Performance Analysis of OFDM Systems with

Adaptive Sub Carrier BandwidthSuvra S. Das, Student Member, IEEE, Elisabeth De

Carvalho, Member, IEEE,and Ramjee Prasad, Senior Member, IEEE

IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 7, NO. 4, APRIL 2008

報告者 :黃聖琪學號 :96325503

Outline

IntroductionSystem DescriptionAnalytical ModelAdaptive Sub Carrier Bandwidth AlgorithmResults and DiscussionConclusion

Introduction

• It is proposed in this work to use dynamically adaptive sub carrier bandwidth (ASB) along with adaptive bit loading to mitigate ICI in such conditions.

• ICI is proportional to the received signal strength and to the square of the residual carrier offset plus Doppler spread while it is inversely proportional to the sub carrier bandwidth.

System Description

• ASB can be implemented in time division multiplexing (TDM) system framework.

• The number of sub carriers in different time slots may be changed to generate different sub carrier bandwidths.

• The guard interval duration (GI) is fixed.

• The power per sub carrier is fixed and equally distributed on all data sub carrier.

• The rate is varied on each sub carrier by means of adaptive modulation.

Analytical Model

• The time domain signal of the transmitted OFDM symbol can be expressed as

• After passing through the channel, the signal can be represented as

• With perfect timing synchronization, but residual carrier frequency offset δfc (Hz), the received OFDM symbol is

ths

• The signal portion without the noise part is

• we can represent as . Therefore, dfjeh 2)( )(h

• The received signal can be computed as

• δfc + fdτ can be termed as effective carrier offset and represented as δf. The relative offset, i.e. the ratio of the effective offset to the sub carrier spacing can be defined as , where is the sub carrier bandwidth

The ICI power at the receiver on sub carrier is

SINR

Adaptive Sub Carrier Bandwidth Algorithm

• The algorithm to dynamically select the appropriate sub carrier bandwidth and bit load per sub carrier to maximize the throughput while satisfying a required BER is presented here.

• The sub carrier bandwidth can be chosen as

• The estimated throughput Thpt( ) in (9) can be written as

• The bit load estimate per sub carrier used in (12) can be expressed as

• The BER associated with the chosen bit load is

The Following Steps are Executed in Sequence

1. Select one sub carrier bandwidth from the available options.

2. Evaluate (15), i.e. SINR at each sub carrier for the selected sub carrier spacing.

3. Use the above in finding bit load for this chosen value of sub carrier spacing following (13).

4. Calculate the associated BER for each sub carrier for the chosen bit load using (14).

5. Use the above calculations of bit load and related BER for each sub carrier in calculating the throughput for the chosen sub carrier bandwidth following (12).

6. Store the value of the estimated throughput along with the value of sub carrier bandwidth and associated bit loads per sub carrier.

7. Repeat all the above steps for all possible values of sub carrier bandwidth.

8. Finally execute (9) to select the sub carrier bandwidth and bit loads per sub carriers which has the highest estimated throughput.

9. Since the rate of change of Doppler condition and average channel quality is much slower compared to the rate of change of channel coefficients, one may consider to adapt the sub carrier spacing at a rate much less than adapting the bit loading. The bit loading should be done once per coherence time of the channel coefficients. i.e. once a sub carrier spacing is selected, it may be used until the Doppler condition or the average signal strength changes significantly and hence step 1 and step 7 may be skipped, and step 8 may be modified to “Finally execute (9) to select bit loads per sub carriers which has the highest estimated throughput for the chosen sub carrier bandwidth”.

Results and Discussion

ASBB

ASBB

ASBB

Conclusion

• ASB avoids the complex compensation or interference cancelation mechanism at the receiver, thereby allowing lower complexity receivers. Thus the advantage of increased throughput with possibility of low complexity receivers makes the proposed ASB a potential candidate for consideration in future systems.

Reference

• [2] M. Speth, S. A. Fechtel, G. Fock, and H. Meyr, “Optimum receiver design for wireless broad-band systems using OFDM, Part I,” IEEE Trans. Commun., vol. 47, no. 11, pp. 1668–1677, Nov. 1999.

• [9] S. T. Chung and A. J. Goldsmith, “Degrees of freedom in adaptive modulation: a unified view,” IEEE Trans. Commun., vol. 49, no. 1, pp. 1561–1571, Sept. 2001.