Technical Foundation of WiMAX (Chapter 6) ENE 490 MON 13:30-16:30 Asst. Prof. Dr. Suwat Pattaramalai

Transcription

1 (Chapter 6) ENE 490 MON 13:30-16:30 Asst. Prof. Dr. Suwat Pattaramalai

2 Orthogonal Frequency Division Multiple Access (OFDMA) Problem: high on-demand data rates in a finite bandwidth Multiple-access: share available bandwidth (fraction of resources) Diverse nature of traffic: VoIP, Data transfer, and video streaming System deployment: mobility, neighboring cells, high required BW OFDM is a modulation technique OFDMA: users share subcarriers and time slots, increase multiuser diversity, increase freedom in scheduling the users, etc. OFDMA costs overhead in both direction which inform its user assigned slot. This chapter: multiple-access with OFDM modulation, key sources of capacity gain in OFDMA, Algorithms for multiuser diversity and adaptive modulation gains, and OFDMA in WiMAX( challenges and opportunities to improve performance)

3 Multiple-Access Strategies for OFDMA Provide orthogonal or non-interfering communication channels Divide frequency in FDMA, divide time in TDMA, and CDMA users can share bandwidth and time slots by orthogonal binary codes Orthogonal is not possible in dense wireless systems: guarantee only in the same cell, imperfect bandpassfiltering and multipath channels and imperfect synchronization Random Access versus Multiple Access Carrier sense multiple access (CSMA): Ethernet and Wireless LAN ALOHA and slotted ALOHA: delay and collision problems CSMA: users listen to channel before transmitting (avoid collision) Random access is inefficient, sophisticated due to contention and collisions: multiple access more prefer in wireless communication

4 Frequency Division Multiple Access (FDMA) Allocate each bandwidth to each users Time Division Multiple Access (TDMA) Allocate each time slot to each users in round robin scheduling

5 Code Division Multiple Access (CDMA) OFDM and CDMA are not fundamentally incompatible

6 Advantages of OFDMA OFDMA: hybrid of FDMA and TDMA, users are dynamically assigned subcarriers (FDMA) in different time slots (TDMA)

7 Advantages of OFDMA Robust multipath suppression and frequency diversity, flexible accommodate many users with widely varying applications, data rates, and QoS requirements Significant advantage: reduce the transmit power and to relax the peak-to-average-power ratio (PAPR) problem

8 Multiuser Diversity and Adaptive Modulation Subcarrier and power allocation based on channel conditions Multiuser diversity: gain available by selecting a user or subset of users having good conditions Adaptive modulation: methods to achieve higher data rates Multiuser Diversity

9 Multiuser Diversity Probability of getting a large channel gain improves as the number of users in creases

10 Multiuser Diversity The capacity and bit error rate are also improve as number of users increases

11 Adaptive Modulation and Coding Transmit as high as a data rate as possible when the channel is good Transmit at a lower rate when the channel is poor (avoid excessive dropped packets) Data rate depends on using number of constellation points and code ratio value 52 configurations of modulation and coding types & rate are possible They are called burst profiles

12 Adaptive Modulation and Coding

13 Adaptive Modulation and Coding (Algorithm)

14 Notations Technical Foundation of WiMAX

15 Resource-Allocation Techniques for OFDMA Algorithms are not specified by WiMAX standard Users estimate and feedback the channel state information (CSI) to a centralized base station where subcarrier and power allocation are determined in resource-allocation procedure Constrained optimization problems: (1) minimize the total transmit power with a constraint on the user data rate (2) maximize the total data rate with a constraint on total transmit power Maximum Sum Rate Algorithm (MSR) Maximize the sum rate of all users (total transmit power constraint) Drawback: users close to base station will be allocated all resources Maximum Fairness Algorithm Allocate the subcarriers and power such that minimum user s data rate is maximized and equalizing the data rates of all users

16 Proportional Rate Constraints Algorithm Maximized the sum throughput with the additional constraint that each user s data rate is proportional to a set of predetermined system parameters Proportional Fairness Scheduling Scheduling algorithms that balance latency and throughput and achieve some degree of fairness are needed PF scheduler adopted in packet date system: HSDPA, 1xEV-DO PF algorithm is used in OFDMA by treating each subcarrier independently Performance Comparison Throughput Fairness Summary of Comparison

17 Throughput Technical Foundation of WiMAX

18 Fairness Technical Foundation of WiMAX

19 Summary of Comparison

20 OFDMA in WiMAX: Protocols and Challenges OFDMA Protocols: subchannelization, mapping messages, and ranging (Chapter 8 and 9) Subchannelization: blocks of subcarriers; spread out over the entire bandwidth (distributed subcarrier permutation) or all in the same frequency range (adjacent subcarrier permutation) Mapping Message: broadcast this information in downlink MAP message and MS must also be informed of the burst profile used in DL and the UL Rangining: synchronize the symbols and equalize the received power levels among the various active MSs Only need synchronize in Uplink (not in Downlink) Uplink power control: (1) improve battery life, (2) reduce spurious other-cell interference, and (3) avoid drowning out faraway users in the same cell who are sharing and OFDM symbol with them

21 Cellular OFDMA Greatly benefits for suppressing or avoiding the interference from adjacent cells Using frequency-hopping pattern for each base station (Flash-OFDM) Better performance can be obtained if the base stations cooperate with one another and make multicell resource-allocation schedules accordingly but computational difficulties involved in processing Simple approaches: neighboring base stations share simple information to make sure they don t assign the same subcarriers to vulnerable users Limited Diversity Gains Total diversity gain will be less than the sum of the diversity gains from the individual techniques

22 Limited Diversity Gains

23 Summary and Conclusions