OFDM basics and trouble shooting technique

OFDM

1. LTE Workshop #5 Covering OFDM Report number: OFDM Date: 10 May 2017

2. OFDM has been around since the 1960’s It was considered as the Physical layer for WCDMA 3G networks, but at the time it was not considered as a mature enough technology. For 4G, the main requirements: High data rates Spectral efficiency Scalable bandwidths (1.4 – 20 MHz) High tolerance of multipath delay & spread These suited a more mature OFDM system perfectly. OFDM has become the standard for not only LTE, but also for WiFi & WiMax . Why OFDM ?

3. Analogy

4. Interference First we need to understand interference

5. Fixed communication path Undisturbed communications Wired Channel PSTN

6. Multipath fading  : One signal with multiple different paths between Tx & Rx Atmospheric ducting, ionospheric reflection & refraction, object reflections Shadow fading (flat fading)  : Shadowing from obstacles affecting the wave propagation. Path 1 Wireless Channel & Multipath Fading Path 3 Path 2

7. Not all frequency components in the signal are affected equally. Multipath fading (frequency selective)

8. Delay Spread & ISI

9. Multipath fading subsequently creates what is known as Delay Spread. Delay spread is problematic as it leads to Inter Symbol Interference (ISI) . Delay Spread & ISI Symbol 1 Symbol 2 t Prolonged symbol ISI

10. Delay Spread & ISI

11. Countering interference

12. Guard intervals Inter Symbol Interference (ISI) is a form of signal distortion where a symbol overlaps the succeeding symbol causing the communication to be less reliable. ISI is prevented by guard intervals , which is a period of time between symbols. Guard intervals accommodate the late arrivals of symbols due to multipath reception. Symbol 1 Symbol 2 t Guard Interval

13. Example: Bandwidth = 1 MHz Center frequency = Fc Delay spread = 3 µsec Bandwidth = 1 MHz Frequency Domain Fc Symbol time ( Ts ) = 1 µsec ( Ts = 1/BW) Time Domain TS = 1 µsec 3 µsec delay spread Combined symbol time = 4 µsec Guard intervals

14. Following our scenario we see that there is a Combined symbol time of 4 µsec. This means that in our scenario there will be a 25% or ¼ reduction in the efficiency of the channel. Scenario Channel Efficiency Reduction ¼ = 25% of channel DELAY SPREAD 25% 25% 75% Delay Spread ¾ = 75% of channel As we can see delay spread is undesirable for the bandwidth and channel usage, but it can be reduced by employing multiple subcarriers with lower bandwidth. Guard intervals

15. Stretch symbol time duration

16. 10 KHZ 10 KHZ In our example: 1 Mhz bandwidth = 100 subcarriers of 10 kHz each. Symbol duration = 100 µsec Meaning a delay 3 µsec per symbol / subcarrier becomes insignificant. 10 KHZ 10 KHZ 10 KHZ 10 KHZ Frequency Domain SC 1 SC 2 SC 3 SC 4 SC 7 SC 8 SC 6 SC 5 SC 100 Time Domain Symbol 1 – SC 1 Symbol 2 – SC 1 symbol duration = 100 µsec symbol duration = 100 µsec delay time = 3 µsec 10 KHZ 10 KHZ 10 KHZ 10 KHZ FDMA & Multi Carrier Wireless Transmission

17. Avoid ISI with longer symbol times ? Example: Symbol time = 50 micro-seconds Use 64 QAM Data rate = ? = (1 / 0.00005) * 6 bits/symbol = 120 kbps

18. Avoid ISI with longer symbol times ? However, if we add sub-carriers to the mix, we have a multiplier affect on the THRP: Thrp = (1 / 0.00005) * 6 bits/symbol * 12 sub-carriers = 1440 kbps

19. OFDM Orthogonal Frequency Division Multiplexing

20. OFDM v OFDMA

21. FDMA Concept Data signals Data modulation & Increase of Bandwidth 1 2 3 1 2 3 Modulated SC separated by guard intervals in the frequency domains Guard Interval Guard Interval SC frequency 1 SC frequency 2 SC frequency 3 channels divided into equal bandwidth spectrums SC 1 SC 2 SC 3

22. O (Orthogonal) FDM Orthogonality in a OFDM system takes place when the peak of a subcarrier (SC) is reached and the other subcarriers (SC) contribution is zero. SC 1 SC 2 SC 3 Peaks of subcarriers (SC) Spectrum Peak SC 3 SC 1 SC 2 SC 3 SC 4 SC 5 SC 6 When subcarriers (SC) 1 – 6 are at their peaks there is no interference from the other subcarriers (SC) due do them being orthogonal.

23. O (Orthogonal) FDM & Bandwidth Because the subcarriers are orthogonal they can use the same bandwidth without any interference. Spectrum Peak SC 1 SC 2 SC 3 SC 4 SC 5 SC 6 Same Bandwidth usage Same Bandwidth usage Same Bandwidth usage When subcarriers (SC) 1 – 6 are at their peaks there is no interference from the other subcarriers (SC) due do them being orthogonal and this allows them to use the same bandwidth.

24. G U A R D G U A R D G U A R D G U A R D G U A R D G U A R D OFDM compared to FDMA FDMA Spectrum OFDM Spectrum Bandwidth Bandwidth Frequency Frequency Saved Bandwidth SC 1 SC 2 SC 3 SC 4 SC 5 SC 6 SC 7 SC 1 SC 2 SC 3 SC 4 SC 5 SC 6 SC 7 Orthogonal subcarriers

25. OFDM - Subcarrier Separation around the DC The subcarriers are then separated on either side of the DC in multiples of 15KHZ. Direct Conversion (DC) is a null subcarrier which is allocated to have a equal frequency to that of the RF center frequency of the transmitting station. SC SC SC SC 15 KHZ 15 KHZ DC DC - SC

26. Channel Bandwidth Usable Bandwidth Subcarrier count 1.4 MHZ 1.08 MHZ 72 3 MHZ 2.7 MHZ 180 5 MHZ 4.5 MHZ 300 10 MHZ 9 HMZ 600 LTE can support variable channel bandwidth as shown below please note that as bandwidth increases so does the number of subcarriers. OFDM - Subcarrier Separation around the DC LTE bandwidth Table. Calculation 1MHZ = 1000 KHZ 1.08 MHZ x 1000 = = 72 SC 2.7 MHZ x 1000 = = 180 SC 4.5 MHZ x 1000 = = 300 SC 9 MHZ x 1000 = = 600 SC  

27. For an example lets take LTE bandwidth of 5 MHZ. OFDM - Subcarrier Separation around the DC Channel Bandwidth Usable Bandwidth Usable Bandwidth In Number Of Subcarriers 5 MHZ 4.5 MHZ 300 Calculation 1MHZ = 1000 KHZ 4.5 MHZ x 1000 = = 300 SC   Remember that the DC is also a subcarrier and there is 0.5 MHZ used for guard bands in the form of subcarriers. 5 MHZ = 5000 KHZ Number of SC used for guard band = = 333 SC Number of SC used for guard band = 333 – 1 (DC subcarrier) – 300 (Usable subcarriers) Number of SC used for guard band = Number of SC used for guard band = 16 SC either side of the DC  

28. 5 MHZ = 5000 KHZ = 333 SC DC = 1 SC (15 KHZ) Guard band 0.5 MHZ = 500 KHZ = 32 SC = 16 SC on each side of the DC D C 15 KHZ SC 30 KHZ SC 45 KHZ SC 60 KHZ SC 75 KHZ SC 90 KHZ SC 105 KHZ SC 120 KHZ SC 135 KHZ SC 150 KHZ SC 165 KHZ SC 180 KHZ SC 195 KHZ SC … 2250 KHZ … – 2250 KHZ - 195 KHZ SC - 180 KHZ SC - 165 KHZ SC - 150 KHZ SC - 135 KHZ SC - 120 KHZ SC - 105 KHZ SC - 90 KHZ SC - 75 KHZ SC - 60 KHZ SC - 45 KHZ SC - 30 KHZ SC - 15 KHZ SC OFDM - Subcarrier Separation around the DC Please note that there is a total of 150 usable subcarriers on either side of the DC so the complete usable KHZ for 5 MHZ is 2250 KHZ on either side of the DC 150 Usable subcarriers of -15 KHZ each 150 Usable subcarriers of 15 KHZ each G U A R D G U A R D Recall that we are using OFDM so all these subcarriers are orthogonal. Furthermore the use of negative frequency is needed because the subcarriers are transmitted below the radio carrier.

29. The drawbacks of OFDMA are specifically: Very sensitive to frequency offsets (Doppler effect of mobility) High Peak to Average power ratio (constructive addition of sub-carriers) Base station equipment can deal with these issues effectively, the equipment requirements are however too complex & expensive for UE’s. SC-FDMA

30. It is for this reason that the LTE Uplink uses SC-FDMA on the physical layer – here we use a single carrier as opposed to N * Sub-carriers. SC-FDMA

31. Time/frequency resources dynamically allocated: Resource allocation example