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Mobile WiMAX Cross-Layer Issues: OFDMA/TDD Optimization

Explore the optimal resolution of cross-layer issues between PHY/MAC layers in mobile WiMAX systems. Learn about subcarrier allocation, frame structure, and channel sounding for efficient broadband network transport. Date: 04/16/2007

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Mobile WiMAX Cross-Layer Issues: OFDMA/TDD Optimization

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  1. PHY/MAC cross-layer Issues in mobile WiMAX Advisor: Dr. Kai-Wei Ke Speaker: Chao-Sung yah Date:04/16/2007

  2. Outline • Introduction • PHY layer in OFDMA/TDD mobile WIMAX • MAC layer in OFDMA/TDD mobile WIMAX • PHY/ MAC cross-layer issues in mobile WiMAX • Conclusions • Reference

  3. Introduction(1/4) • last-mile connections • less costly • non-line-of-sight

  4. Introduction(2/4) • wireless networks are highly dependent on communications channels (dynamic, correlated, unreliable, very expensive) • This is why performance will be highly dependent on how well radio resource management supports quality-of-service (QoS) requirements

  5. Introduction(3/4) • Therefore, several cross-layer issues between the medium access control (MAC) layer and the physical (PHY) layer need to be optimally resolved on the radio resource management side of Mobile WiMAX systems.

  6. Introduction(4/4) • To provide the highest transport efficiency in broadband networks, time division duplex (TDD) is preferred over frequency division duplex (FDD) • because it offers more flexibility in changing the UL and DL bandwidth ratio according to the dynamic traffic pattern.

  7. PHY layer in OFDMA/TDD mobile WIMAX • OFDMA • TDD • OFDMA / TDD Frame Structure • Subcarrier allocation • Distributed subcarrier allocation • Adjacent Subcarrier Allocation

  8. OFDMA(1/2) • Intersymbol interference and frequency selective fading. • Cyclic prefix (CP) Collect multi-path and maintain orthogonality • Inverse Fourier transform • On the DL, a subchannel may be intended for different receivers. • On the UL, a transmitter may be assigned one or more subchannels and several transmitters may transmit simultaneously.

  9. OFDMA(2/2)

  10. TDD • TDD, FDD, H-FDD • Channel sounding • Channel sounding is a signaling mechanism that enables the base station (BS) to estimate BS-to mobile station (MS) channel response based on the UL signals transmitted by the MS.

  11. OFDMA / TDD Frame Structure • The FCH specifies • The subchannel groups used for the segment • The burst profile • The length of the DL-mobile application part (MAP) message

  12. Subcarrier Allocation(1/3) • Three types of subcarriers • Data subcarriers • Data subcarriers are used for data transmissions • Pilot subcarriers • pilot subcarriers are used for channel estimation and various synchronization purposes • Null subcarriers • null subcarriers are used for the direct current (DC) carrier and guard bands transmitting no signals

  13. Subcarrier Allocation(2/3)

  14. Subcarrier Allocation(3/3) • Two main types of subcarrier allocation • Distributed Subcarrier Allocation • In a distributed subcarrier allocation(DSCA),multiple data subcarriers are grouped into a subchannel. • Adjacent Subcarrier Allocation • In the adjacent subcarrier permutation, symbol data within a subchannel is assigned to adjacent subcarriers, and the pilot and data subcarriers are assigned fixed positions

  15. Distributed Subcarrier Allocation • DL-partial usage of subchannels(DL-PUSC) • Full usage of subchannels (FUSC) • optional FUSC(OFUSC) • Tile usage of subchannels (TUSC) • UL-partial usage of subchannels(UL-PUSC) • UL-optional PUSC (OPUSC)

  16. DL-partial usage of subchannels(DL-PUSC) • The default DL subcarrier allocation method. • Subchannels are grouped into six major groups and assigned to three segments (three sectors) of a cell. Assigning two major groups to each segment, the cell can be viewed as frequency reused by a factor of three. • DL-PUSC is designed to minimize the probability of using the same subcarrier in adjacent sectors or cells.

  17. DL-partial usage of subchannels(DL-PUSC) G1 Segment 1 G2 frequency G3 Segment 2 G4 G5 Segment 3 G6 time

  18. Full usage of subchannels (FUSC) • Uses all subchannels and minimizes the performance degradation of fading channels by frequency diversity. • FUSC is also designed to minimize the probability of using the same subcarrier in adjacent sectors or cells. • FUSC pilots are in both variable and fixed positions.

  19. optional FUSC(OFUSC) • Also designed to fully use frequency diversity. • One difference from FUSC is that OFUSC uses a bin structure like band adaptive modulating and coding (AMC).

  20. UL-partial usage of subchannels(UL-PUSC) • The default UL subcarrier allocation method. • It is not necessary to start the UL subframe in the UL-PUSC zone. • UL-PUSC has a tile structure, with each tile comprising four subcarriers by three symbols. The four corner subcarriers are used as pilots, and the remaining eight subcarriers are used as data subcarriers.

  21. (UL-PUSC) corner tile frequency time pilot data

  22. Tile usage of subchannels (TUSC) • For use in the adaptive antenna system (AAS) zone • similar in structure to UL-PUSC.

  23. UL-optional PUSC (OPUSC) • Also has a tile structure, with each tile comprising three subcarriers by three symbols. • The center subcarrier is used as a pilot, and the remaining eight subcarriers are used as data subcarriers.

  24. tile UL-optional PUSC (OPUSC) frequency pilot time

  25. Adjacent Subcarrier Allocation • DSCAs can gain frequency diversity in frequency selective slow fading channels • ASCAs can gain multiuser diversity in frequency non-selective fading channels • symbol data within a subchannel is assigned to adjacent subcarriers, and the pilot and data subcarriers are assigned fixed positions • Band AMC and PUSC-adjacent subcarrier allocation (ASCA) are ASCA techniques.

  26. Band AMC(adaptive modulation and coding) • A bin—the set of nine contiguous subcarriers within an OFDMA symbol—is the basic allocation unit on both the DL and UL. • A group of four rows of bins is called a physical band. • An AMC slot consists of six contiguous bins in the same band • Four types of AMC slots are defined in the IEEE 802.16-2004 standard. • But in Mobile WiMAX, only one type of slot, defined as two bins by three symbols, is used.

  27. Band AMC(adaptive modulation and coding)

  28. PUSC-ASCA • PUSC-ASCA uses distributed clusters for the PUSC mode. The symbol structure uses the same parameters as those of the regular PUSC, and the same cluster structure is maintained • only the subcarrier allocation per cluster is different from that of the regular PUSC.

  29. MAC layer in OFDMA/TDD mobile WIMAX • Service Classes • Network Entry

  30. Service Classes • UGS: periodically generate fixed-size data packets,(VoIP) • rtPS:periodically generate variable-size data packets,(MPEG) • ertPS:A scheduling mechanism that builds on the efficiency of both UGS and rtPS. • nrtPS:ensures that the service flow receives request opportunities even during network congestion(FTP) • BE:Typical BE service is Web surfing.(HTTP)

  31. Network Entry • When an MS wants to enter the network, it follows the network entry process: • (1) downlink channel synchronization • (2) initial ranging • (3) capabilities negotiation • (4) authentication message exchange • (5) registration • (6) IP connectivity • (7) periodic ranging afterwards

  32. Network Entry

  33. PHY/MAC CROSS-LAYER ISSUES IN MOBILEWiMAX • Zone Switch and Frequency Reuse • Multiuser Diversity • Dynamic TDD Usage

  34. Zone Switch and Frequency Reuse • Since the OFDMA PHY layer has many choices of subcarrier allocation methods, multiple zones can use different subcarrier allocation methods to divide each subframe. • One benefit of using zone switching is that different frequency reuse factors (FRFs) can be deployed in a cell (or sector), dynamically.

  35. Multiuser Diversity • multiuser diversity is a very important leveraging factor of resource allocation management • Each MS faces a different fading channel hence, radio resource management can use multiuser diversity to maximize system throughput

  36. Multiuser Diversity • The difficulty lies in the fact that radio resource allocation also should satisfy fairness among subscribers. • Ultimately, radio resource management should follow a combined form of multiuser diversity and fairness scheduling.

  37. Dynamic TDD Usage • UL and DL stream volumes that have been considered symmetrical for conventional voice transmissions are unbalanced, and the ratio is time varying. • TDD is preferred to FDD because it enables real-time adaptation of UL and DL bandwidth according to the dynamic traffic pattern.

  38. Dynamic TDD Usage • FDD channel bands cannot be adjusted dynamically in response to the varying ratio of DL and UL traffic, due to hardware limitations. • It has been assumed that network operators determine the switching points for TDD UL and DL schemes

  39. If the DL/UL ratio is changed dynamically frame by frame and independently cell by cell, co-channel interference can exist in all four cases.

  40. Dynamic TDD Usage

  41. Conclusions • Mobile WiMAX is expected to bring fast, broad, seamless data communications, not only for fixed home and small business subscribers, but also for mobile subscribers. • WiMAX will begin competing in fixed broadband markets to link homes and businesses with worldwide core networks, • To strengthen the market power of Mobile WiMAX, radio resource management that deals with PHY/MAC crosslayer issues needs to be developed accurately and close to optimally.

  42. Reference • [1] J. Yun and M. Kavehrad,”PHY/MAC Cross-Layer issues in Mobile WiMAX.” January 2006 Bechtel Telecommunications Technical Journal • [2] Wireless MAN Working Group (http://www.wirelessman.org/). • [3] IEEE Std 802.16-2004, “IEEE Standard for Local and Metropolitan Area Networks – Part 16: Air Interface for Fixed Broadband Wireless Access Systems,” October 2004. subscribers.

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