WO2009072037A2

WO2009072037A2 - Method and system for detecting an interfering ofdm-based signal at an ultra-wideband module

Info

Publication number: WO2009072037A2
Application number: PCT/IB2008/054980
Authority: WO
Inventors: Charles Razzell
Original assignee: NXP BV
Current assignee: NXP BV
Priority date: 2007-12-03
Filing date: 2008-11-27
Publication date: 2009-06-11
Anticipated expiration: 2010-06-03
Also published as: WO2009072037A3

Abstract

A technique for detecting an orthogonal frequency division multiplexing (OFDM)-based signal at an ultra-wideband (UWB) module is disclosed. The technique involves receiving radio frequency (RF) energy at the UWB module, wherein the RF energy may include a cyclically extended OFDM symbol having a cyclic prefix and an OFDM symbol, downconverting the received RF energy into a baseband signal, autocorrelating the baseband signal to identify a match between the cyclic prefix and the OFDM symbol, and providing an output that indicates the presence of an interfering OFDM-based signal when a match is found between the cyclic prefix and the OFDM symbol.

Description

METHOD AND SYSTEM FOR DETECTING AN INTERFERING OFDM-BASED SIGNAL AT AN ULTRA- WIDEBAND MODULE

The invention relates generally to wireless communications systems, and more particularly, to detecting an interfering orthogonal frequency division multiplexing (OFDM)-based signal at an ultra-wideband (UWB) module.

Radio frequency (RF) wireless technologies are being developed to enable the wireless distribution of rich digital video content within a local environment such as a home or office. For example, the WiMedia Alliance has developed the WiMedia Ultra- Wideband (UWB) Common Radio Platform, which incorporates media access control (MAC) layer and physical (PHY) layer specifications based on Multi-band OFDM (MB-OFDM). The WiMedia UWB Common Radio Platform enables shortrange multimedia file transfers at data rates of 480 Mbit/s and beyond with low power consumption using the 3.1 to 10.6 GHz UWB spectrum. WiMedia UWB Common Radio Platform is optimized for personal computers (PCs), consumer electronic (CE) devices, mobile devices, and automotive applications.

Systems that utilize the WiMedia UWB Common Radio Platform may experience interference from other wireless technologies, especially wide area wireless technologies such as WiMax that utilize OFDM-based signals. If a system that includes a UWB module can detect an interfering OFDM-based signal, the interfering band could be avoided. The process of detecting and avoiding interfering RF signals is generally referred to in the field as Detect and Avoid (DAA). In some cases, the detection of interfering WiMax signals is based on the detection of an uplink signal from a nearby WiMax device. One drawback to this approach is that the power of an uplink signal from a nearby WiMax device is typically very strong relative to the operating power of a UWB module and may overwhelm the dynamic range of the UWB receiver. Another drawback to uplink detection is that the WiMax CPE transceiver may never acquire the downlink transmissions from the WiMax base station due to a raised interference floor that is caused by the UWB transmissions. If the WiMax CPE does not acquire the downlink transmissions, it will not gain entry to the WiMax network and will never make uplink transmissions, thereby avoiding detection. Alternatively, the detection of interfering WiMax signals can be based on detection of a downlink signal from a WiMax base station. Conventional techniques for detecting a downlink WiMax signal include energy detection (e.g., spectrum analysis) and correlation. Energy detection involves identifying the frequency of interfering WiMax signals using the UWB receiver. However, because WiMax downlink signals tend to be relatively weak, this technique requires sensitive detection to distinguish interfering signals from noise. Sensitive detection can be demanding in terms of signal processing resources and power consumption. Correlation involves comparing received RF energy with a standards-based correlation vector to determine if an interfering OFDM-based signal is present. However, this technique requires the UWB module to maintain a library of standards-based correlation vectors that includes a correlation vector for each potentially interfering OFDM-based signal. Additionally, the library would be rendered obsolete or need to be updated as current standards evolve or new standards are developed. A technique in accordance with an embodiment of the invention for detecting an

OFDM-based signal at a UWB module involves receiving RF energy at the UWB module, wherein the RF energy may include a cyclically extended OFDM symbol having a cyclic prefix and an OFDM symbol, downcoverting the received RF energy into a baseband signal, autocorrelating the baseband signal to identify a match between the cyclic prefix and the OFDM symbol, and providing an output that indicates the presence of an interfering OFDM-based signal when a match is found between the cyclic prefix and the OFDM symbol.

Because the technique relies on autocorrelation, there is no need to maintain a library of correlation vectors and the technique can easily be adapted for future OFDM- based signals (e.g., for 4G services).

In an embodiment in accordance with the invention, a UWB module includes a UWB transceiver and an OFDM interference detector. The UWB transceiver is configured to receive RF energy and to downconvert the RF energy to a baseband signal, wherein the RF energy may include a cyclically extended OFDM symbol having a cyclic prefix and an OFDM symbol. The OFDM interference detector is configured to autocorrelate the baseband signal to identify a match between the cyclic prefix and the OFDM symbol and to provide an output that indicates the presence of an interfering

OFDM-based signal when a match is found between the cyclic prefix and the OFDM symbol.

Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

Fig. 1 illustrates two wireless networks, a Wireless Metropolitan Area Network and a Wireless Personal Area Network, which have overlapping areas of influence.

Fig. 2 depicts a host system, such as a person computer, a digital camera, a printer, or a media player, which includes a UWB module.

Fig. 3 illustrates a cyclically extended OFDM symbol that includes an OFDM symbol and a cyclic prefix.

Fig. 4 depicts an embodiment of a UWB module that includes a UWB transceiver and an

OFDM interference detector in accordance with an embodiment of the invention. Fig. 5 depicts a detection system that includes at least a portion of a UWB transceiver and an embodiment of an OFDM interference detector in accordance with an embodiment of the invention.

Fig. 6 depicts a detection system that includes at least a portion of a UWB transceiver and another embodiment of an OFDM interference detector in accordance with an embodiment of the invention.

Fig. 7 is a process flow diagram of a method for detecting an OFDM-based signal at a

UWB module in accordance with an embodiment of the invention.

Throughout the description, similar reference numbers may be used to identify similar elements. Fig. 1 illustrates two wireless networks, a Wireless Metropolitan Area Network

(WMAN) 100 and a Wireless Personal Area Network (WPAN) 102, which have overlapping areas of influence. The WMAN has a coverage area of several kilometers and operates, for example, according to the IEEE 802.16 standard also known as WiMax,

Worldwide Interoperability for Microwave Access. Additional WMAN technologies include, for example, technologies being developed under IEEE 802.20. WiMax utilizes

OFDM and operates in the 2 - 11 GHz range, with target bands in the 2.4 GHz, 3.5 GHz, and 5.8 GHz bands. The WMAN of Fig. 1 includes a base station 104 and a customer premises equipment (CPE) 106 such as a laptop computer. According to WiMax, downlink communications 108 occur from the base station to the CPE and uplink communications 110 occur from the CPE to the base station. The WPAN 102 has a coverage area of up to about 10 meters. The WiMedia

UWB Common Radio Platform utilizes MB-OFDM and operates in the 3.1 GHz - 10.6 GHz band. The WPAN of Fig. 1 includes a UWB host 112 and various UWB devices 114. For example, the UWB host may be a personal computer such as a laptop and the UWB devices may include a digital camera, a printer, and a media player (e.g., a television, movie projector, a music player, DVD player, etc.). Whether acting as a UWB host or a UWB device, the WiMedia UWB Common Radio Protocol is used in conjunction with a host system. Fig. 2 depicts a host system, such as a person computer, a digital camera, a printer, or a media player, which includes a UWB module 122. The UWB module includes the PHY and MAC layer functionality as specified in the WiMedia UWB Common Radio Platform and may act as a UWB host or as a UWB device. Additionally, the UWB module may be integrated into the host system or removably connected to the host system. For example, a UWB module may be embodied as a removable stick or "dongle" that can be inserted into a port of the host system such as a Universal Serial Bus (USB) port. Referring back to Fig. 1, because WiMax and WiMedia networks, WMAN 100 and WPAN 102, operate in similar frequency bands, interference is possible when the area of influence of the wireless networks overlap. In a particular case, UWB modules may experience interference from the much stronger uplink WiMax signals that are transmitted from a WiMax CPE. For example, if a WiMax CPE operates in close proximity to a WiMedia network, the WiMedia communications may be adversely affected by the relatively strong uplink WiMax signals.

One technique for preventing a degradation in WiMedia communications is for a UWB module to implement some form of DAA. For example, a UWB module that functions as a UWB host can be configured to implement DAA so as to avoid WiMedia communications in frequency bands that are actively being used by a nearby WiMax CPE. Implementing DAA involves detecting an interfering signal, for example, an interfering OFDM-based signal from a WiM ax network. In an embodiment of the invention, detecting an interfering OFDM-based signal at a UWB module involves receiving RF energy at the UWB module, wherein the RF energy may include a cyclically extended OFDM symbol having a cyclic prefix and an OFDM symbol, downcoverting the received RF energy into a baseband signal, autocorrelating the baseband signal to identify a match between the cyclic prefix and the OFDM symbol, and providing an output that indicates the presence of an interfering OFDM-based signal when a match is found between the cyclic prefix and the OFDM symbol.

The above-described technique relies on the fact that OFDM-based wireless communications protocols typically utilize a cyclically extended OFDM symbol. As illustrated in Fig. 3, a cyclically extended OFDM symbol 130 includes an OFDM symbol 132 and a cyclic prefix 134, which is a copy of a portion of the OFDM symbol, typically an end portion of the OFDM symbol, prepended to the OFDM symbol. As is known in the field of wireless OFDM communications, a cyclic prefix is used to deal with the problem of intersymbol interference.

Although the length of the cyclic prefix 134 may vary from protocol to protocol, the duration of the useful part of the cyclically extended OFDM symbol 130 is determined by the Fast Fourier Transform (FFT) length. Thus, a decision variable may be generated by autocorrelating the cyclically extended OFDM symbol. In particular, the cyclic prefix 134 is correlated with the OFDM symbol 132 to find a match. When an

OFDM-based signal is present in received RF energy, a match will be found between the cyclic prefix and the OFDM symbol as indicated by the decision variable. In an embodiment, a decision variable is expressed in continuous-time as:

v(0 = jy(t)xy^*(t -A)dt (i) t-δ

where: δ is the sliding integration interval;

Δ is the delay; y{t) is the signal at time t; and y* (t - Δ) is the complex conjugate of the delayed signal at time (t - Δ). In an embodiment, the duration of the sliding integration interval, δ, is chosen to be the length of the minimum cyclic prefix allowed by the protocol with which the OFDM-based signal corresponds. For example, if interfering signals are expected to be WiMax signals, then the sliding integration interval is selected to correspond to the minimum cyclic prefix allowed under WiMax, e.g., 11.43us. In an embodiment, the delay, Δ, is chosen to be equivalent to the useful part of the OFDM symbol excluding the cyclic prefix, e.g., 91.4us.

Although equation (1) is expressed in continuous-time, a discrete-time equivalent of the expression can be readily devised using a two tap Infinite Impulse Response (HR) filter to replace the sliding integration. For example, a discrete-time moving average filter can be expressed as:

\ - Z^~N

H{Z) = - X - Z r -l (2)

where N is the length of the moving average filter in number of samples.

Alternatively, a discrete-time moving average filter can be expressed in a difference equation as: y(n) = x(n) - x(n - N) + y(n - \) (3)

where n is the index of the current sample, and n - 1 is the index of the previous sample, etc.

In an embodiment, the above-described technique for detecting an interfering OFDM-based signal is implemented within a UWB module. Fig. 4 depicts an embodiment of a UWB module 140 that includes a UWB transceiver 142 for transmitting and receiving UWB signals and an OFDM interference detector 144 for detecting an interfering OFDM-based signal using autocorrelation. In the embodiment of Fig. 4, the UWB transceiver includes OFDM receiver components as are known in the field of wireless OFDM communications. These components include, for example, amplifiers, mixers (downconverters/downconverters), filters, analog-to-digital converters (ADCs), FFT modules, symbol detection modules, error correction modules, and decoders. The OFDM interference detector includes components that enable autocorrelation of received RF energy and specifically components that enable autocorrelation of a cyclic prefix and an OFDM symbol of an OFDM-based signal such as a WiMax signal. The OFDM interference detector also includes components that provide an output that indicates the presence of an interfering OFDM-based signal based on the autocorrelation results.

Fig. 5 depicts a detection system 150 that includes at least a portion of the UWB transceiver 142 and an embodiment of an OFDM interference detector 144. The portion of the UWB transceiver that is depicted in Fig. 5 include elements that support the OFDM interference detector, specifically, an antenna 152, mixer/filter modules 154, an oscillator 156, a phase shifter 158, amplifiers 159, ADCs 160, and a combiner 162.

Although Fig. 5 illustrates some elements of the UWB transceiver, the UWB transceiver may include additional elements for WiMedia signal processing. The combiner is provided in Fig. 5 to simplify the depiction and description of the OFDM interference detector. The OFDM interference detector includes delay module 164, complex conjugate module 166, multiplier 168, moving average module 170, threshold comparison module 172, and Boolean logic 174.

In operation, RF energy received at the UWB transceiver 142 is downconverted to baseband signals by the mixer/filter modules 154 and converted to digital data by the ADCs 160. Digital data representative of the baseband signals is then provided to the OFDM interference detector 144 for detection of interfering OFDM-based signals.

Although not illustrated, the same digital data may also be processed through additional UWB transceiver elements for WiMedia communications. With reference to the OFDM interference detector, digital data representative of the baseband signals is delayed by the delay module 164, for example, with a delay of Δ. The delayed baseband signal is then provided to the complex conjugate module 166 and the complex conjugate of the delayed baseband signal is generated. The complex conjugate is then provided to the multiplier 168 and the complex conjugate is multiplied by a delayed portion of the baseband signal.

The multiplier output is provided to the moving average module 170 and a moving average is calculated. In an embodiment, the moving average is calculated over the sample interval, δ, which may be expressed in terms of a number of cyclic prefix samples, N_cp. The moving average is then provided to the threshold comparison module 172 and the moving average is compared to a pre-established threshold. In an embodiment, the pre-established threshold is set to a value that indicates a match between a cyclic prefix and an OFDM symbol and the threshold comparison module generates an output that indicates whether the moving average is above or below the pre-established threshold. Ideally, the threshold is set high enough to avoid excessive false detections and low enough to ensure a high probability of detection. Once way to regulate false detections is to set the threshold to a specified number of standard deviations above the mean of the decision variable under typical noise plus interference conditions. In an embodiment, the threshold comparison module outputs a high signal (e.g., a "1") if the moving average is above the threshold and a low signal (e.g., a "0") if the moving average is below the threshold.

The Boolean logic 174 receives the output from the threshold comparison module and generates a digital output in response. In an embodiment, the Boolean logic generates an output that indicates the presence of an interfering OFDM-based signal when the moving average exceeds the pre-established threshold, that is, when a match is found between a cyclic prefix and an OFDM signal.

Although one embodiment of the OFDM interference detector 144 is described with reference to Fig. 5, other embodiments of an OFDM interference detector are possible. In an embodiment, the complex conjugate is applied to the non-delayed path instead of the delayed path. In still another embodiment, the delay, Δ, is imparted using integer numbers of samples (e.g., a shift register) and/or with a digital filter/interpolator to get a fractional sample delay.

The OFDM interference detector described above with reference to Fig. 5 does not necessarily produce an output that identifies a particular subcarrier of an OFDM- based signal. In an embodiment, the OFDM interference detector is configured to detect interfering OFDM-based signals within specific subcarrier bands that are used by the UWB module. Fig. 6 depicts a detection system 180 that includes at least a portion of a UWB transceiver 142 and an embodiment of an OFDM interference detector 182 that is configured to detect interfering OFDM-based signals within specific subcarrier bands that are used by the UWB module. As depicted in Fig. 6, the OFDM interference detector is connected to receive subcarrier-specific outputs from an FFT module 184 and includes multiple subcarrier- specific OFDM interference detectors 186. In particular, the OFDM interference detector includes a subcarrier-specific OFDM interference detector corresponding to each of N subcarriers that are supported by the UWB transceiver. Each subcarrier-specific OFDM interference detector uses autocorrelation to output a subcarrier-specific indication of the presence or absence of an interfering OFDM-based signal. In the embodiment of Fig. 6, each subcarrier-specific OFDM interference detector 186 is similar to the OFDM interference detector 144 described above with reference to Fig. 5.

A WiMax-compliant OFDM signal will have periodic correlation peaks in the range of one peak every 100 - 150 microseconds (us). As specified by the WiMedia UWB Common Radio Protocol, the MAC layer uses superframes that have 256 Media Access Slots (MASs), with each MAS being 256us in length. Because a WiMax- compliant OFDM signal will result in periodic autocorrelation peaks that repeat every 100 - 150us, it will take only one MAS to detect at least one periodic autocorrelation peak. In accordance with an embodiment of the invention, only one MAS is reserved for interference detection. During the one reserved MAS, no UWB transmissions occur and the above-described detection technique is implemented. Using only one MAS for detection of interfering signals has a very small impact on bandwidth utilization between WiMedia modules. In an embodiment, the reserved MAS can be marked as available for shared use to further preserve MAC resources. Although an interfering signal can be detected using only a single MAS, other embodiments may utilize multiple MASs, either adjacent to each other in a superframe, separated from each other in a superframe, or a combination thereof.

Using the above-described autocorrelation technique to detect an interfering signal eliminates problems that may be contributed from frequency shifts. Specifically, frequency shifts cancel out because the same frequency shift exists in both the baseband signal and the delayed portion of the baseband signal.

To support the above-described detection technique, the accuracy with which the delay, Δ, is represented in discrete-time should be within a fraction of one sample period of the potentially interfering OFDM-based signal, e.g., a WiMax OFDM signal.

Sampling rates in a UWB module are more than adequate to enable sampling within a fraction of one sample period of a WiMax OFDM signal. In particular, a WiMedia clock rate of around 528 MHz makes it relatively easy to get within a fraction of one sample period of a WiMax signal, which has a clock rate of around 11.2 MHz. Additionally, one could consider using interpolation filters to more accurately implement a delay that is not an integer number of sample periods.

Using the above-described technique, a wide variety of potentially interfering OFDM-based signals can be detected simply by adjusting the length of the delay. For example, multiple different OFDM-based signals can be detected by cycling the delay through a range of delay values. Additionally, because the technique relies on autocorrelation, there is no need to maintain a library of correlation vectors and the technique can easily be adapted for future OFDM-based signals (e.g., for 4G services). Fig. 7 is a process flow diagram of a method for detecting an OFDM-based signal at a UWB module in accordance with an embodiment of the invention. At block 702, RF energy is received at the UWB module, wherein the RF energy may include a cyclically extended OFDM symbol having a cyclic prefix and an OFDM symbol. At block 704, the received RF energy is downconverted into a baseband signal. At block 706, the baseband signal is autocorrelated to identify a match between the cyclic prefix and the OFDM symbol. At block 708, an output that indicates the presence of an interfering OFDM- based signal is provided when a match is found between the cyclic prefix and the OFDM symbol.

Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts as described and illustrated herein. The invention is limited only by the claims.

Claims

What is claimed is:

1. A method for detecting an orthogonal frequency division multiplexing (OFDM)- based signal at an ultra- wideband (UWB) module, the method comprising: receiving radio frequency (RF) energy at the UWB module, wherein the RF energy may include a cyclically extended OFDM symbol having a cyclic prefix and an OFDM symbol; downcoverting the received RF energy into a baseband signal; autocorrelating the baseband signal to identify a match between the cyclic prefix and the OFDM symbol; and providing an output that indicates the presence of an interfering OFDM-based signal when a match is found between the cyclic prefix and the OFDM symbol.

2. The method of claim 1 wherein the autocorrelating comprises setting a sampling interval that is equivalent to a minimum length of a standardized cyclic prefix.

3. The method of claim 2 wherein the cyclic prefix is standardized according to the IEEE 802.16 standard.

4. The method of claim 1 wherein the autocorrelating comprises setting a delay to a length of the OFDM symbol.

5. The method of claim 4 wherein the length of the OFDM symbol is specified by the IEEE 802.16 standard.

6. The method of claim 1 wherein the autocorrelating comprises setting a sampling interval that is equivalent to a minimum length of a standardized cyclic prefix and setting a delay to a length of the OFDM symbol.

7. The method of claim 6 wherein the autocorrelating comprises calculating a moving average and comparing the moving average to a pre-established threshold.

8. The method of claim 1 further comprising reserving only one media access slot (MAS) for detecting an OFDM-based signal and performing the detection method based on RF energy received during the reserved MAS.

9. The method of claim 1 wherein the autocorrelating comprises performing subcarrier- specific autocorrelations for multiple different subcarriers that are supported by the UWB module and wherein providing an output that indicates the presence of an interfering OFDM-based signal further comprises indicating the subcarrier within which the interfering OFDM-based signal is present.

10. An ultra- wideband (UWB) module comprising: a UWB transceiver configured to receive radio frequency (RF) energy and to downconvert the RF energy to a baseband signal, wherein the RF energy may include a cyclically extended orthogonal frequency division multiplexing (OFDM) symbol having a cyclic prefix and an OFDM symbol; an OFDM interference detector configured to: autocorrelate the baseband signal to identify a match between the cyclic prefix and the OFDM symbol; and provide an output that indicates the presence of an interfering OFDM- based signal when a match is found between the cyclic prefix and the OFDM symbol.

11. The UWB module of claim 10 further comprising: a delay module to delay the baseband signal; a complex conjugate module to calculate the complex conjugate of the baseband signal; a multiplier to multiply the delayed baseband signal with the calculated complex conjugate and to provide a multiplier output; a moving average module to calculate a moving average in response to the multiplier output; a threshold comparison module to compare the moving average to a pre- established threshold and to generate a threshold comparison output; and logic for generating the output that indicates the presence of an interfering OFDM-based signal in response to the threshold comparison output.

12. The UWB module of claim 10 further comprising a moving average module configured to calculate a moving average over a pre-established sampling interval, wherein the sampling interval is set to be equivalent to a minimum length of a standardized cyclic prefix.

13. The UWB module of claim 12 wherein the cyclic prefix is standardized according to the IEEE 802.16 standard.

14. The UWB module of claim 12 further comprising: a threshold comparison module to compare the moving average to a pre- established threshold and to generate a threshold comparison output; and logic for generating the output that indicates the presence of an interfering OFDM-based signal in response to the threshold comparison output.

15. The UWB module of claim 10 further comprising a delay module configured to delay the baseband signal, wherein the delay is set to a length of the OFDM symbol.

16. The UWB module of claim 10 wherein the OFDM interference detector comprises multiple subcarrier-specific OFDM interference detectors corresponding to different subcarriers that are supported by the UWB transceiver.

17. An apparatus comprising: an ultra-wideband (UWB) module configured to wirelessly communicate with another UWB module using a first orthogonal frequency domain multiplexing (OFDM) signal according to a first service protocol, the UWB module comprising: a UWB transceiver configured to receive radio frequency (RF) energy and to downconvert the RF energy to a baseband signal, wherein the RF energy may include a second OFDM signal according to a second service protocol, the second OFDM signal including a cyclically extended OFDM symbol having a cyclic prefix and an OFDM symbol; an OFDM interference detector configured to: autocorrelate the baseband signal to identify a match between the cyclic prefix and the OFDM symbol of the second OFDM signal; and provide an output that indicates the presence of the second OFDM signal when a match is found between the cyclic prefix and the OFDM symbol.

18. The apparatus of claim 17 wherein the OFDM interference detector comprises a moving average module configured to calculate a moving average over a pre-established sampling interval, wherein the sampling interval is set to be equivalent to a minimum length of a standardized cyclic prefix.

19. The apparatus of claim 17 wherein the OFDM interference detector comprises a delay module configured to delay the baseband signal, wherein the delay is set to a length of the OFDM symbol.

20. The apparatus of claim 17 wherein the OFDM interference detector comprises multiple subcarrier-specific OFDM interference detectors corresponding to different subcarriers that are supported by the UWB transceiver.