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Contents
Orthogonal Radio Carriers ...2.1 Variable Channel Bandwidth...2.2 Factors Affecting Data Rate...2.3 FFT (Fast Fourier Transform) ...2.4 New Air Interface Technologies ...2.5 OFDM Concepts ...2.6 The Cyclic Prefix ...2.7 Subcarrier Assignment...2.8 The ‘Brickwall’ Effect ...2.9 OFDM and OFDMA...2.10 OFDMA Operation ...2.11 SC-FDMA Concept ...2.12 SC-FDMA and OFDMA...2.13 SC-FDMA Operation ...2.14 SC-FDMA Multiple Access...2.15
LTE/SAE Engineering Overview
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OFDMA Principles
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Objectives
At the end of this section you will be able to:
list the new set of technologies employed by E-UTRAN on the air interface, including OFDMA and SC-FDMA
describe the basic concepts that underlie OFDM such as subcarriers, the cyclic prefix and symbols
explain the meaning of the term ‘orthogonality’ with reference to OFDM systems
demonstrate an understanding of the concept of orthogonal carriers and the relationship between carrier spacing and bandwidth
outline the functionality and use of the FFT (Fast Fourier Transform) in OFDM
outline the differences between OFDM and OFDMA and the benefits associated with each system
explain the basic concepts that underlie SC-FDMA as used by the E-UTRA uplink
outline the basic concepts that underlie the SCTP and the reasons for its use in EPS
LTE/SAE Engineering Overview
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OFDMA Principles
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Adjacent Carrier Interference Carrier Spacing
FDM
C C+1 C–1
A OFDM
Centre point of subcarrier C intersects with subcarriers C–1 and C+1
Orthogonal Radio Carriers
In a standard FDM (Frequency Division Multiplexing) system, the radio carriers are spaced sufficiently far apart to minimize the effects of adjacent channel interference. High-capacity FDM systems
therefore require large amounts of bandwidth.
For an OFDM (Orthogonal Frequency Division Multiplexing) system to work efficiently there should be as little interference as possible between adjacent subcarriers that make up the transmitted channel.
To ensure minimum interference, orthogonal radio subcarriers are selected. Orthogonality among a set of adjacent radio subcarriers occurs when the peak of one subcarrier intersects with the point where the signals and harmonics produced by neighbouring subcarriers are passing through zero.
This is shown at point A. The peak power frequency of subcarrier C is also the frequency at which the signals from subcarriers C+1 and C–1 pass through zero and the point at which the secondary harmonics from subcarriers C+2 and C–2 do the same.
The net effect of this is that the signals radiated by adjacent subcarriers contribute no interference to signals radiating on any particular neighbouring subcarrier. Guard bands, in the form of a number of unused subcarriers, are employed at the top and bottom ends of the channel width to ensure that minimal interference is received from services occupying adjacent channels.
Channel width and subcarrier spacing are vitally important in OFDM systems – if the subcarrier spacing is incorrect then adjacent subcarriers will not be orthogonal.
Further Reading: 3GPP TS 36.211, 36.300
LTE/SAE Engineering Overview
Variable Channel Bandwidth
If the number of subcarriers remains fixed but the bandwidth of the overall channel is increased, the separation between the subcarriers can increase. Increased separation means that each subcarrier must increase its radiated bandwidth by a comparable amount; otherwise, the orthogonality between adjacent subcarriers will be lost.
Section (a) shows a set of subcarriers operating orthogonally.
Section (b) shows an example of increasing the bandwidth of the centre subcarrier without increasing the separation between it and its neighbouring subcarriers. The intersection between subcarriers no longer occurs at the ‘zero’ point.
Section (c) demonstrates an increase in separation and a comparable increase in subcarrier bandwidth, which shows that the orthogonality between the subcarriers can be maintained.
One way of increasing the radiated bandwidth of each subcarrier is to increase the number of
modulation symbols transmitted across them, as a channel’s occupied bandwidth is proportional to its modulation rate. If more modulation symbols are transmitted it means that more data is transferred across each subcarrier, which in turn increases system throughput.
The symbol rate of the system must also fit in with the requirements of the receiving equipment. To ensure that all signals are received correctly, the receiver sampling rate should be slightly higher than the bandwidth of the signal used to carry it – for example, an OFDM channel with a bandwidth of 1.75 MHz may be sampled at a rate of 2 MHz. This allows for the inclusion of a symbol guard period.
The sampling frequency for a given channel bandwidth is the fundamental parameter associated with OFDM capacity planning. Once this figure is known, subcarrier spacing and symbol rates can be derived.
Further Reading: 3GPP TS 36.211, 36.300
OFDMA Principles
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Modulation Schemes
Carrier Spacing
Number of Carriers
Symbol Rate Number of Null Carriers Channel
Width
Factors Affecting Data Rate
The data rate achievable by an OFDM system is dependent upon several interrelated factors:
channel bandwidth
channel frequency band
number of orthogonal subcarriers
separation between subcarriers
number of unused or ‘null’ subcarriers
modulation scheme employed on subcarriers
the optimum sample rate employed by OFDM receivers
The E-UTRA OFDM-based air interface avoids the complexities associated with the interrelated dependencies of OFDM parameters by employing a fixed subchannel spacing of 15 kHz. A fixed spacing eliminates the variability between subcarrier width, symbol rate, data rate and sample rate, making it a far simpler implementation of OFDM than that used by, for example, WiMAX.
Further Reading: 3GPP TS 36.211, 36.300
LTE/SAE Engineering Overview
Channels Combined
Waveform Discrete Channels
Data Out
Data In Data Out
Parallel/Serial Conversion
FFT (Fast Fourier Transform)
An OFDM transmitter can be thought of as comprising hundreds of separate radio modulators, all operating in parallel on different radio frequencies. Data is converted from serial to parallel and then fed to each separate modulator to be transmitted across a discrete radio channel. An OFDM receiver can be conceptualized as hundreds of separate radio receivers all demodulating the data from different radio channels. The data is then combined back into the original, serial data stream before being processed.
This conceptual view of an OFDM system is useful for explaining the principles of OFDM, but would be expensive and unwieldy to produce and operate in real life. Real OFDM and DMT (Discrete Multi-Tone modulation) systems make use of a mathematical function known as FFT (Fast Fourier Transforms) to create the ‘parallel’ data signals required by a multicarrier system.
The FFT is a more efficient form of the DFT (Discrete Fourier Transform), which itself provides a way of analysing the frequencies that make up a complex signal. Data is first converted from a serial stream to n parallel virtual branches, n being equal to the number of subcarriers being employed on the RF (Radio Frequency) channel. The modulation scheme to be employed on each subcarrier is selected and the inverse FFT system computes the characteristics of the wideband radio signal that would result if all of the signals were transmitted on a set of parallel radio carriers. This ‘combined’
signal is then transmitted across the radio channel employed by the system.
The OFDM receiver takes the wideband signal it receives and passes it to an FFT. The FFT unit processes the received signal and determines the mix of parallel modulation symbols that would have had to have been combined together to produce a signal with those precise characteristics.
The FFT process can output parallel virtual streams for each carrier, which can then be converted back into a serial stream of received data.
Further Reading: Anders E. Zonst, Understanding FFT Applications: A Tutorial. (Citrus Press, 2000)
OFDMA Principles
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