6. Multiple access. FER-Zagreb, Satellite communication systems 2011/12

Transcription

1 6. Multiple access

2 Topics Introduction Multiplex transmission Duplex FDD, TDD Multiplex TDM, FDM Multiple access FDMA SCPC i MCPC TDMA CDMA DS, FH, TH, Hybrid SDMA 1

3 Topics Packet access ALOHA S-ALOHA CSMA Satellite application PAMA DAMA PRMA 2

4 Introduction For better use of frequency spectrum, multiple transmission and multiple access is used. If allocation of time or frequency segments is fixed then multiple transmission is at hand (Division Multiplex Transmission). Examples of multiple transmissions are FDM i TDM. If allocation of time or frequency segments is variable, then multiple access is at hand (Division Multiple Access). Examples of multiple access are FDMA i TDMA 3

5 Introduction Simplex Information is transmitted only in one direction It is used for broadcasting (TV, radio) Receiver does not have to send any information back Semi-duplex Communication in both ways, but only in one direction at the same time Same frequency is used for transmitting and receiving Command for the end of transmission must be previously agreed. Example walkie-talkie Duplex (M = 2 = D) Simultaneous communication in both ways Frequency FDD (Frequency Division Duplex) Time TDD (Time Division Duplex) Multiplex More (M > 2) two way communications with division by time or frequency. 4

6 Multiple transmission Duplex FDD Frequency Division Duplex Communication with 2 different communication channels; one for receiving and other for transmitting signals Transmitting channel Receiving channel Frequency interval f Two simplex channels at the same time Frequency interval between channels is fixed. 5

7 Multiple transmission TDD Time Division Duplex Communication on same frequency; half time for transmitting and have time for receiving signal Transmitting channel Receiving channel t Time interval Two simplex at the same frequency If time interval is small, it seems that communication is at the same time. 6

8 Multiple transmission Multiplex If M = 2, Duplex transmission If M > 2, Multiplex transmission Multiple transmission is a procedure where a certain number of separate signals is combined into a complex signal for simultaneous transmission over a common communication channel. Allocation of time or frequency segments is fixed. TDM Time Division Multiplex FDM - Frequency Division Multiplex 7

9 Multiple transmission TDM - Time Division Multiplex When signal is sampled with narrow pulses, there are intervals in which other signals can be sampled. Transmission of several signals in one broadband link is enabled. Switches on transmitting and receiving end must be synchronized. 8

10 Multiple transmission TDM Time samples are very narrow and thus have large frequency bandwidth. In order to avoid overlapping of neighboring samples, safety intervals t are introduced between the samples. Every signal occupies the entire frequency band f part of the time. 9

11 Multiple transmission TDM can be synchronous and asynchronous Synchronous TDM All devices equally share time, all the time, regardless if they are transmitting or no. Time slot allocated to one device can not be used by other devices. Number of time slots in the frame is equal to the number of inputs. 10

12 Multiple transmission Asynchronous TDM Maximizes use by multiplexing slow inputs into one fast line. Here not every slot is allocated to a device. Each slot contains the device designation and the message. Number of time slots in a frame is not necessarily equal to the number of input devices. Some device can be allocated with more than one time slot. 11

13 Multiple transmission FDM - Frequency Division Multiplex Signals created by every transmitter modulate different transmission frequencies. These modulated signals are then merged into one complex signal which is transmitted. Transmitting frequencies must be sufficiently placed apart to enable modulation and demodulation. It is used in telephony where 30 khz voice channels are combined into one broadband 4 MHz channel. 12

14 Allocation of frequency or time segments is variable (Division Multiple Access). Multiple access can be by frequency, time, code, space and packet. FDMA TDMA CDMA DS FH TH SDMA Packet access ALOHA S-ALOHA CSMA 13

15 FDMA Frequency division multiplex access Multiple access by frequency division Each channel is allocated to one user at a time. Narrowband channels (30 khz) for voice communications Speed rate is 25 kb/s On receiving side there are band pass filters. 14

16 FDMA Oldest method (analogous systems) Channels share antenna and amplifier at the same time. There could be inter-channel interference. A single carrier per channel (SCPC) B multiple carrier per channel (MCPC) Output power decreases with the number of channels transmitted. 15

17 SCPC Single Channel per Carrier Example Satellite radio Not suitable for satellite internet because the user would pay for the service all the time even when not using the service MSPC Multiple Channels per Carrier More subcarriers is combined into one data bit stream They modulate the carriers and go to FDMA transponder SCPC is simple and reliable, but not appropriate for sending packets. 16

18 FDMA characteristics - There is only one communication at a time - Frequency band is taken even when there is no communication large bandwidth required - Relatively narrow channels (30 khz) voice channels + T Signal >>T Delay small ISI (inter symbol interference) + Complexity of FDMA is smaller than TDMA + FDMA is continuous, no synchronization is necessary; less complex symbols than TDMA + System is flexible from large rural cells to small urban cells - Expenses per cell are larger than for TDMA SCPC demands expensive band pass filters - Relatively small transmission rate - Higher power of transmission needed than for TDMA 17

19 TDMA Time division multiplex access Multiple access by time division Each carrier is divided into several individual channels. Division is made by time. Frame is divided into several time slots. Frequency bandwidth can be narrow (20-30 khz) or wide ( khz). Transmission is not continuous but in bursts. Synchronization is necessary in order to separate one burst from another. Synchronization requires 20-30% more bits altogether. t Code n 1 2 f 18

20 TDMA frame Header contains address and synchronization information. Guard bits are used for synchronization of receiver between different slots and frames. Header Message (Information) Tail bits Slot 1 Slot 2 Slot Slot n Tail bits Synchronization bits Data Guard bits 19

21 TDMA characteristics + More users use single frequency. + Variable data rate is possible frequency bandwidth can be allocated on request. + Transmission is not continuous energy (battery) saving - Inter symbol interference (ISI) larger than with FDMA. - Signal processing adds delay. + Switching from one station to another station can be performed in the moments when there is no signal transmission. - On uplink TDMA demands high power which can be a problem with mobile devices. + On downlink TDMA requires less power than FDMA 20

22 SSMA- Spread Spectrum Multiple Access Multiple access by code At first only military application Energy of narrow band signal (voice channel 10 khz wide) is expanded to much larger band (1-10 MHz) Modulated signals in spread spectrums are wide, appear as noise and are resistant to multipath propagation. t Code channel 1 channel 2... channel n f 21

23 SSMA classification CDMA Code Division Multiple Access Also called DSSS - Direct Sequence Spread Spectrum FH/SS Frequency Hopping Spread Spectrum TH/SS Time Hopping Spread Spectrum Hybrid CDMA 22

24 SSMA - characteristics Each user has unique code sequence, with which the information signal is coded. Receiver, knowing the code sequence, decodes the received signal. This is possible because, overlap between codes from other users is small. Decoding process narrows the spectrum which carries the information. The ratio of transmitted (spread) and information (baseband) bandwidth is called processing gain G P of spread spectrum: B t transmitted bandwidth B i information bandwidth G P =B t /B i Receiver compares the received signal with synchronously generated copy of spread code in order to obtain the original frequency carrier. Receiver must know the code by which the data was coded. 23

25 DS Direct Sequence Modulated signal which carries the information (data) is directly modulated with, discrete in time, coding signal. Data signal can be analogue or digital (more often). If digital, modulation can be omitted and data signal is the multiplied with code signal. Resulting signal then modulates the wideband carrier. Channel coding b(t) digital signal - bits Spectrum f Mixer DS-CDMA transmitter b(t)c(t) RF modulator Code generator c(t) Code bits - chips Spectrum of spread signal f 24

26 DS Direct Sequence Information signal b(t), with speed 1/T is multiplied with unique, fast digital code c(t) which has ~ 100 passes through zero per interval symbol/bit. Spreading code c(t) is periodic with T. Transmitted signal b(t)*c(t) is wideband and has frequency band equal to the spreading code. At transmitter, multiple signals are combined into radiofrequency channel. Each signal appears as noise. Spread signal is at receiver multiplied again with synchronized copy of the same code. For narrowing and obtaining original signal it is valid c 2 (t)=1 for all t! Signals from multiple users are received due to their unique and different codes. 25

27 DS Direct Sequence RF demodulator b(t)c(t) Mixer b(t) digital signal - bits Channel decoding Spread spectrum f f narrow spectrum Code generator c(t) Code bits - chips DS-CDMA receiver Other users f 26

28 DS-CDMA characteristics + Creating of coded signal is simple and can be realized by multiplying. + Frequency synthesizer is simple because there is only one transmission frequency necessary. + Coherent DS signal demodulation is simple. + There is not need for synchronization between users. - It is hard to achieve and keep synchronization of locally created coded signal and received signal. Synchronization should be within the part of chip time. - Due to synchronization, bandwidth is limited to MHz. - Power received by users close to the transmitter is much higher than the power received by users which are very far (near-far effect). 27

29 FH Frequency Hopping Carrier frequency is not constant but periodically changes. Hopping scheme is determined by the spreading code. While with DS, whole frequency spectrum is used, with FH, only part of the spectrum is used, but the band changes in time. Power in a single interval is higher compared to DS, but on average, both methods send same power. If hopping speed is higher than the transfer speed of a symbol (bit), then we have fast FH. One bit of information is send on several different frequencies. If hopping speed is lower than the transfer speed of a symbol (bit), then we have slow FH. In this case, several symbols are transmitted on one frequency. f f FH DS 28 t t

30 FH Frequency Hopping Data is modulated in baseband. Frequency synthesizer, controlled by coded signal, helps in up conversion. At receiver, information is obtained by demodulation in baseband. Synchronization is necessary for harmonizing hopping scheme of local code generator and receive signal, in order to narrow the signal properly. For smooth frequency hopping, it is desirable to lower the transmitting power before hop and to increase it again after the hop. Data Baseband demodulator Up converter Down converter Synchronization Data demodulator Data Code generator Frequency synthesizer Frequency synthesizer Code generator 29

31 FH-CDMA characteristics + Synchronization is easier than compared to DS-CDMA. Spectrum spreading is not achieved with high frequency of hopping but by using a set of frequency hopping. Hop time is longer than DS chip time, so larger error is possible. + Frequency bandwidth used by FH does not have to continuous, because parts of spectrum can be committed. This enables wider spectrum. + The possibility that more users use same frequency bandwidth at the same time is small. Problem near-far is smaller compared to DS because users use different frequencies. + Because of wider frequency bandwidth, decreasing narrowband noise is higher than with DS. - More complex synthesizer is necessary compared to DS. - Sudden signal change when different frequency bandwidth is used leads to spreading of spectrum. This is the reason why it is necessary to turn of and on signal before and after hopping. - Coherent demodulation is difficult because it is hard to keep phase relation during hopping. 30

32 TH Time Hopping With time hopping, data is being sent in fast bursts determined by code allocated to the user. Time axis is divided into frames, with every frame divided into M time slots. During every frame, user transmits in one of time slots. Code signal determines which of the M slots will transmit. TH-CDMA uses whole frequency spectrum part of the time, while FH- CDMA uses part of the spectrum all the time. f TH t 31

33 TH Time Hopping Scheme of TH-CDMA transmitter and receiver buffer buffer data Slow input Fast output Data modulator Demodulator Fast input Slow output data Code generator Carrier generator Code generator Carrier generator 32

34 TH-CDMA characteristics + Application is more simple compared to FH-CDMA. + Method is useful if transmitter is limited with mean power and not peak power because the data is sent by short strong bursts. + As with FH-CDMA the problem near-far is smaller than with DS, because most of the time signal distant from the transmitter emits alone. - Code synchronization takes long time and the time in which synchronization should be performed is short. - If there is multiple transmission, a large portion of data is lost, so the good error correcting method is necessary. 33

35 Hybrid CDMA 2 or more methods of spread spectrum combined DS/FH, DS/TH, FH/TH i DS/FH/TH Using good characteristics of each method DS/FH advantage with multipath propagation of DS combined with the advantage of near-far of FH. Disadvantage complexity of transmitter and receiver Signal is first spread using DS code. Spread spectrum is then modulated at a frequency which hops according to second code. Code clock ensures constant relation between the codes. data Code generator Mixer Code clock Code generator DS/FH transmitter Up converter Frequency synthesizer 34

36 SDMA Space Division Multiple Access SDMA is method which optimizes frequency spectrum use and lowers the expenses by using antenna directivity. There is a possibility of using same frequency in more than one direction. 35

37 Packet access Method of random access developed 1970s on Hawaii, USA. Small packets with little information Long packets Frequent short packets Transmission is with packets. If there is a successful transmission, the return acknowledgment ACK arrives, and if not, negative message NACK will arrive. Characteristics Traffic average number of tried message in a unit of time Throughput average number of successful transmission in a unit of time Delay appears due to the difference between traffic and flow ALOHA S-ALOHA (slotted) CSMA 36

38 Packet access ALOHA protocol User sends message when it is ready. Acknowledgment ACK arrives on a same or neighboring channel if everything is ok. If NACK arrives, after a random time, packet is sent again. With increasing user number, delay increases. Sensitive period time interval in which packets are vulnerable to collisions with transmitted packages of neighboring users. Transmitter 1 Package B Packet C Transmitter 2 Packet A One packet length τ Sensitive period 2τ t 1 +3τ t 37

39 Packet access ALOHA protocol Packet A will not be received if other terminals are transmitting in interval t 1 to t 1 +2τ. Even a small overlap (B and C) will make packet A useless. All packets should have constant length and transmission speed. Packets are send according to Poisson distribution, and mean speed is λ packets per second. Normalized traffic is given as R = λτ Erlang 0 < R < 1 τ packet length [s] λ speed of arrival [1/s] If R > 1, packet overlap has occurred. 38

40 Packet access Normalized traffic R is a number of tried transmissions in interval τ, including new and resend messages. Normalized throughput T is a mean number of successful transmission per interval τ T = R P R where P R is probability of successful packet sending. The possibility that n packets are successfully sent by user during interval 2τ is given by Poisson distribution. n 2R ( 2R) e PR ( n) = n! The probability that there is no overlap is: PR 2 ( 0) = e Throughput is then 2R T = R e R 39

41 Packet access S-ALOHA (Slotted) If packets are synchronized, throughput is increased. Sensitive period with this method is only one packet length, that is, τ. Transmitter always sends at the beginning of each time slot. There are no partial collisions. With more users, delay increases due to the collisions and repeated transmission of lost packets. The possibility that there is no overlap is PR ( 0) = R e Throughput is R T = R e Throughput with ALOHA is 18.4% capacity, while S-ALOHA has 36.8 % capacity. 40

42 Packet access CSMA Carrier sense multiple access Common in Ethernet networks Only for LEO orbits because other orbits would have large delay Each user tries to send a packet at a random time. User (device) checks if the channel is free. If it is, it begins sending a packet. If other device tries to send a packet at the same time, collision appears and they are both discarded. Then, each device waits a random time before transmitting again, until the packet is sent successfully. All stations must be able to feel channel status and transmission by others at any moment, even when they transmit themselves. The highest delay time τ must be final and usually very small. There should not be hidden places. All stations must be able to feel all others. Stations must be synchronized. 41

43 Satellite application PAMA Pre-Assigned Multiple Access Users are allocated a certain frequency in advance. Users use channels all the time, which is suitable for interactive data applications and large traffic. DAMA Demand Assignment Multiple Access Channel number is smaller than number of users. The link is established on demand. PRMA Packet Reservation Multiple Access Increased TDMA in combination with S-ALOHA. It is used for mobile satellite links for one base and several mobile stations. The problem could be a large delay. 42