Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
2000-07-17
2003-12-02
Vo, Don N. (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C375S355000
Reexamination Certificate
active
06658063
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a radio packet communication receiver system using orthogonal multi-carrier modulation system, in particular, relates to such a receiver system which detects reference received timing accurately under severe environment of large carrier frequency offset between a transmit side and a receiver side, and/or multi-path propagation, and/or presence of thermal noise.
An orthogonal multi-carrier modulation system transmits high rate signal by using a plurality of sub-carriers each satisfying orthogonal relation, and called OFDM (Orthogonal Frequency Division Multiplexing) system. An OFDM signal can be modulated or demodulated by using (Inverse) Fast Fourier Transform ((I)FFT) circuit. An OFDM system has a feature that an inter-symbol interference is avoided if multi-path delay is within a guard interval, which is added to each OFDM signal so that it carries an OFDM signal cyclically. Therefore, an OFDM system is excellent to combat multi-path environment in high rate transmission.
Because of the excellent multi-path immunity, a radio computer network communication (radio LAN) is expected to use an OFDM system. In a computer network communication, data length is not fixed, and a packet signal in which received timing is indefinite is used. In radio transmission of such a packet signal, burst reception process is essential for independent synchronization of each received packet, including determination of reference timing of a received symbol.
A technical standard IEEE802.11a is one of the radio LAN's using OFDM modulation system. That technical standard can satisfy high rate transmission higher than 20 Mbit/sec.
FIG. 11A
shows a packet format which uses an OFDM modulation scheme, having a preamble for synchronization, a preamble for channel estimation, and a plurality of OFDM signals OFDM
1
, OFDM
2
, OFDM
3
, , , . An preamble for channel estimation, and each OFDM signals comprises a data following a guard interval (GI
2
, GI). A guard interval has a repetition of waveform at the end of the following OFDM data cyclically. A preamble for synchronization comprises a plurality of repetitive known data patterns called a short interval (in the embodiment,
10
short intervals t
1
through t
10
are provided). A preamble for channel estimation comprises known data patterns T
1
and T
2
following a guard interval G
12
, for demodulating sub-carriers (channel demodulation for coherent detection). Each OFDM signal is used for carrying transmission data. At least one of the OFDM signals (for instance OFDM
1
) is used for showing a property (modulation scheme, transmission rate, length of a packet et al) of succeeding OFDM signals.
A preamble for synchronization (t
1
through t
10
) is used for synchronizing receiver carrier frequency with transmitter carrier frequency, and defining a reference timing of a receiver.
The present invention provides accurately a reference timing of a preamble for synchronization. The reference timing is at a rear end of the short timing t
10
. By using the accurate reference timing of the preamble for synchronization, a carrier frequency of a receiver is synchronized with a carrier frequency of a transmitter, a guard interval is removed from each OFDM signal for Fourier transformation, and coherent detection of each OFDM signal is carried out for demodulating each sub-carriers.
FIG. 11B
shows a block diagram of a prior art of an OFDM packet communication receiver. A received signal R is a radio packet signal shown in
FIG. 11A. A
received signal R is applied to a correlator
301
which has coefficient of a known pattern (short preamble) of a preamble for synchronization, so that the correlator
301
provides a high level of correlation output signal B when the known pattern (short preamble) is received.
The correlation output signal B is high and has period of the short preamble when a preamble for synchronization is received, and said signal B is low when other signals are received. The correlation output signal B is applied to a timing decision circuit
303
, which recognizes the presence of a preamble for synchronization when the correlation output signal B exceeds a threshold level, and recognizes the end of the preamble for synchronization when the correlation output signal B is decreased lower than another threshold level after repetition period of the preamble for synchronization. Then, a reference timing signal D, or a symbol timing signal D is obtained. As a received signal R and an output of the correlator
301
are complex signal having a real part and an imaginary part, the correlation output signal must be converted to a scalar signal when it is applied to the timing decision circuit
303
.
A timing decision circuit
303
in a prior art has a delay circuit
37
having delay time T (T=t
1
=t
2
=- - - =t
10
) coupled with a correlation output signal B, a first threshold circuit
39
coupled with an output of said delay circuit
37
, a second threshold circuit
40
coupled with a correlation output signal B, and a logic circuit
43
coupled with an output of two threshold circuits to provide a symbol timing signal D when the first threshold circuit
39
shows that an output of the delay circuit
37
is higher than a first threshold level and the second threshold circuit
40
shows that a signal B is lower than a second threshold level.
Thus, the correlation circuit
301
and the timing decision circuit
303
compose a symbol timing detection block
10
A. The symbol timing signal D is applied to a frequency offset compensation circuit
20
and a guard interval remove circuit
4
. The frequency offset compensation circuit
20
recognizes repetition signals in a received signal R, or a preamble for synchronization, so that it measures carrier frequency offset between a transmit side and a receiver side by measuring phase rotation between repetition waveforms, and compensates said carrier frequency offset .
An output signal A of the carrier frequency offset compensation circuit
20
is applied to a guard interval removal circuit
4
, which removes a guard interval GI or GI
2
in an OFDM signal in an output A of the frequency offset compensation circuit
20
.
After removal of a guard interval, an OFDM symbol E which has no guard interval is applied to a Fourier transform circuit
5
which provides sub-carrier vectors F of each sub-carriers. The sub-carrier vectors F are applied to a coherent detection circuit
6
for coherent detection of each sub-carriers to provide coherent detected signal G. Further, the signal G is applied to a code decision circuit
22
for deciding a code
0
or
1
to provide a received data G
2
.
By the way, a received signal in radio communication is subject to thermal noise undesirably generated in a receiver amplifier, and/or undesired interference noise. Further, a propagation path is a combination of multi-paths including a direct path and an indirect path reflected by a wall. An OFDM system can provide a high quality transmission even under a multi-path propagation, because of the presence of a guard interval.
However, in an OFDM system in a radio packet communication, a synchronization including decision of a received symbol timing must be established for each packet independently, and the synchronization must be accurate for enjoying to combat multi-path propagation in an OFDM system. Further, a preamble for synchronization in radio packet signal is preferably as short as possible for high rate transmission, and synchronization process is preferably as quick as possible by using a short preamble signal.
A prior burst OFDM receiver has a correlator for recognizing a preamble, and a timing decision circuit for deciding the presence of a preamble in an output of the correlator. However, an accurate decision of a symbol timing would be difficult or a symbol timing would be erroneously decided under large noise environment, and/or multi-path propagation with many delayed waves.
Further, a decided symbol timing is used for compensatin
Kumagai Tomoaki
Mizoguchi Masato
Morikura Masahiro
Onizawa Takeshi
Armstrong Kratz Quintos Hanson & Brooks, LLP
Nippon Telegraph and Telephone Corporation
Vo Don N.
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