(OFDMA)
Single Carrier - Frequency Division Multiple Access
(SC-FDMA)
New Air Interface Technologies
The Long Term Evolution of UMTS requires a number of new technologies to be deployed. Chief amongst these are the technologies used to provide air interface connections.
Although other technologies were considered during the consultation and development phases, 3GPP finally settled on the use of OFDMA on the downlink and SC-FDMA on the uplink.
OFDMA is a well understood and widely used technology that has formed the basis of air interface services for systems such as Wi-Fi (802.11a,g and n), WiMAX (802.16), DAB (Digital Audio
Broadcasting) radio and DVB-T/H/S (Digital Video Broadcasting for Terrestrial Handheld and Satellite) television systems.
SC-FDMA is a more recent variation of OFDMA, which provides a similar service but in a less power-hungry way.
Further Reading: 3GPP TS 36.211, 36.300
LTE/SAE Engineering Overview
OFDM, also known as discrete multi-tone modulation and multi-carrier modulation, is an advanced form of FDM
Instead of transmitting a single modulated radio signal, as would happen in a single carrier system, OFDM transmits hundreds or even thousands of separately modulated radio signals using carriers spread across a wideband channel.
Each radio carrier is known as a subcarrier. Data is sent in parallel across the set of subcarriers, each subcarrier only transporting a part of the whole transmission.
The modulation rate of all subcarriers in the channel is synchronized to a central source, so all modulation symbols should be transmitted at the same points in time on all subcarriers.
The time period occupied by the modulation symbols on all subcarriers is known as an OFDM symbol and represents all the data being transferred in parallel at that point in time.
OFDM symbols can make use of adjustable ‘guard periods’ before the ‘useable’ part of the symbol to provide protection against multipath effects. This guard period is known as a CP (Cyclic Prefix) and is created by repeating part of the modulated RF signal for a specified period of time.
The achievable symbol rate is determined by the bandwidth of the channel and the spacing between the subcarriers.
OFDMA Principles
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Usable Symbol Symbol Period CP
Last x samples of symbol
The Cyclic Prefix
The OFDM cyclic prefix is designed to combat the ISI (Inter Symbol Interference) effects caused by multipaths and other channel impulse response effects. Multipaths cause ‘echoes’ of a previous part of the signal that, having travelled via a longer path than the primary component of the signal, arrive later in time.
The cyclic prefix eliminates or masks the effects of ISI, as long as the cyclic prefix period is longer than the maximum delay spread suffered by the signal.
The cyclic prefix is formed by taking a portion of the ‘useable’ part of each OFDM symbol and copying it onto the beginning of the symbol period.
This is necessary, rather than just using a blank guard period, in order to maintain orthogonality between adjacent subcarriers at all points through the nominal symbol period.
The cyclic prefix ratio has potentially significant consequences for the bandwidth efficiency of a channel, but these tend to be outweighed by the benefits in terms of minimized ISI. The use of the cyclic prefix to manage the effects of ISI is only permissible due to the comparatively long symbol duration enjoyed by OFDM-based systems. LTE’s typical total symbol duration (including the cyclic prefix) of 71.42 μsec compares with 3.69 μsec for GSM and just 0.26 μsec for WCDMA (Wideband Code Division Multiple Access). A cyclic prefix with a duration of around 5 μsec, which would be prohibitively expensive if employed by these other systems, is easily accommodated by an OFDM-based system, meaning that the more complex ISI management techniques employed by other systems are not required.
Further Reading: 3GPP TS 36.211, 36.300
LTE/SAE Engineering Overview
Different subcarriers from across the whole population of subcarriers created by an OFDM channel are assigned to perform different functions.
Most subcarriers will be assigned to carry modulated user data signals. Each data subcarrier will be modulated to carry one part of the entire parallel signal being transmitted across the multitone
channel. The data rate of each data subcarrier is determined by a combination of the symbol rate and the modulation scheme employed.
In some OFDM variants (such as that employed by WiMAX), entire subcarriers are given over to carrying ‘pilot signals’. Pilot subcarriers allow channel quality and signal strength estimates to be made and allow other control functions, such as frequency calibration, to operate.
Pilots are generally transmitted at a higher power level than data subcarriers – typically 2.5 dB higher – which serves to make them more easily acquired by receiving stations. In E-UTRA and other systems, including DVB, the same function is performed by ‘reference signals’. A reference signal, like a pilot, allows a receiving station to recalibrate its receiver and make channel estimates, but instead of occupying an entire subcarrier it is instead periodically embedded in the stream of data being carried on a ‘normal’ subcarrier.
There are two types of ‘null’ subcarrier – guards and the DC carrier. Nothing is transmitted on null subcarriers. Guard subcarriers separate the top and bottom data subcarriers from any adjacent channel interference that may be leaking in from neighbouring channels and, in turn, serve to limit the interference caused by the OFDM channel. The more guard subcarriers that are assigned, the lower the amount of adjacent channel interference that will be caused or detected, but this also has an impact on the data throughput of the channel.
The DC carrier is the centre subcarrier of each OFDM channel – the one that has a 0 Hz offset from the channel’s centre frequency – and is also null.
OFDMA Principles
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Nominal Channel Width
Reference
Data Null
Frequency
Steep Drop Off
The ‘Brickwall’ Effect
The use of null or guard subcarriers at the top and bottom ends of each OFDM channel produces a very sudden drop-off in radiated power. Power reduces far more rapidly than at the edge of a traditional single carrier channel.
This affects the level of interference caused to adjacent channels.
When viewed on a spectrum analyser, the steep drop-off in power seen at the edge of an OFDM channel has been dubbed the ‘brickwall effect’. A representation of this can be seen in the diagram.
The difference between the radiated power level of data subcarriers and pilots can also be seen.
LTE/SAE Engineering Overview
OFDM and OFDMA
The 3GPP E-UTRA specifications refer to the technology employed on the downlink as OFDM, whereas a more commonly accepted term for the technology employed would be OFDMA (Orthogonal Frequency Division Multiple Access).
These two technologies are almost identical; the only substantial difference is in the way capacity is allocated.
Allocation of capacity in OFDM systems is generally based on time division principles. Each user is allocated the full channel and all subcarriers exclusively for a certain number of symbol periods. This allocation method can be inflexible, especially when user connections do not have enough data to send to fill up the space allocated. In these situations network capacity is wasted by carrying padding.
OFDMA systems allocate capacity based on a combination of time and frequency – a certain number of subcarriers for a certain number of symbol periods. This allocation method has two main benefits.
Firstly, OFDMA-based systems assign capacity to connections on the basis of a number of
subcarriers for a number of symbol periods, rather than assigning the entire channel to one user at a time. This allows the amount of capacity assigned to each connection to match more closely the amount of data it has queued ready to transmit.
Secondly, the ability to subdivide the subcarrier population allows the link to serve more than one user at a time.
OFDMA Principles
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S/P
The layout of a typical OFDMA transmitter/receiver pair is shown in the diagram. In most FDM variants, data is introduced in serial and then converted into n parallel streams, n being equal to the number of data subcarriers.
The parallel data streams are then separately passed through error coding, interleaving and modulation stages before being transmitted.
In reality, the process outlined above would take place in a DSP (Digital Signal Processor) rather than in discrete physical processing stages, and the eventual transmitted signal would be created using IFFT (Inverse FFT) techniques.
Further Reading: 3GPP TS 36.211, 36.300
LTE/SAE Engineering Overview
12 x15 kHz spacingSC-FDMA Concept
SC-FDMA, as employed on the E-UTRA uplink, can be viewed as a power-efficient adaptation of OFDM.
SC-FDMA employs subcarriers, FFTs and other OFDM concepts but is designed to provide a better PAPR (Peak to Average Power Ratio) using a narrower channel.
PAPR relates to the ratio between the peak power output of a signal and its average transmit strength – the higher the peaks, the greater the range of power levels over which the transmitter is required to work. Systems such as OFDM, with a high PAPR, are not best suited for use with mobile and other battery-powered devices.
The lower PAPR achieved by a system like SC-FDMA makes devices that employ it much less power-hungry and therefore more suitable for mobile operation.
For SC-FDMA a form of subchannel is used for resource allocation. Each subchannel occupies 180 KHz of spectrum containing 12 subcarriers.
Resource allocation can be flexible, with all subcarriers in a channel being assigned to one user or separate groups of subcarriers being assigned to multiple users.
Further Reading: 3GPP TS 36.211, 36.300
OFDMA Principles
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SC-FDMA FFT
SC-FDMA and OFDMA
The most immediately apparent difference between the operation of the E-UTRA downlink and uplink technologies is that OFDM transmits data in parallel across multiple subcarriers, whereas SC-FDMA transmits data in series, but still employs multiple subcarriers.
Using the basic E-UTRA numerology of 12 subcarriers occupying 180 kHz of bandwidth, during one symbol period on the downlink OFDM will transmit 12 modulation symbols, one on each subcarrier.
On a corresponding uplink channel, SC-FDMA will transmit 12 modulation symbols in series during the same time period – each SC-FDMA modulation symbol in this example therefore has a duration 1/12th that of an OFDM modulation symbol.
In the case of an OFDM channel employing 16QAM (16-state Quadrature Amplitude Modulation), if all subcarriers happened to modulate a high-amplitude symbol at the same time, the aggregate
transmitted power of the channel would be high. If during the next symbol period all subcarriers modulated a low-amplitude symbol the aggregate transmitted power of the channel would drop – the corresponding ratio between the peak power transmitted and its long term average could therefore be high. This is an extreme example and is unlikely to occur often, if at all, when real data is being applied to parallel OFDM subcarriers, but the potential for a large difference between peak and average radiated power remains.
SC-FDMA avoids such large differences by employing an additional processing stage in front of the IFFT process. This additional stage deals with the group of modulation symbols to be transmitted during one SC-FDMA symbol period in a series. An FFT process represents the changes made to the modulated signal during the symbol period as outputs on a set of subcarriers – each modulation symbol results in a set pattern of outputs across the 12 subcarriers that make up an individual uplink LTE channel.
The subcarriers created by this process have a set amplitude, which should remain more or less constant between one SC-FDMA symbol and the next for a given modulation scheme, resulting in little difference between the peak power radiated on that channel and its long-term average.
Further Reading: 3GPP TS 36.211, 36.300
LTE/SAE Engineering Overview
component analysis of modulation symbolsThe typical layout of a SC-FDMA transmitter and receiver is shown in the diagram. Despite its name, the transmitted radio signal for SC-FDMA is an orthogonal multicarrier transmission similar to OFDM.
The term ‘single carrier’ refers to the pre-processing of the baseband data prior to its application to the IFFT.
The data stream is presented in a serial fashion to the radio transmission process. The first stage is modulation symbol mapping, which produces blocks of complex-valued symbols. Modulation mapping may operate on pairs of baseband bits, as shown in the diagram, for QPSK (Quadrature Phase Shift Keying) but if 16QAM or 64QAM are in use then each modulation symbol will represent either four or six serial baseband bits respectively.
The series of modulation symbols is then presented to the FFT, which produces an output representing the frequency components of the modulation symbols. It is then these frequency components that are mapped to the allocated inputs of the IFFT. Note that the FFT output size is always smaller than the IFFT input size. This is because a typical cell’s uplink capacity will generally be greater than 180 kHz, meaning that more than one uplink channel will be available. Individual UEs will be assigned one or more 180 kHz uplink blocks to use, which represents only a portion of the total uplink capacity in the cell. Other UEs will be assigned other groups of subcarriers to use across the uplink channel bandwidth. No two UEs will be assigned the same 180 kHz block to use
simultaneously, unless Multi-User MIMO is in use.
The output of the IFFT and modulator will be a multi-carrier transmission. However, unlike OFDM there is not a direct mapping of each baseband symbol onto individual transmitted subcarriers.
Instead, the frequency components of each baseband symbol are now represented across all the transmitted subcarriers, hence the term ‘single carrier’. The result is a transmitted signal with an improved PAPR compared to an equivalent OFDM transmission.
Further Reading: 3GPP TS 36.211, 36.300
OFDMA Principles
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I
SC-FDMA Multiple Access
Again, in the example above, the channel is being shared by three UEs, each of which has been assigned a different set of subcarriers. From the receiver’s point of view, it receives one signal that occupies the whole channel but which is in reality the sum of three separate orthogonal signals generated by the three UEs.
An overall receive FFT process, operating across the entire channel, recovers the separate subcarrier data streams and individual IFFT processes can then recover the original serial data streams
transmitted by each UE.
Further Reading: 3GPP TS 36.211, 36.300
LTE/SAE Engineering Overview
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LTE/SAE Engineering Overview
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