EVM Degradation in LTE systems by RF Filtering

2. OFDM
 To overcome the effect of multi path fading problem
available in UMTS, LTE uses Orthogonal Frequency
Division Multiplexing (OFDM) for the downlink[2].

3. 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].

4. 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].

5. 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

6. 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].

7. OFDMA
 With the identical number of channels, OFDM occupies
less bandwidth than FDMA by orthogonality between
subcarriers.

8. 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]:

9. 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.

10. 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].

11. 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

12. SC-FDMA
 Large PAPR requires high linearity requirements for PA
and increases power consumption[7]
PA

13. 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.

14. 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.

15. 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.

16. 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].

17. 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.

18. 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.

19. 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.

20. 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.

21. 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

22. 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].

23. Group Delay
 Usually, large group delay variation appears near the
transition region in frequency response, leading to
distortion of the signal waveform[6].

24. 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

25. 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

26. EVM
 Let’s inject a LTE Downlink Signal (with Subcarrier
Modulation = 64QAM, Source Power = 0 dBm) into a
filter[1] :
Filter

27. 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

28. 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

29. 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

30. 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

31. 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.

32. 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

33. 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].

34. 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

35. [18] Receiver Optimization Using Error Vector Magnitude Analysis
[19] Temperature-Compensated Filter Technologies Solve Crowded Spectrum Challenges