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Multiple Access Techniques

Multiple Access Techniques. 2007.10 김영재 / 연구전문그룹. 미래기술연구소. 목차. Multiple Access Techniques Contentionless Multiple Access Contention Multiple Access Hanging Multiple Access MAC Issues MAC Design Issues MAC Layer Issues Technology Trends What is 4G ? Evolution Paths to 4G.

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Multiple Access Techniques

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  1. Multiple Access Techniques 2007.10 김영재 / 연구전문그룹 미래기술연구소

  2. 목차 • Multiple Access Techniques • Contentionless Multiple Access • Contention Multiple Access • Hanging Multiple Access • MAC Issues • MAC Design Issues • MAC Layer Issues • Technology Trends • What is 4G ? • Evolution Paths to 4G

  3. Multiple Access Techniques A Multiple Access Technique is defined as a function sharing a (limited) common transmission resource among (distributed) terminals in a network.

  4. History of Adopted Access Technique [ Multi-carrier techniques for 4G systems] • Robust against frequency selective fading • A lot of know-how obtained through research and development of wireless LANs and digital broadcasting • Synergistic effects when combined with CDMA MIMO-SCM ? MIMO-SCM OFDM ? OFDM 5G systems 2020 CDMA CDM TDM 4G systems 2010 Adopted 3G systems 2000 Not Adopted 2G systems 1990

  5. Difference Between Multiple Access and Multiplexing

  6. Properties of Multiple access protocols (1/2) • Good properties • Shall control the allocation of channel capacity to the users • Be efficiency in terms of channel throughput and the delay of transmissions • The allocation should be fair toward individual users • Be flexible in allowing different types of traffic (e,g., voice and data) • Be stable • In equilibrium state, an increase in load should move to a new equilibrium point • Be robust with respect to equipment failure and changing conditions

  7. Properties of Multiple access protocols (2/2) • In the wireless mobile environment, the protocol should be able to deal with: • The hidden terminal problem • The near-far effect • The effects of multipath fading and shadowing • The effects of cochannel interference in cellular wireless systems caused by the use of the same frequency band in different cells

  8. Classification of Multiple Access Protocol Multiple access protocols Contention (random access) Contentionless (scheduling) Hanging (swimming) Fixed assigned Demand assigned Coding concept Subcarrier concept Repeated Random access Random Access With reservation FDMA TDMA Polling Token passing CDMA OFDMA ALOHA S-ALOHA Implicit Explicit

  9. Contentionless Multiple Access Protocols • Fixed assignment scheduling • The available channel capacity is divided among the users • E.g., TDMA, FDMA • Demand assignment scheduling • A user is only allowed to transmit if he/she is active • Demand assignment with centralized control • Polling (e.g., IEEE 802.11 PCF), SRMA • Demand Assignment with distributed control • Implicit: PRMA • Explicit: R-TDMA

  10. Contention Multiple Access Protocols • Features • No scheduling of transmissions • Should resolve the contention • Repeated random access protocols • P-ALOHA, S-ALOHA, CSMA • Random access with reservation • R-ALOHA

  11. Contention Multiple Access Protocols • ALOHA • (p)ure-ALOHA : users transmit any time they desire. • (s)lotted-ALOHA : users begin their transmission only at the beginning of a slot Vulnerable period for slotted ALOHA Time Vulnerable period for pure ALOHA

  12. Capacity for Contention-based protocols

  13. Brief history of Contention MAC protocols

  14. Hanging Multiple Access Protocols • CDMA type (Spread spectrum )protocols • Direct sequence (DS) CDMA • Frequency hopping (FH) CDMA • Time hopping (TH) CDMA • Subcarrier type protocols • Multi-carrier (MC) CDMA • OFDM-FDMA • OFDM-TDMA • OFDMA • Many others

  15. CDMA protocols • Use coding to achieve their multiple access property • Direct sequence • Frequency hopping • Time hopping • Advantages • Low probability of signal detection and interception • Protection against hostile jamming • Resistance to multipath fading • Graceful performance degradation from interference • Frequency reuse Frequency Direct sequence Frequency hopping Time hopping Time

  16. DS-CDMA • Basic concepts Spectral Density =

  17. DS-CDMA의 다중화 • Down Link : Walsh Code 에 의하여 확산처리 및 채널(통화자) 구분, Short PN Code 에 의하여 기지국 구분 • Up Link : Long PN Code 에 의하여 통화자 구분

  18. DS-CDMA의 다중화 • Allow different users to use the channel simultaneously by assigning different spreading code sequences to them. • Thus there is no physical separation in time or in frequency between signals from different users.

  19. DS-CDMA의 다중화 • The physical channel is divided into many logical channels by the spreading codes. • Unlike TDMA and FDMA, spread signals from different users do interfere each other unless the transmissions from all users are perfectly synchronized and orthogonal codes are used. • The interference from other users is known as multiple access interference (MAI). • Synchronous v.s. Asynchronous

  20. Frequency hopping (FH) CDMA • Basic concepts

  21. Frequency hopping (FH) CDMA • Example: FFH system with 2-FSK modulation, 8 hopping bins, and 2 hops per symbol (L = 2).

  22. Time hopped (TH) CDMA • Spread the spectrum by modulating the data signal by a random pulse-position modulated (PPM) spread signal • TH-SS is used for the conventional UWB communication.

  23. Subcarrier type protocols • Multi-Carrier CDMA • MC-CDMA (OFDM-CDMA) • MC-DS-CDMA • OFDM • OFDM-FDMA • OFDM-TDMA • OFDM-CDMA (MC-CDMA) • OFDMA-FH • OFDMA

  24. Multi-Carrier CDMA • Features of MC-CDMA • The advantages of DS-CDMA systems are its robustness to narrowband interference, multipath diversity, and capability of frequency reuse factor of 1. • But, in high-speed transmission, the increase in the number of the resolvable paths makes it impossible to implement the rake receiver. • The advantages of multicarrier systems are its robustness to frequency selectivity and reduced complexity in equalization of the receiver. • These advantages of multicarrier modulation and flexibility offered by the spread spectrum have motivated the combination of two techniques. • Two schemes exist: • MC-CDMA (OFDM-CDMA) and MC-DS-CDMA. • The MC-CDMA signal is generated by a serial concatenation of DS-CDMA and OFDM. Each chip of the DS spread data symbol is mapped onto a different subcarrier.

  25. MC-CDMA (OFDM-CDMA) • Spread the data in frequency domain and thus has inherent frequency diversity.

  26. MC-DS-CDMA • Signal is generated by serial-to-parallel converting the data symbols into N substreams and applying DS-CDMA on each individual sub-stream. • MC-DS-CDMA system with one subcarrier is identical to a single carrier DS-CDMA.

  27. OFDM의 다중화 방식

  28. OFDM-FDMA and OFDM-TDMA • OFDM-FDMA • Each user occupies a subset of subcarriers for a given time. The frequency bands assigned to a specific user is not changed over the time. • OFDM-TDMA • Each user occupies more than one OFDM symbols, and transmits on different time slots.

  29. OFDMA • Each user occupies a subset of subcarriers for a given time. Users should not be overlapped in frequency domain at any given time. But, the frequency bands assigned to a specific user may change over the time. • Advantages of OFDMA • High speed transmission • No intra-cell interference • Avoidance and averaging the inter-cell interference • Granularity • Multiuser diversity

  30. OFDMA • Multiuser diversity • In wireless system with many users, the achievable data rate of a given resource varies from one user to another. • Such variations make the overall system performance to be maximized by assigning each resource to the user who can exploit it best → multiuser diversity. • For example, consider a single cell with one BS and two users:

  31. OFDMA – multiuser diversity • We have the following assumptions: ① The two users are independent, the channel response are independent, ② The users have perfect CSI information, ③ There is perfect feedback channel from users to BS. ④ The BS collects the channel information from the users and allocates subcarriers based on the channel measurements reports. • For example, the figures shown below are the channel response for each user.

  32. OFDMA – multiuser diversity • Due to interference and noise, some of the subcarriers are in deep fading. • However, since the two users are independent, deep-faded subcarriers for one user may be good for another. • For OFDM-TDMA, the SINR on each subcarrier is the average of two users • For OFDMA with resource allocation, each subcarrier are allocated to the specific user that has the best channel frequency response. Thus the SINR for OFDMA is the maximum of two users.

  33. OFDMA – subchannel allocations • Two types of cases are defined: • Fixed case: • the channel is varying slowly and the channel estimation is accurate. The channel measurement/report and allocation do not have to update very often. The multiuser diversity can be used by resource allocation. • Mobile case: • in fast fading environments, the measurement should be sent back quite often to track the channel. Thus using the multiuser diversity is not feasible. Rather frequency diversity is useful. • Consider an OFDMA system with a total number of N subcarriers and K users. Divide the N subcarriers into L traffic channels, each with M subcarriers. Define cluster in which C consecutive subcarriers exist. For example, (M, C) = (8, 4) is:

  34. OFDMA – subchannel allocations • Fixed case • we can treat the channel as constant and use the multiuser diversity. User’s CSI is periodically reported to the BS, and the BS send back the resource allocation and the adaptive modulation and coding (AMC) scheme to the users. This is feasible since both the TX and RX have the accurate CSI with low overhead.

  35. OFDMA – subchannel allocations • Mobile case: • the channel is varying so fast that it is impractical for BS to allocate the channels to the users. Obtain the frequency diversity through the subcarrier spreading in the subchannel or frequency hopping.

  36. OFDMA – subchannel allocations • Latin Square • An efficient method way of achieving the frequency diversity is the use of the Latin square. • Def: A Latin square of order N is an N × N matrix from a set Q of N distinct elements, say Q = {0,1,L, N −1} such that each row and column contains every element of Q exactly once. • For example 5th degree Latin square is given below. The entries in Q represent different users in the same cell: qij = frequency slot for user j at OFDM symbol time i

  37. OFDMA – subchannel allocations • The user 1 is assigned frequency slots 0, 2, 4, 1, 3 respectively. • Note that ① 5 users divide the 25 resources, ② There is no intra-cell interference, ③ Since every user experiences all subcarrier, the frequency diversity is maximized. • Each BS has its own hopping matrix. The design rule is to have minimum overlap between users of neighboring BSs to minimize the interference. • Two Latin squares are said to be orthogonal if the ordered pair (i, j), where i and j are the entries from the same position in the respective squares, exhaust the N2 possibilities. • Orthogonality of Latin square corresponds to there being exactly one time/subcarrier overlap for every pair of users in different cells.

  38. OFDMA – subchannel allocations • When N is prime, there are N – 1 mutually orthogonal Latin squares. For a =1,L, N −1, we define an N × N matrix Qawith entry: (i, j = 0, 1, L, N −1) • For example, N = 5 can support four cells. Each user has interference from one user per cell. User 1 in 1st cell receives interference from user 3 in 2nd cell, user 5 in 3rd cell, and user 2 in 4th cell.

  39. MAC Design Issues • Many factors • FDD & TDD for MAC • AMC (Adaptive Modulation & Coding) • FEC (Forward Error Correction) • ARQ (Automatic Repeat reQuest) • Hybrid-ARQ • Burst Packet transmission • …

  40. FDD & TDD for MAC

  41. AMC (Adaptive Modulation & Coding) • QPSK to 64QAM

  42. AMC (Adaptive Modulation & Coding)

  43. FEC (Forward Error Correction

  44. ARQ (Automatic Repeat Re quest) • 수신단에서 전송에러의 유무를 CRC를 이용하여 점검 • 정상:ACK, 비정상:NAK으로 수신단에서 회신 • 송신측: ACK 신호 수신 Time out되거나 NACK수신시 재전송 • 사용 프로토콜, 지연, 패킷사이즈, 패킷 수, 버퍼 크기 등에 의하여 성능에 영향 • Burst error형태의 유선환경: TCP • Scatter Error형태의 무선환경: RLP

  45. Hybrid ARQ • ARQ와 FEC의 조합에 의한 에러복구 • 높은 초기 FER값 설정에 의한 전력 효율성 증대=> 통화용량증대, throughput증대 • Chase combining과 Incremental redundancy를 사용하여 효율성 제고 • Chase combining: 에러가 발생한 프레임을 폐기하지 않고 재전송 프레임과 combining Chase combining

  46. Hybrid ARQ • Incremental Redundancy: 재 전송시 마다 채널코딩 이득을 점차 증가시켜 재전송 Incremental Redundancy

  47. Burst Packet transmission

  48. Burst Packet transmission

  49. MAC Layer Issue List (1/4)

  50. MAC Layer Issue List (2/4)

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