Boundary tracking apparatus and related method of OFDM system

A boundary tracking method applied in an OFDM receiver includes generating a plurality of demodulated signal sets corresponding to part of sub-carriers of a packet according to different boundaries with different positions located at the packet, determining the most precise one of the boundaries according to a plurality of inter-symbol interference (ISI) values according to the demodulated signal sets, and calibrating a timing of a currently utilized boundary.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a boundary tracking apparatus, and more particularly, to a boundary tracking apparatus in an orthogonal frequency division multiplexing (OFDM) system and a method thereof.

2. Description of the Prior Art

Please refer toFIG. 1.FIG. 1is a block diagram of a typical orthogonal frequency division multiplexing (OFDM) frame. According to the IEEE 802.11 a standard, the frames of the OFDM system have a short preamble in the beginning, a long preamble following afterwards, and a guard interval GI2between the short preamble and the long preamble. There are also guard intervals GI between each data block to avoid the inter-symbol interference (ISI). The guard interval GI and GI2can be a cyclic prefix of a data block or a cyclic prefix of the long preamble.

In conventional boundary detecting methods, a receiver detects the beginning of the long preamble and the beginning of the data blocks according to a period and the auto-correlation of the short preamble. The guard intervals GI are then removed accordingly to plan a boundary of the data blocks for fast Fourier transforming (FFT). Next, the conventional boundary detecting methods execute a fast Fourier transform on each data block according to the estimated channel responses of all the sub-carriers with different boundaries to determine a proper boundary.

However, the above-mentioned method usually mistakenly estimates the guard interval of the long preamble with a serious delay spread. Therefore, the above-mentioned methods choose a wrong timing to execute the fast Fourier transform. Furthermore, the prior boundary tracking methods are inefficient to compute for several times the FFT of numerous symbols according to every sub-carrier. That is, tracking the boundary by comparing the FFT of each symbol takes a lot of effort.

SUMMARY OF THE INVENTION

It is therefore one objective of the claimed invention to provide a boundary tracking apparatus for reducing the ISI in an OFDM system with less effort than the conventional boundary tracking apparatuses.

According to an embodiment of the claimed invention, a boundary tracking apparatus of an OFDM receiver is disclosed. The OFDM receiver receives at least one of data blocks corresponding to a plurality of sub-carriers according to a current timing boundary, the plurality of sub-carriers comprises a plurality of pilot sub-carriers and a plurality of data sub-carriers. The boundary tracking apparatus comprises: a first data acquisition module for receiving at least one of the data blocks according to a first timing boundary, and generating a first decoded data set according to a plurality of specific sub-carriers, wherein the plurality of specific sub-carriers are a part of the plurality of sub-carriers; a second data acquisition module for receiving at least one of the data blocks according to a second timing boundary distinct from the first timing boundary, and generating a second decoded data set according to the plurality of specific sub-carriers; an interference detecting module coupled to the first and the second data acquisition modules for respectively generating a first inter-symbol interference and a second inter-symbol interference according to the first decoded data set and the second decoded data set; and a timing controller for adjusting the current timing boundary according to the first and second inter-symbol interferences.

According to an embodiment of the claimed invention, a boundary tracking method of an OFDM receiver. The boundary tracking method of an OFDM receiver receives at least one of data blocks corresponding to a plurality of sub-carriers according to a current timing boundary, and the sub-carriers comprises a plurality of pilot sub-carriers and a plurality of data sub-carriers. The boundary tracking method comprises: receiving at least one of the data blocks according to a first timing boundary, and generating a first decoded data set according to a plurality of specific sub-carriers, wherein the plurality of specific sub-carriers are a part of the plurality of sub-carriers; receiving at least one of the data blocks according to a second timing boundary distinct from the first timing boundary, and generating a second decoded data set according to the plurality of specific sub-carriers; respectively generating a first inter-symbol interference and a second inter-symbol interference according to the first decoded data set and the second decoded data set; and adjusting the current timing boundary according to the first inter-symbol interference and the second inter-symbol interference.

DETAILED DESCRIPTION

Please refer toFIG. 2showing a functional block diagram of a first embodiment of a boundary tracking apparatus10applied in an OFDM receiver according to the present invention. It is well-known that each data block includes numerous pilot codes transmitted respectively via the pilot sub-carriers and numerous data codes transmitted respectively via the data sub-carriers, where the pilot codes of each data block corresponding to the same sub-carrier have the same characteristic. The boundary tracking apparatus10therefore estimates the magnitude of the Inter Symbol Interference (ISI) by comparing the pilot codes of two adjacent data blocks and adjusts the timing boundary of Fast Fourier transform (FFT) according to the comparing result. In the present embodiment, the boundary tracking apparatus10includes a boundary detecting module12, a timing controller14, data acquisition modules20,40,60, and interference detecting module80.

The boundary detecting module12detects the boundary of data blocks. According to the detected boundary, the timing controller14outputs a control signal Sctrlto control the data acquisition modules20,40,60. Finally, the interference detecting module80outputs a comparing signal Scmpto the timing controller14in order to adjust the boundary used by the data acquisition modules20,40and60. The data acquisition modules20,40,60respectively demodulate a received packet according to different boundaries in order to generate the decoded data sets R1, R2, R3for computing the related inter-symbol interferences I1, I2, I3. The interference detecting module80therefore computes the inter-symbol interferences I1, I2, I3according to the decoded data sets R1, R2, R3, and outputs the comparing signal Scmprelated to the comparing result of the inter-symbol interference I1, I2, I3.

The data acquisition module20includes a guard interval removing unit22, an FFT unit24, and a multiplexer26. The guard interval removing unit22generates a timing boundary by determining the boundary of data blocks according to the control signal Sctrl, and removes the guard interval GI according to the boundary of data blocks, and then gathers the output data in the data block. In the present embodiment, the FFT unit24generates a plurality of decoded data by executing fast Fourier transform on the data block according to each sub-carrier. Then the multiplexer26gathers some specific decoded data from the plurality of decoded data, and outputs those specific decoded data as a decoded data set R1. In other words, the decoded data set R1is only a part of the numerous of decoded data, and the specific decoded data corresponds to a plurality of predetermined sub-carriers in the present embodiment.

The data acquisition module40includes a delaying unit42, a guard interval removing unit44, and an FFT unit46. The delaying unit42is used for delaying the received packet with a predetermined time W. As a result, the timing boundary corresponding to the guard interval removing unit44exceeds the timing boundary corresponding to the data acquisition module20. Please note that the delaying or exceeding relates to the starting position on the received packet in the present invention. The operation of the guard interval removing unit44is the same with the guard interval removing unit22, and a repeated detailed description of the guard interval removing unit44is therefore abbreviated. However, the FFT unit46generates a plurality of decoded data by transforming the data block according to the specific sub-carriers, instead of generating the decoded data according to all of the sub-carriers.

The data acquisition module60and the data acquisition module40have the same functions with the same name. The difference between is that the delaying unit62advances the received packet with a predetermined time W. As a result, the timing boundary used by the data acquisition module60lags behind the timing boundary used by the data acquisition module20. The data acquisition module60outputs the decoded data set R3by generating the FFT of the data block according to the specific sub-carriers. Please note that, the predetermined time W can be a fixed value or a dynamically adjustable value based on the inter-symbol interference or the environment.

In the present embodiment, the interference detecting module80includes three interference detecting units82,84,86and a comparing unit88. The interference detecting units82,84,86compute respectively the inter-symbol interferences I1, I2, I3according to the decoded data set R1, R2, R3, wherein the inter-symbol interferences I1, I2, I3is related to remaining, delaying, and advancing the received packet. Finally, the comparing unit88chooses the smallest of the inter-symbol interferences I1, I2, I3and outputs a comparing signal Scmpto the timing controller14according to the smallest inter-symbol interference in order to adjust the currently used timing boundary.

For example, assume the OFDM system has 16 sub-carriers, wherein 4 sub-carriers are pilot sub-carriers (i.e., the specific sub-carriers mentioned above) presented as

ⅇ+j⁢2⁢π⁢⁢n·016,ⅇ+j⁢2⁢π⁢⁢n·416,ⅇ+j⁢2⁢π⁢⁢n·816,ⅇ+j⁢2⁢π⁢⁢n·1216.
According to the formula of fast Fourier transform

X⁡[k]=∑n=0N-1⁢⁢x⁡[n]⁢ⅇ-j⁢2⁢π⁢⁢nk16,
the decoded data X[0], X[4], X[8], X[12] corresponding to the 4 pilot sub-carriers are shown as below:

X⁡[0]=(x⁡[0]+x⁡[2]+…+x⁡[15])⁢ⅇ-j⁢2⁢π·016X⁡[4]=(x⁡[0]+x⁡[4]+x⁡[8]+x⁡[12])⁢ⅇ-j⁢2⁢π·016+(x⁡[1]+x⁡[5]+x⁡[9]+x⁡[13])⁢ⅇ-j⁢2⁢π·416+(x⁡[2]+x⁡[6]+x⁡[10]+x⁡[14])⁢ⅇ-j⁢2⁢π·816+(x⁡[3]+x⁡[7]+x⁡[11]+x⁡[15])⁢ⅇ-j⁢2⁢π·1216X⁡[8]=(x⁡[0]+x⁡[2]+…+x⁡[14])⁢ⅇ-j⁢2⁢π·016+(x⁡[1]+x⁡[3]+…+x⁡[15])⁢ⅇ-j⁢2⁢π·816X⁡[12]=(x⁡[0]+x⁡[4]+x⁡[8]+x⁡[12])⁢ⅇ-j⁢2⁢π·016+(x⁡[3]+x⁡[7]+x⁡[11]+x⁡[15])⁢ⅇ-j⁢2⁢π·416+(x⁡[2]+x⁡[6]+x⁡[10]+x⁡[14])⁢ⅇ-j⁢2⁢π·816+(x⁡[1]+x⁡[5]+x⁡[9]+x⁡[13])⁢ⅇ-j⁢2⁢π·1216
wherein the data x[0] . . . x[15] are the output data shown inFIG. 1. The mentioned equation can be implemented by summing the output of a four-tap matching filter. The FFT unit46and66can therefore be implemented by four matching filters instead of 16 matching filters (i.e., center frequency are

(i.e.,center⁢⁢frequency⁢⁢are⁢⁢ⅇ+j⁢2⁢π⁢⁢n·016,ⅇ+j⁢2⁢π⁢⁢n·116⁢…⁢⁢ⅇ+j⁢2⁢π⁢⁢n·1516).
In summary, the apparatus of the present invention reduces the complexity of the FFT unit46and66, and increases the efficiency of the FFT unit46and66.

The present embodiment involves utilizing the FFT unit24to generate a plurality of decoded data according to all of the sub-carriers. However, the FFT unit24can only generate a few of decoded data according to the specific sub-carriers.

Please refer toFIG. 3, which is a schematic diagram of the interference detecting unit82shown inFIG. 2. Please note that the interference detecting units82,84,86have the same electrical structure, so a repeated description of the interference detecting units84,86is omitted for abbreviation. The interference detecting unit82includes a plurality of delaying units202,212,222, a plurality of subtractors204,214,224, a plurality of squaring units206,216,226, and an adder208. The delaying units202,212,222are used to delay the decoded data set R1for a sampling time. In the present embodiment, the decoded data set R1includes decoded data X[0], X[4], X[8], X[12]. As a result, the delaying units202,212,222are used to delay the decoded data X[0], X[4], X[8], X[12] for a sampling time. The subtractors204,214,224subtract the current decoded data from the delayed decoded data, meaning that calculating the difference between a decoded signal in the previous data block and a decoded signal in the current data block transmitted via the same sub-carriers. Then, the squaring units206,216,226output the square values of the difference to adder208, and the adder208sums all the square values to generate the inter-symbol interference I1. Please note that the method of generating the inter-symbol interference by squaring the differences should not be construed as limiting the present invention. The present invention can also be applied to other applications, which generate the inter-symbol interference according to differences, such as summing the absolute values of the differences.

The difference value between the two decoded data corresponding to the same pilot sub-carrier should be zero if there is no inter-symbol interference. In other words, adopting an inappropriate boundary causes a greater difference indicating that worse inter-symbol interference occurs.

Finally, in order to adjust the timing boundary of data acquisition module20, the comparing unit88generates a comparing signal Scmpto the timing controller14according to the minimum of the inter-symbol interferences11,12,13.

Please refer toFIG. 4, which is functional block diagram of a second embodiment of the boundary tracking apparatus110according to the present invention. In the present embodiment, the boundary tracking apparatus110is capable of using data sub-carriers to detect inter-symbol interference, wherein the function and structures of the boundary detecting module112, the timing controller114, and the data acquisition modules120,140,160are the same with the components having the same name inFIG. 2. Therefore, a repeated description of these components is omitted. Please note that the multiplexer126in the data acquisition module120gathers a plurality of decoded data corresponding to a plurality of specific data sub-carriers to generate the decoded data set R1′, and the FFT units146,166in the data acquisition modules140,160execute the fast Fourier transform on the data blocks according to a plurality of specific data sub-carriers and output the decoded data set R2′, R3′.

In addition, the signal compensation module170includes an equalizing unit172, a channel compensation unit174, and a multiplexer176. The compensation module170generates the estimated constellation signal {circumflex over (d)}[k] according to a well-known decision-directed method, and generates the compensated data set ({circumflex over (d)}[k]. Ĥ[k]) according to the estimated channel response Ĥ[k] and the estimated constellation signal {circumflex over (d)}[k]. Finally, the multiplexer176gathers a part of the compensated data set, which corresponds to the plurality of specific data sub-carriers. As a result, all of the decoded data in the decoded data sets R1′, R2′, R3′ corresponds to the plurality of specific data sub-carriers.

Please refer toFIG. 5.FIG. 5is a functional block diagram of the interference detecting unit182inFIG. 4. Please note that the interference detecting units182,184,186have the same electrical structure. Therefore, a repeated description of the interference detecting units184,186is omitted. The interference detecting unit182includes subtractors304,314,324, rotators302,312,322, squaring units306,316,326, and an adder308. The rotators302,312,322rotate the compensated data set with a predetermined phase corresponding to a propagation timing delayed with a certain value. For example, if the propagation timing of the decoded data set R1′ remains constant, the phase shift of the decoded data set R1′ is zero, and if the propagation timing of the decoded data set R2′ is delayed with a predetermined time W, the phase shift of the decoded data set R3′ is

ⅇ-j⁢2⁢π⁢⁢nkWM,
and the propagation timing of the decoded data set R3′ exceeds a predetermined time W, so the phase shift of the decoded data set R3′ is

In the present embodiment, assume the OFDM has 16 sub-carriers, wherein 4 sub-carriers are data sub-carriers. The FFT units146,166generate the decoded data set R2′, R3′ according to the 4 data sub-carriers

ⅇ+j⁢2⁢π⁢⁢n·016,ⅇ+j⁢2⁢π⁢⁢n·416,ⅇ+j⁢2⁢π⁢⁢n·816,ⅇ+j⁢2⁢π⁢⁢n·1216
(i.e., the specific sub-carriers mentioned above). However, the FFT unit124still generates the FFT of the data block according to all of the sub-carriers, and the multiplexer126selects decoded data set R1′ from all of the decoded data. In other words, all of the decoded data sets R1′, R2′, R3′ include the decoded data X′[0], X′[4], X′[8], X′[12] corresponding to the same specific sub-carriers.

Because the compensated data set ({circumflex over (d)}[k]. Ĥ[k]) generated by the data acquisition module120is defined as the correct decoded data, the differences between the compensated data set and the decoded data sets R1′, R2′ and R3′ are caused by the inter-symbol interference. The method of computing the inter-symbol interferences I1′, I2′, I3′ according to the decoded data sets R1′, R2′, R3′ is shown in the following equations:

Finally, the comparing unit188generates a comparing signal Scmpto the timing controller114according to the minimum of the inter-symbol interferences I1′, I2′, I3′ in order to adjust the timing boundary.