SPECIFICATION WIRELESS COMMUNICATION APPARATUS TECHNICAL FIELD [0001] The present invention relates to a wireless communication apparatus for carrying out reception processing by performing frequency conversion on a wireless signal that hops among frequencies. Inparticular, the invention relates to a wireless communication apparatus for carrying out reception processing by performing frequency conversion on a multiband OFDM signal of which center frequency hops at predetermined band intervals. [0002] More specifically, the invention relates to a wireless communication apparatus for carrying out reception processing byperf orming frequency conversion on amultiband OFDM_UWB signal that switches frequencies over a wide band. In particular, the invention relates to a wireless communication apparatus for resolving a self-mixing problem in performing frequency conversion on a received signal by direct conversion in a multiband OFDM_UWB communication scheme. BACKGROUND ART [0003] Canonical standards concerning wireless networks include IEEE (The Institute of Electrical and Electronics Engineers) 802.11, HiperLAN/2, and IEEE802.15.3 . [0004] Further, in recent years, attention is focused on the "ultra-wideband (UWB) communication" as a wireless communication system capable of short-distance, ultrafast transmission. The system performs wireless communication by carrying information on ultrashort pulse waves less than one nanosecond over a very wide frequency band without using carriers . It is expected to put the system into practical use . At present, in IEEE802 .15 .3 and the like, a data transmission scheme having a packet structure including a preamble is being developed as an access control scheme for ultra-wideband communication. [0005] It is anticipated that WPAN (Wireless Personal Access Network) represented by the UWB as short-distance communication will be employed in various kinds of household electrical appliances and CE (Consumer Electronics) devices in the feature, and home networks and P-to-P transmission exceeding 100 Mbps between CE devices are expected to be achieved. If the use of millimeter wave bands becomes widespread, it becomes possible to achieve short-distance wireless communication exceeding 1 Gbps and also an ultra-high-speed DAN (Device Area Network) for short-distance communication including a storage device etc. [0006] A wireless network constructed in a room forms a multipath environment in which a receiver receives a combination of a direct wave and a plurality of reflected/delayed waves. Multipath generates a delay distortion (or frequency-selective fading) to cause a communication error. Further, a delay distortion causes intersymbol interference. [0007] A major countermeasure against delay distortion can be a multicarrier transmission scheme. According to the multicarrier transmission scheme, transmission data is transmitted by dividing it into a plurality of carriers having different frequencies. Each carrier uses a narrow band and is hardly subject to frequency-selective fading. [0008] For example, the OFDM (Orthogonal Frequency Division Multiplexing) scheme, which is a typical multicarrier transmission scheme, configures a frequency of each carrier so that the carriers become orthogonal to each other in a symbol period. During information transmission, the scheme converts serially transmitted information into parallel information at every symbol cycle lower than the information transmission rate . The scheme allocates a plurality of pieces of output data to carriers, modulates the amplitude and the phase for each carrier, transforms the carriers into signals along the time domain while maintaining the orthogonal ity of each carrier along the frequency domain by performing an inverse FFT on the multiple carriers, and transmits the transformed signals. The reception occurs in the reverse order of the transmission. The scheme performs an FFT to transform signals along the time domain into those along the frequency domain and demodulates the carriers in accordance with the modulation of each carrier. The scheme performs parallel-serial conversion to reproduce the information originally transmitted in serial form. [0009] The OFDM modulation scheme is adopted as a wireless LAN standard in the IEEE802.lla/g, for example. The IEEE802.15.3 standardization is also in progress for the UWB communication scheme using the OFDM modulation scheme in addition to the DS-UWB scheme and the impulse-UWB scheme. The DS-UWB scheme increases spread speeds of DS information signals to the utmost limit. The impulse-UWB scheme uses impulse signal sequences having very short periods of several hundred picoseconds to configure information signals for transmission and reception. For example, investigation is being made on multiband OFDM_UWB modulation that performs frequency hopping (FH) for a frequency band of 3.1 through 4.8 GHz into a plurality of 528-MHz-wide subbands anduses IFFT/FFT having 128-point frequency bands (e.g. , see non-patent document 2). [0010] Direct spread (DS) and frequency hopping (FH) are a spread spectrum scheme that performs multiple access by assigning a different spread code to each communication channel and uses a very wide communication bandwidth with respect to information signals. The scheme performs primary modulation such as OFDM and'"secondary modulation with the spread. [0011] FIG. 7 shows frequency allocation stipulated in the multiband OFDM_UWB communication scheme. As shown in FIG. 7, there are group 1 composed of bands #1 to #3 having center frequencies of 3432 MHz, 3960 MHz and 4488 MHz respectively, group 2 composed of bands #4 to #6 having center frequencies of 5016 MHz, 5548 MHz and 6072 MHz respectively, group 3 composed of bands #7 to #9 having center frequencies of 6600 MHz, 7128 MHz and 7656 MHz respectively, group 4 composed of bands #10 to #12 having center frequencies of 8184 MHz, 8712 MHz and 9240 MHz respectively, and group 5 composedof bands #13 and#14 having center frequencies of 9768 MHz and 10296 MHz respectively. It is mandatory to employ the three bands of group #1 out of these groups . The other groups and bands are reserved for future expansion. [0012] FIG. 8 exemplifies a block diagram of a receiver used in a multiband OFDM system (e.g. , see non-patent document 6) . The receiver shown in FIG. 8 employs a direct conversion scheme for frequency conversion of a received signal. The direct conversion scheme which does not have an intermediate-frequency (IF) stage, amplifies a signal received by an antenna and multiplies the amplified signal by a local frequency so as to perform direct frequency conversion on a baseband signal. The direct conversion scheme facilitates a wider band of a receiver and thereby increases the structural f lexibilityof the receiver. [0013] In the example of FIG. 8, local (LO) signals cos (2πfc) and sin(2πfc) of the same frequency as the center frequency of an RF signal are used for frequency conversion of the received signals of the I-axis and the Q-axis. After the frequency conversion, the low frequencies are extracted by low pass filters (LPF) and amplified by variable gain amplifiers (VGA). The amplified signals are converted from analog to digital form. Further, the time-domain signals are transformed into the frequency-domain signals by FFT, and the carriers are demodulated for the reproduction of information originally transmitted in serial form. [0014] The direct-conversion receiver as shown in FIG. 8 requires three local frequencies of 3432 MHz, 3960 MHz and 4488 MHz which are the same frequencies as the center frequencies of RF signals, for example in the case of using the bands of group 1 shown in FIG. 7. [0015] The adoption of the direct conversion scheme facilitates a wider band of a receiver due to no use of an IF filter and thereby increases the structural flexibility of the receiver. However, in the direct conversion scheme, since a received frequency and a local frequency are identical, there has been a problem that a direct-current component, i.e., a DC offset occurs due to a local signal' s self mixing (LO self mixing) (e.g. , see non-patent document 3). [0016] As shown in FIG. 9, LO self-mixing occurs when part of the local signal leaks toward the antenna from the receiver proper, reflects on the antenna, returns to the receiver, and is multiplied by the local signal itself at amixer. Alternatively, there are cases where after part of the local signal is emitted outside through the antenna, the reflected wave is received by the antenna and mixed with the local signal . [0017] For example, assuming that the amplitude of a local signal in FIG. 9 is 0.5V, the total gain of the low noise amplifier (LNA) and the mixer is 30 dB, and the leakage of a local signal reflects on the antenna and returns to point A in FIG. 9 with an attenuation of -70 dB, the DC offset of the output of the mixer is 2.5 mV. On the other hand, since the signal level of a desired wave is a minimum of about -74 dBm, the output of the mixer is -44 dBm =1.4 mVrms . As described, the DC offset is greater than the signal level of a desired wave. [0018] The process of occurrence of a DC offset is expressed by the following equation. In the equation, cos (cot) denotes a local signal , and a and § denote the amplitude and phase of a reflected wave that returns to the mixer, respectively. The first term on the right-hand side of the equation indicates a DC offset, and the second and third terms indicate double -frequency components. It is understood that the DC offset varies with the amplitude and phase of the reflected wave. [0019] [0020] Since frequency hopping (FH) is performed in a multiband OFDM communication system as described above, the frequency of a local signal changes with every frequency hopping. The reflection coefficient of the antenna also varies with a frequency; therefore, the DC offset caused by self-mixing also changes with frequency hopping. Since frequency hopping occurs at the same frequency of 3.2 MHz as an OFDM symbol rate, the DC offset changes stepwise in a cycle of 1/3.2MHz=312.5ns, as shown in FIG. 10. [0021] The elimination of the DC offset is performed generally by using a method for inserting a capacitor in series with the output of the mixer that multiplies a received signal by a local frequency. Inthiscase, asshowninFIG. 11,capacitorCandcircuitimpedance R constitute a primary high-pass filter (HPF). The cutoff frequency of a frequency response is I/ (27TCR) , and the convergence time of a step response is 27ICR. [0022] Since the subcarrier frequency of the multiband OFDM system is 4 .125 MHz, it is desirable that the direct-conversion receiver passes frequencies down to 4.125 MHz. On the other hand, it is desirable to reduce the convergence time of the step response of a DC offset to about 1/10 of the OFDM symbol rate (about 30 ns) . However, if the cutoff frequency is 4.125 MHz, the convergence time of the step response becomes as long as 242 ns (=1/4.125MHz) as shown in FIG. 12. Accordingly, there is a troublesome problem that most of the OFDM symbol time is consumed for the step response. Lowering the cutoff frequency prolongs the response convergence time. Consequently, the DC of f set does not fall to zero within one symbol, thereby affecting the next symbol. [0023] In the case of a time division multiplex wireless system, it is possible to detect and store a DC offset during a period of time other than a time slot assigned to a receiver and perform DC offset elimination by subtracting the stored DC during the time slot assigned to the receiver. [0024] In the case of a frequency divisionmultiplexwireless system, it is possible to detect and store a DC offset during a preamble period of a received frame and perform DC offset elimination by subtracting the stored DC during the other periods. [0025] Conventionally, Bluetooth communication is known as a wireless system for performing frequency hopping. In this case, there is a 72-bit access code before a payload, and the head portion thereof has a 4-bit preamble for detecting a DC offset (see FIG. 13) . Therefore, by switching a time constant with an additional circuit for reducing the charge and discharge time of a capacitor, it is possible to eliminate a DC offset by using the relatively short preamble time. [0026] On the other hand, in the mult iband OFDM communication system, frequency hopping is performed at each symbol and a frequency switching time is only less than 10 ns; therefore, it is necessary that it is within such a short switching time that a DC offset is detected and eliminated. However, it is extremely difficult to detect and eliminate a DC offset within such a short switching time. That is, the problemof aDC of f set inperforming frequency conversion on a received signal by direct conversion is particularly remarkable in performing frequency hopping in the multiband OFDM system. [0027] [Non-patent document 1] NIKKEI ELECTRONICS Mar. 11, 2002, pp. 55-66 "Ultra Wideband: Revolutionary Wireless Technology is Born" [Non-patent document 2] IEEE802.15.3a TI Document