Wireless LANs have attracted a great deal of attention as a system for liberating users from LAN cables of the wired method. According to a wireless LAN, the most part of cables in the work space such as an office can be omitted, so communication terminals such as personal computers (PC) can be moved with relative ease. In recent years, the demand thereof has markedly increased, accompanying the increase speeds and reduced prices of wireless LAN systems. Particularly, in these days, implementation of personal area networks (PAN) has been studied for establishing a small-scale wireless network between multiple electronic apparatuses present around a personal environment to perform information communication. For example, different wireless communication systems and wireless communication apparatuses have been stipulated by using frequency bands not requiring the authorization of a competent authority, such as the 2.4-GHz band, 5-GHz band, or the like.
Examples of the standard specifications relating to a wireless network include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (e.g., see Non-patent Document 1), HiperLAN/2 (e.g., see Non-patent Document 2 and Non-patent Document 3), IEEE802.15.3, and Bluetooth communication. As for the IEEE802.11 standard, expansion standards such as IEEE802.11a (e.g., see Non-patent Document 4), IEEE802.11b, and IEEE802.11g are available according to the differences of wireless communication methods and frequency bands to be used.
With a wireless communication system, multi-bus environment is formed wherein a combination of multiple reflected waves and delayed waves is received at a data reception station in addition to the direct wave from a data transmission station side. Delay distortion (or frequency selective phasing) is caused due to the multi-bus, and error is caused over communication. This results in a problem wherein inter-symbol interference due to delay distortion is caused.
Major countermeasures of delay distortion include a multi-carrier transmission method represented with the OFDM modulation. With the OFDM (Orthogonal Frequency Division Multiplexing) method, the frequency of each carrier is set such that the respective carriers are mutually orthogonal within a symbol zone. When transmitting information, information transmitted in serial is subjected to serial-to-parallel conversion for each symbol cycle slower than the information transmission rate, the multiple converted output data is assigned to each carrier to perform amplitude and phase modulation for each carrier, the data is converted into time axial signals while keeping the orthogonality of each carrier by performing inverse FFT regarding the multiple carriers, and transmitted. Also, when receiving information, the operations opposite of this, i.e., time axial signals are converted into frequency axial signals by performing FFT to perform demodulation corresponding to each modulation method regarding each carrier, and then are subjected to parallel-to-serial conversion to reproduce the original information transmitted with serial signals. The OFDM modulation method has been employed as the standard specifications of a wireless LAN in IEEE802.11a/g, for example.
Also, as another problem in a wireless communication system, there is a problem wherein a linear frequency spectrum is formed by data of 0 through 1 extremely continuing. For example, in the event of employing the OFDM modulation method, a configuration is made up of multiple sub carriers, the difference between average power and peak power is great, resulting in shortage of an power range on the transmission side and on the reception side. To this end, scrambling is usually performed for pseudo-randomizing data over a transmission path.
In general, on the transmission side, scrambling is performed by calculating an exclusive-OR operation between delivery data and pseudo-random bits, the output thereof is taken as transmission data. On the other hand, on the reception side, descrambling is performed by calculating an exclusive-OR operation between received data and pseudo-random bits, whereby the transmission data can be extracted. At this time, the transmission side and the reception side must have the same pseudo-random bit generator, and also have the same initial value thereof.
Examples of a wireless communication system for performing scrambling include IEEE802.11a, which is the technical standard of a wireless LAN. In FIG. 32, a configuration example of a wireless communication apparatus, which is employed for IEEE802.11a. This wireless communication apparatus performs, for example, audio communication as to another wireless communication apparatus (not shown) corresponding to IEEE802.11a. Hereinafter, description will be made regarding the wireless communication operations thereof with reference to FIG. 32.
First, description will be made regarding operation of the transmission system following the flow of signals. In the event of data communication such as being connected to a computer, a data signal such as audio is input to a data input/output processing unit 102, and the signal is converted into an appropriate digital data sequence.
Subsequently, the data sequence is input to a transmission data processing unit 110. If necessary, the transmission data processing unit 110 receives communication control data to be transmitted to a wireless communication apparatus (not shown) serving as an other party of wireless communication from a control unit 104, subjects this to multiplexing as appropriate, and then forms and outputs frame and slot configurations for transmitting this in a wireless zone.
Subsequently, a CRC (Cyclic Redundancy Check) adder 112 adds redundancy for detecting an error on the reception side to the transmission data, and further, a cipher device 114 subjects the transmission data to encryption, and outputs this.
Subsequently, a scrambler 116 subjects the transmission data to scrambling so as to form pseudo-random in accordance with a predetermined algorithm (described later). Also, a header generator 117 generates a PHY (physical layer) header. Subsequently, an encoder 118 subjects the PHY header and the transmission data subjected to scrambling to convolution encoding, and further an interleaver 120 subjects this to interleaving. According to this interleaving processing, the coded bit sequence is rearranged in accordance with a particular rule, so on the reception side, burst errors can be converted into random errors by performing an inverse operation, i.e., de-interleaving (described later).
Subsequently, a modulator 122 subjects the transmission data to mapping to signal points at the time of transmission, and outputs inphase components (I components) and orthogonal components (Q components). A complex IFFT unit 124 subjects the output thereof to inverse FFT, thereby performing the OFDM modulation.
Subsequently, a time-waveform trimming unit 126 provides guard time by adding a cycle prefix, and subjects the output data to window wing processing so as to smooth the rise and decay of the OFDM modulation symbol.
Subsequently, a DA converter 128 converts the transmission data from a digital waveform to an analog waveform, and further, an RF transmitter 130 subjects the transmission data to filtering, vector modulation using the I components and Q components, a frequency conversion to an appropriate transmission frequency channel, control of transmission power, amplification, and so forth.
The transmission signal up-converted by the RF transmitter 130 is input to an antenna 134 via an antenna duplexer 132, and finally transmitted from the antenna 134 as an electromagnetic wave. This transmission signal is received by a other party of wireless communication (not shown).
Note that the antenna duplexer 134 is used for separating a transmission signal and a reception signal, an antenna switch is employed in the TDD method and the FDD/TDMA method, and a duplexer is generally employed in the other methods. Now, let us say that an antenna switch is employed, since the example here is IEEE802.11a of the TDD method.
Next, description will be made regarding the operation of the reception system in detail. Now, let us say that a wireless communication apparatus 100 receives a transmission signal generated by another wireless communication apparatus serving as an other party of wireless communication (not shown) performing the same processing as the transmission system in the above IEEE802. 11a.
The transmission signal from the other party of wireless communication is received at the antenna 134 as an electromagnetic wave. The signal is separated from the own transmission signal at the antenna duplexer 132, following which is input to an RF receiver 140. The RF receiver 140 subjects the reception signal to amplification, attenuation of unnecessary frequency components, selection of a desired frequency channel, frequency conversion, reception signal amplitude level control, vector detection process for separating the I components and the Q components, band limit, and the like, and thus the I components and the Q components of the reception signal are extracted.
An AD converter 142 converts the reception signal down-converted by the RF receiver 140 from an analog waveform to a digital waveform. Subsequently, a synchronization circuit 144 subjects the reception data to frame synchronization, frequency error correction, and the like. Now, in the event of searching a communicable communication other party immediately after power is turned on or the like, detection of a synchronous signal or initial synchronization is performed using this synchronization circuit 144. Various arrangements have been proposed regarding initial synchronization, frame synchronization, frequency error correction, and the like, but these are not directly associated with the essence of the present invention, so further description will not be made in the present specification.
Subsequently, a time-waveform trimming unit 146 subjects the reception data to time waveform trimming so as to remove guard time provided by adding a cycle prefix, following which a complex FFT unit 148 subjects the reception data to FFT to perform the OFDM demodulation.
Subsequently, an equalizer 150 performs equalization using estimation of a transmission path and estimation results. In some cases, the equalizer 150 inputs the information of the synchronization circuit 144, and uses this for estimation of a transmission path, or the like. Note that various arrangements have been proposed as an equalizer, but these are not directly associated with the essence of the present invention, so further description will not be made in the present specification.
The output of the equalizer 150 is input to a demodulator 152, and is subjected to signal point determination to output a reception bit estimation value. Subsequently, the reception data is input to a de-interleaver 154, and is subjected to de-interleaving for rearranging the coded bit sequence in accordance with a particular rule. Subsequently, a decoder 156 performs decoding of error correction codes subjected on the transmission side.
Subsequently, a descrambler 158 subjects the decoded reception data to descrambling, which is inverse conversion of scrambling performed on the transmission side. Also, a header extractor 157 extracts a PHY header from the decoded reception data. Further, a decipher device 160 deciphers encryption subjected on the transmission side, following which a CRC checking unit 162 outputs the reception data of which a CRC is removed, and the result of a CRC check regarding the reception block.
Subsequently, in the event that determination is made that the result of the CRC check of the reception block has no error, a reception data processing unit 164 removes the frame configuration and slot configuration subjected for transmission in a wireless zone. Subsequently, in the event of data communication such as connected with a computer, the data input/output processing unit 102 converts the reception data into a data signal, and outputs this.
In the event that the reception data includes communication control data transmitted from an other party of wireless communication (not shown), that portion is extracted by the reception data processing unit 164, and is input to the control unit 104 via a reception system control line 106. Subsequently, the control unit 104 interprets the received control data, and performs operation control of each unit within the wireless communication apparatus 100 in accordance with the received instruction.
Each unit of the transmission system is connected to the control unit 104 via a transmission system control line 108. Accordingly, the control unit 104 can perform various operation control and monitoring of the transmission system such as on/off control of the transmission system, operation control and status monitoring of the RF transmitter 130, fine adjustment of transmission timing, modification of an encoding method or signal point mapping method, control of retransmission, and the like via the transmission system control line 108.
Also, each unit of the reception system is connected to the control unit 104 via the reception system control line 106. Accordingly, the control unit 104 can perform operation control and monitoring of various reception systems such as on/off control of the reception system, operation control and status monitoring of the RF receiver 140, fine adjustment of reception timing, modification of a decoding method or signal point demapping method, control of retransmission, and the like via the reception system control line 106.
FIG. 33 illustrates the configuration of the scrambler 116 disposed in the transmission system of the wireless communication apparatus 100. The scrambler 116 shown in the drawing is made up of a 7-stage shift register, and an arrangement is made wherein X1 is the lowest bit, X7 is the highest bit, and the value of each bit is shifted to the adjacent higher bit X2 through X7 in order. As for the highest bit X7, an exclusive-OR operation between the output from the X4 and the output from the X7 is calculated, and the result is input to the lowest bit X1. At the same time, an exclusive-OR operation between the result and input data is calculated, and this result is output as data following scrambling.
Data other than 0000000 (i.e., all zeroes) is employed for the X1 through X7 shown in FIG. 33. This is because all-zero data cannot serve as a scrambler. In other words, the total number of a bit sequence, which can be employed, is 27−1=127, and any number of these may be employed. A scrambling pattern, which occurs through scrambling, can be changed by modifying the initial value of scrambling.
Also, FIG. 34 illustrates the configuration of the descrambler 158 disposed in the reception system of the wireless communication apparatus 100. The descrambler shown in the drawing has completely the same configuration as the scrambler shown in FIG. 24, stores the initial value provided from the transmission side to the X1 through X7, and calculates an exclusive-OR operation between the stored initial value and received input data, thereby performing descrambling.
FIG. 35 illustrates the format of the OFDM signal stipulated by IEEE802.11a. As shown in the drawing, the Preamble is transmitted first, and subsequently, the SIGNAL field is transmitted with one OFDM symbol, and further subsequently, the DATA field is transmitted.
The PHY header of IEEE802. 11a comprises the above-described SIGNAL field, and the Service field made up of 16 bits on the MSB side of the DATA field. FIG. 36 illustrates the configuration of the PHY header in detail. As shown in the drawing, the SIGNAL field comprises the modulation method, RATE information of 4 bits determined from the encoding rate of error correction codes, a reserved bit of one bit, LENGTH information of 14 bits indicating the length of a transmission packet, PARITY information of one bit for detecting a bit error of the SIGNAL field, and TAIL bits of 6 bits for terminating a convolution code. Here, the PARITY bit is set such that the number of “ones” included in the bit sequence made up of the RATE information, the reserved bit, the LENGTH information, and the PARITY bit becomes even.
Following the SIGNAL field, the Service field of 16 bits continues, and of these 16 bits, 7 bits from the MSB side are used for transmission of the initial value of scrambling, i.e., notification, and this is equivalent to the initial value provided for descrambling on the reception side. Incidentally, the residual 9 bits of the Service field are reserved.
The SIGNAL field is transmitted by BPSK R1/2 of which the required Eb/No is the lowest, of the modulation modes stipulated by IEEE802.11a. The 24 bits of the SIGNAL field are equivalent to 48 (sub carriers)×½ (encoding rate)=24 bits in the event of transmitting data carrier and 48 sub carriers by BPSK R1/2 using the OFDM transmission, and are transmitted with one OFDM symbol. This SIGNAL field is not subjected to scrambling.
The subsequent data field hereafter is transmitted in a state of being subjected to scrambling with the modulation mode indicated by the RATE field within the SIGNAL field.
Now, description will be made further in detail regarding handling of the scrambling initial value on the transmission side, and the descrambling initial value on the reception side. Scrambling and descrambling are realized by combining a shift register and an exclusive-OR circuit according to a generator polynomial representing pseudo-random number sequence (e.g., see Patent Document 1).
FIG. 37 illustrates the configuration of around the scrambler 116 on the transmission side in detail. Transmission data is scrambled by calculating an exclusive-OR between the transmission data and a scrambling pattern generated with a later-described method at an EXOR 116b within the scrambler 116 following the transmission data being encrypted at the cipher device 114. The output thereof is subjected to error correction encoding at the encoder 118.
When starting scrambling, a scrambling pattern generator 116a receives notification from the control unit 104 regarding a scrambling initial value at the time of generating a scrambling pattern. The scrambling initial value notified is set within the register of the scrambler body made up of a shift register such as shown in FIG. 33, and a scrambling pattern is generated while the value within the register is shifted for each clock.
In particular, as for the first 7 bits of the Service field disposed in the headmost of the DATA field for transmitting a scrambling pattern, “0” data of 7 bits is entered at the time of outputting from the cipher device 114, and the first 7 bits of the Service field are generated by calculating an exclusive-OR operation between this data and a scrambling pattern to be generated from the scrambling initial value, which is set, for each bit.
However, the input data was all “0”, so the first 7 bits of the output of the EXOR 116b are the same as the scrambling initial value, which is set. On the reception side, which received this data, the 7 bits of this field can be used as a descrambling initial value.
FIG. 38 illustrates the configuration of around the descrambler 158 on the reception side in detail. Reception data is descrambled by calculating an exclusive-OR between the reception data and a descrambling pattern generated by descrambling pattern generator 158a with a later-described method at an EXOR 158b within the descrambler 158 following the reception data being subjected to error correction decoding at the decoder 156. The output thereof is deciphered at the decipher device 160.
When starting descrambling, the 7 bits from the MSB side of the Service field disposed in the headmost of the received DATA field are extracted as the initial value thereof, and are set within the register of the descrambler body made up of a shift register such as shown in FIG. 34, and a descrambling pattern is generated while the value within the register is shifted for each clock.
Thus, with IEEE802. 11a, the initial value 7 bits of scrambling are configured so as to be transmitted using the Service field within the PHY header section, which is not scrambled, and thus, the same scrambling initial value and the same descrambling initial value can be shared between transmission and reception, thereby performing scrambling and descrambling correctly. However, a method for transmitting the scrambling initial value thus using the data section is equivalent to a method for storing data different from the user data, which a user actually wants to send, in the data section and transmitting this, so transmission efficiency is deteriorated by the difference thereof, which is not preferable.
Also, with the transmission format of IEEE802. 11a, one OFDM symbol for transmitting the SIGNAL field employs the BPSK modulation of which required S/N is low, and encoding rate=½, but the DATA field including a scrambling initial value tends to be transmitted with a modulation method of which required S/N is higher than the above S/N, and encoding rate higher than the above rate, so bit error is readily caused.
Also, as for the SIGNAL field and the entire data, error correction is performed at a physical layer using convolution codes. As for a decoding method of convolution codes, the Viterbi decoding method has been known.
On the other hand, even if such error countermeasures are performed, it is difficult to completely correct an error, and accordingly, it is necessary to request the transmission device side of retransmission in the event of detecting an uncorrectable error on the reception device side. Increase of such repetitions of retransmission may affect communication speed. For example, when performing streaming transmission of moving image data or the like such as QoS (Quality of Service), it becomes difficult to reserve a communication band and assure a constant speed. Accordingly, in order to deal with the QoS function, as for MAC (Medium Access Control) sublayer data, i.e., the PSDU of data, it can be conceived to implement error correction processing as a MAC sublayer. Block codes such as Reed-Solomon product codes can be employed for such error correction.
However, even if error correction corresponding to the QoS is implemented to the MAC sublayer, synchronization between scrambling and descrambling is not normally performed in the event that countermeasures are not performed regarding the Service, the PSDU may not be descrambled normally on the reception device side. In the event that descrambling is not normally performed, consequently the transmission side is requested of retransmission of data, and even if an error correction corresponding to the QoS is implemented, the effect thereof cannot be obtained. As described above, it can be conceived that transmission in a modulation mode having high error tolerance is not assured regarding the Service, so the Service has high possibility to cause an error as compared to the SIGNAL.
As one method for preventing deterioration of transmission efficiency due to notification of a scrambling initial value, a technique for referring to an MAC address, and employing a part thereof as a scrambling initial value has been proposed (e.g., see Patent Document 2). In this case, it is not necessary to bother to transmit a scrambling initial value using the data section.
However, with this method, the MAC address itself cannot be scrambled, which causes a problem from the perspective of concealment. Also, that the MAC address itself cannot be scrambled means that if “0” and “1” within the data form a biased distribution during transmission of the MAC address, it cannot be converted into a random bit sequence, and consequently, the possibility that linear components occur in the spectrum of that zone remains.
Further, with IEEE802.11a, the MAC header including the PHY header, the MAC address, and the like is clearly separated, and in the event of applying the above scrambling notification method to such a wireless communication system, in addition to the MAC address being described in the DATA section, further a part of the MAC address is used for the PHY header section, which causes redundancy.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 2000-269944
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 8-107414
[Non-patent Document 1]
International Standard ISO/IEC 8802-11: 1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
[Non-patent Document 2]
ETSI Standard ETSI TS 101 761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part1: Basic Data Transport Functions
[Non-patent Document 3]
ETSI TS 101 761-2 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part2: Radio Link Control (RLC) sublayer
[Non-patent Document 4]
Supplement to IEEE Standard for Information technology-Telecommunications and information exchange between systems-Local and metropolitan area networks-Specific requirements-Part11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHZ Band