Multiple Access, Duplexing, and Rake Receiver Techniques in Telecommunications

1. مخابرات سیّارروشهای دسترسی چندگانه نیمسال اوّل 95-94 افشین همّتیار

2. Multiple users want to use the same media • to transmit and receive information. • In wired systems, the media can be an • Ethernet cable. • In wireless systems, the media is free • space. Although might have to pay a lot of • money to get the right of using it. What is Multiple Access?

3. Multiplexing: One wants to use one • media to transmit and receive information • of many users. • All of the users in each side of the channel • are connected to one node. • Main multiplexing schemes are: • FDM: Frequency Division Multiplexing • TDM: Time Division Multiplexing • CDM: Code Division Multiplexing Multiple Access vs Multiplexing

4. Duplexing: How transmit and receive paths • are separated from each other. • Duplexing is still an issue for single user • scenarios in which Multiple Access is not • required. • Main duplexing schemes are: • FDD: Frequency Division Duplexing • TDD: Time Division Duplexing Multiple Access vsDuplexing

5. Transmit and receive links use different • frequency channels, so each duplex • channel consists of two simplex channels • in two frequencies. • Example: GSM • Tx: 890-915MHz • Rx: 935-960MHz • Tx and Rx frequencies for each link are separated by 45MHz. • In order to separate Tx and Rx paths, circuits known as duplexer are required at analog front end of each transceiver. Frequency Division Duplexing

6. Transmit and receive links use same • frequency channel, but occupy different • time slots, so each duplex channel consists • of two simplex channels at two time slots. • Simpler RF circuits (no duplexer), • but delays should be handled properly •  not easy to implement for highly • moving users. • Examples: • Cordless phones (DECT) • WLAN (802.11) Time Division Duplexing

7. Fixed: used for circuit switched application • such as voice. • Frequency Division MA (FDMA) • Time Division MA (TDMA) • Code Division MA (CDMA) • Orthogonal Frequency Division MA (OFDMA) • FDMA used for analog systems. • TDMA, CDMA and OFDMA used for digital Systems. • MA and Duplexing are two different issues. • A system such as GSM can be TDMA but FDD. • Statistical: used for packet switched • applications such as data Multiple Access Techniques (1)

8. FDMA • Example: old AMPS systems with analog 30KHz channels. • TDMA • One of important issues is synchronization • Special pattern in each frame used to correlate • and synchronize named Preamble • Overhead such as guard bits between frames • and coding bits should be taken into account to • compute throughput. • Example: • for GSM each time slot consists of 6 trailing bits, • 26 training bits, 116 information bits, and equals to 8.25 bits guard time so: • bT = 8*(6+8.25+26+116) = 1250bits • bOH = 8*(6+8.25+26) = 322bits • Efficiency = (1-bOH/bT)*100% = 74% Multiple Access Techniques (2)

9. CDMA • Based on Direct Sequence Spread Spectrum • technique • Use more BW • Frequency is the most valuable asset in a • wireless system!, but • More resistance to multipath and fading • effects • Multiplexing many users on the same BW, • using proper codes • Another Spread Spectrum technique is • Frequency Hopping Multiple Access Techniques (3)

10. DSSS vs FH

11. Example Code for A = <1,-1,-1,1,-1,1> Code for B = <1,1,-1,-1,1,1> Code for C = <1,1,-1,1,1,-1> DSSS (1)

12. PN code generation DSSS (2)

13. DSSS Tx DSSS Rx DSSS (3)

14. An integral part of DSSS systems Rake Receiver (1)

15. DSSS removes most of the energy from multipath. • The received signal components typically experience fading. The system normally synchronizes to the strongest multipath component. • A Rake receiver has N branches that synchronize • to N different multipath components. • Different multipath components are combined • using • Scanning • Selection • Equal Gain • Maximal Ratio • Rake is a diversity combining technique, with • diversity branches provided by the environment. Rake Receiver (2)

16. Channel Impulse Response • When the chip time Tc is much less than the rms • delay spread, each branch has independent fading • (assuming uncorrelated scattering), and Rake • provides diversity gain. • When chip time Tc is greater than the rms delay • spread, multipath components can not be • resolved, and there is no diversity gain. Rake Receiver (3)

17. Performance in fading channel Rake Receiver (4)

18. Spread Spectrum originally aimed at single user • applications (mainly for military purposes). • But later found to be useful for multiple access • schemes where we use different codes for different • users.  CDMA • Other users appear as noise •  System highly interference limited • Duplexing can be either FDD or TDD CDMA (1)

19. Design Goals • Make the interference look as much like Gaussian noise as possible: • Spread each user’s signal using a pseudo-noise • random sequence • Tight power control for managing interference • within the cell • Averaging interference from outside the cell • as well as fluctuating voice activities of users • 2) Apply point-to-point design for each link • extract all possible diversity in the channel CDMA (2)

20. Point-to-point link design • Very low SINR per chip: can be less than -15dB • Diversity is very important at such low SINR. • Time Diversity is obtained by interleaving across different coherence times. • Frequency diversity is obtained by Rake combining of the multi-paths. • Transmit diversity in 3G-CDMA systems (multiple base stations and antennas). CDMA (3)

21. Power control • In the reverse path, signals coming from different users will experience wide range of variations. • Since, cross-correlation of codes of different users is not completely zero, we will experience large interference if interferer’s signal is strong. • This will lead to the so-called “Near-far” problem. • The solution to Near-far problem is using proper “Power Control” algorithms, to ensure received power at base station coming from different users is almost the same. CDMA (4)

22. Power control • Maintains equal received power for all users in the cell. • Tough problem since the dynamic range is very wide (users’ attention can be differ by many 10’s of dB). • Consists of both open-loop and closed–loop. • Closed loop is needed since IS-95 is FDD. • Consists of 1-bit up-down feedback at 800Hz. • Not cheap: consumes about 10% of capacity for voice. • Power control is one the most difficult parts of CDMA systems. • For a long time it was believed to be impossible, but “Qualcomm” proved that it works. CDMA (5)

23. Interference averaging • The received signal-to-interference-plus-noise ratio for a user is defined as: • In a large system, each interferer contributes a small fraction of the total out-of-cell interference. • This can be viewed as providing interference diversity. • Same interference-averaging principle applies to voice activity and imperfect power control. CDMA (6)

24. Strengths • Multipath friendly • Using spread spectrum techniques (DS or FH) creates some sort of frequency diversity that will improve system performance against deep fading. • In addition, in time domain, different paths can be resolved and properly added together to improve performance • Since some paths can be in fade, but others not in fade, Rake receiver improves performance by proper combining of paths (some sort of time diversity). CDMA (7)

25. Strengths • Soft Capacity • Unlike FDMA and TDMA that have fixed number of slots in time or frequency domain and therefore put a hard limit on system capacity, in CDMA number of users can be increased without hard limits. • In this case, more users will show up as additional noise and decrease system performance gradually. CDMA (8)

26. Strengths • Soft Handoff • Unlike FDMA and TDMA in which neighbor base stations can not transmit at the same frequency, in CDMA neighbor stations can use same frequencies and talk to a mobile at the same time. • In this way, when a user cross the boundary of two stations, it can simultaneously talk to two stations and even add their signals together in the same way they combine multipath signals. • Therefore, instead of switching one base to another, during handoff multiple signals are used and some sort of macro-diversity is achieved. • This procedure is known as “Soft Handoff” and will improve handoff quality. CDMA (9)

27. Example: IS-95 • Most wide-spread 2G CDMA system • Channel Bandwidth: 1.25MHz • Processing Gain: 128 • Bit Rate: 9.6Kbps • Data rate reduced to 1.2Kbps during silence times, so Tx power can be reduced during vacant bits. • FDD used with 45MHz separation • Forward link band: 869 – 894MHz • Reverse link band: 824 – 849MHz CDMA (10)

28. Example: IS-95 Forward link • In forward link 64 Walsh-Hamard codes are used to differentiate up to 63 users. • Each code is multiplied by I and Q PN codes, unique for each base station, to ensure spreading of signals and to reduce interference from neighbor cells. • Orthogonality preserved since users are added synchronously at base. • However, multipath can cause un-orthogonal signals arriving at receiver to be combined by Rake receiver. • Pilot signal used to ensure users can properly use coherent detection and also detect proper base signals during handoffs. CDMA (11)

29. Example: IS-95 Forward link CDMA (12)

30. Example: IS-95 Reverse link • 6 Symbols mapped to one of 64 Walsh codes which are the same for all users, used for modulation and spreading. • Then, user specific codes of length 242-1 used to separate users and base stations from each other. • In this way, the robostness to in-cell interference increases compared with a short code. • Open-loop and fast, closed loop power control used to control transmit power of each user. • Fast closed loop power control essential in fading environments. • A 800bps forward channel used for closed loop signals sent back to the mobile (1dB step changes). • -50 to 23dBm dynamic range, accuracy = 1.5 – 2dB • 3 finger Rake used at base station CDMA (13)

31. Example: IS-95 Reverse link CDMA (14)

32. Issues • Main advantages • Allows interference averaging across many users • Soft capacity limit • Allows soft handoff • Simplified frequency planning • Challenges • Very tight power control to solve the near-far problem • Keeping orthogonality only for users in the cell • More sophisticated coding/signal processing to extract the information of each user in a very low SINR environment • Synchronization issues CDMA (15)

33. Synchronization issues • Carrier vs. Chip synchronization • Acquisition vs. Tracking • Matched filter vs. Correlator for acquisition • Early-Late gates for tracking • PLL vs. DLL CDMA (16)

34. Frequency selective channel with no equalizer • Earlier saw that this is possible with DSSS and • Rake receiver. • Another option: Multicarrier or OFDM modulation Multicarrier Modulation (1)

35. Breaks data into N non-overlapping substreams • Substream modulated onto separate carries • Substream bandwidth is BN = B/N for B total • bandwidth • BN < Bc implies flat fading on each subcarrier • (no ISI) • Use BPF of width BN to separate signals at • receiver Multicarrier Modulation (2)

36. It is quite clear how an ideal OFDM system works in frequency domain if we use continuous time non-overlapping subcarriers. • Actually, in this case, these functions (cos(2πfit)) are “eigen-functions” of the LTI system. • But, one main problem with this approach is that in reality infinite-length subcarriers can not be used. • Also it is not practical to generate continuous time signals and multiply them with input signals. • So, the main question is how to implement an OFDM system practically in discrete-time domain? OFDM (1)

37. If we only transmit a finite number of N symbols, d[0] to d[N-1], sinusoids are no longer eigen-functions of the system. • One way to restore this property is by adding a cyclic prefix (CP) to the symbols (of length L-1): • x = [d[N-L+1], . . . d[N-1], d[0], . . . d[N-1]] • Now, if only look at channel output for m = L to N+L-1 and define channel vector of length N as • h = [h0, h1, . . . hL-1, 0, . . . 0], then it can be easily verified that the new output is equal to “cyclic convolution” of d and h. OFDM (2)

38. Consequently, for cyclic convolution in time domain, the DFT of output is given by multiplication of DFT of d and DFT of channel vector: • Yi = Di.Hi • So, by using the cyclic prefix and transmitting the DFT of signal through channel, we again get each component separately at the output (eigen-functions are back). • In this way channel will be transformed into a set of independent parallel channels. OFDM (3)

39. Efficient IFFT structure at transmitter: • Reverse structure (remove CP and use FFT) at receiver. OFDM (4)

40. Challenges • Peak-to-average power ratio • Adding multiple substreams can result in high peak signal values • Impacts amplifier efficiency • Solutions include clipping, coding, and tone reservation. • Inter-carrier Interference • Subcarrier orthogonality compromised by timing jitter, frequency offset, and fading. • Frequency and timing offset causes interference between carriers. OFDM (5)

41. High PAR • For single carrier we have: • In multicarrier, assuming coherent addition of subcarriers, peak power increases linearly with N2, while average power added over all subcarriers increases linearly with N. • It can be shown that PAR increases approximately linearly with number of subcarriers N. OFDM (6)

42. High PAR OFDM (7)

43. High PAR OFDM (8)

44. Inter-carrier interference • Mismatched oscillators, Doppler shift or errors in timing synchronization cause subchannel interference (loss of subcarrier orthogonality). • Mitigating by minimizing number of subchannels and using pulse shapes robust to timing errors. OFDM (9)

45. Effects of phase/frequency imperfections OFDM (10)

46. Fading across subcarriers • Leads to different BERs for subcarriers • Performance limited to worst subcarrier status • Compensation techniques • Frequency equalization • Noise enhancement • Precoding: compensate channel variations at transmitter • Accurate channel estimate • Power inefficient • Coding across subcarriers • Works well for small BC (large delay spread) in order to use codes over many subchannels and recover data • Adaptive loading (power and rate) • For small subcarrier bandwidths Doppler spread becomes important (higher user mobility): • Should have fD<<BN in order to ignore Doppler spread • Current OFDM-based wireless systems: 802.11a, 802.11g, 802.16a, 802.20 OFDM (11)

47. Example: Flash OFDM • Bandwidth = 1.25MHz • Number of data subcarriers=113 • OFDM symbol = 128 samples = 100μS • Cyclic prefix = 16 samples = 11μS delay spread OFDM (12)

48. We have seen OFDM as a point-to-point modulation scheme, converting the frequency-selective channel into a parallel channel. • It can also be used as a multiple access technique called “OFDMA”. • By assigning different time/frequency slots to users, they can be kept orthogonal, no matter what the multipath channels are. Multiuser OFDM (1)

49. In-cell Orthogonality • The basic unit of resource is a virtual channel: • a hopping sequence • Each hopping sequence spans all the subcarriers to get full frequency-diversity. • Coding is performed across the symbols in a hopping sequence. • Hopping sequences of different virtual channels in a cell are orthogonal. • Each user is assigned a number of virtual channels depending on their data rate requirement. Multiuser OFDM (2)

50. In-cell Orthogonality Multiuser OFDM (3)