JPH066325A - Wireless transmission system - Google Patents

Abstract

(57) [Abstract] [Purpose] When transmitting a frequency division multiplexed signal and a spread spectrum signal in the same frequency band, to prevent deterioration of transmission characteristics due to difference in frequency characteristics of received signals in different radio transmission paths. With the goal. On the transmitting side, a frequency division multiplexed signal and a spread spectrum signal in the same frequency band are combined by a combiner (COMB), and the combined signal is transmitted from a common antenna (AT). On the receiving side, the two signals are received by separate antennas or a common antenna.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wireless transmission system for transmitting a frequency division multiplexed signal and a spread spectrum signal in the same frequency band.

[0002]

2. Description of the Related Art Frequency division multiplexing is often used in wireless transmission systems. In this method, as shown in FIG. 3, the bandwidth W given to the system is divided into N channels and multiplexed with a bandwidth of W / N per channel. It should be noted that although one channel in this figure may be time-division-multiplexed, all of them will be referred to as frequency-division multiplexed signals below.

As described above, there is known a method of simultaneously using spread spectrum signals in the same frequency band in which frequency division multiplexed signals are used. The spread spectrum signal forms one signal in the entire frequency band of W as shown in FIG. As a concrete method, the spread spectrum signal is simultaneously used in the frequency band in which the frequency division multiplexed signal is used as shown in FIG. In this example, the first transmitting station FD for transmitting the frequency division multiplexed signal
T, a first receiving station FDR that receives it, a second transmitting station that sends a spread spectrum signal and a second receiving station SSR that receives it. There are usually N or more first receiving stations corresponding to N channels, but here it is assumed to be one station. Similarly, there may be a plurality of second receiving stations, but here it is assumed to be one station. The first transmitting station FDT and the second transmitting station may be housed in the same building, but different antennas are used.

The signal from the first transmitting station in the first receiving station is P ₁ and the signal from the second transmitting station is P ₁₂ . Further, the signal from the second transmitting station in the second receiving station is P ₂ , and the signal from the first transmitting station is P ₂₁ . In such a conventional method, both signals can be satisfactorily transmitted when the frequency characteristic of the transmission line is flat and the propagation loss is almost the same.

As a concrete example of transmitting two signals, a system band W = 10 MHz and the number of channels N = 1000, that is, a case where a spread spectrum signal is simultaneously transmitted to a transmission line of a frequency division multiplexing system of 10 kHz per channel will be briefly described. explain. Since the frequency division multiplexed channel has only a band of 10 kHz, the signal rate is 10
The upper limit is about kb / s. As the spread spectrum signal, for example, a high speed signal of 100 kb / s is transmitted. At this time, the process gain is 10 MHz / 100 kb / s = 100
Therefore, it becomes 20 dB. Assuming that the signal-to-noise ratio when despreading is 5 dB, the frequency division multiplexed signal is CNR 5d when demodulating the spread spectrum signal.
It can be regarded as B noise. At this time, the ratio of the power spectrum of the frequency division multiplexed signal to the spread spectrum signal is L = 20-5 = 15 dB. Therefore, when demodulating the frequency division multiplexed signal, the spread spectrum signal can always be regarded as CNR 15 dB noise, and the transmission characteristic hardly deteriorates.

However, in order for the above-mentioned two signals to coexist at the same time, it is necessary that the frequency characteristic of the wireless transmission line be flat and the power spectrum ratio L be preserved on the receiving side. Generally, since the wireless transmission path is a multipath propagation path, the transmission path has frequency selective fading, that is, frequency characteristics, and at this time, good transmission characteristics as described above cannot be obtained. In the conventional system as shown in FIG. 4, since the transmitting antennas are different, the signals P ₁ , P ₁₂ , P ₂ and P ₂₁ reach the receiving station through different propagation paths. Therefore, the transmission lines have different frequency characteristics. Specific frequency characteristics are shown in FIG. FIG. 10A shows an example of frequency characteristics of a received signal in the first receiving station FDR. Frequency f ₁ or f
_In the area of ₂ , the spectrum of the desired wave P ₁ is sufficiently larger than the spectrum of the interference wave P ₁₂ , so that the frequency division multiplexed signal can be received well. However, in the region of f ₃ , the spectrum of the interference wave P ₁₂ is larger than that of the desired wave P ₁ . That is, since the desired wave is greatly dropped due to fading, the frequency division multiplexed signal cannot be demodulated.

On the other hand, FIG. 1B shows an example of the frequency characteristic of the received signal in the second receiving station SSR. Since the desired wave P ₂ is much lower than the interference wave P ₁₂ by L = 15 dB or more over the entire band, the spread spectrum signal cannot be demodulated due to the interference by the frequency division multiplexed signal.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a wireless transmission system which solves the problem that the transmission characteristics are deteriorated due to the difference in frequency characteristics of received signals in different wireless transmission paths.

[0009]

The feature of the present invention for achieving the above object is that, on the transmitting side, a frequency division multiplexed signal of the same frequency band and a spread spectrum signal are combined by a combiner, and the output is There is a wireless transmission system in which both signals are transmitted from a common transmitting antenna, and on the receiving side, a frequency division multiplexed signal and a spread spectrum signal are received by an individual antenna or a common antenna.

[0010]

The present invention is characterized mainly in that the frequency division multiplexed signal and the spread spectrum signal in the same frequency band are input to the combiner and the combined signal is sent out from the transmitting antenna. Unlike the conventional technique, a combiner is provided and the transmitting antenna is the same, and the conventional problems associated with multipath propagation are eliminated.

[0011]

FIG. 1 shows an embodiment of the present invention. # 1 to #N
The signals up to are multiplexed and output by the frequency division multiplexer FD. The frequency division multiplexed signal is a combiner COM
Input to B. On the other hand, the signal input from the input terminal I _S is converted into a spread spectrum signal by the spread spectrum device SS and input to the combiner COMB. The combined signal is transmitted from the transmission antenna AT. In the combined signal, the power spectrum density of the spread spectrum signal is lower than that of the frequency division multiplexed signal by LdB (L> 0). Although there is one transmitting antenna AT in this figure,
After the combiner, the combined wave may be divided and transmitted from a plurality of transmitting antennas. Even if the signals are transmitted from a plurality of such antennas, two signals may be transmitted from the same antenna.

The transmitted signal is demodulated by the respective receivers into a frequency division multiplexed signal and a spread spectrum signal in the same manner as in FIG. However, the received spectrum is significantly different from the conventional scheme when there is selective fading.

FIG. 2 shows an example of the spectrum at the first receiving station. Since the transmission lines of the desired wave P ₁ and the interference wave P ₁₂ are completely the same, they exhibit the same frequency characteristics. Therefore, the signal is received while maintaining the difference LdB in the power spectral density of the combined signal at the transmitting antenna AT. Therefore, f
Not only in ₄ , but also in the frequency region of f ₅ , if the frequency division multiplexed signal level is sufficiently higher than the noise level, good demodulation is performed. Similarly, in the second receiving station SSR, the frequency characteristic is as if P ₁ in FIG. ₂ is replaced with P ₂₁ and P ₁₂ in FIG. ₂ is replaced with P ₂ . That is, since the signals are received while maintaining the LdB level difference, the spread spectrum signal can be well demodulated.

As will be understood from the following description, according to the present method, two signals can be satisfactorily transmitted even in the presence of selective fading.

[0015]

As described above, since two signals can be used in the same band as described above, there is an advantage that the radio frequency can be effectively used.

[Brief description of drawings]

FIG. 1 shows a wireless transmission system according to the present invention.

FIG. 2 is a spectrum explanatory diagram of a received signal in the present invention.

FIG. 3 is an explanatory diagram of a spectrum of a transmission signal.

FIG. 4 is a diagram showing an example of a conventional method.

FIG. 5 is an explanatory diagram of a spectrum of a received signal in the conventional method.

[Explanation of symbols]

 FDE Frequency division multiplexed signal transmitting station (first transmitting station) FDR Frequency division multiplexed signal receiving station (first receiving station) SST Spread spectrum signal transmitting station (second transmitting station) SSR Spread spectrum signal receiving station (second receiving station) Station) AT1 first transmitting antenna AT2 second transmitting antenna AR1 first receiving antenna AR2 second receiving antenna FD frequency division multiplexer SS spread spectrum apparatus COMB combiner

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Kota Kinoshita 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nippon Telegraph and Telephone Corporation

Claims (1)

[Claims] 1. A transmitting side combines a frequency division multiplexed signal and a spread spectrum signal in the same frequency band by a combiner, and outputs the output from a common transmitting antenna for both signals, and a receiving side performs frequency division. A wireless transmission system characterized in that a multiplexed signal and a spread spectrum signal are received by an individual antenna or a common antenna.