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`OFDM Orthogonal Frequency Division Multiplexing
`Tutorial
`
`- OFDM: Orthogonal FrequencyDivision Multiplexing, is a form of signal modulation that
`divides a high data rate modulating stream placing them onto many slowly modulated
`narrowband close-spaced subcarriers, and in this wayis less sensitive to frequency
`selective fading.
`
`OFDM TUTORIAL INCLUDES
`
`OFDM basicstutorial (ofdm-basics-tutorial.php)
`
`Cyclic Prefix, CP (cyclic-prefix-cp.php)
`
`Synchronization (synchronization-timing-frequency.php)
`
`OFDMA / OFDM CDMAcomparison (ofdma-cdma.php)
`
`Orthogonal Frequency Division Multiplexing or OFDM is a modulation format that is being used for many of the
`latest wireless and telecommunications standards.
`
`OFDM has been adoptedin the Wi-Fi arena where the standards like 802.11a, 802.11n, 802.11ac and more. It has
`also been chosen for the cellular telecommunications standard LTE / LTE-A, and in addition to this it has been
`adopted by other standards such as WiMAX and many more.
`
`Orthogonal frequencydivision multiplexing has also been adopted for a number of broadcast standards from DAB
`Digital Radio to the Digital Video Broadcast standards, DVB. It has also been adopted for other broadcast systems
`as well including Digital Radio Mondiale used for the long medium and short wave bands.
`
`MTel., Exhibit 2002, ARRIS v. MTel., Page 1, IPR2016-00765
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`
`Although OFDM, orthogonal frequency division multiplexing is more complicated than earlier forms of signal
`format, it provides some distinct advantages in terms of data transmission, especially where high data rates are
`needed along with relatively wide bandwidths.
`
`What is OFDM? - The concept
`OFDM is a form of multicarrier modulation. An OFDM signal consists of a number of closely spaced modulated
`carriers. When modulation of any form - voice, data, etc. is applied to a carrier, then sidebands spread out either
`side. It is necessary for a receiver to be able to receive the whole signal to be able to successfully demodulate the
`data. As a result when signals are transmitted close to one another they must be spaced so that the receiver can
`separate them using a filter and there must be a guard band between them. This is not the case with OFDM.
`Although the sidebands from each carrier overlap, they can still be received without the interference that might be
`expected because they are orthogonal to each another.This is achieved by having the carrier spacing equal to the
`reciprocal of the symbol period.
`
`Receiver
`filter passband:
`one signal iss elected
`
`hdultiple signak
`
`Traditional view of receiving signals carrying modulation
`
`To see how OFDM works,it is necessary to look at the receiver. This acts as a bank of demodulators, translating
`each carrier downto DC.The resulting signal is integrated over the symbol period to regenerate the data from that
`carrier. The same demodulator also demodulates the other carriers. As the carrier spacing equal to the reciprocal
`of the symbol period means that they will have a whole number of cycles in the symbol period and their
`contribution will sum to zero - in other words thereis no interference contribution.
`
`Carrier 1
`
`Carriern
`
`
`
`Contributions
`from other
`signals cancel
`
`
`
`OFDM Spectrum
`
`One requirement of the OFDM transmitting and receiving systems is that they must be linear. Any non-linearity will
`cause interference between the carriers as a result of inter-modulation distortion. This will introduce unwanted
`signals that would causeinterference and impair the orthogonality of the transmission.
`
`In terms of the equipment to be used the high peak to average ratio of multi-carrier systems such as OFDM
`requires the RF final amplifier on the output of the transmitter to be able to handle the peaks whilst the average
`power is much lower and this leads to inefficiency. In some systems the peaksare limited. Althoughthis introduces
`distortion that results in a higher level of data errors, the system can rely on the error correction to remove them.
`
`Data on OFDM
`
`MTel., Exhibit 2002, ARRIS v. MTel., Page 2, IPR2016-00765
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`
`The data to be transmitted on an OFDM signal is spread acrossthe carriers of the signal, each carrier taking part of
`the payload. This reduces the data rate taken by each carrier. The lower data rate has the advantage that
`interference from reflections is much lesscritical. This is achieved by adding a guard band time or guardinterval
`into the system. This ensures that the data is only sampled when the signal is stable and no new delayed signals
`arrive that would alter the timing and phaseofthe signal.
`
`Direct signal
`
`Reflection 1
`
`Last reflection
`
`Sampling
`window
`
`'
`
`|
`
`Guard Interval
`
`BP
`
`The distribution of the data across a large number of carriers in the OFDM signal has some further advantages.
`Nulls caused by multi-path effects or interference on a given frequency only affect a small number ofthe carriers,
`the remaining ones being received correctly. By using error-coding techniques, which does mean adding further
`data to the transmitted signal, it enables manyor all of the corrupted data to be reconstructed within the receiver.
`This can be done becausethe error correction code is transmitted in a different part of the signal.
`
`OFDM advantages & disadvantages
`OFDM advantages
`
`OFDM has been usedin manyhigh data rate wireless systems because of the many advantagesit provides.
`
`Immunity to selective fading: One of the main advantages of OFDM is that is more resistant to frequency
`selective fading than single carrier systems because it divides the overall channel into multiple narrowband
`signals that are affected individually as flat fading sub-channels.
`
`Interference appearing on a channel may be bandwidth limited and in this
`Resilience to interference:
`waywill not affect all the sub-channels. This means that not all the data is lost.
`
`Spectrum efficiency: Using close-spaced overlapping sub-carriers, a significant OFDM advantageis thatit
`makesefficient use of the available spectrum.
`
`Resilient to ISI: Another advantage of OFDM is thatit is very resilient to inter-symbol and inter-frame
`interference. This results from the low data rate on each of the sub-channels.
`
`Resilient to narrow-band effects: Using adequate channel coding and interleavingit is possible to recover
`symbols lost due to the frequency selectivity of the channel and narrow band interference. Notall the data is
`lost.
`
`Simpler channel equalisation: One of the issues with CDMA systems was the complexity of the channel
`equalisation which had to be applied across the whole channel. An advantage of OFDM is that using multiple
`sub-channels, the channel equalization becomes much simpler.
`
`OFDM disadvantages
`
`Whilst OFDM has been widely used, therearestill a few disadvantagesto its use which need to be addressed when
`consideringits use.
`MTel., Exhibit 2002, ARRIS v. MTel., Page 3, IPR2016-00765
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`
`¢ High peak to average power ratio: An OFDM signal has a noise like amplitude variation and has a
`relatively high large dynamic range, or peak to average power ratio. This impacts the RF amplifier efficiency
`as the amplifiers need to be linear and accommodatethe large amplitude variations and these factors mean
`the amplifier cannot operate with a highefficiencylevel.
`
`e Sensitive to carrier offset and drift: Another disadvantage of OFDM is that is sensitive to carrier
`frequency offset and drift. Single carrier systems are less sensitive.
`
`OFDM variants
`
`There are several other variants of OFDM for which the initials are seen in the technical literature. These follow the
`
`basi c format for OFDM, but have additional attributes or variations:
`
`e COFDM: Coded Orthogonal frequency division multiplexing. A form of OFDM whereerror correction coding
`is incorporated into the signal.
`
`Flash OFDM: This is a variant of OFDM that was developed byFlarion and it is a fast hopped form of OFDM.
`It uses multiple tones and fast hopping to spread signals over a given spectrum band.
`
`e OFDMA: Orthogonal frequency division multiple access. A scheme used to provide a multiple access
`capability for applications such as cellular telecommunications when using OFDM technologies.
`
`e VOFDM: Vector OFDM. This form of OFDM uses the concept of MIMO technology.It is being developed by
`CISCO Systems. MIMO stands for Multiple Input Multiple output and it uses multiple antennas to transmit
`and receive the signals so that multi-path effects can be utilised to enhance the signal reception and
`improve the transmission speeds that can be supported.
`
`WOFDM: Wideband OFDM. The conceptof this form of OFDM is that it uses a degree of spacing between
`the channels that is large enough that any frequency errors between transmitter and receiver do not affect
`the performance. It is particularly applicable to Wi-Fi systems.
`
`Each of these forms of OFDM utilise the same basic concept of using close spaced orthogonal carriers each
`carrying low data rate signals. During the demodulation phase the data is then combined to provide the complete
`sign al.
`
`OFDM, orthogonal frequency division multiplexing has gained a significant presence in the wireless market place.
`The combination of high data capacity, high spectral efficiency, and its resilience to interference as a result of
`multi-path effects means thatit is ideal for the high data applications that have become a major factor in today's
`communications scene.
`
`By Ian Poole (https://plus.google.com/104687638 164370436625?rel=author)
`
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