Binary offset carrier modulation

Binary Offset Carrier (BOC ) is a special coding process for frequency spreading by applications in the digital communication equipment in the so -called code division multiplex. It can be distinguished more to be transmitted Nutzdatenfolgen by different pseudo-random code sequences ( PRN). BOC is a special form of digital modulation technique that still is part of current research.

Motivation

Conventional CDMA modulate the chips (PRN ) are indicated as the single discrete -state values ​​at the output of a pseudo-random data generator, in most applications, by means of analog phase shift of the RF carrier signal ( BPSK) to transfer data, as it can be a radio channel. BOC is additionally inserted between the chips of the PRN and the analog modulation of the RF carrier signal, a further, discrete logic, which leads, depending on the parameters of the BOC method, an additional band spreading.

The aim is to minimize the mutual interference of different codes are used by different code multiplexed on a shared medium such as a radio channel in the frame. This is especially important when using code division multiplexing meet different code classes and the mutual interference of these different classes of code should be minimal. As an example, BOC allows a higher mutual immunity with simultaneous use of special PRN generators such as Gold sequences with different generator polynomials.

BOC not change while the respective PRN generators underlying features, such as the respective generator polynomials, the start values ​​or code phase shifts. BOC has almost a type of "intermediate layer" between different code generators for the purpose of improved code division through various code classes dar.

Method

To represent the function of the BOC and its parameters is most easily understood by the so-called chip rate. This rate is the number of chips of the PRN generator provides a second. From this chip rate, the fundamental frequency f0 of the BOC systems is derived. The factor by which the fundamental frequency f0 is less than the chip rate is usually referred to in the literature as m:

The binary subcarrier, Eng. Binary Offset Carrier, of which the name of this method is derived, a binary sequence {1, -1} with a fixed frequency fs dar. This frequency is a multiple of the fundamental frequency f0; the factor between them is in the literature usually denoted by n:

The output signal is formed by a logical XOR operation of the intermediate carrier with the chip sequence. From the factors outlined above n and m, the classification of the BOC method derives; This is written in the literature, mainly in the form of BOC ( n, m). n and m may be arbitrary real values ​​greater than or equal to 1.

In the spectral range, the two parameters n and m are interpreted equivalent and somewhat clearer:

  • The n parameter specifies the factor by which, multiplied by the chip rate, the center frequency of the transmission spectrum is offset. N is for example 1 and the chip rate of 1 Mchip per second, the transmission spectrum of the code string is shifted spectrally by 1 MHz. In this case, both sidebands occur, that is, BOC spectrum is reflected symmetrically about the carrier center frequency. This spectral shift allows using frequency-division multiplex several different codes on the same transmission frequency by BOC accommodate.
  • The m parameter specifies the factor by which the transmission spectrum of the output sequence is expanded. M is equal to 1, the spectrum of the BOC - sequence is not dilated, m is equal to 5, the transmission spectrum is 5 times as wide as the original code sequence. This factor is therefore in addition to the chip sequence, a further band-spreading and spectral broadening dar. The additional spreading of the code-division multiplexing of different code sequences is facilitated, which need not necessarily be orthogonal to one another in code space, that is, with minimal cross-correlation of the code sequences to each other. Without BOC as a kind of coding intermediate layer, the PRN codes would therefore have a stronger mutual interference.

Examples

  • BOC ( 1, 1) represents the simplest form, that the fundamental frequency of the subcarrier is equal to the chip - code rate. Per period of the intermediate carrier, a chip is transmitted exactly. This clearly means that undergoes an inversion in the time sequence of each chip in half the transmission time. This additional inverting the bit rate is doubled at the output of the BOC and thus achieving a spectral shift by the center frequency, similarly to the Manchester encoding. The BOC sequence at the output The spectrum is spectrally offset only to the chip rate at the entrance, but not additionally widened.
  • BOC (5, 3): The fundamental frequency of this system is 1 /3 of the chip rate. There are per period of the fundamental frequency f0 transferred exactly 3 chip bits. The carrier frequency at which the PRN chip sequence is XORed is exactly five times the fundamental frequency. Due to the non-integer divisibility by 5 to 3 arise in chip changes within the period of the fundamental frequency spectral frequency multiples, which cause the complex band spreading. The BOC - sequence at the output spectrum is shifted by 3 times the chip rate is symmetrical about the center frequency and spectrally widened 5 times as much as the voltage applied at the input of the chip sequence.
  • The chip rate is not necessarily a whole number multiple of the fundamental frequency at BOC. Thus, methods such as Boc (15, 2.5 ), where each period of the fundamental frequency of 2.5 chips are transmitted, it is possible. Currently, this special coding methods found in practice, however, hardly been used and are still the subject of a research.

Applications

Applies BOC in digital code division multiplexing transmissions, as it finds use in satellite-based navigation of the newer generation. Thus, the new satellites of the GPS system using BOC -based transmission techniques, in combination with the older and more usual transmission techniques with C / A code and P / Y - code. BOC is also used in the nascent structure in the European navigation system Galileo in the field of digital modulation of the navigation signal.

Multiplexed binary offset carrier

BOC various methods can be multiplexed to achieve a further reallocation of spectral signal power. While a combination of a BOC (1,1 ) and a BOC ( 6,1 ) signal is in talks for Galileo and GPS L1C signal newer. Here, the BOC (1,1) signal gets 10/11 of the total signal energy with the following spectral power density Φ:

If a time -division multiplexing in an interview is for the L1C signal ( TMBOC ). This is 30 symbols long at full power, the BOC (1,1) modulated signal, and 3 symbols long at full power, the BOC ( 6,1 ) modulated signal sent. The spectrum is the same in both cases. If only short symbol sequences analyzed, but the behavior is identical. The technical implementation of the encoding and decoding are different.

The advantage of the MBOC method over the conventional method is both a targeted possible spectral deformation to avoid interference from other signals, and on the other hand, the selectability of the individual signal components. Thus, e.g. a Galileo receiver which only BOC (1,1 ) signals also supports decoding MBOC signals.

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