OFDM

Verwenden von OFDM-Signalen für die drahtlose Kommunikation

OFDM (Orthogonal Frequency-Division Multiplexing) ist eine weit verbreitete Methode für die digitale Modulation bei der drahtlosen Kommunikation, wie WLAN, LTE, DVB-T und 5G. OFDM gehört zur Klasse der Mehrträger-Modulationsschemas. OFDM zerlegt das Übertragungs-Frequenzband in eine Gruppe schmalerer zusammenhängender Teilbänder (Träger) und jeder Träger ist individuell moduliert. Eine solche Modulierung kann leicht durch eine inverse Fast-Fourier-Transformation (IFFT) implementiert werden. Mithilfe schmaler orthogonaler Sub-Träger wird die Robustheit des OFDM-Signals über einen frequenzselektiven Fading-Kanal erhöht und das Übersprechen zwischen benachbarten Sub-Trägern wird eliminiert. Auf der Empfängerseite kann das OFDM-Signal mit einer Fast-Fourier-Transformation (FFT) demoduliert und leicht mit einer komplexen Signalverstärkung für jeden Sub-Träger entzerrt werden. Mehrere neue Arten von OFDM sind für 5G-Anwendungen vorgeschlagen worden, wie CP-OFDM, F-OFDM, W-OFDM, GFDM, UFMC und FBMC.

Einträger-Modulation und OFDM im Zeit- und Frequenzbereich.

Communications Toolbox™WLAN Toolbox™LTE Toolbox™, and 5G Toolbox™ bieten verschiedene OFDM Funktionen. Dies Toolboxen bieten allgemeine oder standardkonforme Funktionen für Simulation, Analyse und Tests der OFDM-Wellenformen. Die Toolboxen bieten auch End-to-End-Systemmodelle für Sender/Empfänger mit konfigurierbaren Parametern und verschiedenen Drahtloskanal-Modellen, mit denen die Drahtlossysteme evaluiert werden können, die OFDM-Wellenformen verwenden. Insbesondere können Sie diese OFDM-Funktionen beim Entwurf drahtloser Kommunikationssysteme verwenden, um Folgendes zu analysieren: Verbindungsleistung, Robustheit, Systemarchitekturoptionen, Kanaleffekte, Kanalschätzung, Kanalentzerrung, Signalsynchronisierung und Sub-Träger-Modulationsoptionen.

The Principles of OFDM

An OFDM signal aggregates the information in orthogonal single-carrier frequency-domain waveforms into a time-domain waveform that can be transmitted over the air. The subcarriers use QPSK or QAM as the primary modulation method.

The inverse discrete Fourier transform equation for this is:

$$f(x) = { 1 \over N} \sum_{t=0}^{N-1} F(t) e^{i \frac{2 \pi xt}{N}} $$

In OFDM, when the amplitude of each subcarrier reaches the maximum, the carriers are arranged at intervals of 1 / symbol time so that the amplitude of other subcarriers is 0, thereby preventing interference between symbols.

Frequency domain representation of orthogonal subcarriers in an OFDM waveform.

Moreover, OFDM of a multicarrier transmission is effective in multipath environments because the influence of multipath is concentrated on specific subcarriers compared with a single-carrier transmission. In the case of a single-carrier transmission, the multipath affects the whole.

The arrival time difference between the direct wave and the reflected wave increases when the signal is transmitted over a long range. In that situation, the number of subcarriers is larger than in a smaller service range.

Ideal OFDM waveform and OFDM waveform influenced by multipath.

OFDM Technology in 5G Systems

During the specification of the 5G standard, various technologies based on OFDM had been considered. CP-OFDM (cyclic prefix OFDM) is used in LTE and was also selected for the 3GPP Release 15 standard. This technique adds an upper-level signal called a cyclic prefix to the beginning of the OFDM symbol. CP-OFDM suppresses intersymbol interference (ISI) and intercarrier interference (ICI) by inserting the data for a certain period of time from the trailing end of the OFDM symbol as the cyclic prefix at the beginning of the OFDM symbol. 

Pros and Cons of OFDM

Advantages of OFDM

Multiple users can be assigned to OFDM subcarriers. Frequency can be efficiently used by orthogonal (1 / symbol time interval). It is resistant to transmission distortion due to multipath, making demodulation possible by error correction without using a complicated equalizer.

Disadvantages of OFDM

Because the amplitude of the signal changes significantly, it is necessary to design an amplifier that has a higher peak-to-average power ratio, smaller-than-average transmit power allowed by the amplifier, or an amplifier with a wide dynamic range. Particularly when the carrier interval is narrow, the effect of OFDM becomes weaker against the Doppler shift, so it is preferable to use an amplifier with a wide dynamic range.

OFDM Using MATLAB

MATLAB® and related toolboxes, including Communications Toolbox™, WLAN Toolbox™, LTE Toolbox™, and 5G Toolbox™,  provide functions to implement, analyze, and test OFDM waveforms and perform link simulation. The toolboxes also provide end-to-end transmitter/receiver system models with configurable parameters and wireless channel models to help evaluate the wireless systems that use OFDM waveforms. Specifically, as a part of wireless communication system design, you can use these OFDM capabilities to analyze link performance, robustness, system architecture options, channel effects, channel estimation, channel equalization, signal synchronization, and subcarrier modulation selections.

MATLAB functions and Simulink® blocks for OFDM modulation provide adjustable parameters such as training signal, pilot signal, 0 padding, cyclic prefix, and points of FFT.  

It is also possible to generate and analyze standard-compliant and custom OFDM waveforms over the air by using the Wireless Waveform Generator app in Communications Toolbox with Instrument Control Toolbox™ to connect MATLAB to RF test and measurement instruments.

OFDM Modulator and OFDM Demodulator blocks and block parameters.

OFDM generation using the Wireless Waveform Generator app. Generated waveforms may be used for simulation or over-the-air tests with Instrument Control Toolbox.



Siehe auch: 5G wireless technology development, massive MIMO, RF system, wireless transceiver