CN101166167B - Channel Estimation Method and Device for Orthogonal Frequency Division Multiplexing 802.11 System - Google Patents

Abstract

The invention relates to a reflexive channel estimation method and its realization system at the receiving end of an Orthogonal Frequency Division Multiplexing (OFDM) 802.11 system, belonging to the field of wireless communication technology. The receiving end of the 802.11-OFDM system constructs a new extended training sequence after encoding and modulating the decoded information sequence, which is used for a new and more accurate channel after the FCS verification proves that the reception is correct. estimate. The result of the channel estimation will be used for signal detection or transmission of subsequent physical frames. The implementation system of reflexive channel estimation is composed of sequence divider, adder, delay register, counter and divider. While decoding the information sequence, the receiving end inputs it into the realization system to calculate the channel estimation. Finally, after the information sequence is verified to be correct by the FCS, the divider outputs the calculated channel estimation result to the receiving end for use.

Description

Channel Estimation Method and Device for Orthogonal Frequency Division Multiplexing 802.11 System

technical field

The invention belongs to the technical field of wireless communication, in particular to a channel estimation method and device for an Orthogonal Frequency Division Multiplexing (OFDM) wireless local area network system.

Background technique

With the increasing demand for information in social life, the network has increasingly become an indispensable part of people's daily life. Wireless local area network has broad development prospects because of its advantages such as flexible access and no need for wiring.

The IEEE802.11 working group has proposed its wireless local area network solution to the application requirements of the wireless local area network, which is the 802.11 wireless local area network standard. So far, there are four main standards for the physical layer of 802.11 wireless LANs, namely 802.11, 802.11b, 802.11g and 802.11a, and the standards for the media access (MAC) layer mainly include 802.11 and 802.11e. In terms of physical layer standards, 802.11 defines three working modes: frequency hopping spread spectrum, direct sequence spread spectrum and infrared; 802.11b expands the direct sequence spread spectrum working mode of 802.11 to make the highest data rate of the physical layer reach 11Mbps (the former 802.11g further expands 802.11b. On the basis of being compatible with 802.11b, OFDM working mode is added, and the maximum data rate of the physical layer can reach 54Mbps. Both 802.11g and 802.11b work at 2.4G Frequency band; the 802.11a standard works in the 5G frequency band, adopts the OFDM working method, and has a maximum data rate of 54Mbps at the physical layer; the OFDM working method of 802.11g adopts exactly the same implementation method as 802.11a, and the difference lies in the working frequency band. In terms of MAC layer standards, 802.11 defines the ways in which working stations occupy channels in 802.11 networks, that is, occupying channels through random competition (DCF) and APs occupying channels through central control (PCF). 802.11e is mainly for 802.11 networks. The solution to the user quality of service problem is proposed, and the HCF working method is proposed to realize the user service quality in LAN communication.

In the 802.11-OFDM (including 802.11a and 802.11g OFDM) system, the channel estimation at the receiving end is realized through the long training symbol in the header of the physical frame. Since each channel estimation can only be realized by two samples on each subcarrier channel, when the channel condition is poor, there will be a large deviation in the channel estimation, which will affect the performance of the receiving end to decode the data packet.

Contents of the invention

A channel estimation method for an OFDM 802.11 system provided by an embodiment of the present invention includes:

The receiving end uses the long training sequence symbols in the physical frame to perform channel estimation on the received physical frame;

Equalizing the data symbols in the received physical frame based on the channel estimation result, and then performing demodulation and decoding to obtain an information sequence;

Performing a frame check sequence FCS check on the information sequence;

After the verification is correct, re-encode and modulate the information sequence to obtain a new training sequence;

Divide the coded and modulated information sequence and the received subcarrier symbol sequence to obtain the current channel estimation result, and the channel estimation results of each OFDM symbol are accumulated and then averaged to obtain the total channel estimation result , when the FCS check is correct, the total channel estimation result is output.

An embodiment of the present invention provides a channel estimation device for an OFDM 802.11 system, including:

Sequential dividers, adders, delay registers, counters and dividers;

The sequence divider is used to divide the received subcarrier symbol sequence and the corresponding elements of the coded and modulated subcarrier symbol sequence to obtain the channel estimation result of the current Orthogonal Frequency Division Multiplexing OFDM symbol, wherein the coded and modulated The subcarrier symbol sequence is a new training sequence obtained by the method according to claim 1;

The adder adds the channel estimation result of the current OFDM symbol to the cumulative value of the channel estimation results of the previous OFDM symbols to generate a new cumulative value;

The delay register is used to store the cumulative value of the OFDM symbol channel estimation result up to the current moment, and it has a trigger terminal. After being triggered, the delay register uses the rising edge or falling edge of the trigger to output the current register data;

The counter is used to store the number of symbols of OFDM symbols that have participated in channel estimation;

The divider divides the accumulated value of the channel estimation and the number of OFDM symbols that have participated in the channel estimation to obtain the result of the channel estimation, which has an output control terminal, and performs the output of the divider according to the result of the FCS check. control;

When the FCS check is correct, the channel estimation result of the divider is output.

Preferably, in the Viterbi decoding at the receiving end, when the information bits decoded from an OFDM symbol are all located before the current backtracking position, the OFDM symbol is used for channel estimation.

To sum up, in the channel estimation scheme proposed by the present invention, after receiving the long training symbol, the receiving end uses it to perform channel estimation on each subcarrier channel of OFDM, and obtains the channel coefficient of each subchannel for channel equalization. The receiving end performs equalization on the received data symbols (and SIGNAL symbols), and then demodulates and decodes them to obtain the information sequence (and the information sequence in the SIGNAL symbol) transmitted by the data symbols. The received information sequence is checked by FCS (the information sequence in the SIGNAL symbol is checked with the 1-bit check bit contained in it), and after it is proved that the reception is correct, it is re-encoded and modulated as a new The extended training sequence is used for channel estimation, so that a more accurate estimation of channel coefficients of each subcarrier channel can be obtained. In this way, when the transmitting station continuously sends data packets, the channel estimation result can be used to equalize the subsequent data packets; or when the closed-loop working mode is adopted between the transmitting station and the receiving station, the receiving station can feed back the channel estimation result For the transmitting site, used for the transmission of subsequent data packets by the transmitting site.

Description of drawings

Figure 1802.11-OFDM system physical frame frame structure;

Fig. 2 implements the reflexive channel estimation method provided by the embodiment of the present invention;

Fig. 3 implements the reflexive channel estimation provided by the embodiment of the present invention;

FIG. 4 is a schematic diagram of the relationship between the received subcarrier symbol sequence, the information sequence, and the coded and modulated subcarrier symbol sequence in the embodiment of the present invention.

Detailed ways

The present invention considers the use of data symbols in transmitted data packets for reflexive channel estimation. When the transmitting station continuously sends data packets to the receiving station, the channel can be considered to remain unchanged for a period of time. Therefore, the data of the current data packet can be used as a training sequence to perform reflexive channel estimation on the current channel for subsequent data Packet signal detection. On the other hand, when the closed-loop working mode is adopted in the physical layer, the receiving end can obtain a relatively accurate channel estimation value through reflexive channel estimation, and then feed it back to the transmitting end. In the 802.11-OFDM system, in order to improve the performance of data transmission at the physical layer, a closed-loop working method may be considered.

The present invention considers the use of data symbols in transmitted data packets for reflexive channel estimation. When the transmitting station continuously sends data packets to the receiving station, the channel can be considered to remain unchanged for a period of time. Therefore, the data of the current data packet can be used as a training sequence to perform reflexive channel estimation on the current channel for subsequent data Packet signal detection. On the other hand, when the closed-loop working mode is adopted in the physical layer, the receiving end can obtain a relatively accurate channel estimation value through reflexive channel estimation, and then feed it back to the transmitting end for subsequent data transmission. In the 802.11-OFDM system, in order to improve the performance of data transmission at the physical layer, a closed-loop working method may be considered.

Figure 2 shows the implementation method of reflexive channel estimation in the 802.11-OFDM system. Its working principle is: after the decoded information sequence is verified as correct by FCS, it is re-encoded and modulated as a new extended The training sequence of is used for more accurate channel estimation. FCS is the English acronym for Frame Check Sequence, and the method of FCS check has been defined in the 802.11 protocol.

An 802.11-OFDM physical frame consists of three parts: frame header, SIGNAL symbol, and data symbol. The frame header is composed of short training symbols and long training symbols, as shown in Figure 1. Short training symbols are used for frame synchronization, timing synchronization, carrier coarse synchronization, etc. The subcarrier sequence of the long training symbol is a fixed symbol sequence containing 52 non-zero elements, which is used for channel estimation, carrier fine synchronization, etc. When it is used for channel estimation, each non-zero element corresponds to a channel estimate for one OFDM subcarrier channel. Since the long training symbol part of the 802.11-OFDM physical frame header only contains two long training symbols, when using it for channel estimation of OFDM subcarrier channels, only two samples can be used to realize each subcarrier channel The channel estimate of , when the channel condition is poor, there will be a large deviation in the channel estimate.

The implementation process of the reflexive channel estimation proposed by the present invention is shown in FIG. 2 . After receiving the long training symbol, the receiving end uses it to perform channel estimation on each OFDM subcarrier channel, and obtains the channel coefficient of each subchannel for channel equalization. The receiving end performs equalization on the received data symbols (and SIGNAL symbols), and then demodulates and decodes them to obtain the information sequence (and the information sequence in the SIGNAL symbol) transmitted by the data symbols. The received information sequence is checked by FCS (the information sequence in the SIGNAL symbol is checked with the 1-bit check bit contained in it), and after it is proved that the reception is correct, it is re-encoded and modulated as a new The extended training sequence is used for channel estimation, so that a more accurate estimation of channel coefficients of each subcarrier channel can be obtained. In this way, when the transmitting station continuously sends data packets, the channel estimation result can be used to equalize the subsequent data packets; or when the closed-loop working mode is adopted between the transmitting station and the receiving station, the receiving station can feed back the channel estimation result For the transmitting site, used for the transmission of subsequent data packets by the transmitting site.

The invention obtains the information sequence, that is, the MAC frame transmitted by the current data packet, by equalizing, demodulating and decoding the received data symbols. If the information sequence is proved to be correct by the FCS check in the MAC frame, it will be coded and modulated to construct a new lengthened training sequence for channel estimation of the current channel. Since the new training sequence is often much longer than the long training symbol, when it is used together with the long training symbol for channel estimation, the deviation of the channel estimation will be greatly reduced. The transmission and detection of the physical frame can significantly improve the performance of data transmission in the physical layer of the 802.11-OFDM system.

An embodiment of the present invention proposes a digital circuit implementation device for implementing reflexive channel estimation at a receiving end. In this implementation device, the input signal performs channel estimation on each sub-carrier channel in units of OFDM symbols, and the estimation results of each OFDM symbol are accumulated and then averaged to obtain a total channel estimation result. After the information sequence is verified to be received correctly by the FCS, the result of the channel estimation is output, and the receiving end completes the reflexive channel estimation.

The digital circuit implementation device of the reflexive channel estimation proposed by the embodiment of the present invention is shown in FIG. 3 . The implementing device performs channel estimation on OFDM sub-carrier channels in units of OFDM symbols. In Fig. 3, it is assumed that the number of OFDM symbols used for channel estimation is K (in the physical frame received by the receiving end, if the last OFDM symbol is not filled with transmitted data, it may not be used for reflexive channel estimation).

For any OFDM symbol k (k=1, 2, . . . , K), the relationship between the received subcarrier symbol sequence and the coded and modulated subcarrier symbol sequence is shown in FIG. 4 . Among them, the information sequence is the MAC frame transmitted by the OFDM data symbol, and its correctness can be verified by the FCS check; the equalizer works according to the channel coefficient provided by the channel estimation; the demodulator decodes the equalized 802.11-OFDM signal For demodulation, decoding, etc., it is composed of demodulator, deinterleaver, decoder, descrambler and other parts; the coding modulator performs coding and modulation operations on the information sequence according to the provisions of the 802.11-OFDM protocol. The transmitted symbol sequence is reconstructed, which is composed of a scrambler, an encoder, an interleaver, a modulator, and the like.

The digital circuit realization device of reflexive channel estimation shown in Fig. 3 is composed of a sequence divider, an adder, a delay register, a counter and a divider. Among them, the sequence divider realizes the division of the corresponding elements of the two input sequences, and the delay register controls the output by the rising or falling edge of the trigger pulse, so there is a delay between its input and output. The input of the sequence divider is the received subcarrier symbol sequence and the coded and modulated subcarrier symbol sequence, and the output is the channel estimation result of the current OFDM symbol. While outputting the current channel estimation result, the sequence divider also outputs a trigger pulse. On the one hand, the trigger pulse is output to the trigger end of the delay register, so that it outputs the current register content to the adder, and on the other hand, it outputs to the counter The input terminal makes the counter generate a count. The delay register only gives an output when the edge of the trigger pulse arrives, and then closes the output. The delay register stores the cumulative value of the channel estimation results obtained by each OFDM symbol for channel estimation up to the current moment, and the counter stores the number of OFDM symbols that have participated in the reflexive channel estimation, which is obtained by dividing the two values The quotient (that is, the result of division in the divider) is the result of channel estimation up to the current moment. Each time a pair of "received subcarrier symbol sequence-coded and modulated subcarrier symbol sequence" sequence pair is input to the input terminal of the sequence divider, an updated channel estimation value will be obtained in the subsequent divider. When the information sequence is verified to be correct by the FCS, the channel estimation value in the divider is allowed to be output. In the device for implementing channel estimation, the initial values of the delay register and the counter are both set to 0. If the estimation result of the long training symbol is to be included in the reflexive channel estimation, two " The received long training symbol sequence-long training symbol sequence" sequence pair is first input to the sequence divider, so that the result of the previous channel estimation is generated in the divider at the output end of the circuit.

Using the implementation device shown in FIG. 3 , the reflexive channel estimation can be performed at the receiving end while detecting the received sequence. When using a Viterbi (Viterbi) decoder to decode the received sequence, in order to save the buffer, a backtracking technique is generally used when making a decision on the signal. Assuming that the backtracking length is L, the Viterbi decoder also makes judgments on information bits that are L away from the current information bit when calculating the probability of the current information bit and each possible path. Therefore, at each moment of Viterbi decoding, the information bit before its backtracking position has actually been determined. In reflexive channel estimation, if we judge that all the information bits of a certain OFDM symbol are located before the current backtracking position, we can use this OFDM symbol for reflexive channel estimation. The advantages of this real-time channel estimation are: (1) it can reduce the calculation delay; (2) it can reduce the buffer required by the receiving end and save the hardware cost of the receiving end. When the transmitting station and the receiving station work in a closed-loop mode, the receiving station may be required to transmit channel information to the transmitting station in response to the current MAC frame. At this time, the calculation of the reflexive channel estimation cannot have excessive delay.

The device for implementing reflexive channel estimation shown in Figure 3 obtains the result of channel estimation under the least squares criterion. The output shown is multiplied by a constant coefficient to obtain the corresponding estimated result.

The above-described embodiments are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the within the protection scope of the present invention.

Claims (3)

1. a channel estimation methods that is used for OFDM 802.11 systems is characterized in that, comprising:

Receiving terminal utilizes the long training sequence symbol in the physical frame that the physical frame that receives is carried out channel estimating;

Based on described channel estimation results data symbol in the physical frame that receives is carried out equilibrium, carry out demodulation sign indicating number, acquired information sequence again;

Described information sequence is carried out Frame Check Sequence FCS verification;

Coded modulation is carried out again to described information sequence in the correct back of verification, obtains new training sequence;

The sub-carrier sequence that receives and the information sequence after the coded modulation be divided by obtain current channel estimation results, the channel estimation results of each orthogonal frequency division multiplex OFDM symbol remakes after adding up on average, obtain total channel estimation results, when FCS is verified as when correct, this total channel estimation results is output.

2. a channel estimating apparatus that is used for OFDM 802.11 systems is characterized in that, comprising:

Sequence divider, adder, delay time register, counter and divider;

Described sequence divider, being used for the sub-carrier sequence that will receive and the corresponding element of the sub-carrier sequence after the coded modulation is divided by, obtain the channel estimation results of current orthogonal frequency division multiplex OFDM symbol, wherein the sub-carrier sequence after the coded modulation is the new training sequence that adopts the described method of claim 1 to obtain;

Described adder is carried out addition with the channel estimation results of current OFDM symbol and the accumulated value of the channel estimation results of each OFDM symbol before this, produces new accumulated value;

Described delay time register is used to be stored in the accumulated value of OFDM symbol estimated result till the current time, and it has a trigger end, after being triggered, and rising edge that the delay time register utilization triggers or the current register data of trailing edge output;

Described counter is used to deposit the symbolic number of the OFDM symbol that has participated in channel estimating;

Described divider is divided by the accumulated value of channel estimating and the OFDM symbolic number that participates in channel estimating, obtains the result of channel estimating, and it has an output control terminal, according to the result of FCS verification, the output of described divider is controlled;

When FCS is verified as when correct, the channel estimation results of described divider obtains output.

3. device according to claim 2 is characterized in that, in the Viterbi Viterbi of receiving terminal decoding, when the information bit that is solved by an OFDM symbol all be positioned at current recall the position before the time, this OFDM symbol is used for channel estimating.

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