阅读说明：本技术 用于对磁共振成像（mri）系统进行无线通信同步的系统和方法 (System and method for wireless communication synchronization for Magnetic Resonance Imaging (MRI) systems ) 是由 A·赖高斯基 P·雷德 T·奥尔蒂斯 G·R·丁辛 于 2016-12-12 设计创作，主要内容包括：磁共振成像(MRI)系统(100、400、500)包括与感测从进行MRI检查的对象发出的磁共振(MR)信号的一个或多个RF线圈相关联的无线RF站(20、320、420、520、620)。所述无线RF站将表示感测到的MR信号的数字数据传送给MRI控制器(124)以供进一步处理,所述MRI控制器可以包括显示器。所述无线RF站中的内部时钟(2202、3202)与MRI控制器时钟(108、2101、3101)精确地同步,具有对诸如伪随机数(PRN)序列的预定义的码序列的载波相位同步和码相位跟踪。(A Magnetic Resonance Imaging (MRI) system (100, 400, 500) includes a wireless RF station (20, 320, 420, 520, 620) associated with one or more RF coils that sense Magnetic Resonance (MR) signals emanating from a subject undergoing an MRI examination. The wireless RF station transmits digital data representing the sensed MR signals to an MRI controller (124), which may include a display, for further processing. An internal clock (2202, 3202) in the wireless RF station is precisely synchronized with the MRI controller clock (108, 2101, 3101), with carrier phase synchronization and code phase tracking of a predefined code sequence, such as a pseudo-random number (PRN) sequence.)

1. A system (100, 400, 500) comprising:

a first wireless communication station (310, 410, 510, 610) comprising:

a first local clock (6110);

a first pseudo-random number (PRN) sequence generator (6112) configured to generate a first PRN sequence (6113) using the first local clock; and

a first transmitter (6910) configured to transmit a first transmit signal (1119, 4119, 6119) comprising the first PRN sequence; and

a second wireless communication station (320, 420, 520, 620) configured to receive via a wireless channel (10) a phase shifted first transmission signal (1229, 4229, 6229) comprising the first transmission signal phase shifted by a first channel phase shift, the first channel phase shift being dependent on a time delay of the wireless channel, wherein the second wireless communication station comprises:

a second local clock (6210);

a second PRN sequence generator (6212) configured to generate a second PRN sequence (6213) using the second local clock;

a recovered first clock (6201) recovered from the received phase-shifted first transmit signal; and

a processor (6250) configured to:

recovering the first PRN sequence from the received phase-shifted first transmit signal using the recovered first clock; and is

Comparing the second PRN sequence to the recovered first PRN sequence to determine a code phase shift error between the first wireless communication station and the second wireless communication station.

2. The system (100, 400, 500) of claim 1, wherein the second communication station further comprises a second transmitter (7910) configured to transmit a second transmit signal (4219, 6219) comprising the second PRN sequence, and

wherein the first wireless communication station is configured to receive via the wireless channel a phase-shifted second transmission signal (4129, 6129) comprising the second transmission signal phase-shifted by a second channel phase shift, the second channel phase shift being dependent on the time delay of the wireless channel, wherein the first communication station further comprises:

a recovered second clock (6120) recovered from the received phase-shifted second transmit signal; and

a processor (6150) configured to:

recovering the second PRN sequence from the received phase-shifted second transmit signal using the recovered second clock; and is

Comparing the first PRN sequence to the recovered second PRN sequence to determine a code phase shift error between the first wireless communication station and the second wireless communication station.

3. A system (100, 400, 500) according to claim 1, wherein the processor comprises a correlator (800) configured to compare the recovered first PRN sequence with at least three time-shifted copies of the second PRN sequence and to determine which of the at least three time-shifted copies of the second PRN sequence has the highest correlation with the recovered first PRN sequence, and wherein the system is configured to adjust the timing of one of the first and second PRN sequence generators to synchronize the first and second PRN sequence generators with each other in dependence on which of the at least three time-shifted copies of the second PRN sequence has the highest correlation with the recovered first PRN sequence.

4. The system (100, 400, 500) of claim 1, in which the wireless channel is a multipath channel, wherein the second wireless communication station comprises:

a plurality of correlators (1212-1.. 1212-M), each configured to correlate the received phase shifted first transmitted signal with a corresponding time delay (1112-1.. 1112-M) selected to correspond to one path in the multipath channel;

a signal processor (1250) configured to process correlation outputs of the plurality of correlators by:

weighting the output of each correlator by a corresponding factor (1214-1.. 1214-M) corresponding to the relative strength of a corresponding path in the multi-path channel;

adding the weighted outputs of the correlators to produce a sum;

integrating the sum over a bit period of the received phase shifted second transmit signal; and is

Comparing the sum to a threshold to determine a value of a bit of a second transmit signal for the received phase shift in the bit period.

5. The system (100, 400, 500) of claim 1, wherein the first transmitted signal comprises a synchronization sequence (6113), wherein the second wireless communication station comprises:

a matched filter for the synchronization sequence, the matched filter configured to detect the received phase-shifted first transmit signal and generate a synchronization output signal; and

a peak detector configured to detect a peak in the synchronization output signal (1310),

wherein the second local clock is adjusted in response to the timing of the detected peak in the synchronization output signal.

Technical Field

The present system generally relates to a Magnetic Resonance Imaging (MRI) system having a wireless-type Radio Frequency (RF) coil portion and a method of operating the same.

Background

Magnetic Resonance Imaging (MRI) is an imaging method that typically uses frequency and phase encoding of protons for image reconstruction. Recently, MRI systems have begun to use one or more wireless type RF coils to sense magnetic resonance signals emanating from a subject undergoing an MRI examination. In particular, wireless RF coils acquire analog MR information during an acquisition session, and then associated RF coil units (also referred to herein as wireless RF stations) convert the analog MR information to form digitized information, e.g., digitized raw data (k-space) information. Thereafter, the wireless RF station communicates the digitized information to the system controller for further processing and/or display on a display of the MRI system.

Here, the wireless RF station relies on an internal clock to properly synchronize with the system clock (e.g., master clock) of the MRI system. However, it is often difficult to accurately synchronize the wireless RF station internal clock to the MRI system clock using conventional wireless communication methods due to the wireless nature of the wireless RF coils and the induced RF jitter and phase drift. For example, line-of-sight (LOS) paths, blocked LOS paths, and/or non-LOS paths between the RF station and the rest of the communication stations of the MRI system bound to the master clock may have varying time delays, which may be caused by motion between the transmitter and receiver and/or variations in the channel model. This time delay over time can cause drift between the internal clock of the RF station and the master clock.

Unfortunately, when the wireless RF station internal clock is not accurately synchronized with the MRI system clock, due to the nature of the encoding method used, phase noise of the wireless RF station internal clock can cause image artifacts in the reconstructed images, especially during long acquisition times. For example, it can be shown that if the clock-induced Root Mean Square (RMS) phase error in the raw image data is required to remain below 1 degree, the RMS time jitter should be controlled to remain less than 44 picoseconds (ps) at 64MHz and less than 22ps at 128 MHz.

Disclosure of Invention

Accordingly, it is desirable to provide systems and methods for wireless communication for MRI systems. It is also desirable to provide a wireless communication system and method for an MRI system that facilitates clock recovery and synchronization of the internal clock of the wireless RF station to the master clock of the MRI system that can compensate for varying time delays in the communication path between the RF station and the communication stations of the rest of the MRI system. Furthermore, it is desirable to provide systems and methods for wireless RF stations to synchronize the wireless RF station internal clock with the MRI system clock based on MRI system transmissions received via multipath propagation.

In one aspect, the invention can provide a method comprising: transmitting a first transmit signal from a Radio Frequency (RF) transmitter of a first wireless communication station of a Magnetic Resonance Imaging (MRI) system, the first transmit signal comprising a first baseband signal upconverted with a first carrier signal, the first carrier signal having a first carrier frequency, the first carrier frequency being a product of a first value and a first Local Oscillator (LO) frequency of the first wireless communication station, the first carrier signal synchronized with the first LO of the first wireless communication station; receiving, at an RF receiver of a second wireless communication station of the MRI system, a phase-shifted first transmit signal via a wireless channel, the phase-shifted first transmit signal comprising the first transmit signal phase-shifted by a first channel phase shift, the first channel phase shift equal to the first carrier frequency multiplied by a time delay of the wireless channel; multiplying, at the second wireless communication station, a second baseband signal with a correction signal to produce a corrected second baseband signal and transmitting a second transmit signal, the second transmit signal comprising the corrected second baseband signal upconverted with a second carrier signal, the second carrier signal having a second carrier frequency, the second carrier frequency being a product of a second value and a second LO frequency of a second LO of the second wireless communication station, wherein the second transmit signal has a second transmit carrier frequency, the second transmit carrier frequency being a product of the second value and the first LO frequency and the second transmit carrier frequency being synchronized in frequency with the first LO of the first wireless communication station; receiving, at an RF receiver of the first wireless communication station, a phase-shifted second transmit signal via the wireless channel, the phase-shifted second transmit signal comprising the second transmit signal phase-shifted by a second channel phase shift equal to the second transmit carrier frequency multiplied by the time delay of the wireless channel; and ascertaining the time delay of the wireless channel at the first wireless communication station from the received phase-shifted second transmission signal.

Typically, the second LO frequency will have a frequency offset relative to the first LO frequency. Advantageously, the second wireless communication station adjusts the frequency offset when transmitting the second transmission signal back to the first wireless communication station. However, phase shifts or errors due to the wireless channel still exist. The first wireless communication station is able to detect the total phase error, determine the phase correction term, and encode the phase correction term in a message it sends back to the second wireless communication station. The second wireless communication station can then use the phase correction term to generate a third signal synchronized with the first LO of the first wireless station. In some embodiments, the third signal has a frequency of: the frequency being a second value (N)₁) The product with the first LO frequency. In some embodiments, the third signal may pass through N₁A divider/prescaler to reach the original first LO frequency.

Another aspect of the present invention can provide a system including a first wireless communication station and a second wireless communication station. The first wireless communication station includes: a first Local Oscillator (LO) configured to generate a first LO signal having a first LO frequency; a first Radio Frequency (RF) transmitter; and a first RF receiver, wherein the first RF transmitter is configured to transmit a first transmit signal comprising a first baseband signal upconverted with a first carrier signal having a first carrier frequency, the first carrier frequency being a product of a first value and the first LO frequency, the first carrier signal being synchronized with the first LO. The second wireless communication station includes: a second LO configured to generate a second LO signal having a second LO frequency; a second RF transmitter; and a second RF receiver, wherein the second RF receiver is configured to receive a phase-shifted first transmit signal via a wireless channel, the phase-shifted first transmit signal including the first transmit signal phase-shifted by a first signal phase shift equal to the first carrier frequency multiplied by a time delay of the wireless channel, wherein the second wireless communication station is configured to multiply a second baseband signal with a correction signal to produce a corrected second baseband signal, wherein the second RF transmitter is configured to transmit a second transmit signal including the corrected second baseband signal upconverted with a second carrier signal, the second carrier signal having a second carrier frequency that is a product of a second value and a second frequency of a second LO of the second wireless communication station, wherein the second transmit signal has a second transmit carrier frequency that is a product of the second value and the first LO frequency and that is synchronized in frequency with the first LO of the first wireless communication station but has a variable phase due to a first channel phase shift equal to the second carrier frequency multiplied by a time delay of the wireless channel, wherein the first RF receiver is configured to receive a phase-shifted second transmit signal via the wireless channel, the phase-shifted second transmit signal including the second transmit signal with a second channel phase shift equal to the second transmit carrier frequency multiplied by the time delay of the wireless channel, and wherein the first wireless communication station ascertains the time of the wireless channel from the received phase-shifted second transmit signal And (4) delaying.

Yet another aspect of the present invention can provide a system including a first wireless communication station and a second wireless communication station. The first wireless communication station may include: a first local clock; a first pseudo-random number (PRN) sequence generator configured to generate a first PRN sequence using the first local clock; and a first transmitter configured to transmit a first transmit signal including the first PRN sequence. The second wireless communication station is configured to receive via a wireless channel a phase-shifted first transmit signal comprising the first transmit signal phase-shifted by a first channel phase shift, the first channel phase shift being dependent on a time delay of the wireless channel. The second wireless communication station includes: a second local clock; a second PRN sequence generator configured to generate the first PRN sequence using the second local clock; a recovered first clock recovered from the received phase-shifted first transmit signal; and a processor configured to: recovering the first PRN sequence from the received phase-shifted first transmit signal using the recovered first clock; and comparing the second PRN sequence to the recovered first PRN sequence to determine a code phase shift error between the first wireless communication station and the second wireless communication station.

Drawings

The present invention will be more readily understood from the following detailed description of exemplary embodiments taken in conjunction with the accompanying drawings.

Fig. 1 illustrates an exemplary embodiment of a Magnetic Resonance Imaging (MRI) system 100.

Fig. 2A illustrates an example of local clocks at two unsynchronized communication stations.

Fig. 2B illustrates an example of local clocks at two synchronized communication stations.

Fig. 3 is a diagram for illustrating conditions for clock synchronization between two wireless communication stations communicating via a wireless channel having a slowly varying path delay.

Fig. 4 shows a conceptual diagram illustrating a process for carrier phase synchronization in a wireless communication system.

Fig. 5 shows a functional block diagram illustrating an arrangement for carrier phase synchronization in a wireless communication system.

Fig. 6 shows a functional block diagram of a clock management system for a master station or base station that wirelessly communicates with a remote station or mobile station.

Fig. 7 shows a functional block diagram of a clock management system for a remote station or mobile station in wireless communication with a master station or base station.

Fig. 8 shows a functional block diagram of an arrangement for phase alignment between a received pseudorandom noise (PRN) code and a locally generated PRN code.

Fig. 9 illustrates a multipath propagation phenomenon that may occur in a wireless communication system for an MRI system.

Fig. 10 illustrates an example of an impulse response for a wireless communication channel characterized by multipath propagation.

Fig. 11 illustrates a model of a wireless communication channel characterized by multipath propagation.

Fig. 12 illustrates an example embodiment of a receiver architecture that may be used to receive a wireless signal in a communication channel characterized by multipath propagation.

Fig. 13 illustrates an example autocorrelation of a synchronization sequence that may be transmitted by a wireless communication station.

Detailed Description

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided as teaching examples of the invention.

Fig. 1 illustrates an exemplary embodiment of a Magnetic Resonance Imaging (MRI) system 100.

The MRI system 100 comprises a static magnet 1, a gradient coil 2, a gradient power supply 103, a patient table or patient bed 104, a patient table controller 105, an RF coil unit 106a, a wireless RF station 106b, a transmitter 107, a clock generator 108, an RF/gradient field controller 109, a driver 110, a radio unit 111, a reconstruction front-end 115, a reconstruction system 120, a storage device 121, a display 122, an input unit 123, a main controller 124 and a data generator 125.

In some embodiments, components other than the wireless RF station 106b are included in a master unit separate from the wireless RF station 106 b. Furthermore, the main unit may be divided into a rack and a processing system. In this case, for example, the static magnet 101, the gradient coil 102, the gradient power supply 103, the patient table 104, the patient table controller 105, the RF coil unit 106a, the transmitter 107, the RF/gradient field controller 109, and the radio unit 111 may be provided in the gantry, while the clock generator 108, the driver 110, the reconstruction front end 115, the reconstruction system 120, the storage device 121, the display 122, the input unit 123, and the main controller 124 may be provided in the processing system.

The static magnet 101 has a hollow cylindrical shape, and generates a uniform static magnetic field in its inner space. For example, a permanent magnet or a superconducting magnet is used as the static magnet 101.

The gradient coil 102 has a hollow cylindrical shape, and is disposed inside the stationary magnet 101. The gradient coil 102 may include a combination of three coils corresponding to X, Y, and Z axes that are orthogonal to each other. When the three kinds of coils are supplied with currents from the gradient power supply 103, respectively, the gradient coil 102 generates gradient magnetic fields whose strengths are inclined along the X-axis, the Y-axis, and the Z-axis. In addition, the Z-axis is in the same direction as, for example, the direction of the static magnetic field. The gradient magnetic fields of the X-axis, Y-axis, and Z-axis correspond to, for example, a slice selection gradient magnetic field Gs, a phase encoding gradient magnetic field Ge, and a readout gradient magnetic field Gr, respectively. The slice selection gradient magnetic field Gs is used to determine a given imaging section. The phase encoding gradient magnetic field Ge is used to change the phase of the magnetic resonance signals according to spatial position. The readout gradient magnetic field Gr is used to vary the frequency of the magnetic resonance signals according to spatial position.

The subject 20 is inserted into the inner space (imaging space) of the gradient coil 102 when mounted on the top plate 104a of the patient table 104. The patient table 104 moves the top plate 104a in the longitudinal direction (left-right direction in fig. 1) and the vertical direction under the control of the patient table controller 105. Typically, the patient table 104 is mounted such that the longitudinal direction is parallel to the central axis of the static field magnet 101.

The RF coil unit 106a includes one or more coils housed in a cylindrical housing. The RF coil unit 106a is provided inside the gradient magnetic field coil 102. The RF coil unit 106a is supplied with a high-frequency pulse (RF pulse) from a transmitter 107 to generate a high-frequency magnetic field.

The RF station 106b may be mounted on the ceiling 104a, embedded in the ceiling 104a, or attached to the subject 20. At the time of imaging, the wireless RF station 106b is inserted into the imaging space together with the subject 20, and receives or senses a magnetic resonance signal emitted from the subject 20 as an electromagnetic wave, and generates digital data representing the sensed magnetic resonance signal in response thereto. The wireless RF station 106b may comprise or be attached to one, two or more receiving RF coil units, which may comprise any kind of coils for sensing magnetic resonance signals emanating from the subject 20. The wireless RF station 106b includes functionality to wirelessly transmit as electrical signals (e.g., as digital signals) that represent magnetic resonance signals received from the subject 20.

The transmitter 107 supplies the RF coil unit 106a with RF pulses corresponding to the larmor frequency.

The clock generator 108 generates a first clock signal having a predetermined frequency. This first clock signal may be used as a system clock that serves as a reference for the timing of the overall operation of the MRI system 100.

The RF/gradient field controller 109 changes the gradient magnetic field according to a desired pulse sequence under the control of the main controller 124, and controls the gradient power supply 103 and the transmitter 107 so that an RF pulse can be transmitted. In addition, the RF/gradient field controller 109 is supplied with a first clock signal after the level of the signal is appropriately adjusted by the driver 110. The RF/gradient field controller 109 performs a pulse sequence in synchronization with the first clock signal.

The radio unit 111 receives the magnetic resonance signal digitally and wirelessly transmitted from the wireless RF station 106 b. The radio unit 111 digitally demodulates the received digital magnetic resonance signals and then outputs the demodulated signals to the reconstruction front end 115. The radio unit 111 modulates the first clock signal by the data signal output by the data generator 125, thereby wirelessly transmitting the data signal and the first clock signal as a first transmission signal to the RF station 106 b.

The reconstruction front-end 115 subjects the magnetic resonance signals provided from the radio unit 111 to gain control, frequency conversion, and quadrature detection. The reconstruction front-end 115 also decompresses the amplitude of the magnetic resonance signals compressed in the wireless RF station 106 b.

The reconstruction system 120 reconstructs an image of the subject 20 based on at least one of the magnetic resonance signals processed in the reconstruction front-end 115.

The storage device 121 stores various types of data, for example, image data indicating an image reconstructed in the reconstruction system 120.

The display 122 displays images reconstructed in the reconstruction system 120 or various types of information including various types of operation screens for a user to operate the MRI system 100, under the control of the main controller 124. Any convenient display device (e.g., a liquid crystal display) can be used as the display 22.

The input unit 123 accepts various commands and information inputs from an operator of the MRI system 100. The input unit 123 may include a pointing device such as a mouse or a trackball, a selection device such as a mode switch, and/or an input device such as a keyboard.

The main controller 124 has a CPU, a memory, and the like, which are not shown, and controls the entire MRI system 100.

The data generator 125 generates a data signal for communication with the RF station 106b via the radio unit 111 under the control of the main controller 124.

The general operation of an MRI system or device is well known and therefore will not be repeated here.

The wireless RF station 106b relies on an internal clock for proper synchronization with the clock generator of the MRI system. However, due to the wireless nature of the wireless RF coils and induced RF noise, it is often difficult to accurately synchronize the receiver clock to the system clock using conventional wireless communication methods.

In the following discussion, reference is made to a first communication station (in particular a first wireless communication station) and a second communication station (in particular a second wireless communication station). In some embodiments, the first wireless communication station may be considered a base station and the second wireless communication station may be considered a remote station or a mobile station. In some embodiments, a portion of the main unit (specifically including the RF coil unit 106a, the transmitter 107, the radio unit 111, the clock generator 108, the driver 110, and the main controller 124) of the MRI system 100 may correspond to a first wireless communication station described below, and the wireless RF station 106b may correspond to a second wireless communication station described below.

MRI systems need to maintain an ecosystem that can coexist in a stable and well-controlled medical facility to function properly and without interfering with other electromagnetic equipment. This is due to the high sensitivity of weak patient signals to strong MRI transmitters. These requirements have necessitated that the MRI system be positioned within a limited and limited RF shielded room. MRI location and ecosystem require specific and unusual conditions for wireless communication. The wireless communication protocol should maintain a high level of quality of service in an environment with unpredictable MRI conditions, including high power spurious emissions, dense multipath channel conditions with large variations in signal propagation in location, frequency, and time without affecting the MRI signal or signal-to-noise ratio (SNR).

To address one or more of these issues, in some embodiments, the first wireless communication station of the main unit of the MRI system 100 may communicate with the wireless RF station 106b according to a communication protocol that conforms to the ultra-wideband (UWB) communication standard, wherein short pulses (e.g., less than a few nanoseconds) of Phase Shift Keying (PSK) modulated signals are spread over a wide spectrum. Such short pulse UWB technology may also be referred to as direct sequence UWB (DS-UWB) or impulse radio UWB (IR-UWB). Unlike conventional narrowband technologies (bluetooth, WiFi, etc.) or orthogonal frequency division multiplexing UWB (OFDM-UWB), which are strongly affected by signal propagation conditions, pulsed UWB has been developed explosively in multipath environments such as may be found in MRI rooms. It should be understood here that for a given power emission mask, UWB means a transmission with a bandwidth greater than 500MHz, and for short pulses PSK UWB this means that the energy per bit (Eb) has a spreading factor equal to the channel bandwidth greater than 500 MHz. Because Eb is spread across the UWB channel, there is zero-mean fading. Short pulses also benefit from the timing of the reflected path delay being greater than the transmission period. The probability density function for a short pulse UWB channel can be greater than the free space performance in a multipath environment. The spreading factor and emission limits also mean that the probability of short pulse UWB interference and interception is low, which is necessary for coexistence in environments that generate strong MRI frequencies and harmonic spurs. The UWB standard allows transmissions in the frequency range of 3.1GHz to 10.6GHz, which allows precise frequencies to be selected to avoid heavily congested spectra, such as 2.4GHz and 5.8 GHz.

One of the challenges of clock synchronization in wireless MRI system communication is the time variation of the propagation delay of the wireless clock synchronization signal. Such temporal variations may be caused by movements of the patient, the patient table or an operator in the MRI room.

Fig. 2A and 2B show a simple example of emphasizing the clock synchronization challenge with optical speed swap signals. In particular, fig. 2A illustrates an example of local clocks 2101 and 2202 at two unsynchronized communication stations, and fig. 2B illustrates an example of local clocks 2101 and 2202 being synchronized. Here, it is assumed that the local clock 2101 is a (first) local clock of a first communication station (e.g., a base station) and the local clock 2202 is a (second) local clock of a second communication station (e.g., a remote station or a mobile station), and the timing of the local clock 2202 may be controlled or constrained by the second communication station based on the timing of a signal received at the second communication station from the first communication station.

In this example, the two local clocks 2101 and 2202 in question are separated by five optical minutes. This is the case, for example, if one clock is placed on earth and another clock is placed on a mars orbit. By replacing "minutes" with "nanoseconds", this paradigm can be easily extended to an MRI environment, since light travels about 30cm per nanosecond, whereas the radio channel length in an MRI environment would be on the order of 30cm to 3 m.

Fig. 2A shows an example in which a local clock 2201 at a first communication station is 10 minutes different from a recovered clock 2102 recovered from a signal received by the first communication station from a second communication station, and a local clock 2202 at the second communication station is perfectly aligned with the recovered clock 2102 recovered from the signal received by the second communication station from the first communication station. Since we know that the channel propagation delay is 5 minutes, we can conclude that the local clocks 2101 and 2202 are not synchronized.

Fig. 2B shows an example where the difference between the local clock 2101 and the recovered clock 2102 at the first communication station and the difference between the local clock 2202 and the recovered clock 2201 at the second communication station are both five minutes, which corresponds to a propagation delay. It can be easily shown that if the time difference measured at the two locations is the same, the two clocks 2101 and 2202 are synchronized and will remain synchronized as long as the channel propagation delay is constant or changes at a much lower rate than the clock update rate.

Fig. 3 is a diagram for illustrating a condition for clock synchronization between the first wireless communication station 310 and the second wireless communication station 320 communicating via the wireless channel 10 with a slowly varying path delay. Specifically, as shown in fig. 3, a first wireless communication station 310 (e.g., a master station or a base station) includes a (first) local clock 2102 and a recovered clock 2101 recovered from a signal received by the first communication station 310 from a second communication station 320, where the phase difference between the first local clock 2102 and the recovered clock 2101 isThe second wireless communication station 320 (e.g., a remote station or a mobile station) includes a (second) local clock 2202 and a recovered clock 2201 recovered from a signal received by the second communication station 320 from the first communication station 310, wherein a phase difference between the second local clock 2202 and the recovered clock 2201 isWhen in useClocks 2101, 2202, 2102, and 2201 are synchronized.

To address the challenge of synchronizing the local clock of the second wireless communication station (e.g., RF station 106b of MRI system 100) with the local clock of the first wireless communication station (e.g., the main unit of the MRI system including RF coil unit 106a, transmitter 107, radio unit 111, clock generator 108, driver 110, and main controller 124), the first and/or second wireless communication stations may employ carrier phase tracking, embodiments of which are described below. Here, carrier phase tracking refers to a method of measuring and tracking the phase of a recovered carrier for a received wireless signal. A phase measurement may be established by comparing the phase of the recovered carrier with a local reference signal synchronized to the local clock and hence with the local time. The local reference is in turn used to generate a carrier for the corresponding transmitted signal. It would be beneficial to do this for both the first wireless communication station and the second wireless communication station.

By continuously tracking the carrier phase in both the first and second wireless communication stations and exchanging messages with the phase information, the phase of one local reference (advantageously the phase of the second wireless communication station) can be adjusted so that the carrier phase measurements in both the first and second wireless communication stations produce the same result. Once the carrier phase measurements in both the first and second wireless communication stations produce the same result within a desired accuracy target or threshold, the clocks are considered synchronized.

Fig. 4 shows a conceptual diagram illustrating a procedure for carrier phase synchronization in a wireless communication system 400, the wireless communication system 400 comprising a first wireless communication station 410 and a second wireless communication station 420 communicating via a wireless channel 10. Fig. 5 shows a more detailed functional block diagram illustrating an arrangement for carrier phase synchronization in a wireless communication system 500 comprising a first wireless communication station 510 and a second wireless communication station 520. The wireless communication system 500 may be an embodiment of the wireless communication system 400.

The first wireless communication station 410 comprises an output signal e^jω0·tAnd the second wireless communication station 420 comprises an output signal e^{j(ω0·t+Δω0·t)}And a second local oscillator 4215. That is, typically at the first LO frequency (ω) of first LO 4115₀) Second LO frequency (ω) with second LO 4215₀+Δω₀) There is a frequency difference Δ ω therebetween₀。

In operation, the first wireless communication station 410 processes a first baseband signal 4111 including data to be transmitted to the second wireless communication station 420 (S)₁(t)) to generate a first transmit signal 4119 (S) transmitted by the first wireless communication station 410 over the wireless channel 10₁(t)·e^jN0(ω0·t)). Beneficially, the portion of the first baseband signal 4111 may be a synchronization sequence (e.g., Barker code) whose pattern is known a priori by the second wireless communication station 420. Another portion of the first baseband signal 4111 contains information about the carrier phase difference measured at the first wireless communication station 410.

In the wireless communication system 500, the first wireless communication station 510 utilizes the second value (e.g., N) as the first value₀)5125 and a first LO frequency ω of first LO 4115₀First baseband signal 4111 is upconverted by first carrier signal 5111 at the first carrier frequency of the product to generate first transmit signal 4119. Beneficially, having a first carrier frequency (N)₀·ω₀) Is synchronized with the first LO 4115.

First transmitted signal 4119 is delayed by propagation delay Δ t over wireless channel 10 and received as delayed first transmitted signal 4229 by second wireless communication station 420 (S)₁(t)·e^{jN0(ω0·t+ω0·Δt)}). In response to the received delayed first transmitted signal 4229, the second wireless communication station 420 generates a recovered baseband signal 4228 (S)₁(t)·e^{jN0((-Δω0·t+ω0·Δt)})。

In the wireless communication system 500, the second wireless communication station 520 utilizes the second value (e.g., N) as the first value₀)5225 and a second LO frequency (ω) of a second LO 4215₀+Δω₀) Down-converted signal 5211 at the frequency of the product down-converts received delayed first transmitted signal 4229 to produce recovered baseband signal 4228.

Here, it can be seen that recovered baseband signal 4228 comprises identifying a frequency difference Δ ω between a first LO frequency of first LO 4115 and a second LO frequency of second LO 4215₀And the time delay deltat through the radio channel 10. This information can be used to provide information about the frequency difference Δ ω₀And correction of the time delay at to synchronize the second LO 4215 with the first LO 4115, as explained below.

The second wireless communication station 420 converts the second baseband signal S₂(t) and a correction signal (e)^{jN1(-Δω0·t+ω0·Δt)}) Multiplied to produce a corrected second baseband signal 4211 (S)₂(t)·e^{jN1(-Δω0·t+ω0·Δt)}). Here, it can be seen that the frequency and phase of the correction signal are the frequency difference Δ ω, between the first LO frequency of the first LO 4115 and the second LO frequency of the second LO 4215, respectively₀And the time delay at through the wireless channel 10, which are present in the recovered baseband signal 4228. Advantageously, the second baseband signal S₂The portion of (t) may be a synchronization sequence (e.g., Barker code) whose pattern is known a priori by the first wireless communication station 410. In some embodiments, the second baseband signal S₂(t) may be the first baseband signal S₁(t) a recovery and cleaning version. At S₁(t) and S₂Having the same message content in (t) allows bit error estimation at the first wireless communication station 410. This is done by comparing the message content S of the received signal₂(t) message content S with the signal transmitted by the first wireless communication station 410₁(t) comparing to complete.

In response to the corrected second baseband signal 4211, the second wireless communication station 420 generates a second transmission signal 4219 transmitted by the second wireless communication station 420 through the wireless channel 10 (S)₂(t)·e^{jN1(ω0·t+ω0·Δt)})。

In the wireless communication system 500, the second wireless communication station 520 has the second value (e.g., N)₁)5215 and a second LO frequency (ω) of a second LO 4115₀+Δω₀) The corrected second baseband signal 4211 is upconverted by a second carrier signal 5213 at the frequency of the product to produce a second transmit signal 4219. Beneficially, having a second carrier frequency (N)₁·(ω₀+Δω₀) ) is synchronized with the second LO 4215.

As a result of this process, the second transmit signal 4219 has a second value (N)₁) With the first LO frequency ω₀And a second transmit carrier frequency (N) that is frequency synchronized with first LO 4115₁·ω₀). However, due to channel 10, second transmit signal 4219 also has a phase shift (N)₁·Δω₀Δ t). Since this phase shift will vary with time, it still needs to be evaluated and compensated for. An example of a method for the phase shift correction will be described next.

The second transmitted signal 4219 is delayed by the propagation delay deltat over the wireless channel 10 and is received by the first wireless communication station 410/510 as a delayed second transmitted signal 4129 (S)₂(t)·e^{jN1(ω0·t+2·ω0·Δt)}). In response to the received delayed second transmission signal 4129, the first wireless communication station 410/510 generates a recovered baseband signal 4128 (S)₁(t)·e^{jN1(2·ω0·Δt)})。

In the wireless communication system 500, the first wireless communication station 510 utilizes the second value (e.g., N)₁)5115 and a first LO frequency ω of first LO 4115₀Down-converted signal 5113 at the frequency of the product down-converts received delayed second transmitted signal 4129 to produce recovered baseband signal 4128.

Here it can be seen that the recovered baseband signal 4128 includes information identifying the time delay at through the wireless channel 10. This information can be used to provide corrections to the carrier phase of the first LO 4115 and/or the second LO 4215 to synchronize the LO and clocks of the first wireless communication station 410/510 and the second wireless communication station 420/520 with one another. For example, in some embodiments, the second wireless communication station 420/520 may adjust the phase of its own local oscillator and clock to account for the propagation delay Δ t through the wireless channel 10. In some embodiments, the second wireless communication station 420/520 may transmit a signal (e.g., a synchronization pulse or timing data) to the first wireless communication station 410/510 to cause the first wireless communication station 410/510 to account for the propagation delay Δ t through the wireless channel 10.

With respect to propagation delay at and/or carrier phase shift at ω₀Can be added as message content to the first baseband signal 4111 (S)₁(t)) and fromA wireless communication station 410/510 transmits to the second wireless communication station 420/520. The information regarding the propagation delay at and/or the carrier phase shift at the second wireless communication station 420/520₀Following the information of (e), the second wireless communication station 420/520 may form a third baseband signal 4237 (e)^{jN1(-ω0·Δt)}). The second wireless communication station 420/520 may then utilize the second value as the second value (e.g., N)₁)5215 and a second LO frequency (ω) of a second LO 4115₀+Δω₀) The third baseband signal 4237 is upconverted with a second carrier signal 5213 of the frequency of the product of (e) to produce a carrier-only third transmit signal 4239 (e)^jN1(ω0·t)). The frequency of the carrier-only third transmit signal 4239 is exactly N of the frequency of the first LO 4115 of the first wireless communication station 510₁And (4) doubling. Using a flow having a ratio N₁A frequency divider (prescaler) 4241 having a recovered clock signal 4243 (e) with the same frequency and phase as the first LO 4115 of the first wireless communication station 410/510 (e)^jω0·t) Recoverable by the second wireless communication station 420/520, the second wireless communication station 420/520 is free of any frequency offset due to the LO of the second wireless communication station 420/520 and is also free of any variable phase offset due to the variable propagation delay at of the wireless channel 10.

Carrier phase tracking as described above allows clock synchronization between remotely located wireless communication stations, but provides only relative time measurement and synchronization. A further refinement of the method is to use a known Pseudo Random Number (PRN) sequence as the data stream. By applying a similar phase tracking method to the PRN sequence (also referred to herein as "code phase tracking"), an absolute time measurement can be established and an absolute clock time can be synchronized between the two wireless communication stations.

Fig. 6 shows a functional block diagram of a clock management system for a first communication station 610 (e.g., a master station or a base station), and fig. 7 shows a functional block diagram of a clock management system for a second communication station 620 (e.g., a remote station or a mobile station) that can communicate wirelessly with the first communication station 610. The first communication station 610 may be an embodiment of the first communication station 310, 410, and/or 510 of fig. 3, 4, and 5. The first communication station 610 may be one embodiment of a wireless communication station of a main unit of the MRI system 100 that includes the clock generator 108. The second communication station 620 may be an embodiment of the second communication stations 320, 420 and/or 520 of fig. 3, 4 and 5. The second communication station 620 may be an embodiment of the RF station 106b of the MRI system 100.

The first communication station 610 includes: a (first) local clock 6101, a recovered second clock 6102, a local code Numerically Controlled Oscillator (NCO)6110, a (first) transmit PRN code generator 6112, a local carrier NCO 6115, a transmit time register 6116, a transmit or up-convert mixer 6117, a transmit PRN sequence register 6118, a recovered code NCO 6120, a recovered second PRN code generator 6122, a cross-correlator 6124, a recovered carrier NCO 6125, a receive time register 6126, a receive or down-convert mixer 6127, a received PRN sequence register 6128, a processor 6150 with associated memory, a transmitter 6910 and a receiver 6920.

In operation, the (first) transmit PRN code generator 6112 generates a first transmit PRN sequence 6113 according to a known generator polynomial based on the timing of the local code NCO 6110, which local code NCO 6110 is in turn synchronized with the local clock 6101. The transmit time register 6116 stores the transmit time of the first transmit PRN sequence 6113, and the first transmit PRN sequence 6113 is stored in the transmit PRN code register 6118. The transmission mixer 6117 mixes the first transmission PRN sequence 6113 with a local carrier generated by the local carrier NCO 6115 in synchronization with the local clock 6101, and outputs a first transmission signal 6119 to be transmitted by the transmitter 6910 to the second wireless communication station 620 via a wireless channel. Meanwhile, the receiver 6920 receives a second transmitted signal 6129, the second transmitted signal 6129 comprising such second transmitted signal received from the second wireless communication station 620 via the wireless channel: the phase of the second transmit signal is shifted by a second channel phase shift that depends on the time delay of the wireless channel. The receive mixer 6127 mixes the received phase-shifted second transmit signal 6129 with the down-converted signal generated by the recovered carrier NCO 6125 in synchronization with the recovered second clock 6102, and outputs a received PRN sequence 6123 generated at the second wireless communication station 620 according to the same known generator polynomial, which is used by the first wireless communication station 610 to generate the first transmit PRN sequence 6113. A cross-correlator 6124 correlates the received PRN sequence 6123 with a PRN sequence generated by a second PRN code generator 6122 based on the timing of the recovered code NCO 6120, which in turn is synchronized with the recovered second clock 6102. The receive time register 6126 stores the receive time of the received PRN sequence 6123, and the received PRN sequence 6123 is stored in the sequence register 6128.

The processor 6150 may compare the timing of the transmit PRN sequence 6113 to the timing of the received PRN sequence 6123 to determine the code phase shift error between the first wireless communication station 610 and the second wireless communication station 620.

The second communication station 620 includes: a (second) local clock 6202, a recovered second clock 6201, a local code Numerically Controlled Oscillator (NCO)6210, a (first) transmit PRN code generator 6212, a local carrier NCO 6215, a transmit time register 6216, a transmit or up-conversion mixer 6217, a transmit PRN sequence register 6218, a recovered code NCO 6220, a recovered second PRN code generator 6222, a cross correlator 6224, a recovered carrier NCO 6225, a receive time register 6226, a receive or down-conversion mixer 6227, a received PRN sequence register 6228, a processor 6250 with associated memory, a transmitter 7910, and a receiver 7920.

In operation, the (second) transmit PRN code generator 6212 generates a second transmit PRN sequence 6213 based on the timing of the local code NCO 6210, which in turn, local code NCO 6210 is synchronized with local clock 6202. The transmit time register 6216 stores the transmit time of the first transmit PRN sequence 6213, and the first transmit PRN sequence 6213 is stored in the transmit PRN code register 6218. The transmit mixer 6217 mixes the second transmit PRN sequence 6213 with the local carrier generated by the local carrier NCO 6215 in synchronization with the local clock 6201, and outputs a second transmit signal 6219 to be transmitted by the transmitter 7910 to the first wireless communication station 610 via a wireless channel. Meanwhile, the receiver 7920 receives a phase shifted second transmit signal 6229, the phase shifted second transmit signal 6229 comprising a first transmit signal 6119 received from the first wireless communication station 610 via the wireless channel, the first transmit signal 6119 being phase shifted by a first channel phase shift that depends on the time delay of the wireless channel. Receive mixer 6227 mixes the received phase-shifted first transmitted signal 6129 with the down-converted signal generated by recovered carrier NCO 6225 in synchronism with recovered first clock 6201 and outputs a received PRN sequence 6223. The cross-correlator 6224 correlates the received PRN sequence 6223 with a PRN sequence generated by a second PRN code generator 6222 based on the timing of the recovered code NCO 6220, which recovered code NCO 6220 is then synchronized with the recovered first clock 6201. The receive time register 6226 stores the receive time of the received PRN sequence 6223, and the received PRN sequence 6223 is stored in the sequence register 6228.

The processor 6250 can compare the timing of the transmitted PRN sequence 6213 with the timing of the received PRN sequence 6223 to determine a code phase shift error between the first wireless communication station 610 and the second wireless communication station 620.

In some embodiments, both the first wireless communication station 610 and the second wireless communication station 620 track the difference in carrier phase and code phase between the first wireless communication station 610 and the second wireless communication station 620. In some embodiments, messages may be exchanged between the first wireless communication station 610 and the second wireless communication station 620 to correct for any deviations in carrier phase and/or code phase. In some embodiments, one wireless communication station (e.g., the first wireless communication station 610, which may be an embodiment of a wireless communication station of the main unit of the MRI system 100 that includes the clock generator 108) will serve as a clock reference, while another station (e.g., the second wireless communication station 620, which may be an embodiment of the RF station 106b of the MRI system 100) will adjust its local clock and code phase according to the exchanged messages.

As described above, both the first wireless communication station 610 and the second wireless communication station 620 may generate the same PRN sequence according to a known generator polynomial. Comparing the locally generated PRN symbol string with the received string by means of the cross-correlation block allows for accurate phase alignment between the received PRN sequence and the locally generated PRN sequence. Once the code phase is found, the received PRN code can be tracked using the arrangement shown in fig. 8.

Fig. 8 shows a functional block diagram of an arrangement 800 for phase alignment between a received pseudorandom noise (PRN) code and a locally generated PRN code. In some embodiments, the arrangement 800 may be included in the first wireless communication station 610 and/or the second wireless communication station 620. Arrangement 800 includes a code NCO 820, a PRN code generator 822, an I channel cross correlator 824A, Q, a channel cross correlator 824B, a recovered carrier NCO 825, a transmit time register 826, a down converter 827, a shift register 830, and a processor 850 with associated memory.

The I-channel cross correlator 824A includes multipliers 8242-L, 8242-P and 8242-E and accumulate and dump elements 8244-L, 8244-P and 8244-E. The Q-channel cross correlator 824B may have the same configuration as the I-channel cross correlator 824A.

In some embodiments, the processor 850 with associated memory can include the processor 6150 and associated memory or the processor 6250 and associated memory.

In operation, downconverter 827 receives via a wireless channel a receive signal 829 comprising a PRN sequence generated at another wireless communication terminal. A downconverter receives the downconverted signal from recovered carrier NCO 825 and uses the downconverted signal to downconvert received signal 829 to in-phase ("I") and quadrature ("Q") channel baseband signals, which are supplied by the downconverter to corresponding I-channel cross-correlator 824A and Q-channel cross-correlator 824B. At the same time, the recovered PRN code generator 822 generates a local PRN sequence 823, the recovered PRN code generator 822 supplies the local PRN sequence 823 to the shift register 830, and the recovered PRN code generator 822 stores the generation time of the local PRN sequence 823 to the transmit time register 826. The shift register generates three time-shifted copies 823 (one earlier copy, one immediate copy, one later copy) of the PRN sequence and supplies these copies to multipliers 8242-L, 8242-P and 8242-E, respectively. Multipliers 8242-L, 8242-P, and 8242-E multiply the in-phase channel baseband signal from down-converter 827 with three time-shifted copies of the PRN sequence 823 to produce three in-phase correlation values or results I_E、I_PAnd I_LWhich are supplied to a processor 850. Similarly, a Q-channel cross-correlator 824B correlates the quadrature-phase channel baseband signal from the down-converter 827 with three time-shifted copies of the PRN sequence 823 to produce three quadrature-phase correlation values or results, which are also supplied to the processor 850.

Thus, in the arrangement 800, the incoming PRN code in the received signal 829 is successively compared (i.e., correlated) with three locally-generated copies of the same PRN sequence 823 (one earlier copy, one immediate copy, and one later copy). When properly adjusted, the correlation for the immediate copy of the PRN sequence 823 should have a significantly higher correlation value than the correlation for the earlier and later copies. The correlation results for the earlier and later sequences should also have approximately the same absolute value. The output signals from the earlier correlator, the immediate correlator, and the later correlator can be used to generate a discriminator signal that is applied as feedback to a locally recovered PRN sequence (e.g., PRN code generator 6122 in fig. 6). This feedback will ensure that the recovered PRN sequence is in phase with the received PRN sequence.

As described above, in some embodiments, the first wireless communication station 610 and the second wireless communication station 620 generate two copies of the PRN sequence consecutively. One copy represents the PRN sequence recovered from the received signal stream and the other copy represents the PRN sequence transmitted to the respective other station. Similar to carrier phase tracking, code phase tracking requires clock synchronization if both the received PRN code and the transmitted PRN code at both ends have the same phase.

If there is a difference in code phase between the first wireless communication station 610 and the second wireless communication station 620, one end will adjust the phase of the PRN sequence it transmits so that the code phase of the first wireless communication station 610 and the second wireless communication station 620 are the same. Once the first wireless communication station 610 and the second wireless communication station 620 are locked and properly track the carrier phase and the code phase, the transmitted PRN sequences on both stations can be used as an indication of time. For example, if it is assumed that the total length of the PRN sequence is 1ms and the duration of each individual symbol is 1ns, this arrangement will allow time measurements with a resolution of 1ns and a clock period of 1 ms. Higher time resolution can be achieved by measuring time as a fraction of the symbol duration. Longer periodicity can be achieved by using longer PRN sequences.

In an MRI system, such as MRI system 100, a sampling clock is used to generate and sample various analog signals required to produce an MRI image. These sampling clocks must be synchronized with each other with very high accuracy. In the case of RF sampling clocks, the maximum drift of these clocks should be less than 22ps to keep the phase error in the raw image data below 1 degree. Meanwhile, 22ps is the time taken for the wireless signal to travel about 7 mm.

In the case of a wireless digital receiver for MRI coils (e.g., wireless RF station 106b), the sampling clock internal to such a receiver should be synchronized with the rest of the MRI system (e.g., clock generator 108) by means of a wireless synchronization signal. In the context of MRI systems, wireless transmission of synchronization signals has several associated challenges. One of these challenges is multipath propagation.

Fig. 9 illustrates a multipath propagation phenomenon that may occur in a wireless communication system for an MRI system. Fig. 9 illustrates a wireless transmitter 910 (e.g., of a wireless communication station of the main unit of the MRI system 100) and a wireless receiver 920 (e.g., of the wireless RF station 106 b). Here, the wireless transmitter 910 transmits a wireless signal to the wireless receiver 920 through a wireless channel in the presence of the plurality of reflectors 902. The wireless signal travels from the wireless transmitter 910 to the wireless receiver 920 via various paths including a main line of sight (LOS) path 905 and a plurality of additional paths 915a, 915b, 915c, and 915 d. Further, one or more of the additional paths 915a, 915b, 915c, and 915d may change over time, for example, due to movement of one or more of the reflector 902 and/or the wireless transmitter 910 and/or the wireless receiver 920. This effect causes "multipath fading": the signals on the additional paths 915a, 915b, 915c and 915d arrive at the radio receiver 920 with different delays, amplitudes and phases, thus partially cancelling or augmenting the signal received through the LOS path 905. In the case of wireless signals having very short symbol durations, the multipath signal may even carry information from previous symbols, which may be completely independent of the current symbol.

Fig. 10 illustrates an example of an impulse response 1000 for a wireless communication channel characterized by multipath propagation. In particular, fig. 10 illustrates a typical impulse response function for an ultra-wideband (UWB) transmission channel having a channel bandwidth in excess of 500 MHz. Due to the very large channel bandwidth, the various delayed echoes of the original pulse are clearly distinguishable in time. The delay spread of the impulse response function depends on the properties surrounding the radio channel. Typical delay spreads inside an MRI scanner are on the order of about 10ns, which corresponds to an increase in the delayed signal by a travel distance of about 10 feet. Accordingly, it would be beneficial to provide one or more multipath propagation mitigation techniques that address the characteristics of such environments.

Fig. 11 illustrates a model 1100 of a wireless communication channel characterized by multipath propagation. Here, the transmitted signal 1119 propagates along multiple paths (LOS path and additional paths), each depicted as having a particular delay τ₁...τ_M1112-M and having a complex gain G₁(t)...G_MComplex multipliers 1114-1, 1114-2.. 1114-M of (t). All of these paths are combined by combiner 1116 with the further impairment of Additive Gaussian Noise (AGN)1120 and multiple access interference 1130 to produce a single received signal r (t) 1229.

As mentioned above, all clocks within an MRI system involving mixed signal processing should be synchronized with high accuracy. If this is not done, errors are introduced in the signal chain, which appear as noise on top of the signal or as artifacts due to coding errors. If part of the signal chain is done wirelessly, the clock needs to be recovered from the wireless signal.

However, wireless signals suffer from multipath fading, varying channel delays, unwanted blocking signals, and other factors. Each of these impairments can negatively impact the quality of the recovered clock signal. Various measures can be taken to combat these impairments, including the use of ultra-wideband (UWB) wireless signals (e.g., transmission bandwidths >500MHz), the use of special signal coding based on pseudo-random noise (PRN) codes such as Barker codes or gold codes, the use of short-pulse radar (IR) type signals, the use of matched filters and/or Rake receivers at the receiving end, and/or the periodic characterization of the channel by measuring the time and/or frequency response of the channel.

One way to combat the effects of multipath channel degradation is to probe the channel carefully and then apply corrections to the received signal. For example, the impulse response of a channel can be recorded at a much higher rate than the time variation of the channel and this information is used for a "Rake receiver" at the receiving end. The channel impulse response can be found by applying a short pulse radar (IR) type pulse in the time domain or by sweeping the channel bandwidth using a chirped signal in the frequency domain.

Fig. 12 illustrates an example embodiment of portions of a receiver 1200 that may be used to receive a wireless signal 1229 in a communication channel characterized by multipath propagation. Receiver 1200 includes a plurality of correlators 1212-1, 1212-2.. 1212-M and a signal processor 1250 including variable gain elements 1214-1, 1214-2.. 1214-M, a combiner 1216, an integrator 1218, and a decision block 1220.

In operation, received wireless signal 1229 is provided to correlators 1212-1, 1212-2.. 1212-M, each of which correlates received wireless signal 1229 with a corresponding time delay selected to correspond to one path in a multipath channel. Signal processor 1250 processes the correlation outputs of the plurality of correlators by: weighting the output of each correlator by a corresponding factor α 1,. α M corresponding to the relative strength of the corresponding path in the multi-path channel; adding the weighted outputs of the correlators to produce a sum; integrating the sum over one bit period of the received phase shifted second transmit signal; and compares the sum to a threshold to determine a value for a bit of received wireless signal 1229 in a bit period and outputs multipath compensated output signal 1239.

Multipath echoes in the received signal may also be reduced by using a UWB signal having a long code length as a synchronization pulse. The encoded signal may be selected such that it has a strong autocorrelation peak with low lobes. Fig. 13 illustrates an example autocorrelation of a synchronization sequence that may be transmitted by a wireless communication station to combat multipath propagation effects, exhibiting a strong autocorrelation peak 1310. An example for such a signal is a Pseudo Random Noise (PRN) code, e.g. a Barker code or Gold code.

Although preferred embodiments have been disclosed herein, many variations are possible which remain within the spirit and scope of the invention. Such variations would become clear to one of ordinary skill in the art after inspection of the specification, drawings and claims herein. Accordingly, the invention is not limited except as by the scope of the appended claims.