Multicarrier spread spectrum transmission

The invention relates to a method according to the preamble of claim 1 and an arrangement according to the preamble of claim 12.

The "Multi Carrier Code Division Multiplex Access" (MC-CDMA) technology for air traffic control is an adaptation of the conventional MC-CDMA procedure to the requirements of civil aviation radio in the VHF (Very High Frequency) area. The method can be used for any type of digital radio transmission (e.g. voice, audio, video, text, etc.). The MCCDMA method is based on a combination of OFDM (Orthogonal Frequency Division Multipexing) a special multi-carrier method and the spread spectrum technology DS-CDMA (Direct Sequence Code Division Multiple Access).

1 shows the typical structure of a digital communication system using the spread spectrum method. The higher layers of the communication system are combined in the digital message source or message sink. With the spreading method, the spread spectrum modulation is added to a conventional digital modulation block. The basic principle of this additional block is to spread the information over a larger bandwidth in order to obtain, among other things, increased frequency diversity. Of the various spread spectrum methods, only the direct sequence spread spectrum method is explained below.

2 shows the basic structure of a direct sequence spread spectrum modulator. The binary input data sequence d_t with the symbol rate R_s = 1 / T_S. is with the spread spectrum signal c_tmultiplied (T_s = Signal duration). The chip rate of the code R_c = 1 / T_C. is many times higher than the data rate of the information source (T._c = Chip duration). The result of the multiplication is a spread of the base bandwidth B._D. (with B_D.~ R_S.) to the bandwidth of the code ß_ss (with B_SS~ R_C.). In the receiver, compression to the data rate takes place in the corresponding demodulation block by repeated multiplication with the spread spectrum code c_f. FIG. 3 shows, in a greatly simplified manner, the bandwidth ratios at prominent points in the direct sequence spread spectrum transmission.

The basis of the spread spectrum technique is the channel capacity C according to CE Shannon, A mathematical theory of communication., Bell System Technical Journal, Vol. 27, 379-423 and 623-657, 1948.

C = B_ss * Id ß_SSN₀ + Pj = B_ss * id (l + SNR)

[Bit / sec] (1)

The channel capacity delivers the maximum data rate R._D.<C for the relationship between the SNR in the transmission channel and the transmission bandwidth β_ss for error-free transmission in an AWGN channel. It follows from this that you can transmit error-free in a channel that is disturbed in addition to the white noise if the bandwidth used is ß_S.s is increased to such an extent that the AWGN term (ßssNo) over the interference term (P_;) dominates. The disturbance term is therefore negligible.

C - B_ss * Id ßss ^ o ⁺ ßy) _{> ca ßss} J_{1 +} ßss ^ o (2)

The efficiency of a spread spectrum system depends very much on the choice of the spread spectrum code (J. Meel, “Spread Spectrum,“ Studiedag Spread Spectrum, 1999). The properties of the spreading code, such as the periodic and aperiodic correlation functions, the length N_c of the code, the frequency spectrum and their cross-correlation properties determine the quality of the CDMA system. Among others, the following codes are for use as

Spread signals suitable in CDMA systems: M-series, Gold-series, Hadamard-Walsh codes, etc.

A key parameter of a spread spectrum system is the process gain; it can be represented as the ratio of the transmission bandwidths and determines the power reduction of a narrow-band interferer within the data bandwidth by the spreading method.

AT 41 2 249 B

G_P. =

<img file="AT412249B_D0001.tif" />

(3)

This interference reduction mechanism is based on the double multiplication of the Spread5 spectrum signal c₍ with the data signal d_tbefore data detection takes place. The second multiplication accumulates the energy within the data bandwidth, while an interfering signal is only multiplied for the first time by the spread-spectrum signal and its energy is therefore distributed over the spread-spectrum bandwidth. By means of a filter, only the energy within the data bandwidth is taken into account, thus cutting away significant portions of the interference energy.

(see Fig. 3)

For reasons of separation of the individual participants, only the direct sequence spread spectrum transmission described above is used for multi-user systems. Analogous to FDMA (Frequency Division Multiple Access) and TDMA (Time Division Multiple Access), this procedure, where users are differentiated by their code, is Code Division Multiple

Called Access (CDMA). The CDMA signals are differentiated by correlation. This requires that the signal family (code family) c, von / '= 1 ... n used must be orthogonal, that is:

c^k * c 'itk 1 i = ki, k = 1 ... n (4)

The following describes the basics in the field of multicarrier methods necessary for understanding MC-CDMA.

The special feature of radio channels is that these channels are generally both time-selective and frequency-selective. The frequency selectivity, caused by multipath propagation with large transit time differences, causes strong linear distortion of the received signal, which decreases with decreasing bandwidth. In the case of multi-carrier methods, the available spectrum is now B_v divided into numerous narrow sub-channels. The data transmission takes place simultaneously on / V carriers with the frequencies ή, f₂, ... Zn- Each sub-channel has a bandwidth of 30

Since the sub-channels are narrow, there is constant attenuation and group delay within a channel. Correction is simple or usually not necessary at all. The effects of time selectivity (statistical changes in the channel parameters over time) become greater within a symbol interval, the longer the symbol duration, ie the narrower the modulation signal. The priority is therefore the appropriate definition of the number of subcarriers for given channel statistics.

In order to obtain a spectrally efficient multi-carrier method, the distance between the individual sub-carriers should be minimized without violating the orthogonality of temporally consecutive symbols or of spectrally adjacent symbols. OFDM (Orthogonal Frequency Division Multiplexing) is such a bandwidth-efficient multicarrier method with a simple pulse shaping concept (see FIG. 4). BR Saltzberg, "Performance of an Efficient Parallel Data Transmission System.", IEEE Trans, on Communication Technology, Vol. COM 15, 805-811, 1967. SB Weinstein, "Data Transmission by Frequency Division Multiplexing Using the Discrete Fourier Transform." , IEEE Trans, on Communication Technology, Vol. COM 19, 628-634, 1971.

The principle of OFDM works as follows: The bit sequence to be transmitted is divided into blocks in the transmitter, converted serially / parallel and then assigned to the corresponding sub-carriers.The inverse discrete Fourier transformation (IDFT) or inverse fast Fourier transformation (IFFT) converts the signal in the time domain and the following parallel / serial converter adds up the individual components of the IDFT to the desired baseband signal. Finally, the pulse is shaped and mixed into the corresponding frequency band (see FIG. 5).

It is the inverse discrete Fourier transformation that transforms the signals in such a way that the values ​​from the serial / parallel converter generate a signal that modulates individual ones

AT 41 2 249 B

Sub-carrier corresponds. All output values ​​of the IDFT generate an OFDM symbol. Therefore, the symbol duration of the OFDM symbol is N times longer than the symbol duration of the data.

The equivalent OFDM baseband signal is described by the following equation:

(6) &lt;(t = 0 with

<img file="AT412249B_D0002.tif" />

fe [0, T_s] otherwise (7) and

<img file="AT412249B_D0003.tif" />

k = 0 ... N-1 (8) d_n, i (is the symbol on the k *^en Sub-carrier in the n^te OFDM symbol is transmitted.

• N is the number of OFDM sub-carriers • f_k is the frequency of the Λ * ® sub-carrier, where f₀ corresponds to the lowest frequency used

A demodulation based on the orthogonality of the individual Sub25 carriers g_k(t) take place:

(n + l) 7s ρι (Φ * (^ = 7₅· Δ (/ τ-ΐ) ^ 0_ηΛ Js (T) g; (_T) cfT (9)

R. ^S. nT_s

Where S (t) is the received signal and g ^ T) is the carrier frequency of the sub-channel k.

There are several known forms of multi-carrier CDMA systems, all on one

Combination of CDMA and OFDM techniques are based, such as “multicarrier (MC) -CDMA“, “multicarrier DS-CDMA“ and “multitone (MT-) CDMA“. Reference is made to

- N. Yee, JP. Linnartz and G. Fettweis, “Multicarrier CDMA in Indoor Wireless Radio Networks,“ Proc. of IEEE PIMRC 93, 109-113, 1993.

- K. Fazel and L. Papke, “On the Performance of Convolutionally-Coded CDMA / OFDM for Mobile Communication System,“ Proc. of IEEE PIMRC 93, 468-472, 1993.

- V. DaSilva and ES Sousa, “Performance of Orthogonal CDMA Codes for Quasi40 Synchronous Communication systems,“ Proc. of IEEE ICUPC 93, 995-999, 1993.

- Vandendope, “Multitone Direct Sequence CDMA System in an Indoor Wireless Environment,“ Proc. f IEEE First Symposium of Communication and Vehicular Technology, pp. 4.1.1-4.1.8, 1993.

The signals from all the multicarrier CDMA methods listed can be sent and received very easily using the Fast 45 Fourier Transformation (FFT) without significantly increasing the complexity of the transmitter or receiver.

For use in air traffic control, the MC-CDMA method serves as the basis of the approaches mentioned above. For this technique, N. Yee, JP. Linnartz and G. Fettweis, “Multicarrier CDMA in Indoor Wireless Radio Networks,“ Proc. of IEEE

PIMRC 93, 109-113, 1993 already proposed some modifications and improvements, such as studies regarding the influence of power control on the capacity of a cell-oriented network according to D. Kim and F. Adachi, “Capacity Estimation of Overlaid Multiband CDMA Systems with SIR- Based Power Control, “IEICE Trans, on Communication, Vol. E83-B, 1454-1464, 2000 Investigation of different spreading codes (Walsh codes, orthogonal gold

Codes and Zandoff-Chu Codes) on the system performance as well as on the crest factor of the

AT 41 2 249 B according to H. Bogucka, “Effectivness and Performance Analysis of Various Spreading Codes Applied in Multi-carrier CDMA Wireless Systems,“ Wireless Communications and Networking Conference 2000, pp. 681-685 and BM Popovic, “Spreading Sequences for Multicarrier CDMA Systems, “IEEE Trans, on Communication, Vol. 47, 918-926, 1999, study by

Time and frequency based equalizer (such as Equal Gain Combining, Maximum Ratio Combining, Controlled Equalization) methods according to N. Yee and JP Linnartz, "Multi-Carrier CDMA in an Indoor Wireless Radio Channel," report of the MICRO project, and WG Jeon, KH Chang and YS Cho, “An Equalization Technique for OFDM and MC-CDMA in a time-varying multipath fading Channel“, 2529-2532, 1997, various strategies for channel coding in multi-carrier CDMA io methods according to RA Stirling-Gallacher and GJR Povey, “Different Channel Coding Strategies for OFDM-CDMA,“ Vehicular Technology Conference, Vol. 2, 845-849, 1997 and shaping the frequency spectrum of an MC-CDMA system by means of pulse shaping in the time domain and sub-carrier allocation in the Frequency range according to R. Li and G. Sttete, “Waveform Shaped MCM for Digital

Microwave Radio, "IEEE International Conference on Communications, Vol. 3, 1695-1699, 1995.

The known MC-CDMA method is a combination of the techniques described above

CDMA and OFDM, with this method spreading in the frequency range. Each data symbol is transmitted simultaneously over N narrowband modulated sub-carriers. The type of modulation used for the sub-carriers can be, for example, BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying) or OQPSK (Offset Quadrature Phase Shift Keying). Of the

The frequency spacing between the individual carriers is F / T_ö Hz, where F is an integer. T_bcorresponds to the symbol duration of the data with which each sub-carrier is modulated. As shown in the transmitter model in Fig. 6, each of the N sub-carriers is modulated with the product of the data symbol and a single chip of the spreading code, which is constant in time. The length of the spreading code c¹ corresponds to the number of sub-carriers A /_c = N. The actually transmitted MC25 CDMA signal for the /?^e Symbol a [k] reads s (f) = a [kgc_m[/] i = 0 cos

<td></td><td>&lt;fy</td>

<td>2π</td><td>f_c + - / f</td>

<td></td><td>l T_b</td>

w

[t-KT_b] (10) where f_c corresponds to the carrier frequency, {c_m} is the spreading code with the alphabet {+1, -1} and w (t) is the window function which forms the frequency spectrum in order to minimize adjacent channel interference.

Different users now use the same set of sub-carriers but different, mutually orthogonal spreading codes in the frequency space. There are therefore two levels of orthogonality. On the one hand, the sub-carriers are orthogonal to one another (see OFDM basics) and there is also an orthogonality between the spreading codes of the individual users.

In contrast to the DS-CDMA signal, the MC-CDMA signal is not significantly affected by the delay spread due to the multipath propagation in the channel. Since, due to the synthesis from many narrowband channels, the symbol duration is significantly longer than with other broadband methods. Thus, the individual sub-bands in the transmission channel are only slightly distorted linearly and can be adjusted in the receiver with very simple equalizer methods (e.g. EGC or MRC according to N. Yee and JP Linnartz, "Multi-Carrier CDMA in an Indoor Wireless Radio Channel," report of the MICRO project.

The MC-CDMA method has both the properties of an OFDM signal and those of the DS-CDMA method and is therefore very well suited as a basic method for use in air traffic control. The necessary adaptations and modifications of MC-CDMA for air traffic control are explained below.

The VHF band in the range of 118-137 MHz is currently used in civil air traffic control for ground-to-air communication. This band is divided into 760 25 KHz half-duplex channels, each of these channels being amplitude modulated (AM) with a band-limited 4.5 KHz speech signal. 7 shows the idealized frequency spectrum of several aeronautical radio channels. The radio channel is used like a kind of “party line”, ie one user (controller or pilot) speaks and all other users who have “keyed in” this frequency listen.

AT 412 249 B

The MC-CDMA method according to the invention should meet the requirements of the known systems and should be able to be operated as trouble-free as possible in this VHF band, which is disturbed by many narrow-band AM transmitters, without any loss of operational reliability.

US Pat. No. 5,726,978 A relates to a method for the adaptive allocation of data symbols to a subset of sub-carriers of an OFDM system. An OFDM system is used as the basis and not an MC-CDMA system. The main difference between these systems is that MC-CDMA has two levels of orthogonality. On the one hand, the sub-carriers are orthogonal to one another and, furthermore, there is an orthogonality between the spreading codes of the individual users. For this reason, a dynamic, individual assignment of the carriers for each link in accordance with US Pat. No. 5,726,978 A would destroy the orthogonality between the users of different links who use the same sub-carriers.

In contrast to the method of US Pat. No. 5,725,978 A, the method according to the invention is a radio method with a possible special type of communication link - the so-called “party line”. This means that all users of a defined service volume share a communication channel and thus form a conference, the connections between which are implemented through a single half-duplex channel. Access to this channel is via radio signaling (PTT). With such a type of communication, the algorithms for the allocation of individual subcarriers from document D1 are not sufficient, since on the one hand the measured values ​​of a mobile transceiver unit would affect all users of a service volume and the dynamic change in the allocation possibly resulting from this type of channel access would be undefined Causes a state which does not allow any communication.

In contrast to US Pat. No. 5,726,978 A, the MC-CDMA system according to the invention is operated in parallel with the conventional system, by means of an overlay technique, in the same VHF band.

The two systems must meet the high requirements for operational safety without interference and without mutual interference. This means that the MC-CDMA system must not influence the conventional system and this fact must therefore be taken into account when allocating the subcarriers. The method according to US Pat. No. 5,488,632 A also uses an overlay technique, but is based on OFDM and therefore does not take into account the specific MC-CDMA properties mentioned above. Furthermore, it is a non-safety-critical broadcast method with simplex connections between a stationary transmitting unit and several mobile or stationary receiving units. The frequency ranges of the location-dependent jammers are not taken into account in the frequency allocation in order to achieve interference-free transmission.

The proposed method is used in areas (preferably civil air traffic control) where the ground infrastructure does not have the corresponding units, as described in US Pat. No. 5,726,978 A.

FAZEL, K. “Narrow-Band Interference Rejection in Orthogonal Multi-Carrier Spread-Spectrum Communications.” In: Universal Personal Communications, 3rd Annual International Conference.

New York: IEEE, 1994, pages 46-50, describes a “soft erasure” method which evaluates severely disturbed chips of a spreading code using the channel status information of the corresponding subcarrier and thereby achieves a higher system performance with regard to the bit error rate. This method, which corresponds to a kind of soft decoding of the spreading code, can be used in addition to the method according to the invention for unexpected, non-local AM jammers.

DE 196 47 833 A1 describes a general expansion of the standard MC-CDMA system through a combination of several allocation methods for chips that apply spreading codes from different or identical users to a set of subcarriers. However, this method, which is regarded as the starting point of the invention, does not take into account:

· The suppression of unexpected or known AM jammers, • the special type of connection architecture and access to the channel by means of radio signaling, • the quasi-static sector-dependent allocation of the subcarriers taking operational reliability into account and according to the invention

AT 41 2 249 Β

Procedure.

The aim of the invention is, inter alia, to replace the conventional transmission method in the long term, but for this purpose it is necessary to be able to operate the conventional transmission method and the transmission method according to the invention in parallel, without interference and without mutual interference.

According to the invention, this is achieved in a method of the type mentioned at the beginning with the features of the characterizing part of claim 1. An arrangement according to the invention is characterized by the features of claim 12.

With the procedure according to the invention, it is possible to reduce transmission interference, since subcarriers on frequency bands can be masked out by jammers either from the start or in a dynamic manner if necessary. These jammers are local AM transmitters used by conventional air traffic control, conventional AM transmitters that transmit unexpectedly, and aircraft with conventional AM transmitters. The system according to the invention can be implemented in the currently used transceiver units without great difficulty. All the possibilities that the current transmission possibilities make available can also be implemented in the transmission system according to the invention, for example a “party line” transmission. According to the invention, a transmission can also take place with the specification of priorities for the transmissions to be received.

As stated in claim 2, it is only necessary to determine the number of local jammers and to exclude from the transmission the subcarriers lying in the frequency ranges used by these jammers. This is advantageously done when implementing the system according to the invention. If the conditions should change in the area of ​​the stationary transceiver unit, the number and frequency values ​​of the subcarriers that are not to be taken into account in the transmission can be changed accordingly. The mobile transceiver units are controlled by the subcarriers or subcarriers that are excluded from transmission. Frequency ranges informs what can be done via its own transmission channel as soon as the mobile transceiver unit approaches this stationary transceiver unit or when the mobile transceiver unit stores the respective characteristic transmission data of the stationary transceiver unit and transfers its transceiver unit accordingly to the stationary one Sender / receiver unit adjusted.

To improve the quality of the transmission, the features of claim 3 can be implemented. Depending on the circumstances, the number of pilot carriers or protective tape sub-carriers is selected.

The features of claim 4 are advantageous in order to enable the transmission to be set up quickly without having to transmit or exchange parameters for the transmission itself beforehand. It is useful if these features are met in advance for a plurality of stationary transceiver units and for all the mobile transceiver units that come into contact with these stationary transceiver units. It is also advantageous if the features of claims 5 and 6 are implemented.

The features of claim 8 are advantageous for evaluating an incoming transmission, since this can result in a simple elimination of the subcarriers that are not to be transmitted or a rearrangement of the data symbols which have been subjected to spreading.

According to the features of claim 9, the quality of the transmission is improved, provided that jamming transmitters that have not been taken into account from the outset and become active unexpectedly start their operation; Such jammers can be, for example, local emergency transmitters of known frequency or

the transmitters of aircraft transmitting in the same frequency range. In this case, the transmissions arriving on these frequency ranges are checked with regard to the transmission power and, if necessary, such subcarriers are amplified, weakened or excluded.

The features of claims 10 and 11 enable the implementation of the types of communication currently customary for transmission between a ground station and an aircraft.

Advantageous embodiments of the invention emerge from the following description, the patent claims and the drawings.

Show it:

1: a block diagram of a digital communication system.

Fig. 2: schematically a spread spectrum modulator.

3: an explanation of the bandwidth ratios in a spread spectrum system.

AT 412 249 Β

Fig. 4: the spectrum of four sub-channels of an OFDM system.

5: a schematic block diagram of an OFDM transmitter.

6: a schematic block diagram of an MC-CDMA transmitter.

7: shows an idealized frequency spectrum of an aeronautical VHF band.

Fig. 8: a frequency spectrum of the AM signal and the MC-CDMA subcarriers.

9: a schematic block diagram of an MC-CDMA transmitter adapted for aeronautical radio.

10: schematically an uplink communication, that is to say the transmission process from a mobile transmitter / receiver unit to a stationary transmitter / receiver unit.

11: schematically, a downlink communication from a stationary transceiver unit of a mobile transceiver unit.

In general, it is noted that the procedure according to the invention or the arrangement according to the invention is intended for transmission between a stationary transceiver unit and a mobile transceiver unit; the stationary one is advantageous

The transceiver unit is formed by an air traffic control ground station, and the mobile transceiver unit is located on aircraft. In the same way, the stationary transceiver unit could be a port control center and the mobile transceiver units could be mounted in ships.

In the following, the invention is explained using the example of an air traffic control ground station and the cell of this air traffic control ground station approaching or departing aircraft.

10 and 11 schematically represent the respective transmitting units and receiving units in the ground station and in the aircraft Transmission to the receiving unit 2 of the ground station takes place. 11 explains the downlink communication in which a transmission takes place from the transmitting unit 1 of the ground station to the receiving unit 4 on the aircraft.

The language to be transmitted and / or the data to be transmitted are processed, in particular compressed, or in a processing unit 12 before they are sent to the transmission unit.

the resulting bits become data symbols d_Odi ... d_nb-1 linked which data symbols are brought into parallel representation in a serial / parallel converter 11. For the transmission it can be provided that one or more data symbols are transmitted at the same time.

The individual data symbols d are fed to an interleaver 5, at the input of which the number N_b present data symbols d each in a number N_c which number of the number N_c the chips of the spreading code c¹ is equivalent to.

At the output of the interleaver 5, the data symbols lie parallel in N_c Arranged in groups in which the N_b Data symbols to be transmitted are contained once in each group, in particular one after the other. Each of these groups is linked in a linking unit 6 with one and the same chip of the spreading code, and these coded data symbols are fed to a mapper 7. The mapper 7 outputs the coded data symbols of a predetermined number N of subcarriers, care being taken that the mapper 7 does not take into account subcarriers which are to be excluded from the transmission. The mapper 7 receives from a mapping control unit 16, which receives corresponding information either from the ground station or from stored memories not to use certain subcarriers for the transmission. The coded data symbols are then supplied by the mapper 7 to the IFFT (inverse discrete Fast Fourier Transformation) block 13, which transforms the signals present at the input so that it corresponds to a modulation of individual subcarriers with the signals present at the input. The individual discrete coefficients of the MC-CDMA symbol to be transmitted are at the N outputs of the IDFT block 13 and are fed to the antenna 14 after parallel / serial and digital / analog / conversion and possible filtering.

In the receiver 15 of the receiving unit 2, after an analog / digital / conversion and the transformation of the MC-CDMa symbol into its discrete frequency coefficients by the FFT block 13 ', the subcarriers 7 and a mapper 7 are assigned to the N inputs

Assignment of the N received distributed over N subcarriers_c x N_b Subcarrier to the N_c x N_b

AT 412 249 Β

Outputs of the mapper 7. Subcarriers that have not been sent are discarded and the incoming subcarriers, in particular according to their sequence, are sent to the N_c x N_b Outputs of mapper 7 created. Connected to the mapper 7 is a linking unit 8 in which the incoming coded data symbols, which are in N_c-Groups are available at the output of the mapper 7, each group containing each received data symbol 1x, linked to one and the same chip of the spreading code and thus the decoded transmitted data symbol is restored.

As such, the transmitted data symbol is subject to interference due to the transmission, which is why each subcarrier or each data symbol is fed to an equalizer control unit 18 in which the respective channel is dynamically checked by means of a channel estimator. The channel estimator checks the transmission power arriving on the subcarriers and can amplify, weaken or eliminate these subcarriers. An interleaver 5 is connected to the equalizer control unit 18, at whose output the data symbols in N_c strong groups in a number N_b are present. The individual on N_c-Sub-carriers incoming and dynamically checked

Data symbols d are summed in a summing unit 9 and fed to the evaluation unit 17 after a parallel / serial conversion.

The procedure for sending and receiving both in the ground station and on the aircraft is essentially identical; the process of sending and receiving is essentially the reverse of the other process, with the exception that when

Receipt a channel estimate is made by means of the equalizer control unit 9.

The VHF band of 118-137 MHz is expediently divided into two frequency bands {B_u; B._O} divided, with a protective band ß between the bands_s(guard band) which is to prevent interference emissions into the respective other band is kept free. The lower band is used for uplink transmission (aircraft to ground station), the upper one for downlink transmission. Of the

The frequency spacing of the uplink and downlink channel should always be the same for a transmission. The two frequency bands are now divided into a channel grid. The bandwidth of each channel is B KHz and depends on the number of channels required for network planning, as well as on the number and spacing of the sub-carriers of the MC-CDMA method used. The bandwidth B of a channel is determined as follows

-N_

T_s (11) where N corresponds to the number of sub-carriers and Ts indicates the symbol duration of the data with which the individual sub-carriers are modulated. The length of the symbol duration and the need for a guard interval T_G strongly depends on the properties of the transmission channel. Ts and T_Gshould be chosen so that both the conditions for slow fading (equation 12) and for shallow fading (equation 13) are fulfilled for a sub-channel.

«(R_{s +}rJ.f_D.__ «1 (12)

Tg&gt; T_Max (13) f_Dmgx is the maximum occurring Doppler frequency; t ™ the largest value of the delay spread. The number of sub-carriers / V is composed as follows.

N = N_c* N_b+ N_f+ N_G (¹⁴)

Nf = Nam * B_{AT THE}* T_s (¹⁵) where N_c is the length of the spreading code and N_b corresponds to the number of bits transmitted in an MC-CDMA symbol; N_p is the number of pilot carriers used for synchronization

AT 412 249 Β become; N_G corresponds to the number of sub-channels that must be left free due to the guard bands to the adjacent channels; N_f is the equivalent number of sub-carriers that cannot be occupied because local AM stations broadcast their signal in the same frequency range. Here, N corresponds to_{AT THE} the number of local AM stations that are in the same frequency range as the

MC-CDMA signal are located; B._{AT THE} the bandwidth of the AM signal, i.e. about 9 KHz and T_s the symbol duration of the MC-CDMA symbol.

The two equations 14 and 15 describe the essential factors that are responsible for the transmitted frequency spectrum. By a predefined, static non-assignment of those sub-carriers that are in the same frequency range as the N._{AT THE} local AM transmitter, the mutual influence of the MC-CDMA method and the analog AM radio is minimized (see Figure 8).

The number N is advantageously specified. The same applies to the number N_c, that is to say the chips Cq · · · of the spreading code to be used or for N_b, that is, the number of data symbols to be transmitted simultaneously. It is entirely possible to change these quantities dynamically; in this case, however, these variables or the corresponding parameters would have to be coordinated before a transmission is carried out between the ground station and the aircraft. This requires considerable effort, so it is preferable if the values ​​N, N_b and N_cbe determined.

The value of N_f is specified and takes into account the jammers in the area of ​​the ground station that are considered to be disruptive. For this reason, the value N is advantageously made larger than N_cx N_b + N_fto have some possibility, N_f to enlarge when additional jammers are added. At the same time, a larger number N of sub-carriers also offers that N to the number N_p and N_G can be adapted or taken into account accordingly by other sub-carriers.

As can be seen in equation 14, several data symbols are transmitted simultaneously, ie in one MC-CDMA symbol (see N._b in equation 14). So that narrow-band interferers do not interfere with several neighboring chips of a symbol, the data symbols are evenly allocated to the available subcarriers after the serial / parallel conversion in the interleaver 5 (see FIG. 9).

The allocation rule is as follows:

N_v= y + N_b* Cj j = 0 ... N_b-1 i = 0 ... N_c-1 v = O ... N_C. * N_b-1 (16)

N_v is the sub-carrier at position v of the MC-CDMA system. The totality of the subcarriers of N_v (with v = 0... = N_c * N_b -1) is the amount of subcarriers available for the transmission of data; N_b corresponds to the number of data symbols transmitted simultaneously in an MC-CDMA symbol; c, is the i^te Chip of the spreading code c 'and with j the /^e Symbol.

In downlink transmission, orthogonal codes with a variable spreading factor can be used in order to guarantee that particularly sensitive data (such as control information) are transmitted securely with a high spreading factor. In order to implement a “party line”, only one spreading code can be used for voice communication in the uplink transmission for all participants (aircraft) within a sector. In addition, because of the frequency separation of the uplink and downlink channels, the ground station must act as a relay station and retransmit the signal received from a subscriber in the corresponding downlink channel. This also makes it possible to prioritize radio calls in the ground station.

With the downlink transmission, all active AM interferers of the MappingControl 16 are transmitted in the control channel, of which the respective ground network has information and which are in the frequency range of the MC-CDMA system. This information is used in the mobile receivers, on the one hand, to update the status information of the channel CSI (channel state information) and, on the other hand, to adapt the allocation for the subcarriers of the own transmission channel.

The CSI is used further for the equalization of the received signal and is determined in the following way. As already mentioned, the CSI receives a priori information about known AM interferers from the control channel. The additional channel estimator of the Equalizing Control10

AT 41 2 249 B

Unit 18 uses an estimate of the individual subcarriers to determine whether certain subcarriers are interfered with by external AM transmitters. External AM stations are those stations for which the local network operator has no information about their status (active, passive). This gives the recipient the option of disrupted sub-carriers from the data

Decision to exclude.

The MC-CDMA method adapted according to the invention enables the simultaneous use of two modulation methods, namely the analog narrowband AM radio and the broadband MC-CDMA method in the same frequency range, and thus a problem-free transition from conventional analog narrowband radio to digital broadband Technology, although it is necessary to minimize the mutual influence of the two methods, which is achieved by corresponding static non-assignment of those subcarriers that are in the same frequency range as the local AM transmitters or by transmitting the status of the network operator's known AM transmitters via the control channel or adapting the subcarrier assignment in the uplink direction. Furthermore, the influence of unknown or suddenly active AM transmitters is estimated via the received transmission power of the individual subcarriers. A “party line” can be implemented by using appropriate codes for the voice communication of all mobile participants and the use of the ground station as a relay station for transmissions.