Utilizzo di segnali OFDM nella comunicazione wireless

L’OFDM, orthogonal frequency-division multiplexing, è un metodo di modulazione digitale ampiamente utilizzato nelle comunicazioni wireless, come WLAN, LTE, DVB-T e 5G. L’OFDM appartiene alla classe degli schemi di modulazioni multiportanti. L’OFDM scompone la banda di frequenza di trasmissione in un gruppo di sottobande contigue più strette (portanti) e ciascuna portante viene modulata individualmente. Tale modulazione può essere implementata facilmente da una trasformata di Fourier veloce inversa (IFFT). Utilizzando sottoportanti ortogonali strette, il segnale OFDM guadagna robustezza in un canale fading selettivo di frequenza ed elimina crosstalk di sottoportanti adiacenti. Al termine della ricezione, il segnale OFDM può essere demodulato con una trasformata di Fourier veloce (FFT) ed equalizzato facilmente con un guadagno complesso in ciascuna sottoportante. Sono state proposte diverse forme nuove di OFDM per applicazioni 5G, come CP-OFDM, F-OFDM, W-OFDM, GFDM, UFMC e FBMC.

Modulazione a portante singola e OFDM nei domini del tempo e della frequenza.

Communications Toolbox™, WLAN Toolbox™ e LTE Toolbox™ e 5G Toolbox™ offrono varie funzionalità OFDM. Questi toolbox forniscono funzioni generali o conformi agli standard per la simulazione, l’analisi e il test delle forme d’onda OFDM. I toolbox forniscono anche modelli di sistemi di trasmettitori/ricevitori end-to-end con parametri configurabili e diversi modelli di canali wireless per aiutare a valutare i sistemi wireless che utilizzano forme d’onda OFDM. Nello specifico, come parte della progettazione di sistemi di comunicazione wireless, è possibile utilizzare queste funzionalità OFDM per analizzare performance di collegamento, robustezza, opzioni di architettura del sistema, effetti del canale, stima del canale, equalizzazione del canale, sincronizzazione del segnale e selezioni di modulazioni sottoportanti.

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.



Vedere anche: 5G wireless technology development, massive MIMO, RF system, wireless transceiver