OFDMA principles

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COPYRIGHT © 2015 ALCATEL-LUCENT. ALL RIGHTS RESERVED.
Welcome to this topic that explains the principles of the Orthogonal Frequency Division Multiplexing
Amplitude (or OFDMA).

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Let’s first review the modulation used in LTE.

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A baseband signal cannot be sent directly to an antenna. The signal is not broadcast over the air interface.
The modulation allows one to mix the message and the carrier. The baseband signal or “message” is
carried by a carrier over the air interface. The carrier is modulated by the baseband signal by the
transmitter and demodulated by the receiver
There are 3 ways to modulate the carrier:
• The amplitude: the receiver can identify the bit by analyzing the amplitude
• The frequency
• The phase

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The 3G LTE uses 3 Quadrature Amplitude Modulations (QAMs) depending on the radio quality.
QAM is a modulation method modifying the phase and the amplitude of the carrier signal.
QAM symbols are represented by the carrier signal being transmitted with specific phase / amplitude
(dictated by the message), for finite periods of time.
One symbol is identified by a Q and an I value.
Transmission channels with a limited bandwidth limit the amount of symbols per second (Baud rate) that
can be transmitted.
To increase the bit per second capacity of a channel, while keeping the Baud rate at the low values
imposed by the channel bandwidth, the symbols carry (represent) more than one single bit.
Symbols will represent a number of n bits, increasing the channel capacity by a factor of n.
The price paid is the presence of multiple symbols in the channel, increasing the probability of incorrect
symbol identification at the receiver.

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The LTE supports in DL and UL the following modulations:
• QPSK (which is equivalent to 4-QAM), the most robust but the less efficient
• 16-QAM
• 64-QAM, the less robust but the most efficient
The QPSK (or 4-QAM) is the most robust modulation. It can be represented by a constellation:
• The radius represents the amplitude.
• The angle represents the phase.
There are 1 amplitude but 4 phases to 4 different states. 2 bits can be coded with 1 QPSK symbol.

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The 16-QAM can modulate 4 bits per symbol.

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The 64-QAM can modulate 6 bits per symbol.

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In reception, it may be difficult to make the distinction between 2 states, that is to say 2-bit sequence. If
the wrong state is selected, there are errors of reception.
The use of higher-order modulation provides the possibility for higher bandwidth utilization, that is the
possibility to provide higher data rates within a given bandwidth.

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Before processing the data (bit stream) to send it on the air interface, the transmitter performs the
encoding, to be able to detect or correct errors of reception. The amount of parity bits is defined by a
rate, called coding rate
The typical coding rates are ½, 2/3, ¾. If the coding rate = ½, the number of bits transmitted on
the air interface is multiplied by 2.
The coding methods are called convolutional and turbo methods.

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The combination of the modulation and the channel coding (identified by its rate) forms one of
the possible Modulation and Coding Schemes.
The link adaptation is done by selection of the best combination of modulation and code rate for
the current radio conditions (CQI)
The 3GPP defines 15 CQI from 1 (worst radio conditions) to 15 (best radio conditions).
The CQI (Channel Quality Indicator) indicates the highest Modulation and Coding Scheme (MCS) level that
can be supported with a 10% BLER on the first H-ARQ process
The most efficient modulation does not always give the best performance.
If Radio quality is bad, it is more efficient to select a more robust modulation like QPSK.
If Radio quality is good, you can expect to reach good performances to select a more efficient modulation
like 64 QAM.

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Let’s explain the basic concepts of OFDMA.

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The LTE PHY is a highly efficient means of conveying both data and control information between an
enhanced base station (eNodeB) and mobile user equipment (UE). The LTE PHY employs some advanced
technologies that are new to cellular applications. These include Orthogonal Frequency Division
Multiplexing (OFDM) and Multiple Input Multiple Output (MIMO) data transmission.
Although the LTE specs describe both Frequency Division Duplexing (FDD) and Time Division Duplexing
(TDD) to separate UL and DL traffic, market preferences dictate that the majority of deployed systems will
be FDD.

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The FDD frequency bands are paired to allow simultaneous transmission on two frequencies. The bands
also have a sufficient separation to enable the transmitted signals not to unduly impair the receiver
performance.
If the signals are too close then the receiver may be "blocked" and the sensitivity impaired. The separation
must be sufficient to enable the roll-off of the antenna filtering to give sufficient attenuation of the
transmitted signal within the receive band.
With the interest in TDD LTE, there are several unpaired frequency allocations that are being prepared for
LTR TDD use. The TDD LTE allocations are unpaired because the uplink and downlink share the same
frequency, being time multiplexed.

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Spectral efficiency is increased by up to four-fold compared with UTRA, and improvements in architecture
and signaling reduce round-trip latency. Multiple Input / Multiple Output (MIMO) antenna technology
should enable 10 times as many users per cell as 3GPP’s original W CDMA radio access technology.
To suit as many frequency band allocation arrangements as possible, both paired (FDD) and unpaired (TDD)
band operations are supported. LTE can co-exist with earlier 3GPP radio technologies, even in adjacent
channels, and calls can be handed over to and from all 3GPP’s previous radio access technologies.
Note that bands 20, 24 and 25 are already used in LA4 and LA5.

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In addition to the bands currently defined for LTE Release 8, TR 36.913 identifies new bands
for Release 10, as listed in this slide.

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Today, there are 4 different bands: 700 (US Digital Dividend), 900 (reshuffling of GSM band but not yet
used in FDD), 800 (Europe DD) and 1900 (PCS) are already used. We can also mention band 1600 that is
already used.

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There are several ways to transmit over the frequency band and to share the resource between several
devices:
• With TDMA, the users are separated by the time. This method if used by the GSM.
• With CDMA, the users are separated by the codes. They receive data at the same time at the
same frequency. This method if used in the CDMAOne, CDMA200 and WCDMA
• And with FDMA, the users are separated by the frequency. The 4G LTE used an improved
FDMA called OFDMA.

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In FDM, the sub-carriers are separated in the frequency domain to avoid interference between the sub-
channels. It results in a loss of spectrum efficiency because the frequency guard band can not be used to
send data.
In OFDM, the sub-carrier frequencies are chosen so that the sub-carriers are orthogonal to each other,
meaning that cross-talk between the sub-channels is eliminated and inter-carrier guard bands are not
required. This greatly simplifies the design of both the transmitter and the receiver; unlike conventional
FDM a separate filter for each sub-channel is not required. There are more sub-carriers, so more
symbols are sent at the same time. The orthogonality brings a better spectrum efficiency.
The orthogonality requires that the sub-carrier spacing is Δf = k/(TU) Hertz, where TU seconds is the
useful symbol duration (the receiver side window size), and k is a positive integer, typically equal to 1.
Therefore, with N sub-carriers, the total passband bandwidth will be B ≈ N·Δf (Hz).

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This brings us to the representation of a sub-carrier. The duration of the symbol depends on the width of
the sub-carrier.
It is inversely proportional. The shorter the symbol, the wider the sub-carrier and vice-versa. The
frequency center of the sub-carrier is linked to the frequency of the carrier.

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Basically, the channel bandwidth is divided into multiple subchannels to reduce ISI and frequency-
selective fading.
A single wideband signal is transformed into multiple narrow band signals transmitted on orthogonal sub-
carriers. There is one single stream at high rate.
Each symbol occupies the whole bandwidth.
Symbol duration is very short to ensure high rate.
Here, the inter-channel (or inter sub-carrier) interferences are cancelled because they are located in a
such way that when there is the peak for a given sub-carrier, the adjacent subcarriers are null.
OFDM allows high density of carriers, without generating Inter-Channel Interference (ICI).

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There are different kinds of sub-carriers:
• Data sub-carrier
• Pilot Sub-carrier
• DC sub-carrier
• Guard Sub-carrier
A Direct Current sub-carrier has no information sent on it. This is an important subcarrier in OFDM based
systems. It is used by the mobile device to locate the center of the OFDM frequency band. So, if LTE does
not have a DC subcarrier, it would be a big deal.

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What is the multipath?
Due to the signal propagation phenomena, like reflection or diffraction, a receiver can receive several
delayed versions of the same signal.
This creates Inter-Symbol Interference (ISI).
The multi-path impact is an overlapping of 2 symbols, called Inter-Symbol Interference (ISI).
The modulation is based on the amplitude and on the phase, so in case of overlapping, there are 2
different amplitudes and phases. The receiver is not able to decode the state of the symbol.

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The problem can be fixed by adding a guard time between each symbol to absorb channel effect and avoid
ISI.
The ISI is still present but is not disturbing for the receiver.

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The guard time is called the Cyclic Prefix (CP). It permits to facilitate demodulation.
The cyclic prefix transforms the classical channel convolution into a cyclic convolution which permits easy
demodulation after FFT.

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Now, let’s review a basic OFDMA transmitter and receiver.

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In the downlink, OFDM is selected to efficiently meet E-UTRA performance requirements. With OFDM, it is
straightforward to exploit frequency selectivity of the multi-path channel with low complexity receivers.
This allows frequency selective in addition to frequency diverse scheduling and one cell reuse of available
bandwidth.
Furthermore, due to its frequency domain nature, OFDM enables flexible bandwidth operation with low
complexity. Smart antenna technologies are also easier to support with OFDM, since each sub-carrier
becomes flat faded and the antenna weights can be optimized on a per sub-carrier (or block of sub-
carriers) basis.
In addition, OFDM enables broadcast services on a synchronized single frequency network (SFN) with
appropriate cyclic prefix design.
This allows broadcast signals from different cells to combine over the air, thus significantly increasing the
received signal power and supportable data rates for broadcast services.
The Fourrier Transformation (FT) changes the time domain in frequency domain (since any function f(t)
can be expressed as a sum of sinusoids. The algorithm able to perform very quickly this transformation is
the FFT (Fast Fourrier Transformation).
The iFFT is the inverse algorithm able to transform a discrete sequence of sinusoids (frequency domain) in
a function in time domain.
The FFT also plays the role of equalizer (change the frequency weighting according to a policy) and filter
(suppress the frequencies out of a given range).

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The FFT (Fast Fourrier Transformation) transforms a function of time into a sequence of sinusoids
(frequency domain).

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Let’s move on to another OFDM process: Single-Carrier FDMA or SC-FDMA.

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Main OFDM disadvantage when compared to single carrier systems: as the number of subcarriers increases,
the composite time-domain signal starts to look like Gaussian noise, which has a high peak-to-average
ratio (PAR) that can cause problems for amplifiers. Allowing the peaks to distort is unacceptable because
this causes spectral regrowth in the adjacent channels. Modifying an amplifier to avoid distortion often
requires increases in cost, size and power consumption. There exist techniques to limit the peaks (such as
clipping and tone reservation) but all have limits and can consume significant processing power while
degrading in-channel signal quality

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The undesirable high PAR of OFDM led 3GPP to choose a different modulation format for the LTE uplink.
This difference contributed to use SC-FDMA, a new hybrid modulation scheme that cleverly combines the
low PAR of single-carrier systems with the multipath resistance and flexible subcarrier frequency
allocation offered by OFDM. SC-FDMA improves the peak-to-average power ratio (PAPR) compared to OFDM
with reduced power amplifier cost for mobile and reduced power amplifier back-off, which improves
coverage.
In DL, OFDM is used together with some PAPR reduction techniques (“clipping and filtering”, “tones
reservation”, etc.)
In UL, we have to find an alternative to OFDM combining some of OFDM’s advantages, but with a PAPR
equivalent to single carrier’s one: DFT-Spread OFDM (DFT-SOFDM), also known as Single-Carrier FDMA (SC-
FDMA).

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The figure shows how a series of QPSK symbols are mapped into time and frequency by the two different
modulation schemes OFDMA and SC-FDMA. For clarity, the example here uses only four (N) subcarriers over
two symbol periods with the payload data represented by QPSK modulation. Real LTE signals are allocated
in units of 12 adjacent subcarriers (180 kHz) called resource blocks that last for 0.5 ms and usually contain
seven symbols whose modulation can be QPSK, 16QAM or 64QAM.
On the left side of the Figure, N adjacent 15 kHz subcarriers are each modulated for the OFDMA symbol
period of 66.7 μs by one QPSK data symbol. In this example, four symbols are taken in parallel. These are
QPSK data symbols so only the phase of each subcarrier is modulated and the subcarrier power remains
constant between symbols. After one OFDMA symbol period has elapsed, the CP is inserted and the next
four symbols are transmitted in parallel. For visual clarity, the CP is shown as a gap; however, it is actually
filled with a copy of the end of the next symbol, meaning the transmission power is continuous but has a
phase discontinuity at the symbol boundary. To create the transmitted signal, an inverse FFT is performed
on each subcarrier to create N time-domain signals that are vector summed to create the final time-
domain waveform used for transmission.
The main difference between the two schemes is that OFDMA transmits the four QPSK data symbols in
parallel, one per subcarrier, while SC-FDMA transmits the four QPSK data symbols in series at four 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. Note that OFDMA and
SC-FDMA symbol lengths are the same at 66.7 μs; however, the SC-FDMA symbol contains N “sub-symbols”
that represent the modulating data. It is the parallel transmission of multiple symbols that creates the
undesirable high PAR of OFDMA. By transmitting the N data symbols in series at N times the rate, the SC-
FDMA occupied bandwidth is the same as multi-carrier OFDMA but - crucially - the PAR is the same as that
used for the original data symbols.

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Let’s explain the application of OFDMA in LTE.

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The width of a Sub-carrier is 15 kHz whatever the bandwidth. Reduced subcarrier spacing of 7.5 KHz for
MBSFN operation is also supported. The symbol duration is always the same whatever the bandwidth.
There are 2 times more sub-carriers in 10 MHz than in 5 MH:
 2 times more symbols can be sent or received at the same time.
 The capacity is multiplied by 2.
Center subcarriers (DC subcarriers) are not used to allow for direct conversion receiver implementation.

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Flexible bandwidth allocation is supported by OFDM.
• Different RF filters are still required.
• The Frame structure is always the same.
• Sampling frequency is a transmitter and receiver implementation issue.
• Smallest bandwidth that is supported was modified recently and needs to be updated.
For the 5 MHz, there are 512 sub-carriers of 15 kHz. The total band is 7.68 MHz. It is larger than the 5 MHz
band!
But only 301 sub-carriers are used (Pilot, DC, data), the other ones are guard sub-carriers:

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The symbol duration depends on the sub-carrier width.
2 Cyclic Prefixes are defined by the 3GPP:
• Long CP: 16.67 micro seconds
• Normal CP: 4.69 micro seconds
The total duration of a symbol is calculated from the useful duration and the cyclic prefix.

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OFDMA principles

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