user continues istening to the channe during its own transmission. If senses a coision, it stops transmission. CSMA with Coision Avoidance (CA): If the channe is busy, the user waits (backs off) for a random amount of time (why random?).

57 Random Access Protocos Carrier Sense Mutipe Access with Coision Avoidance (CSMA/CA) Inter-Frame Space DIFS: distribution coordination function IFS SIFS: short IFS DIFS SIFS DIFS SIFS DIFS Termina A Packet A Termina B Packet B Termina C Packet C AP ACK ACK Backoff Interve Packet Arriva Residua Backof Time

58 Random Access Protocos Reservation Protoco: The transmission time is divided into sots. Unike previous contention based random access protocos, here a user can reserve sots for transmission. One can think of it as a midde ground between mutipe access schemes and other random access protocos. Speciay good for transmission of high priority packets and for transmission without interrupt (continuous transmission). Reservation and some other protocos (such as Poing) are sometimes caed controed-access protocos. WiFi uses CSMA/CA with reservation.

59 Capture Effect and Hidden Termina Often the cosest transmitter is abe to capture a receiver because of sma path oss. This is caed near-far (or capture) effect. Good because many packets of this terminas arrive at the receiver despite coision. Bad because a strong transmitter may make it impossibe for the receiver to detect a much weaker transmitter which is attempting to communicate to the same receiver (hidden termina).

60 Exampe: IEEE Wireess LAN b GHz unicensed spectrum up to 11 Mbps a 5-6 GHz range up to 54 Mbps g GHz range up to 54 Mbps n: mutipe antennae GHz range up to 200 Mbps v v a use CSMA/CA for mutipe access a have base-station and ad-hoc network versions

61 Wireess LAN Architecture Internet v wireess host communicates with base station base station = access point (AP) AP hub, switch or router v Basic Service Set (BSS) (aka ce ) in infrastructure mode contains: wireess hosts BSS 1 AP access point (AP): base station ad hoc mode: hosts ony BSS 2

62 IEEE Mutipe Access avoid coisions: 2 + nodes transmitting at same time : CSMA - sense before transmitting don t coide with ongoing transmission by other node : no coision detection as in Ethernet! difficut to receive (sense coisions) when transmitting due to weak received signas (fading) can t sense a coisions in any case: hidden termina, fading goa: avoid coisions: CSMA/C(oision)A(voidance)

63 IEEE MAC Protoco: CSMA/CA sender 1 if sense channe ide for DIFS then transmit entire frame (no CD) 2 if sense channe busy then start random backoff time timer counts down whie channe ide transmit when timer expires if no ACK, increase random backoff interva, repeat receiver DIFS sender data ACK receiver SIFS - if frame received OK return ACK after SIFS (ACK needed due to hidden termina probem)

64 IEEE MAC Protoco: CSMA/CA idea: aow sender to reserve channe rather than random access of data frames: avoid coisions of ong data frames sender first transmits sma request-to-send (RTS) packets to BS using CSMA RTSs may sti coide with each other (but they re short) BS broadcasts cear-to-send CTS in response to RTS CTS heard by a nodes sender transmits data frame other stations defer transmissions avoid data frame coisions competey using sma reservation packets!