Multiple Access Techniques

1. Multiple Access Techniques

2. Multiple Access Techniques • The transmission from the BS in the downlink can be heard by each and every mobile user in the cell, and is referred as broadcasting. Transmission from the mobile users in the uplink to the BS is many-to-one, and is referred to as multiple access. • Multiple access schemes to allow many users to share simultaneously a finite amount of radio spectrum resources. • Should not result in severe degradation in the performance of the system as compared to a single user scenario. • Approaches can be broadly grouped into two categories: narrowband and wideband.

3. Multiple Access Techniques • Multiple Accessing Techniques : with possible conflict and conflict- free • Random access • Frequency division multiple access (FDMA) • Time division multiple access (TDMA) • Spread spectrum multiple access (SSMA) : an example is Code division multiple access (CDMA) • Space division multiple access (SDMA)

4. Multiple Access Techniques -- Narrowband Systems • Transmission experiences nonselective fading. This means that when fades occur, all of the information (i.e. the whole channel) is affected. • Channel system : generally total spectrum is divided into a number of relatively narrow radio channels (e.g. FDMA). Occurrence of call blocking if channels are all being used. Unused bandwidth in each channel cannot be used by other users.

5. Narrowband Systems (Interleaving + coding techniques are used) • Data generated in source 1 2 3 4 5 … 8 9 10 11 12 … 15 16 17 18 … • Transmitted data 1 8 15 22 29 … 2 9 16 23 … 3 10 17 24 … • Received data 1 8 15 22 29 … 2 9 16 23 30 … 3 10 17 24 31 … • with the shaded data bits undergo deep fade • Deinterleaved data 1 2 3 4 5 … 8 9 … 15 16 17 18 …

6. Multiple Access Techniques -- Wideband Systems • The main feature of wideband systems is that either all the spectrum available (e.g. CDMA, TDMA) or a considerable portion of it is used by each user (e.g. TDMA+FDMA). • The advantage of wideband systems is that the transmission bandwidth always exceeds the coherence bandwidth for which the signal experiences only selective fading. That is, only a small fraction of the frequencies composing the signal is affected by fading. • Signal can be distorted and therefore equalization is needed but unlikely that a total signal fade will occur.

7. Duplexing • For voice or data communications, must assure two way communication (duplexing, it is possible to talk and listen simultaneously). Duplexing may be done using frequency or time domain techniques. • Forward (downlink) band provides traffic from the BS to the mobile • Reverse (uplink) band provides traffic from the mobile to the BS.

8. Reverse Channel Forward Channel frequency fc,,F fc,R Frequency separation Frequency separation should be carefully decided Frequency separation is constant Frequency division duplexing (FDD) • Provides two distinct bands of frequencies for every user, one for downlink and one for uplink. • A large interval between these frequency bands must be allowed so that interference is minimized.

9. Time division duplexing (TDD) • In TDD communications, both directions of transmission use one contiguous frequency allocation, but two separate time slots to provide both a forward and reverse link. • Because transmission from mobile to BS and from BS to mobile alternates in time, this scheme is also known as “ping pong”. • As a consequence of the use of the same frequency band, the communication quality in both directions is the same. This is different from FDD.

10. Slot number 0 1 2 3 4 5 6 7 … F R F R F R F R …. channel Reverse Channel Forward Channel time Ti+1 Ti Time separation Time division duplexing (TDD)

11. Random Access -- Aloha systems • Aloha is a packet-switching system. The time interval required to transmit one packet is called a slot. • When transmissions from two or more users overlap, they destroy each other, whether it is complete overlap or partial overlap – collision takes place. • The maximum interval over which two packets can overlap and destroy each other is called the vulnerable period. • The mode of random access in which users can transmit at anytime is called pure Aloha. In this case, the vulnerable period is two slot times. • A version in which users are restricted to transmit only from the instant corresponding to the slot boundary is referred to as slotted Aloha. The alignment of transmission to coincide with the slot boundary means that packets can only experience complete overlap, so the vulnerable period is only one slot time.

12. Random Access -- Aloha systems

13. Throughput of Aloha systems • In the Aloha systems, a transmitting user upon hearing a collision, backs off for a random delay interval and transmits again, until success is achieved. This is known as collision resolution. • We assume that the total traffic generated for transmission (including re-transmission) obeys a Poisson distribution. • The transmission is successful when there is no other packet transmission during the vulnerable period.

14. Throughput of Aloha systems

15. Throughput of Aloha systems • P (success) =P (no other packet transmission occurs within a vulnerable period) • Let S be the throughput, defined as the successfully transmitted traffic load, and G be the total offered channel traffic load. Assuming that traffic generated for transmission obeys a Poisson distribution, then P (no other packet transmission occurs) = exp(−τG) where τ is the vulnerable period. Hence P (success) = exp (−τG) By definition P (success) = S /G, therefore S = G exp (−τG)

16. Throughput of Aloha systems

17. Delay • The delay experienced by a packet in the system is measured from the instant of the packet’s arrival until the instant the sender receives confirmation. • It is a function of the number of transmission, the retransmission delay and the time required for the sender to receive confirmation of successful transmission.

18. Delay • The mean number of transmission until success • The average packet delay for pure Aloha • The average packet delay for slotted Aloha • The mean re-transmission delay depends on the collision resolution algorithm used.

19. Delay

20. FDMA • In FDMA, each user is allocated a unique frequency band or channel. During the period of the call, no other user can share the same frequency band.

21. FDMA • All channels in a cell are available to all the mobiles. Channel assignment is carried out on a first-come first- served basis. • The number of channels, given a frequency spectrum BT , depends on the modulation technique (hence Bw or Bc ) and the guard bands between the channels 2Bguard . These guard bands allow for imperfect filters and oscillators and can be used to minimize adjacent channel interference. • FDMA is usually implemented in narrowband systems.

22. Main features • Continuous transmission : the channels, once assigned, are used on a non-time-sharing basis. This means that both subscriber and BS can use their corresponding allotted channels continuously and simultaneously. • Narrow bandwidth : Analog cellular systems use 25-30 kHz. Digital FDMA systems can make use of low bit rate speech coding techniques to reduce the channel band even more. • If FDMA channels are not in use, then they sit idle and cannot be used by other users to increase capacity. • Low ISI : Symbol time is large compared to delay spread. No equalizer is required (Delay spread is generally less than a few μs – flat fading).

23. Main features • Low overhead : Carry overhead messages for control, synchronization purposes. As the allotted channels can be used continuously, fewer bits need to be dedicated compared to TDMA channels. • Simple hardware at mobile unit and BS : (1) no digital processing needed to combat ISI (2) ease of framing and synchronization. • Use of duplexer since both the transmitter and receiver operate at the same time. This results in an increase in the cost of mobile and BSs. • FDMA required tight RF filtering to minimize adjacent channel interference.

24. Nonlinear effects in FDMA • In a FDMA system, many channels share the same antenna at the BS. The power amplifiers or the power combiners, when operated at or near saturation are nonlinear. • The nonlinear ties generate inter-modulation frequencies. • Undesirable harmonics generated outside the mobile radio band cause interference to adjacent services. • Undesirable harmonics present inside the band cause interference to other users in the mobile system.

25. FDMA

26. TDMA • TDMA systems divide the channel time into frames. Each frame is further partitioned into time slots. In each slot only one user is allowed to either transmit or receive. • Unlike FDMA, only digital data and digital modulation must be used. • Each user occupies a cyclically repeating time slot, so a channel may be thought of as a particular time slot of every frame, where N time slots comprise a frame.

27. Features • Multiple channels per carrier or RF channels. • Burst transmission since channels are used on a timesharing basis. Transmitter can be turned off during idle periods. • Narrow or wide bandwidth – depends on factors such as modulation scheme, number of voice channels per carrier channel. • High ISI – Higher transmission symbol rate, hence resulting in high ISI. Adaptive equalizer required.

28. Features • High framing overhead – A reasonable amount of the total transmitted bits must be dedicated to synchronization purposes, channel identification. Also guard slots are necessary to separate users.

29. One TDMA Frame Preamble Information Trail Bits Slot 1 Slot 2 Slot 3 … Slot N Guard Bits Sync Bits Control Bits Information CRC One TDMA Slot A Frame repeats in time TDMA Frame

30. Features • A guard time between the two time slots must be allowed in order to avoid interference, especially in the uplink direction. All mobiles should synchronize with BS to minimize interference.

31. Features • The use of digital technology permits the inclusion of several facilities in the mobile unit, increasing its complexity. One example is the use of slow frequency hopping to counteract multi-path fading. • Flexible data rates by assigning multiple time slots to different users based on their demand. • No duplexer used since uses different time slots for transmission and reception. Even if FDD is used, a switch rather than a duplexer is required. • Lower BS cost : the use of multiple channels per carrier channel provides a proportional reduction of the equipment at the BS.

32. Features • Efficient power utilization : FDMA systems require a 3- to 6-dB power back off in order to compensate for inter-modulation effects. • Efficient handoff : TDMA systems can take advantage of the fact that the transmitter is switched off during idle time slots to improve the handoff procedure. An enhanced link control, such as that provided by mobile assisted handoff (MAHO) can be carried out by a subscriber by listening to neighboring base station during the idle slot of the TDMA frame.

33. Efficiency of TDMA • Efficiency of TDMA is a measure of the percentage of bits per frame which contain transmitted data. The transmitted data include source and channel coding bits. • bOHincludes all overhead bits such as preamble, guard bits, etc.

34. TDMA/TDD and TDMA/FDD • In TDMA/TDD system, half of the time slots in the frame information message would be used for the forward link channels and half would be used for reverse link channels. Same channel conditions. • In TDMA/FDD systems, same frame structure can be used for both forward and reverse transmission but carrier frequencies used are different.

35. TDMA/TDD and TDMA/FDD

36. Cellular Systems and MAC

37. Synchronous and asynchronous TDMA • Previously we mentioned the same slot of all the frames are assigned to the same user. The slot assignment can actually be fixed or dynamic. • If the assigned slot is fixed from frame to frame for the duration of the connection, the users have to synchronize to their respective assigned slots. This mode of TDMA is referred to as STDMA. • With packet-switched transmission, it is more efficient to allow a user to transmit only when it has packets to send. Transmission slots are dynamically assigned from frame to frame. This mode is known as ATDMA. • A TDMA is implemented using a reservation access mechanism.

38. SSMA • Spread spectrum systems : The desired signal is transmitted over a bandwidth which is much larger than the Nyquist bandwidth. It is first developed for military applications for • Security • Undetectability: minimum probability of being detected • Robust against intentional jammers

39. Applications • Security • Robust against unintentional interference • It is not bandwidth efficient when used by a single user but has the capability to overcome narrowband jamming signals (cannot overcome AWGN or wideband jamming signal) and multi-path. • Providing multiple access • If many users can share the same spread spectrum bandwidth without interfering with one another, bandwidth efficient improved but will affect the capability to overcome jamming.

40. Spread Spectrum Access Two techniques Frequency Hopped Multiple Access (FHMA) Direct Sequence Multiple Access (DSMA) Also called Code Division Multiple Access – CDMA

41. Frequency Hopping (FHMA) Digital muliple access technique A wideband radio channel is used. Same wideband spectrum is used The carrier frequency of users are varied in a pseudo-random fashion. Each user is using a narrowband channel (spectrum) at a specific instance of time. The random change in frequency make the change of using the same narrowband channel very low.

42. Frequency Hopping (FHMA) • The sender receiver change frequency (calling hopping) using the same pseudo-random sequence, hence they are synchronized. • Rate of hopping versus Symbol rate • If hopping rate is greather: Called Fast Frequency Hopping • One bit transmitted in multiple hops. • If symbol rate is greater: Called Slow Frequency Hopping • Multiple bits are transmitted in a hopping period • GSM and Bluetooth are example systems

43. Code Division Multiple Access (CDMA) In CDMA, the narrowband message signal is multiplied by a very large bandwidth signal called spreading signal (code) before modulation and transmission over the air. This is called spreading. CDMA is also called DSSS (Direct Sequence Spread Spectrum). DSSS is a more general term. Message consists of symbols Has symbol period and hence, symbol rate EE 552/452 Spring 2007

44. Code Division Multiple Access (CDMA) • Spreading signal (code) consists of chips • Has Chip period and and hence, chip rate • Spreading signal use a pseudo-noise (PN) sequence (a pseudo-random sequence) • PN sequence is called a codeword • Each user has its own cordword • Codewords are orthogonal. (low autocorrelation) • Chip rate is oder of magnitude larger than the symbol rate. • The receiver correlator distinguishes the senders signal by examining the wideband signal with the same time-synchronized spreading code • The sent signal is recovered by despreading process at the receiver.

45. CDMA Advantages Low power spectral density. Signal is spread over a larger frequency band Other systems suffer less from the transmitter Interference limited operation All frequency spectrum is used Privacy The codeword is known only between the sender and receiver. Hence other users can not decode the messages that are in transit Reduction of multipath affects by using a larger spectrum

46. CDMA Advantages • Random access possible • Users can start their transmission at any time • Cell capacity is not concerete fixed like in TDMA or FDMA systems. Has soft capacity • Higher capacity than TDMA and FDMA • No frequency management • No equalizers needed • No guard time needed • Enables soft handoff

47. CDMA Principle Represent bit 1 with +1 Represent bit 0 with -1 One bit period (symbol period) 1 1 Data 0 1 1 1 0 1 0 1 1 1 1 1 0 1 0 1 1 Coded Signal Chip period Input to the modulator (phase modulation)

48. Processing Gain • Main parameter of CDMA is the processing gain that is defined as: Gp: processing gain Bspread: PN code rate Bchip: Chip rate R: Data rate • IS-95 System (Narrowband CDMA) has a gain of 64. Other systems have gain between 10 and 100. • 1.228 Mhz chipping rate • 1.25 MHz spread bandwidth

49. Near Far Problem and Power Control • At a receiver, the signals may come from various (multiple sources. • The strongest signal usually captures the modulator. The other signals are considered as noise • Each source may have different distances to the base station B pr(M) M M M M

50. Near Far Problem and Power Control • In CDMA, we want a base station to receive CDMA coded signals from various mobile users at the same time. • Therefore the receiver power at the base station for all mobile users should be close to eacother. • This requires power control at the mobiles. • Power Control: Base station monitors the RSSI values from different mobiles and then sends power change commands to the mobiles over a forward channel. The mobiles then adjust their transmit power.