1. Field of the Invention
The present invention relates to an orthogonal frequency division multiplexed signal transmitting apparatus, an orthogonal frequency division multiplexed signal receiving apparatus and an orthogonal frequency division multiplexed signal transmitting/receiving system, which employ an orthogonal frequency division multiplexing system and also employ the transmitted data having the packet structure to execute the data transmission like a burst and, more particularly, an insertion of a reference signal used to compensate amplitude and phase characteristics of the spatial transmission line.
2. Description of the Related Art
In recent years, the remarkable development of the multimedia network with the Internet taking the key media is attained and information revolutions containing IT (Information Technology) as the central revolution are in progress in all the countries of the world.
At present, the multimedia network is advanced mainly by using the computers that are coupled via the cable. In order to achieve the user's convenience, the coupling of the networks via the radio is important. Thus, the products a part of terminals of which is connected by utilizing the radio have been developed, and are available on the market.
Then, initially the character data are main as the information that are dealt with by the multimedia network, and shortly the image data, the video data, the sound data are also transmitted. As for the video data, the request to transmit the video data having high resolution like the normal TV image is increased.
In such circumstances, the realization of the radio network system that has the transmission rate of 20 Mbit/second or more is studied.
In this system, the OFDM (Orthogonal Frequency Division Multiplexing) system is employed in the sub-millwave band of 5 GHz, for example, and the packet type suitable for the radio LAN (Local Area Network), i.e., the burst communication system is employed to transmit the information.
As the burst communication system, for example, there are IEEE 802.11a (Institute of Electrical and Electronics Engineers 802.11 activities) that is standardized recently, HYPER LAN 2 of ETIS (European Telecommunications Standards Institute), MMAC (Multimedia Mobile Access Communication; wideband mobile access) system that is discussed in the interior, etc. The studies of all systems are put forward to realize the communication system that is suitable for the radio transmission of all multimedia.
This OFDM system is the digital modulation system that is employed in the digital terrestrial broadcastings in Japan and Europe, and is known to have the strong transmitting characteristic with respect to the multipath. In addition, it is possible to transmit the information signal having the higher transmission rate by executing the sufficient estimation of the transmission line to compensate the characteristic of the spatial transmission line.
Such estimation of the transmission line is carried out, for example, by providing the preamble data period to the head of the data formulated in the packet structure that is transmitted at a burst, then defining a part of the preamble data, which are transmitted together with a plurality of packets, as the symbol assigned to the reference signal, and then executing the compensation of the transmission line characteristic on the receiver side based on the reference signal transmitted in this manner (referred to as the “reference symbol” hereinafter).
Then, as the second method of executing the compensation of the transmission line characteristic, there is the method of defining the particular sub-carrier of a plurality of sub-carriers, which transmit the data formulated in the packet structure, as the pilot carrier transmitting carrier, then transmitting the data of the packet structure, then examining the characteristic of the transmitted and received pilot carrier on the receiver side, and then compensating of the transmission line characteristic based on the resultant examined characteristic.
In addition, as the third method of executing the estimation of the transmission line characteristic, the method of transmitting the reference symbol every predetermined period if the period in which the data are transmitted is long, etc. may be employed.
FIG. 1 shows major parameters of OFDM data symbols for the multimedia mobile access, in which the data packeted by the OFDM system whose discussion is in progress in IEEE 802.11a, HYPER LAN 2, MMAC, etc., are transmitted like a burst.
In the main parameter specifications of these OFDM data symbols, the OFDM signal is generated by using IFFT (Inverse Fast Fourier Transform) whose order is 64.
The transmitted information signal that is formulated as the packet structure is supplied to IFFT as the burst signal, the header signal called the preamble being attached to the head of the burst signal, and the reference signal called the reference symbol and used to decode the data being inserted into a part of the preamble signal.
IFFT generates 53 sub-carriers consequently. Then, 48 sub-carriers of 53 sub-carriers are assigned as the data carrier, and then these data carriers are digital-modulated by BPSK (Bi-Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 16 QAM (16-Quadrature Amplitude Modulation) or 64 QAM based on the information signal to be transmitted.
In the above respective standards, it is decided that the reference signal is inserted in the preamble period and also transmitted in the data period subsequent to the preamble period. That is, particular four sub-carriers of 53 sub-carriers are defined as the pilot carrier (reference carrier), and the decoding reference signal is transmitted by using the pilot carrier. In this case, as shown in FIG. 1, the remaining one sub-carrier is the zero (center) carrier.
FIG. 2 shows a configuration of a sub-carrier transmitted from the transmitting apparatus.
As described above, 4 sub-carriers of 53 sub-carriers are used as the pilot carrier, in other words the decoding reference signal is always transmitted, and the data carrier is decoded based on the transmitting characteristic, which is transmitted by the decoding reference signal, on the receiver side.
By the way, IFFT outputs 53 sub-carriers as 64 time-series sampling point signal. At this time, in order to reduce the multipath distortion in the transmission line, the last 8 or 16 sample signal of the 64 sample signal is inserted prior to the 64 sample signal as the guard interval signal.
FIG. 3 shows the relationship between the signal generated by IFFT and the guard interval.
Meanwhile, in the transmission of the decoding reference signal by using the reference symbol and the pilot carrier in the apparatus in the prior art, if respective data carriers have the transmission characteristic with the frequency selectivity due to the multipath characteristic of the spatial transmission line, or if the transmitting amplitude characteristic with the frequency selectivity is varied with the time, or if the transmitted burst data length is long, it is difficult to execute the sufficient estimation of the transmission line by using these reference symbols and four pilot carriers. Thus, the false data are caused frequently depending on the characteristics of the transmission line.
Then, in HiSWAN (High Speed Wireless Access) as one standard specification in the MMAC promoting conference, it is defined that, in the downward communication executed from the master apparatus to the terminal apparatus, the reference symbol signal in addition to the above pilot carrier is applied periodically. Thus, since the transmission line compensation for all carriers can be carried out with respect to the data signals having the long burst period every predetermined interval by employing the inserted reference symbol signal, the good transmission characteristic can be obtained.
However, if all sub-carriers which are transmitted in the particular symbol period are used as the reference signal, the reduction in the symbol period used in the data transmission is caused, which brings about the reduction in the data transmission efficiency with the reduction of the symbol period.
In summary, in the transmission of the reference signal in the prior art, the data transmission with the predetermined performance can be carried out in the good spatial transmission line that is not subjected to the influence of the multipath distortion, however the data error rate is increased under the circumstances in which the multipath distortion characteristic of the spatial transmission line is bad, so that it is impossible to assure the sufficient communication path.
In such manner, there is the method of inserting the reference symbols periodically to execute the sufficient compensation of the transmission characteristic even in the spatial transmission line that has the large multipath distortion. In this case, the data error rate can be improved, but there is the drawback that reduction in the data transfer rate is brought about.