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`OFDM Orthogonal Frequency Division Multiplexing
`Tutorial
`
`- OFDM: Orthogonal Frequency Division 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 way is less sensitive to frequency
`selective fading.
`
`OFDM TUTORIAL INCLUDES
`
`OFDM basics tutorial (ofdm-basics-tutorial.php)
`
`Cyclic Prefix, CP (cyclic-prefix-cp.php)
`
`Synchronization (synchronization-timing-frequency.php)
`
`OFDMA / OFDM CDMA comparison (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 adopted in 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 frequency division 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, lPR2016-00765
`http://www.radio-electronics.com/info/rf-technology-design/ofdm/ofdm-basics-tutorial.php
`
`1/10
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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.
`
`R eeeitrer
`filter passband:
`onesignal isseleeted
`/
`
`Multiple signals
`
`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 down to 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 there is no interference contribution.
`
`Carrier 1
`
`Carrier n
`
`
`
`Ba nirib Iltions
`from other
`sig nal can: el
`
`
`
`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 cause interference 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 peaks are limited. Although this introduces
`
`distortion that results in a higher level of data errors, the system can rely on the error correction to remove them.
`
`Data 0 n O F D M
`
`MTel., Exhibit 2002, ARRIS v. MTel., Page 2, lPR2016-00765
`http://www.radio-electronics.com/info/rf-technology-design/ofdm/ofdm-basics-tutoria| .php
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`2/10
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`What is OFDM | Orthogonal Frequency Division Multiplexing | Radio-Electronics.com
`
`The data to be transmitted on an OFDM signal is spread across the 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 less critical. This is achieved by adding a guard band time or guard interval
`
`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 phase of the signal.
`
`Directsianal
`
`I
`R e11 e cticu n 1
`I
`_Ll—”|_
`Last re 11 ecticu n
`
`lFIb-l
`
`Sampling
`I
`
`I windowl
`
`MT
`
`Guard Interval
`
`IIIIII
`
`.I.
`
`Guard
`interval
`
`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 of the carriers,
`
`the remaining ones being received correctly. By using error-coding techniques, which does mean adding further
`
`data to the transmitted signal, it enables many or all of the corrupted data to be reconstructed within the receiver.
`
`This can be done because the error correction code is transmitted in a different part of the signal.
`
`OFDM advantages & disadvantages
`
`OFDM advantages
`
`OFDM has been used in many high data rate wireless systems because of the many advantages it 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.
`
`. Resilience to interference:
`
`Interference appearing on a channel may be bandwidth limited and in this
`
`way will 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 advantage is that it
`
`makes efficient use of the available spectrum.
`
`- Resilient to 151: Another advantage of OFDM is that it 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 interleaving it is possible to recover
`
`symbols lost due to the frequency selectivity of the channel and narrow band interference. Not all 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, there are still a few disadvantages to its use which need to be addressed when
`
`MTel., Exhibit 2002, ARRIS v. MTel., Page 3, lPR2016-00765
`conSIderIng '5 use.
`http://www.radio-electronics.com/info/rf-technology-design/ofdm/ofdm-basics-tutoria| .php
`
`3/10
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`What is OFDM | Orthogonal Frequency Division Multiplexing | Radio-Electronics.com
`
`- 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 accommodate the large amplitude variations and these factors mean
`
`the amplifier cannot operate with a high efficiency level.
`
`. 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
`
`basic format for OFDM, but have additional attributes or variations:
`
`- COFDM: Coded Orthogonal frequency division multiplexing. A form of OFDM where error correction coding
`
`is incorporated into the signal.
`
`- Flash OFDM: This is a variant of OFDM that was developed by Flarion and it is a fast hopped form of OFDM.
`
`It uses multiple tones and fast hopping to spread signals over a given spectrum band.
`
`. 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.
`
`. 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 concept of 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
`
`signal.
`
`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 that it is ideal for the high data applications that have become a major factor in today's
`communications scene.
`
`By Ian Poo/e (https://p/us.google. com/7 04687638 7 64370436625?re/=author)
`
`.. . .
`
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`MTel., Exhibit 2002, ARRIS v. MTel., Page 4, lPR2016-00765
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