OFDM(A) for wireless communication - Unik

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OFDM(A) for wireless communication6 ConclusionsIn this report we have explained how transmission and multiple access basedon OFDM and OFDMA is designed, with special attention to the Mobile WiMAXstandard and the Evolved UTRA (also known as LTE) standard, both which arebased on Scalable OFDMA.One major advantage of OFDM is that it can efficiently handle multipathtransmission without complex receivers. A simple one-tap equalized is enough.This also opens for efficient and simple support of multiple antennas. The use ofFast Fourier Transform (FFT) techniques is “simple” and cost-efficient. Changingthe FFT size also makes it easily scalable with respect to bandwidth. On thedownside, the OFDM-signal possesses the property of having a high so-calledPeak-to Average Power Ratio (PAPR), which increases with the number of subcarriersused, however this is also present in WCDMA on a smaller scale. This isunfavourable because to ensure linear behaviour a large backoff is needed inthe amplifier. On the base station transmitter, this may not be a majorproblem, but on the user terminal transmitter, this implies higher cost andpossibly higher power consumption and shorter battery life.OFDM used as a multiple access technique, commonly known as OFDMA,benefits from the scalability in assigning resources to different users in aflexible manner whether FDD or TDD operation is assumed. Since OFDM utilizestime- and frequency diversity, channel dependent scheduling is possible.Main advantages of OFDMA:• Scalability• Effective implementation using FFT (favourable for MIMOimplementation)• Opportunistic frequency schedulingMain disadvantage of OFDMA• Larger PAPRThe two major standards for mobile broadband are Mobile WiMAX, based onIEEE 802.16e, and the Long Term Evolution (LTE) standard from 3GPP, formallynamed Evolved UTRA (E-UTRA). There are no major differences between thesetwo standards on the physical layer. Both are designed for the same range ofbandwidths. The basic design is fairly similar, however details may differ. Basedon slightly different choices of values for the OFDM-parameters we might saythat Mobile WiMAX may have a better performance in “heavy” multipath, sincethe symbols and cyclic prefix are longer than for E-UTRA. On the other hand,this gives smaller sub-carrier distance which puts higher demands on frequencyaccuracies and smaller tolerance for Doppler spread in the channel. MobileWiMAX uses more resources for pilots or reference signals which at least intheory should give more robust performance in difficult radio conditions. On theother hand, it consumes more radio resources and power than E-UTRA, whichcan result in loss in efficiency.Conclusively on Mobile WiMAX vs. E-UTRA, there is no clear winner when itcomes to technical performance. E-UTRA has however the advantage of beingdesigned for smoother co-existence with and migration from WCDMA/HSPA.Radio planning OFDMA networks is more equal to GSM planning than WCDMAplanning. In an interference-limited scenario, intra-cell interference is basically50 - Telenor R&I R 7/2008
OFDM(A) for wireless communicationeliminated due to the fact that users are orthogonal to each other in eithertime- or frequency domain. This is similar to GSM, where there is orthogonalityin the time-domain within the cell. It is different from WCDMA, where intra-cellinterference on the uplink is a major factor when planning for high capacity.Main interference is from neighbour cells and the problem is to estimatereasonable interference margins.A few examples of link budgets for Mobile WiMAX and E-UTRA show that thereare some differences in the interference margin estimates.Additionally, Scalable OFDMA opens a few new ideas on frequency reusemethods. In WCDMA, a frequency reuse factor of 1 (same carrier in all cells) isoften used, but frequency planning with e.g. 3 frequencies are also possible. InOFDMA it is possible to adjust the frequency reuse factor between 1 and e.g. 3using fractional frequency reuse which opens up the possibility for dynamicfrequency reuse and also dynamic capacity planning. Dividing each cell in aninner part using all frequencies (reuse 1) and outer part using 1/3 of thefrequencies (reuse 3) makes it possible to tune the different cell capacitiescooperatively, e.g. by moving capacity from one cell to another dependent onthe time of day.Telenor R&I R 7/2008 - 51
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