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EVM Degradation in LTE systems by RF Filtering



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EVM Degradation in LTE systems by RF Filtering

  1. 1. OFDM  To overcome the effect of multi path fading problem available in UMTS, LTE uses Orthogonal Frequency Division Multiplexing (OFDM) for the downlink[2].
  2. 2. OFDM  That is, from the base station to the terminal to transmit the data over many narrow band careers of 180 KHz each instead of spreading one signal over the complete 5MHz career bandwidth i.e. OFDM uses a large number of narrow sub-carriers for multi-carrier transmission to carry data[2].  OFDM, is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method.  Besides, The basic LTE physical resource can be also seen as a time-frequency grid[2].
  3. 3. OFDM  The OFDM symbols are grouped into resource blocks. One resource block has a total size of 180kHz in the frequency domain and 0.5ms in the time domain. Each user is allocated a number of so-called resource blocks in the time and frequency grid. The more resource blocks a user gets, and the higher the modulation used in the resource elements, the higher the data-rate[2].
  4. 4. OFDM  As mentioned above, a resource block (RB) is the smallest unit of resources that can be allocated to a user. The RB is 180 kHz wide in frequency and 0.5 ms in time. In frequency, the RB contains 12 x 15 kHz subcarriers[4]. Time Unit Value Frame 10 ms Half-frame 5 ms Sub-frame 1 ms Slot 0.5 ms Symbol (0.5 / 7) ms
  5. 5. OFDM  The bandwidths defined by the standard are 1.4, 3, 5, 10, 15, and 20 MHz. For downlink signals, the DC subcarrier is not transmitted, but is counted in the number of subcarriers. For uplink, the DC subcarrier does not exist because the entire spectrum is shifted down in frequency by half the subcarrier spacing and is symmetric about DC[4,5].
  6. 6. OFDMA  With the identical number of channels, OFDM occupies less bandwidth than FDMA by orthogonality between subcarriers.
  7. 7. OFDMA  To achieve high radio spectral efficiency as well as enable efficient scheduling in both time and frequency domain, a multicarrier approach for multiple access was chosen[7].  For the downlink, OFDMA (Orthogonal Frequency Division Multiple Access) was selected[7,8].  Several division multiple access scenarios are as below[13]:
  8. 8. OFDMA  In OFDM, the user are allocated on the time domain only while using an OFDMA system the user would be allocated by both time and frequency.  This is useful for LTE since it makes possible to exploit frequency dependence scheduling. For instance, it would be possible to exploit the fact that user 1 might have a better radio link quality on some specific bandwidth area of the available bandwidth.
  9. 9. OFDMA  What is the difference between OFDM and OFDMA[8]?  OFDM support multiple users (Multiple Access) via TDMA basis only, while OFDMA support either on TDMA or FDMA basis or both simultaneously.  OFDMA supports simultaneous low data rate transmission from several users, but OFDM can only support one user at given moment.  Further improvement to OFDMA over OFDM robustness to fading and interference since it can assign subset of subcarrier per user by avoiding assigning bad channels.  OFDMA allows these subcarriers to be shared between multiple users, but OFDM doesn’t[7].
  10. 10. SC-FDMA  But whether OFDM or OFDMA, one of the most difficult engineering concerns in the RF section of is handling very large peak-to-average power ratios (PAPRs). A peak in the signal power will occur when all, or most, of the sub-carriers align themselves in phase. In general, this will occur once every symbol period[10-12]. Average Power Peak Power Time OFDM Symbol Power
  11. 11. SC-FDMA  Large PAPR requires high linearity requirements for PA and increases power consumption[7] PA
  12. 12. SC-FDMA  Consequently, Single Carrier Frequency Division Multiple Access(SC-FDMA) transmission technique is used for Uplink[13].  SC-FDMA, variant of OFDM, reduces the PAPR[13]:  Combines the PAPR of single-carrier system with the multipath resistance and flexible subcarrier frequency allocation offered by OFDM.  It can reduce the PAPR between 3- to 9dB compared to OFDMA.
  13. 13. SC-FDMA  OFDMA transmits the data symbols in parallel, one per subcarrier[14].  SC-FDMA transmits the data symbols in series at several times the rate, with each data symbol occupying N x 15 kHz bandwidth.  Visually, the OFDMA signal is clearly multi-carrier and the SC-FDMA signal looks more like single-carrier, which explains the “SC” in its name.
  14. 14. SC-FDMA  The value of the PAPR is directly proportional to the number of carriers, and is given by: where N is the number of carriers  As shown below, with the identical CCDF, the more subcarriers are, the larger PAPR will be.
  15. 15. SC-FDMA  Hence, SC-FDMA has smaller PAPR than OFDMA due to merely single carrier. Higher Peak  As shown below, SC-FDMA actually has smaller peak than OFDMA[16].
  16. 16. Group Delay  Clearly we cannot have a filter output appearing before its input, so the signal must have a positive delay[6] : Input Output Time Filter  Besides, any signal contains harmonics. That is, any signal is composed of several signals with different frequencies. If all these signals don’t have the identical delay, there will be group delay.
  17. 17. Group Delay  In terms of the relationship between phase and frequency, Group delay is:  A measure of device phase distortion.  The transit time of a signal through a device versus frequency.  The derivative of the device's phase characteristic with respect to frequency.
  18. 18. Group Delay  As shown above, the phase characteristic of a device typically consists of both linear and higher order (deviations from linear) phase-shift components. Linear phase-shift component: Higher-order phase-shift component: Represents average signal transit time. Represents variations in transit time for different frequencies. Attributed to electrical length of test device. Source of signal distortion.
  19. 19. Group Delay  The linear phase shift component is converted to a constant group delay value (representing the average delay).  The higher order phase shift component is transformed into deviations from constant group delay (or group delay ripple).  The deviations in group delay cause signal distortion, just as deviations from linear phase cause distortion.
  20. 20. Group Delay  As mentioned above, Group delay depicts the amount of time it takes for each frequency to travel through the device.  As mentioned above, Group delay depicts The derivative of the device's phase characteristic with respect to frequency. Phase Frequency  Thus, if group delay is zero, it means that phase is constant over frequency, and each frequency takes the same amount of time to travel through the device. No Group Delay
  21. 21. Group Delay  But, actually, there must be group delay. The phase is never constant over frequency, and each frequency never takes the same amount of time to travel through the device. Phase Frequency Slope = Group Delay  Nevertheless, what really matters is not only group delay, but also group delay variation, which will cause distortion of the signal waveform[6].
  22. 22. Group Delay  Usually, large group delay variation appears near the transition region in frequency response, leading to distortion of the signal waveform[6].
  23. 23. EVM  The total EVM of an LTE signal is calculated as[1] :  EVM is the rms EVM across all RBs in the LTE signal  EVMi is the EVM measured across the i th RB  N is the number of RBs in the LTE signal
  24. 24. EVM  Using this method the EVM of the i th RB can be calculated as follows[1]:  ∆α is the effective magnitude ripple across the i th RB of the filter’s pass band.  ∆ø is the effective phase ripple across the i th RB of the filter’s pass band, Filter
  25. 25. EVM  Let’s inject a LTE Downlink Signal (with Subcarrier Modulation = 64QAM, Source Power = 0 dBm) into a filter[1] : Filter
  26. 26. EVM  By far the worst result was the 15 MHz bandwidth case due to the fact that the bandwidth of the signal (15 MHz) was greater than the bandwidth of the filter (14.6 MHz), causing part of the signal’s spectrum to be rejected by the filter. As a result a higher EVM reading is not surprising[1]. 15 MHz 14.6 MHz
  27. 27. Green = wideband Filter Red = legacy Filter Blue = predistorted waveform EVM  During pre-distortion, the signal bandwidth will increases. If the filter’s bandwidth is not wide enough, the pre-distorted waveform will be truncated, as marked yellow in the photo below, thereby distorting waveform and leading to EVM issue[17]. PA Real PA DPD Predistorter
  28. 28. EVM  Except 15 MHz, the EVM results for the other signal bandwidths show a clear trend: the wider the bandwidth of the signal, the lower the measured EVM rise[1].  Because a narrowband LTE signal, a greater proportion of the signal’s RBs lies near the band edge of the filter, where the group delay variation is greatest, leading to distortion of the signal waveform. As a result the average RB EVM level will be higher, leading to a higher EVM level for the signal as a whole[1]. Bandwidth (MHz) 1.4 3 5 10 15 EVM (%) 0.39 0.22 0.17 0.15 1
  29. 29. EVM  For instance, a 1.4 MHz bandwidth signal is near band edge. If 3 RBs are contaminated by large group delay variation, it means that 50% RBs (3/6 = 50%) have poor EVM, thereby making EVM of the whole signal poor. 1.4 MHz Pass band  Conversely, a 10 MHz bandwidth signal is near band edge. Even though 5 RBs are contaminated by large group delay variation, it means that only 10% RBs (5/50 = 10%) have poor EVM, thereby making EVM of the whole signal still good[1]. 10 MHz Pass band RB with good EVM RB with poor EVM
  30. 30. EVM  As mentioned above, we know that if a signal with narrow bandwidth is near band edge of filter, the EVM aggravates[1]. EVM Rise of LTE Downlink Signal vs. Carrier Frequency (Signal Bandwidth = 1.4 MHz, Source Power = 0 dBm)  As shown below, near the band edge of the filter’s pass band, the group delay variation was more severe. Consequently the EVM rise in this frequency range was somewhat higher.
  31. 31. EVM  In terms of RX signal, the higher the EVM is, the higher symbol error rate will be, thereby aggravating sensitivity[18].  Consequently, the filter should be wideband. Even though the high channel, it’s still NOT near band edge. Pass band
  32. 32. EVM  Nevertheless, the filter’s frequency response will shift in high temperature. That is, even though the high channel is not near band edge in normal temperature. But in high temperature, the high channel may be near band edge, thereby aggravating EVM. Normal Temperature High Temperature  Consequently, when selecting filter, pay attention to not only bandwidth, but also its frequency response variation in high temperature[19].
  33. 33. Reference [1] EVM Degradation in LTE Systems by RF Filtering [2] LTE OFDM Technology [3] UMTS Long Term Evolution(LTE) - Technology Introduction, Application Note, Rohde & Schwarz [4] LTE Physical Layer Overview, Keysight [5] Synchronization Signals (PSS and SSS) [6] Group Delay Explanations and Applications [7] The Mobile Broadband Standard, 3GPP [8] Difference Between OFDM and OFDMA [9] LTE Uplink Transmission Scheme [10] The OFDM Challenge [11] OFDM and Multi-Channel Communication Systems, National Instruments [12] 4G Broadband-what you need to know about LTE [13] LTE Radio Interface (OFDM,OFDMA,SC-FDMA) [14] 3GPP LTE - Evolved UTRA - Radio Interface Concepts [15] PAPR Reduction in MIMO-OFDM Systems Using PTS Method [16] PAPR Reduction Method for OFDM Systems without Side Information [17] QFE1100 PA Power Management IC Training Slides, Qualcomm
  34. 34. [18] Receiver Optimization Using Error Vector Magnitude Analysis [19] Temperature-Compensated Filter Technologies Solve Crowded Spectrum Challenges