Orthogonal frequency-division multiplexing

OFDM ( orthogonal frequency-division multiplexing, orthogonal frequency division multiplexing German ) as a specific implementation of multi-carrier modulation, a modulation method using a plurality of orthogonal carrier for digital data transmission. Thus the method is a special form of FDM wherein the orthogonality of the carrier by a cross-talk between signals is reduced, the adjacent carriers are modulated.

The payload to be transmitted at a high data rate is first divided into several partial data streams with lower data rate. These partial data streams are each modulated with a conventional modulation for methods such as quadrature amplitude modulation with low bandwidth, and then adds the modulated RF signals. In order to distinguish the various signals in the demodulation in the receiver, it is necessary that the support are orthogonal to each other in the working space. This has the effect that the sub-data streams influence as little as possible with each other.

The advantage of OFDM is that so that the data transmission to the specifics of a transmission channel, such as a radio channel by a fine granulation can be easily adapted. If within the OFDM signal spectrum to a narrow-band interference, affected carriers may be affected by data transmission of the disorder. The entire data transfer rate drops accounts for only a small part. With a broadband quadrature amplitude modulation with only one carrier, however, can make the complete data transfer impossible narrow-band interference in the transmission channel. Also destructive interference by multipath are mainly single carrier.

Modulation method

Each beam is first modulated separately. Depending on the used of three free parameters of frequency, amplitude and phase for it transmits information of one or more bits per symbol step. Be per symbol, for example, in a DAB bit / carrier, transferred eight bits / carrier for DVB-T two, four, or six bits / carrier and DVB- T2.

The waveform of a symbol consists in the total of all OFDM modulated carrier. Therefore, a very large number of bits is transmitted in parallel with OFDM. If, for example, as used in practical applications, around 7,000 per carrier and carrier transmit four bits, so has a symbol an information content of a maximum of 28,000 bits, which are transmitted in parallel in one step icon. In practice, the number of bits is slightly lower, as some carrier frequencies are used for synchronization, and a pilot tone for the operation. Also, the channel coding for forward error correction reduces the amount of user data.

To each other as the low spectral distance of the carriers is modulated with only low bandwidth. Therefore, the symbol duration is much longer in OFDM over single-carrier. Thus resulting in a total bandwidth of 8 MHz and at 7000 carrier frequencies as a rough guide, a symbol duration of 875 microseconds, which corresponds to a symbol rate of 1143 baud. Consequence, the resulting maximum bit rate is around 32 Mbit / s For exact configurations, various other parameters such as the maximum delay spread must be considered when multipath reception.

OFDM signals are generated by the complex computing the inverse discrete Fourier transform ( IDFT ). The IDFT requires that all subcarrier frequencies are orthogonal to each other. The block length of the IDFT corresponds to the number of subcarriers. IDFT can be fully realized in digital technology with digital signal processors, so that the high frequency part of the circuit is relatively simple.

Orthogonality if and only if there is true:

Reception

On the receiver side, the single carrier must be separated from the composite signal. This could be done with individual filters, which at more than a handful of frequencies is, however, too expensive. Therefore, it is today used a Fast Fourier Transform (FFT ) for all OFDM decoders, which makes the IFFT at the transmitter back. The input data to the FFT, the digitized values ​​of the signal from an analog -to-digital converter (ADC).

Problematic and costly in an OFDM receiver is the synchronization to the received signal, as the receiver has no direct feeding of the transmit clock. Usually run to more synchronization stages from behind the other. First, the sampling clock of the ADC, and the frequency of the RF carrier needs to be adjusted so that all the carriers fall exactly on the FFT frequencies (corresponding to a stretching / compression and displacement of the spectrum). By the presence of many echoes there is a time at which the impulse response has the greatest energy. From this point can be concluded that the time received in the echo and overlap consecutive symbols. He is with an auto - correlation found on certain reference symbols or pilot carrier. Last you need extracted for quadrature amplitude modulation ( QAM) necessary phase reference (so-called channel estimation ).

Depending on OFDM various supplementary signals this synchronization. With Digital Audio Broadcasting ( DAB) you transfer to a symbol long no energy ( null symbol ) and then a so-called phase reference symbol for the exact frequency and time synchronization. DVB- T uses a systematic via the carrier hinwegwanderndes pattern of pilot tones. Using these pilot tones, the phase change on the frequency and period of time to be determined.

COFDM

Coded Orthogonal Frequency Division Multiplexing ( COFDM) is a transmission method for digital information, which is added to the modulation method OFDM, a forward error correction within the symbol.

The strengths of COFDM lie in the resistance to the general interfering multipath and its echoes, and the resulting ability to operate several spatially adjacent channels on the same transmission frequency as a so called single-frequency network. It is also suitable for mobile reception so that signals transmitted.

COFDM as a transmission method is used especially of Digital Audio Broadcasting ( DAB), Digital Radio Mondiale (DRM ) and the European digital TV standard DVB -T.

By the direct wave mode or in multipath reception occurs within the period of a symbol to constructive and destructive interference, which leads to extinction or amplification of individual carrier frequencies. However, since many are within the channel carrier frequencies available in parallel and interference are frequency selective, only some carriers are at particular spatial reception points actually extinguished or reinforced.

In OFDM consist principally the same physical problems as with single-carrier, however, these disturbing influences of the interference by two methods greatly reduce, since the symbol duration in OFDM over single-carrier is much longer.

Adjacent to the outer forward error correction, the information to be transmitted becomes redundant with the COFDM distributed on a plurality of carrier frequencies. This allows the COFDM receiver even when extinction of individual carrier frequencies by interference reconstruct the correct payload data and a single-frequency transmitters operating with overlapping zones of the individual channels is possible.

A guard interval ensures that between two transmitted symbols a " rest period" is observed, so that it does not come to -symbol interference successive symbols. Typical protection time is between 1/32 symbol duration up to 1/4 symbol duration. The length of the guard interval determines the possible inter-symbol interference - free distance difference to the transmitters. At a rest period of 33 microseconds disturb differences in distance of ten kilometers, which allows channel spacing of about 20 km, because extinction requires similar field strengths.

OFDMA

In Orthogonal Frequency Division Multiple Access ( OFDMA ), the OFDM sub-carriers are distributed to more than one user channel. Requirement for the process is bi-directional radio communication, wherein, in contrast to the unidirectional channel may be measured. By constantly measuring the transmitter is aware of the reception quality of the subcarriers for the individual users. Based on this knowledge, it can optimize the use of sub-carriers, and hence the spectral efficiency.

Application Examples

• Digital Audio Broadcasting ( DAB) with 192-1536 media ( at about 1.5 MHz bandwidth)
• Digital Radio Mondiale (DRM ) with 88-460 members ( approximately 4 to 20 kHz bandwidth)
• DVB- T with 2048, 4096, or 8192 carriers - depending on the mode 2k, 4k ( only for DVB -H) or 8k (to about 6.5 to 7.5 MHz bandwidth)
• WLAN, IEEE 802.11a, IEEE 802.11g and IEEE 802.11n
• ADSL ( Asymmetric Digital Subscriber Line) with 32 carriers for the up-and 190 for the downstream (each 4.3125 kHz above about 1 MHz bandwidth; see also DMT)
• VDSL
• G.fast
• Called 3GPP Long Term Evolution ( LTE), or Beyond 3G
• WiMAX IEEE 802.16.2-2004 in NLOS connections with 256 carriers ( by the WiMAX Forum recommended) or 2048 carriers
• CWUSB, Bluetooth 3.0 and WiNet, all of which build on the ECMA -368 standard

The following table lists the typical key data from OFDM or COFDM based on some systems are summarized:

1/4, 3/8 or 1/2

1/2, 2/ 3, 3/ 4, 5/ 6 or 7/8

1/2, 2/ 3, 3/ 4, 5/ 6 or 7/8

0.4, 0.6 or 0.8

1/2, 2/3 or 3/4

Others

OFDM stands for Optical Frequency - Division Multiplexing, which is a synonymous term for wavelength division multiplexing. The term "Optical frequency-division multiplexing " emphasizes, however more strongly that it is a known electrical communication engineering from the frequency-division multiplexing technology in this optical technique.