OFDM

To overcome the effect of multi path fading problem

available in UMTS, LTE uses Orthogonal Frequency

Division Multiplexing (OFDM) for the downlink[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].

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].

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

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].

OFDMA

With the identical number of channels, OFDM occupies

less bandwidth than FDMA by orthogonality between

subcarriers.

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]:

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.

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].

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

SC-FDMA

Large PAPR requires high linearity requirements for PA

and increases power consumption[7]

PA

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.

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.

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.

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].

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.

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.

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.

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.

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

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].

Group Delay

Usually, large group delay variation appears near the

transition region in frequency response, leading to

distortion of the signal waveform[6].

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

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

EVM

Let’s inject a LTE Downlink Signal (with Subcarrier Modulation = 64QAM, Source Power = 0 dBm) into a

filter[1] :

Filter

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

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

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

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

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.

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

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].

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

[18] Receiver Optimization Using Error Vector Magnitude Analysis

[19] Temperature-Compensated Filter Technologies Solve Crowded Spectrum Challenges