Methods and apparatuses for channel estimation for wideband ofdm communication systems

The present invention generally relates to wireless communication systems, specifically channel estimation in an Orthogonal Frequency Division Multiplexed wireless communication system.

Technology related to information and communication has undergone rapid development over the past few decades. For example, in the last 20 years, wireless communication technology has shifted from offering products that are initially regarded as novel items to offering products that are the basic means of mobile communications. Perhaps the most influential of these wireless technologies were mobile phone systems and products. Mobile phone technology has emerged to bring mobile scalability to existing wired communications systems, providing users with anywhere coverage using traditional circuit-switched wireless paths. However, recently, wireless communication technology has replaced wired connections in almost all areas of communication. Wireless Local Area Networks (WLANs) are rapidly gaining popularity as an alternative to traditional wired networks in both homes and offices.

Many of today's WLAN systems operate according to IEEE 802.11 standards, such as the IEEE 802.11a or 802.11g standards. These standards specify the transmission of signals using Orthogonal Frequency Division Multiplexing (OFDM). In an OFDM system, the signal is divided into multiple narrowband channels, each transmitted at a different frequency. Recently, OFDM has been proposed as an access methodology for ultra-wideband (UWB) systems. For example, multiband OFDM has been proposed for use in UWB systems that spread data over a very wide bandwidth, say about 500MHz, at high data transfer rates with low power, for example up to 480Mb / s. This combination of the use of a wider spectrum and even lower power improves speed and reduces interference in other radio spectra.

Like other wireless communication systems, UWB systems need to address certain technical challenges. One such challenge involves channel estimation. Channel estimation generally describes a technique used by digital receivers to estimate the effect of a radio channel on a transmitted signal, so the receiver is at the same time one of the processes for recovering transmitted data. As a part, those effects can be compensated.

As described in detail below, an exemplary UWB system includes a channel estimation sequence as part of the preamble transmitted at each frame. One way to perform channel estimation in a UWB system is to determine the frequency response characteristics associated with the received version of the channel estimation sequence and take the average of the two such frequency response characteristics as the current channel estimation. Is to use. However, this technique results in a mounting loss of about 1.7 dB, which adversely affects overall system performance. However, using more complex channel estimation techniques for channel estimation sequences results in very high data rates in UWB systems (and the need for high levels of processing power to perform complex calculations). Given that, it does not provide a viable solution.

<p> Therefore, it would be desirable to provide techniques and equipment for performing channel estimation of UWB transmitters and receivers that provide satisfactory performance without the problems of conventional techniques.</p><p> The systems and methods according to the invention address this and other needs by providing channel estimation methods, systems, and devices that determine coarse and fine channel estimates. Coarse channel estimation is determined based on the channel estimation sequence sent to the receiver and can then be used to detect header symbols. The header symbol may be used to calculate additional channel estimates that can be combined with coarse channel estimates to determine fine channel estimates.</p>

<p> According to one exemplary embodiment of the invention, the method of wireless communication is a step of receiving a frame of data, the frame including a preamble portion, a header portion, and a data portion, and a preamble of the frame. One step is to use the channel estimation symbol received in the part to determine the coarse channel estimation, and the other is to process the header part of the frame of the data using the coarse channel estimation to generate the received header symbol. Includes steps to determine fine channel estimates based on coarse channel estimates and header symbols, and steps to equalize the data portion of the frame using fine channel estimates.</p><p> According to another exemplary embodiment of the invention, the transmitter / receiver is a receiver that receives a frame of data, the frame being a receiver that includes a preamble portion, a header portion, and a data portion, and a frame. The channel estimation symbol received in the preamble part is used to determine the coarse channel estimation, and the coarse channel estimation is used to process the header part of the frame of the data to generate the received header symbol, and the coarse channel It includes a processor that determines fine channel estimates based on the estimated and received header symbols and an equalizer that uses fine channel estimates to equalize the data portion of the frame.</p><p> The accompanying drawings illustrate exemplary embodiments of the invention.</p><p> Subsequent detailed description of the present invention will refer to the accompanying drawings. The same reference numbers in different drawings indicate the same or similar elements. Moreover, the following detailed description does not limit the present invention. Instead, the scope of the invention is defined by the appended claims.</p>

To partially illustrate the context of this description, an exemplary WLAN system is first described with respect to FIG. However, those skilled in the art will appreciate that the present invention is not limited to implementation in WLAN systems. Among them, the wired network 10 (eg, an Ethernet network) has a file server 12 and a workstation 14 connected thereto. Those skilled in the art will appreciate that a typical wired network serves a large number of fixed workstations 14, but only one is shown in Figure 1 for brevity. Wired network 10 is also connected to WLAN 16 via router 18. The router 18 interconnects the access point (AP) of the WLAN 16 with the wired network, which can communicate with, for example, the file server 12 through the wired network. In the example WLAN system in Figure 1, three cells 20, 22, 23 (sometimes BSS-Basic Service Set-) or Basic Service Elea (BSA-Basic Service). Area-)) is shown with each AP, but again those skilled in the art will appreciate that more or fewer cells may be provided to WLAN16. Within each cell, each AP serves a large number of radio stations (W) over a wireless connection. Note that the radio station W may be, for example, a personal computer, a personal digital assistant, a camera, a cell phone, or any other device capable of communicating with the system via a UWB connection.

According to an exemplary embodiment according to the invention, transmission of a signal between the AP and each radio station W is performed using an OFDM signal, eg, in accordance with IEEE P802.15-03 / 268r2 in November 2003. Will be done. Those skilled in the art will appreciate that this particular standard specification is merely used as an example of an OFDM physical layer that can be used in conjunction with the present invention, and that other radio standards or formats may be adopted with it. There will be. Devices and methods according to exemplary embodiments of the invention provide techniques for receiving such OFDM signals and performing channel estimation on them.

An exemplary format for data transmission according to an exemplary embodiment of the invention is shown in FIGS. 2 (a) and 2 (b). In FIG. 2 (a), an OFDM frame format generally including a preamble 30, a header 32, and a variable length payload (data) section 34 is shown. Details on these various fields can be found in the IEEE standard proposals mentioned above. Figure 2 (b) shows the preamble section 30 in detail. Among them, channel estimation sequence 36 has six OFDM symbols C.₀~ C₅It can be seen that it contains. Three channel hopping sequences (ie, periodic changes in subcarriers over the same logical channel) are used in this exemplary UWB system. Therefore, for a given frame of data, depending on the particular subcarrier adopted to transmit a particular frame, two of the six channel estimation symbols (eg C).₀And C₃, C₁And C₄, Or C₂And C₅) Can be used to perform channel estimation. These channel estimation symbols are predefined and proactively known by the receiver. Therefore, as mentioned above, one technique for performing channel estimation in a receiving frame of data with the formats shown in FIGS. 2 (a) and 2 (b) is to receive the appropriate set of channel estimation sequence symbols. Determining the associated frequency response characteristics and performing averaging on those frequency response characteristics to determine the channel estimation. An example of the use of averaging to perform channel estimation is found in US Pat. No. 5,432,816, which is incorporated herein by reference. However, as mentioned above, the use of this technique alone can result in a mounting loss of about 1.7 dB.

Therefore, according to an exemplary embodiment of the invention, channel estimation can be performed using another technique described below in conjunction with the flow diagram of FIG. Among them, the OFDM frame is first parsed into its component parts, such as the preamble 30, header 32, and payload data 34. Due to this specification, the description of packet and frame synchronization is not particularly relevant, and therefore the processing associated with these parts of the preamble 30 shown in FIG. 2 (b) is not described further. To determine the channel distortion based on the comparison between the received version of the appropriate channel estimation symbol set and the stored known values of this channel estimation symbol set, the rest of the preamble 30 is, for example, a least squares technique. Is processed in block 40 as described above. Alternatively, other techniques (eg, LMS-Least Mean) to perform channel estimation in block 40 Square-) averaging, maximum likelihood, etc.) may be used. Regardless of the particular technique used to analyze the reception of the channel estimation sequence portion of the preamble 30, block 40 results in the channels shown herein as "coarse channel estimates" for this particular OFDM frame. It brings the output of the estimation function 40. Coarse channel estimates are then used to decode the header symbols and to generate finer channel estimates as described below.

Header section 32 is first processed in blocks 42-46. Among them, the cyclic prefix (CP-Cyclic Prefix-) removal function 42 removes the CP associated with the OFDM signal. The phase distortion associated with, for example, the frequency offset is then compensated by the phase rotation function 44. The header section 32 is then transformed into the frequency domain by the Fast Fourier Transform function 46 so that it can be multiplied by a coarse channel estimate in block 48. The frequency domain output of the FFT46 is also supplied to another multiplier 50, as described below. When the header section 32 is compensated for the channel effect by the multiplier 50, the information contained therein is deinterleaved at block 52 and decoded / demodulated using, for example, the Viterubi decoder 54. Resulting information, such as the speed at which data section 34 is transmitted, is used for further processing of received frames, so it uses MAC-Media Access. Transferred to the Control-) layer.

For channel estimation according to an exemplary embodiment of the invention, header section 32 is then recoded, reinterleaved, modulated and mapped to its subcarriers by blocks 58, 60, 62, and 64, respectively. Will be returned. This has the effect of returning header section 32 to its transmission format, even if the information contained therein is compensated for the channel effect by coarse channel estimation. All functions performed by blocks 58-64 are transmitter functions, and therefore the hardware associated with converting the decrypted version of header section 32 back to the transmitting version of header section 32 and / Or note that the software may simply be reused from the transmitter chain of transmitters and receivers, i.e. no new hardware / software needs to be added to perform this process for channel estimation purposes. The output of the carrier mapping function 64 is input to the multiplier 50 so as to be multiplied by the output of the FFT 46. With reference to Figure 2 (a), the resulting frequency domain information output from the FFT46 contains six frequency domain symbols (one for each bit field), any two of which are in the hopping sequence. Depending on which of the three subcarriers is being used to transmit this particular frame, it will be available for multiplication at multiplier 50. Therefore, the multiplier 50 performs a mathematical comparison between the transmit version of header section 32 compensated for using coarse channel estimation and the receive version of header section 32 uncompensated for channel effects. Therefore, assuming that the value associated with header section 32 was detected correctly, the output of multiplier 50 is then used in the averaging function 66 to improve coarse channel estimation and generate fine channel estimation. Provides two additional channel estimates.

The effect of the noise term associated with the channel estimation calculation is reduced by averaging over a larger number of symbols, so that the channel estimation of the two symbols in the channel estimation sequence and from header section 32 Averaging the channel estimates of the two additional symbols improves the accuracy of the fine channel estimates over the coarse channel estimates. This improvement is estimated to reduce the implementation loss associated with channel estimation from about 1.7 dB to less than about 1 dB by reusing existing hardware / software from the transmit chain with little increase in complexity. To. Fine channel estimates may then be supplied to the equalizer 68, for example, to compensate the data section 34 for the channel effects experienced during transmission.

As mentioned above, the use of header section 32 to calculate fine channel estimates is based on the assumption that header section 32 is correctly detected by the receiver. As shown in FIG. 2 (a), the header section 32 contains information about, for example, bandwidth expansion, which identifies which mode is used to send data section 34, for example 55, Includes the speed at which data section 34 is being transmitted, such as one of 80, 110, 160, 200, 320, 480 Mb / s. Transmission rates also affect other transmission criteria associated with data section 34, such as modulation and coding rates, in UWB systems according to exemplary embodiments of the invention. Given the nature of the information carried in header section 32, the receiver cannot decrypt data section 34 without the correct reception of this information. Thus, exemplary embodiments of the invention use symbols in header section 32 to estimate channels, even if the information transmitted in header section 32 is not foreseen to the receiver. For the purposes of execution, treat the header symbols as known information by assuming the correct reception of header section 32. This assumption is performance-wise because the system is unable to decrypt data section 34 of this frame anyway, even if it is not received correctly, and corrective action must be taken, such as retransmission. There is no adverse effect.

FIG. 4 is a flow chart showing a method of wireless communication according to an exemplary embodiment of the present invention. In step 100, a frame of data including a preamble portion, a header portion, and a data portion is received. Coarse channel estimation is determined in step 102 using the channel estimation symbols received in the preamble portion of the frame. Then, in step 103, the header portion is processed to determine the symbols received there using coarse channel estimation. Fine channel estimation is determined in step 104 using coarse channel estimation and header symbols. The data portion of the frame can then be equalized in step 105 using fine channel estimation to output the detected data symbols.

The exemplary embodiments described above relate to using the header section of the frame to improve channel estimation, but the invention is not so limited. Rather, channel estimation can be extended to perform channel estimation using data symbols in addition to (or instead of) header symbols. Physically, the transmitter / receiver according to the invention can include a receiver chain, a transmitter chain, and a processor that processes the data associated with the receiver chain and the transmitter chain.

The above exemplary embodiments are intended to be exemplary, but not limiting, in all respects of the invention. Accordingly, the present invention can be modified in many ways by those skilled in the art in detailed implementations that can be derived from the description contained herein. All such modifications and modifications are deemed to be in accordance with the scope and spirit of the invention as defined by the appended claims. No element, action or instruction used in the description of the present application should be construed as decisive or essential to the present invention unless explicitly stated. Also, as used herein, the article "a" is intended to include one or more items.

<figref num="1">It is a figure which shows the WLAN system which can implement this invention.</figref><figref num="2(a)">FIG. 5 illustrates an exemplary OFDM frame format that can be used in conjunction with exemplary embodiments of the present invention.</figref><figref num="2(b)">It is a figure which shows the preamble part of the OFDM frame format of FIG. 2 (a) in detail.</figref><figref num="3">FIG. 5 is a process diagram illustrating channel estimation of a transmitter / receiver according to an exemplary embodiment of the present invention.</figref><figref num="4">It is a flow chart explaining the method of performing the channel estimation by the exemplary embodiment of this invention.</figref>