Resource A llocation in Multiuser OFDM Systems

1. Resource Allocation in Multiuser OFDM Systems Presented by Wong Wang Kit Nov. 7th, 2013

2. OUTLINE • Introduction to Multiuser OFDM • Fix and Dynamic Allocation • MAO: Adaptive Subcarrier, Bit and Power Allocation • Performance Gain • Conclusion

3. Introduction to Multi-user OFDM • OFDM: Encoding digital data on multiple sub channels • Modulation output are transformed into time domain by IFFT • Cyclic extension is then added between subcarriers • Multi-user OFDM Systems: • Add multiple access to OFDM systems • Allow a number of users to share OFDM symbols

4. Resource Allocation Problem in MUOFDM • Limit resources • Users are sharing limited number of sub channels • There is total transmit power constraint at transmitter • Allocation Problem: • How to allocation sub carriers to users • How to assign power to each sub carrier

5. Existing resource allocation schemes • Fixed resource allocation • Time division multiple access(OFDM-TDMA) • Frequency division multiple access(FDMA) • Dynamic resource allocation

6. Fixed resource allocation • OFDM-TDMA • The total bandwidth (all subcarriers) is allocated to one user for the duration of several OFDM symbols whithin a downlink or uplink frame • OFDM-FDMA • An unambiguous subset of all carriers is assigned to each user

7. Fix resource allocation • Fix allocation in an independent dimension • Simple and of low computational cost • Are fixed regardless of the current channel condition • Time-varying nature of the wireless channel • For a specific sub channel, different users have different instantaneous channel gain • Not optimal • A Waste of resources (an allocated subchannel may not be used by the user)

8. Dynamic resource allocation • Allocates resources adaptively to the users based on their channel condition and requirement • Makes use of multiuser diversity (for a specific sub channel, different users have different instantaneous channel gain) • Jointly allocated subchannels and transmit power • Achieve higher performance • Difficult to design

9. MAO: Adaptive subcarrier, bit and power allocation • A resource allocation scheme proposed by[1] • Design consideration: • Multiuser OFDMsytstems in selective fading environments • The subcarriers which appear in deep fade to one user may not be in deep fade for other user as the fading parameters for different users are mutually independent an adaptive multiuser subcarrier allocation scheme • Assuming that the transmitter knows the channel transfer functions of all user, it is beneficial to choose modulation adaptive to channel gain  bit allocation • Based on different modulation schemes and required Qos different transmit power need to be allocated to the subcarrierrs

10. Design consideration (cont’d) • The system has K users • All users transmit in all the time slots • The frequency band is divided into N subchannels • The kth user has a data rate requirement • The objective is to minimize the overall transmit power under the user data rate constraints

11. System model

12. System model • The transmitter collects the instantaneous channel gains on all subcarries of all the users via a feedback channel Using the channel information, the transmitter applies the combined subcarrier, bit and power allocation algorithm to assign different subcarriers to different users and the number of bit/OFDM symbol to transmitted on each subcarrier. Depending on the number of bit assigned to a subcarrier, the adaptive modulator will use a corresponding modulation scheme The transmit power level will be adjust according to the assigned subcarriers and modulation scheme

13. System model • denotes the number of bit of the kth user that is assigned to nth subchannel • Each subcarrier can only used by one user, it follows that: • Since, the number of different modulation scheme is limited, can only take the value in set

14. System model • denotes the required received power in a subcarrier for reliable reception of c information bits/symbol • depends on subcarrier n, rate c and channel gain of user k on subcarrier n • In order the success in receive at receiver, the transmit power allocated to the nth subcarrier for the kth user must be equal to

15. System model • Properties of : • It is a convex function • . No power is needed when on bit is sent • is an increasing function in c • is increasing in c, additional power required to transmit one more bit increases with c

16. Problem formulation

17. Bit Allocation Algorithm For Single User Channel • Bit allocation problem for single user system can be solved optimally be a greedy algorithm, due to: • The power need to transmit a certain number of bits in a subcarrier is independent of the numbers of bits allocated to other subcarriers. • The addiction power needed to transmit one more bit is increasing function of c, i.e. Subcarrier, bit and power allocation for multiuser bit allocation for single user (power allocation is implied by bit allocation) =

18. Greedy Bit Allocation Algorithm For Single User Channel • Addition power required to transmit one more bit: • Greedy algorithm (basic idea): • Assigns bits to the subcarriers one bit at a time • Each time assign one bit to the subcarrier that requires the least additional power, i.e.

19. Greedy Bit Allocation Algorithm For Single User Channel Alogrithm 1

20. Multiuser Subcarrier and Bit Allocation • Convex relaxation: the hardness of the problem comes from integer constraints on • Relax it by introducing and

21. Multiuser Subcarrier and Bit Allocation Let F be the set of subchannels, |F|=N. Solve the relaxed optimization problem and use the rounding scheme to get the subset of subchannels that assigned to user k Given for each user k, the problem reduced to bit allocation problem for single user system. Use the greedy bit allocation algorithm given by algo 1 to assign bit and power

22. Performance gain

23. References • Shen, Zukang, Jeffrey G. Andrews, and Brian L. Evans. "Adaptive resource allocation in multiuser OFDM systems with proportional rate constraints." Wireless Communications, IEEE Transactions on 4.6 (2005): 2726-2737. • Wong, Cheong Yui, et al. "Multiuser OFDM with adaptive subcarrier, bit, and power allocation." Selected Areas in Communications, IEEE Journal on 17.10 (1999): 1747-1758. • Rohling, Hermann, and R. Gruneid. "Performance comparison of different multiple access schemes for the downlink of an OFDM communication system." Vehicular Technology Conference, 1997, IEEE 47th. Vol. 3. IEEE, 1997.

24. Thank you!