The basic principle of COFDM

This article describes the COFDM scheme based on two main principles. We studied the total power spectral density of COFDM signal, and benefits of moving to the Fourier transform.

Keywords: channel codec, the COFDM modem, quadrature differential phase modulation, KFM-PCM-F

In modern systems, radio and television digital broadcasting transmission principle SCPC-PCM-transformed into FM transmission method broadband multi-frequency signals, each of which is an ensemble consisting of a large number of orthogonal frequency-divided carrier (hence the name OFDM). Thus, when the parallel transmission bit stream occurs with frequency diversity to the carrier frequency when the signal is simultaneously transmitted on K carriers, where for each transmission speed is reduced (consequently, the duration of a clock interval τ is increased) K times.

«Orthogonal» part of the title in OFDM indicates that there are exact mathematical relationship between carrier frequencies. In systems with PCM-FDM-FM carrier frequencies are located at a distance (from each other), which can allow using conventional filters and demodulators. In such receivers must be placed between the carrier frequency guard intervals, whose introduction in the frequency domain significantly reduces the efficiency of spectrum use.

It is possible to arrange the bearing so that the sidebands adjacent carriers overlap, but still maintain the ability to receive information without interference. To implement this bearing must be mathematically orthogonal. dense «packing» the spectrum of multi-frequency signal can be realized only in this case (Fig. 1.). The frequency interval (the distance between adjacent carriers -) may be, depending on the transmission mode 1 to 8 kHz. Carriers are linearly independent (i.e., orthogonal), if they are spaced a multiple of 1 / T_u, where T_u- useful part of OFDM-symbol interval equal to the processing duration (integration) signal. The width of the signal spectrum, Orthogonal FDM estimated approximate relation.

Δf_opt = K / Т_u.

Fig. 1. Orthogonal multicarrier OFDM signal

In the technical literature elementary OFDM-signal called OFDM-symbol. Each symbol duration T_s is formed by modulation to the carrier. Interval T_s is composed of two components: a «useful» interval during which the actual input data and transmitted, i.e. Useful information and a guard interval (also referred to). The guard interval is a copy, or a cyclic repetition of the desired interval, which is inserted before useful. Since the OFDM-signal comprises many separately modulated carriers, each symbol of the OFDM-can in turn be considered as being divided into cells, each of which corresponds to the modulation of a single carrier during one symbol.

It goes on coding an Orthogonal Frequency Division (COFDM), is based on two basic principles. The first of these (OFDM) is parallelizing information: the information transmitted is spread over a large number (sub) carrier centered in a densely packed group (structure). Each of the modulated carrier signal 4 RPM with a low bit rate. As a result, each of the carriers becomes non-selective channel. In multipath conditions, due to frequency-selective fading, the signal received at some carriers, amplified, and on the other — is reduced. For this reason, some of the information transmitted with high quality, while the other may be significantly distorted or even lost.

The second principle exploit multipath propagation between transmitter and receiver by using the fact that the signals sufficiently spaced in frequency and time may not be identical distorted. At the same time, information items transmitted at different times on the carrier spaced in frequency, are interconnected by encoding the original information sequence. This usually allows the receiver to recover the lost information due to its correlation (due to coding) the information received is correct. The basis of the second basic principle COFDM are convolutional encoding, decoding on the basis of maximum likelihood (Viterbi algorithm) with soft decision, as well as interleaving in frequency and time. Modulation system involves redistribution interleaved data stream elements in time and frequency, whereby the successive samples of the transmitted signal undergo independent fading. The efficiency is maximum in the Viterbi algorithm channel is independent errors and interleaving provides error de-correlation. Note that for stationary reception separation in the frequency domain is the only means to ensure successful reception as a temporary separation in this case does not help.

Thus, in accordance with the basic proposals g. EBU 1988, COFDM system is:

– Transmitting information parallelization, i.e. Distribution information transmitted by multi-carrier, each of which provides a low-speed digital transmission stream, thereby increasing the chip length which greatly exceeds the value of delay spread echoes in multipath reception; in this case the frequency selectivity of the channel does not lead to inter-symbol interference (ISI);

– Frequency-time interleaving, carried out in N narrowband channels when each audio or a television channel arranged on (under) the carriers are uniformly distributed and interleaved over the transmission channel;

– The introduction of a guard interval provides an exception of influence of the ISI in the processing of the reception signal; wherein any echo signal returned to the receiver during the time of the guard interval not only

It causes ISI, but the received power increases;

– The use of double FMR each orthogonal subcarrier of a powerful error-correcting coding in combination with a Viterbi decoder in the receiver provides a sufficiently high frequency and the energy efficiency of the system.

In addition, the noise immunity of the signals increases substantially if the correction capacity of error-correcting code is used not evenly throughout transmitted digital stream and selectively providing unequal, greater protection for those characters that need it — in accordance with the properties of the applied modulation system. «Canal + COFDM modem codec» Therefore, the system can be viewed as a signal-code construction of frequency-spaced orthogonal carriers with their differential quadrature phase modulation and Unequal Protection.

The total spectral power density COFDM signal can be found as the sum of the power spectral densities of the individual carriers. It could be very close to a constant in the frequency band occupied by the carriers but the duration of the transmitted OFDM-symbol is greater than the reciprocal distance between the carriers at the guard interval size.

If the transition to the Fourier transform has not been made, the OFDM modulation would have small chances of practical implementation of the advantages of OFDM system because it manifested in a very large number of carriers (amounts to many hundreds and even thousands), and in this case, the direct hardware generation OFDM signal It would require a huge cost circuitry in the form of thousands of generators and modulators in the transmitter and the same number of detectors in the receiver. It is unlikely that such a scheme would be implemented. A direct and inverse discrete Fourier transform in the last decade designed fast and efficient algorithms (FFT) and inverse Fourier transform (IFFT) and FFT processors designed as a large-scale integrated circuits.

All of the above parameters DVB-T signal transmission system are specified in table 1:

Table 1

Key Features COFDM signals DVB-T standard