Patent Publication Number: US-2020292666-A1

Title: Multi-chip synchronization for digital radars

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the filing benefits of U.S. provisional application, Ser. No. 62/816,941, filed Mar. 12, 2019, which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention is directed to radar systems, and in particular to digital radar systems. 
     BACKGROUND OF THE INVENTION 
     The use of radar to determine location and velocity of objects in an environment is important in a number of applications including, for example, automotive radar, industrial processes, and gesture detection. A radar system typically transmits radio signals and listens for the reflection of the radio signals from objects in the environment. By comparing the transmitted radio signals with the received radio signals, a radar system can determine the distance to an object, and the velocity of the object. Using multiple transmitters and/or receivers, or a movable transmitter or receiver, the location (angle) of an object can also be determined. 
     A radar system consists of transmitters and receivers. The transmitters generate a baseband signal which is up-converted to a radio frequency (RF) signal that propagates according to an antenna pattern. The transmitted signal is reflected off of object or targets in the environment. The received signal at each receiver is the totality of the reflected signal from all targets in the environment. The receiver down-converts the received signal to baseband and compares the baseband received signal to the baseband signal at one or more transmitters. This is used to determine the range, velocity, and angle of targets in the environment. 
     A MIMO radar system includes a plurality of transmitters and a plurality of receivers. Each of the plurality of transmitters is coupled to a corresponding antenna, and each of the plurality of receivers is coupled to a corresponding antenna. The transmitter and receiver antennas are used to form a first set of virtual antenna locations. The more virtual antennas the better the angular resolution. 
     SUMMARY OF THE INVENTION 
     Embodiments of the present invention provide methods and a system for synchronizing multiple radar chips so that they can be used together to improve performance and/or angular resolution (MIMO radar systems). By synchronizing the chips (each with separate radar systems), all the transmitters (TX&#39;s) of each chip can be aligned so that all TX&#39;s from different chips are synchronized. In addition, all receivers (RX&#39;s) can be aligned so that all RX&#39;s from different chips are synchronized, and therefore, more TX&#39;s and RX&#39;s can be used (even if on different radar system chips) to increase performance and/or angular resolution. Such synchronization is achieved by using available high frequency TX/RX, as well as a controller for controller the synchronization of the TX&#39;s and RX&#39;s. 
     A multi-chip MIMO radar system in accordance with an embodiment of the present invention includes a first integrated circuit chip and a second integrated circuit chip. The first integrated circuit chip includes a first plurality of transmitters and a first plurality of receivers. The second integrated circuit chip includes a second plurality of transmitters and a second plurality of receivers. The first integrated circuit chip is a master chip and the second integrated circuit chip is a slave chip. The first integrated circuit chip includes a synchronization clock operable to synchronize the operation of each of the first and second integrated circuit chips. 
     A method for synchronizing a multi-chip MIMO radar system in accordance with an embodiment of the present invention includes powering up the chips of the radar system. Transmitters and receivers of each chip are synchronized on an intra chip basis such that all the transmitters and receivers of each chip are synchronized. Roughly synchronizing a plurality of chips of the radar system, such that the plurality of chips are synchronized to within 10-100 ns. Performing a 2 GHz chip synchronization using a 2 GHz chip scan on a master chip of the plurality of chips. Performing a fine-tuned inter-chip synchronization by computing a sub-chip misalignment using inter-rangebin interpolation between the master chip and each slave chip of the plurality of chips. The sub-chip misalignment is corrected via pulse swallowing a required number of pulses, in 62.5 ps increments, to align the transmission scans. Finally, the synchronization is validated to ensure the transmission scans are synchronized to a desired sub-chip accuracy. As necessary, the method repeats the 2 GHz chip synchronization and the fine-tuned inter chip synchronization steps. 
     In an aspect of the present invention, an internal sync signal or START signal is used by each chip to synchronize their respective pluralities of transmitters and pluralities of receivers. The internal sync signal is used such that all transmitter dividers and receiver dividers will transition on a same edge of an LO 16 GHz input clock. 
     In another aspect of the present invention, for the rough inter chip synchronization, a 2 GHz signal may be used between the plurality of chips to synchronize internal timers. Roughly synchronized chips are synchronized to within 10-100 ns. 
     In a further aspect of the present invention, 2 GHz chip synchronization includes sending a small scan, such as one pulse repetition interval, using a known pattern from a master chip of the plurality of chips and correlating that pattern on the master chip and the slave chips of the plurality of chips. Using the correlation output, internal timer offsets of each slave chip may be adjusted such that subsequent scans will start with a desired clock boundary. 
     In another aspect of the present invention, fine-tuned inter chip synchronization includes the use of inter-range bin interpolation to compute a sub-chip misalignment between the master chip and the slave chips. With this sub-chip misalignment, pulse swallowing (in 62.5 ps increments) is used to remove the required number of clock pulses to ensure that the subsequent scans are aligned to the desired degree of synchronization. 
     In an aspect of the present invention, data converters are configured to operate at a divide down factor from 16 GHz LO. 
     In another aspect of the present invention, a single LO clock distribution network is used. 
     In yet another aspect of the present invention, local dividers in each chip are synchronized. 
     In a further aspect of the present invention, a first portion of a plurality of chips are used in a first scan, while a second portion of the plurality of chips are used for a second scan. 
     In another aspect of the present invention, a first portion of a plurality of chips performs a first portion of post processing of received data. A second portion of the plurality of chips performs a second portion of the post processing of the received data. 
     These and other objects, advantages, purposes and features of the present invention will become apparent upon review of the following specification in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an automobile equipped with a radar system in accordance with the present invention; 
         FIGS. 2A and 2B  are block diagrams of radar systems in accordance with the present invention; 
         FIG. 3  is a block diagram illustrating a radar system using a single chip with a plurality of receivers and a plurality of transmitters in accordance with the present invention; 
         FIG. 4  is a block diagram illustrating a radar system with multiple chips, each with a plurality of receivers and a plurality of transmitters in accordance with the present invention; 
         FIG. 5  is a diagram illustrating the local oscillator (LO) distribution from either a master chip or an alternative source to all chips to be synchronized in accordance with the present invention; 
         FIG. 6  is a diagram illustrating the use of a free running system timer which can be used for rough clock synchronization in accordance with the present invention; 
         FIG. 7  is a diagram illustrating the use of mini radarscans (e.g. for a single pulse repetition interval) which is provided through either a dedicated TX or a coupled TX and forwarded through a direct connect or over the air to the other slave chips in accordance with the present invention; 
         FIG. 8  is a diagram illustrating a perfect match with a known correlation peak output as well as an interpolated peak with a known correlation peak; 
         FIG. 9  is a diagram illustrating the hardware components for “pulse swallowing” to control the alignment across chips; and 
         FIG. 10  is. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described with reference to the accompanying figures, wherein numbered elements in the following written description correspond to like-numbered elements in the figures. Methods and systems of the present invention result in better performance from a radar system. An exemplary radar system provides an exemplary chip operating frequency for a multi-chip MIMO radar system. The MIMO radar system includes a plurality of transmitters and a plurality of receivers. Each of the plurality of transmitters is coupled to a corresponding antenna, and each of the plurality of receivers is coupled to a corresponding antenna. The transmitter and receiver antennas are used to form a set of virtual antenna locations. 
       FIG. 1  illustrates an exemplary radar system  100  configured for use in a vehicle  150 . In an aspect of the present invention, a vehicle  150  may be an automobile, truck, or bus, etc. The radar system  100  may utilize multiple radar systems (e.g.,  104   a - 104   d ) embedded in the vehicle  150  (see  FIG. 1 ). Each of these radar systems may employ multiple transmitters, receivers, and antennas (see  FIG. 3 ). These signals are reflected from objects (also known as targets) in the environment and received by one or more receivers of the radar system. A transmitter-receiver pair is called a virtual radar (or sometimes a virtual receiver). As illustrated in  FIG. 1 , the radar system  100  may comprise one or more transmitters and one or more receivers ( 104   a - 104   d ) for a plurality of virtual radars. Other configurations are also possible.  FIG. 1  illustrates the receivers/transmitters  104   a - 104   d  placed to acquire and provide data for object detection and adaptive cruise control. As illustrated in  FIG. 1 , a controller  102  receives and then analyzes position information received from the receivers  104   a - 104   d  and forwards processed information (e.g., position information) to, for example, an indicator  106  or other similar devices, as well as to other automotive systems. The radar system  100  (providing such object detection and adaptive cruise control or the like) may be part of an Advanced Driver Assistance System (ADAS) for the automobile  150 . 
     An exemplary radar system operates by transmitting one or more signals from one or more transmitters and then listening for reflections of those signals from objects in the environment by one or more receivers. By comparing the transmitted signals and the received signals, estimates of the range, velocity, and angle (azimuth and/or elevation) of the objects can be estimated. 
     There are several ways to implement a radar system. One way, illustrated in  FIG. 2A , uses a single antenna  202  for transmitting and receiving. The antenna  202  is connected to a duplexer  204  that routes the appropriate signal from the antenna  202  to a receiver  208  or routes the signal from a transmitter  206  to the antenna  202 . A control processor  210  controls the operation of the transmitter  206  and the receiver  208  and estimates the range and velocity of objects in the environment. A second way to implement a radar system is shown in  FIG. 2B . In this system, there are separate antennas for transmitting ( 202 A) and receiving ( 202 B). A control processor  210  performs the same basic functions as in  FIG. 2A . In each case, there may be a display  212  to visualize the location of objects in the environment. 
     A radar system with multiple antennas, transmitters, and receivers is shown in  FIG. 3 . Using multiple antennas  302 ,  304  allows an exemplary radar system  300  to determine the angle (azimuth or elevation or both) of targets in the environment. Depending on the geometry of the antenna system different angles (e.g., azimuth or elevation) can be determined. 
     The radar system  300  may be connected to a network via an Ethernet connection or other types of network connections  314 , such as, for example, CAN-FD and FlexRay. The radar system  300  may also have memory ( 310 ,  312 ) to store software used for processing the signals in order to determine range, velocity, and location of objects. Memory  310 ,  312  may also be used to store information about targets in the environment. There may also be processing capability contained in the application-specific integrated circuit (ASIC)  300  apart from the transmitters  302  and receivers  304 . 
     The description herein includes an exemplary radar system in which there are N T  transmitters and N R  receivers for N T ×N R  virtual radars, one for each transmitter-receiver pair. For example, a radar system with eight transmitters and eight receivers will have 64 pairs or 64 virtual radars (with 64 virtual receivers). When three transmitters (Tx 1 , Tx 2 , Tx 3 ) generate signals that are being received by three receivers (Rx 1 , Rx 2 , Rx 3 ), each of the receivers is receiving the transmission from each of the transmitters reflected by objects in the environment. 
     There are several different types of signals that transmitters in radar systems employ. A radar system may transmit a pulsed signal or a continuous signal. In a pulsed radar system, the signal is transmitted for a short time and then no signal is transmitted. This is repeated over and over. When the signal is not being transmitted, the receiver listens for echoes or reflections from objects in the environment. Often a single antenna is used for both the transmitter and receiver and the radar transmits on the antenna and then listens to the received signal on the same antenna. This process is then repeated. In a continuous wave radar system, the signal is continuously transmitted. There may be an antenna for transmitting and a separate antenna for receiving. 
     Another classification of radar systems is the modulation of signal being transmitted. A first type of continuous wave radar signal is known as a frequency modulated continuous wave (FMCW) radar signal. In an FMCW radar system, the transmitted signal is a sinusoidal signal with a varying frequency. By measuring a time difference between when a certain frequency was transmitted and when the received signal contained that frequency, the range to an object can be determined. By measuring several different time differences between a transmitted signal and a received signal, velocity information can be obtained. 
     A second type of continuous wave signal used in radar systems is known as a phase modulated continuous wave (PMCW) radar signal. In a PMCW radar system, the transmitted signal from a single transmitter is a sinusoidal signal in which the phase of the sinusoidal signal varies. Typically, the phase during a given time period (called a chip period or chip duration) is one of a finite number of possible phases. A spreading code consisting of a sequence of chips, (e.g., +1, +1, −1, +1, −1 . . . ) is mapped (e.g., +1→0, −1→p) into a sequence of phases (e.g., 0, 0, p, 0, p . . . ) that is used to modulate a carrier to generate the radio frequency (RF) signal. The spreading code could be a periodic sequence or could be a pseudo-random sequence with a very large period, so it appears to be a nearly random sequence. The spreading code could be a binary code (e.g., +1 or −1). The resulting signal has a bandwidth that is proportional to the rate at which the phases change, called the chip rate f chip , which is the inverse of the chip duration, T chip =1/f chip . In a PMCW radar system, the receiver typically performs correlations of the received signal with time-delayed versions of the transmitted signal and looks for peaks in the correlation as a function of the time-delay, also known as correlation lag. The correlation lag of the transmitted signal that yields a peak in the correlation corresponds to the delay of the transmitted signal when reflected off an object. The round-trip distance to the object is found by multiplying that delay (correlation lag) by the speed of light. 
     In some radar systems, the signal (e.g. a PMCW signal) is transmitted over a short time period (e.g. 1 microsecond) and then turned off for a similar time period. The receiver is only turned on during the time period where the transmitter is turned off. In this approach, reflections of the transmitted signal from very close targets will not be completely available because the receiver is not active during a large fraction of the time when the reflected signals are being received. This is called pulse mode. 
     The radar sensing system of the present invention may utilize aspects of the radar systems described in U.S. Pat. Nos. 10,261,179; 9,971,020; 9,954,955; 9,945,935; 9,869,762; 9,846,228; 9,806,914; 9,791,564; 9,791,551; 9,772,397; 9,753,121; 9,689,967; 9,599,702; 9,575,160, and/or 9,689,967, and/or U.S. Publication Nos. US-2017-0309997; and/or U.S. patent application Ser. No. 16/674,543, filed Nov. 5, 2019, Ser. No. 16/259,474, filed Jan. 28, 2019, Ser. No. 16/220,121, filed Dec. 14, 2018, Ser. No. 15/496,038, filed Apr. 25, 2017, Ser. No. 15/689,273, filed Aug. 29, 2017, Ser. No. 15/893,021, filed Feb. 9, 2018, and/or Ser. No. 15/892,865, filed Feb. 9, 2018, and/or U.S. provisional application, Ser. No. 62/816,941, filed Mar. 12, 2019, which are all hereby incorporated by reference herein in their entireties. 
     Digital frequency modulated continuous wave (FMCW) and phase modulated continuous wave (PMCW) are techniques in which a carrier signal is frequency or phase modulated, respectively, with digital codes using, for example, GMSK. Digital FMCW radar lends itself to be constructed in a MIMO variant in which multiple transmitters transmitting multiple codes are received by multiple receivers that decode all codes. The advantage of the MIMO digital FMCW radar is that the angular resolution is that of a virtual antenna array having an equivalent number of elements equal to the product of the number of transmitters and the number of receivers. Digital FMCW MIMO radar techniques are described in U.S. Pat. Nos. 9,989,627; 9,945,935; 9,846,228; and 9,791,551, which are all hereby incorporated by reference herein in their entireties. 
     Multi-Chip Synchronization: 
     The present invention provides a method for synchronization of multiple digital radar ASICs of a multi-chip MIMO radar system where each ASIC (chip) can support multiple transmitters and multiple receivers. By synchronizing the different ASICs, and therefore ensuring that all TXs can transmit exactly at the same time and all RXs can receive exactly at the same time, the overall performance of the radar system can be improved. Additional improvements which are possible with a multi-chip synchronized radar system include, e.g.
         different ASICs can be used to process a different range region, or   improve the total number of virtual receivers (angular resolution), or   operate different scans on different ASICs, or   boost the TX power, or   provide more processing power by splitting up the RDC3 post processing on different ASICs.       

     As discussed herein, all transmitters and all receivers of an exemplary radar system operate in lockstep. 
     More specifically, all transmitters transmit their corresponding chips at the same time with respect to the START of the scan, and all receivers sample the received data at the same time with respect to the START of the scan. These conditions are grouped into two synchronization requirements for the multichip system:
         Intra-Chip synchronization (synchronization within the ASIC) and   Inter-Chip synchronization. (synchronization between the ASIC).
 
Both are satisfied to better than 62.5 ps alignment accuracy to not appreciably affect the performance of the radar system. Processing/control commands can be passed between different ASICs e.g. using an Ethernet connection/switch.
       

     Intra-chip synchronization is used to synchronize all n RX&#39;s and all m TX&#39;s on each chip ( FIG. 3 ). The m TX channels and n RX channels on each chip have independent clock dividers, which locally generate the corresponding sample clocks, derived from the common 16 GHz LO clock ( FIG. 8 ). All individual dividers associated with the TX and RX channels are synchronized. Since these dividers cannot be synchronized by simply releasing them from reset at exactly the same time, a system was put in place which achieves synchronization by using a centralized clock gating method. 
     The first “START” signal will release the clock gating and ensure that all TX&#39;s and RX&#39;s are aligned for subsequent radar scans. There are other methods for intra-synchronization. In an aspect of the present invention, inter chip delays are controlled in increments smaller than a sample rate. 
     In addition to the exemplary intra-chip synchronization discussed herein, all TXs and RXs can also be delayed by a programmable sub-clock value to account for the different TX/RX routing on the board and chip. This delay can be achieved either through individual FIFOs (for a number of clocks) and through the use of, e.g., inverters to specify sub-clock delays on a per RX/TX basis. 
     Inter-chip synchronization: even though the intra-chip synchronization described above ensures that all 12 TX channels and 8 RX channels on each individual chip are in lockstep, one chip with respect to another can have a completely random timing relationship. Since each chip generates its own START of scan signal, the inter-chip timing relationship corresponds fundamentally to the inter-chip START timing relationship. The uncertain relative timing between STARTs is caused by three categories of uncertainty mechanisms:
         1. START of scan uncertainty caused by inter-chip communication—a very large error, order of μs.   2. FIFO clock-domain-synchronizers—order of a few chip delays.   3. TX/RX channel clock generation on each chip resulting from dividing down from the 16 GHz LO clock—divider start uncertainty for the sample rate which can be e.g. 1 or 2 GHz.       

     Accordingly, the inter-chip synchronization will have three types of synchronization processes:
         1. SW sync (getting closer to 1-2 μs accuracy)   2. Coarse sync (getting closer to 1 clock accuracy)   3. Fine sync (sub-clock accuracy).       

     The multi-chip arrangement, illustrated in  FIG. 4 , is as follows:
         There is one ASIC chip assigned as “master;”   Each ASIC chip can contain multiple TX antennas and RX antennas; and   Each ASIC chip provides an internal synchronization scheme (intra TX/RX synchronization) to ensure the TX and RX&#39;s of each ASIC chip are properly aligned.       

     The multi-chip arrangement for clock distribution, illustrated in  FIG. 5 , is as follows:
         There is one ASIC chip assigned as “master.”   Exemplary embodiments use either a “master,” which provides the 16 GHz reference clock to all slaves and the master device itself, or alternatively, an external LO will be used.   Wilkinson dividers are used to split the LO and to provide it to the different ASICs.   The distribution of the 16 GHz clock needs to be balanced so that all chips are phase aligned. The alignment impacts both the 80 GHz carrier phase alignment and the inter-chip VRX alignment.
           The phase alignment requires matching lengths to within 0.93 mm (dk=4) for a better than 180° alignment at 80 GHz. The matching requirement is directly related to the beam-forming calibration range.   
               

     The multi-chip arrangement for rough clock synchronization, illustrated in  FIG. 6 , is as follows:
         There is one ASIC chip assigned as “master.”   Each ASIC chip contains a free running system timer for controlling the timing of the “START” of a scan. The free running system timer is started when the chip is released from reset. The start of a scan is scheduled relative to the internal free running system timer. For example, scheduling the “START of a Scan at z” means that the START will be issued when the internal free running timer reaches value z.   After reset, the system needs to identify the relative offset between each free running system timer across all chips.       

     In an exemplary process, the synchronization process follows the steps listed below:
         Each ASIC chip can generate an output signal A at a programmable time relative to the free running system timer.   Each ASIC chip can monitor an input signal B and copy the current value of the free running system timer into an internal register, e.g. at the rising edge of the input signal.   In order to perform a rough clock synchronization, the “master” chip will assert the output signal A at a specific time in the future and all slave chips (can also include master) are programmed to copy their system timer value as soon as they detect a rising edge on the input signal B.   Based on the difference of the retrieved input copy of each ASIC, the Master chip can compute the relative offset of each free running system counter compared to the master free running system counter.   This allows synchronization of all chips within 1-2 μs accuracy using a normal GPIO signal which can operate at a lower frequency (e.g. 200 MHz).   Other synchronization methods can be used, e.g., Ethernet based synchronization, PTP. The more accurate the SW synchronization, the faster the convergence to a fully lock-stepped system.       

     The multi-chip arrangement for coarse/fine synchronization, illustrated in  FIG. 7 , is as follows:
         There is one ASIC chip assigned as “master.”   Exemplary embodiments use either a direct path from the “master” TX to all RX of all chips in the multichip system or spill-over. A direct path could be one dedicated TX to one RX pin of each of the slave devices.   The Master ASIC will perform a small scan for one PRI, e.g. transmit 10,000 chips at 2 GHz chip rate with, for example, a known PRBS pattern. The slave chips are programmed to capture a SCAN, for example, 2.5 μs before the master starts transmitting (5,000 chips/2 GHz=2.5 μs based on the known offsets from the free running system timer achieved through.   Each slave chip is configured to receive, for example, 10,000 chips and correlates against the known PRBS pattern which was sent out by the master chip.   Depending on the range bin where the correlation peak from the master TX was detected, the “offset” for each slave chip can be further refined, since the range bin provides the exact delay between the master TX and the RX on the slave chips.   Based on this synchronization, the “offset” for each slave chip can now be properly adjusted (2 GHz). Assume the original offset between master and slave chip  1 , in regard to the system timer, was −1,000 and the correlation result places the TX pattern at range bin  15  on the slave chip  1 . That means the slave chip  1  started the correlation 15 chips too early and the corrected offset should have been −1000+15=−985. So, if the master schedules a scan to be performed at time X, slave chip  1  should start its “START” scan at time Y=X−985 (see  FIGS. 6 and 7 ).   If the correlation peak cannot be observed because it fell outside the correlation window, the chip rate shall be reduced to half of the initial frequency and the previous step repeated. This situation may appear if software synchronization is based on Ethernet as opposed to the hardwired method illustrated in  FIG. 6 . Alternatively, the chip rate can initially be started low, for example, 125 MHz, and increased to 2 GHz after the offset was adjusted by the amount identified by the correlation peak range bin.       

     The correlation output from an exemplary coarse/fine synchronization illustrated in  FIG. 7  can also be used for fine synchronization. The Fine Synchronization step ensures that the data converters sampling clocks are synchronous across all chips in the system to a relative time accuracy smaller than the sampling clock period, a.k.a. sub-clk synchronization. 
     If, for a given slave chip, the master and slave clocks are in perfect phase alignment, the correlation peak output should reach the maximum possible magnitude with respect to the adjacent range bin correlation magnitudes. The adjacent range bin magnitudes are smaller and equal to one another. If the master and slave clock phase alignment is off by a small fraction, the adjacent correlation range bin magnitudes are imbalanced. In that case, the sub range bin offset can be extracted from the correlation output through, for example, parabolic or quadratic interpolation. 
     The fractional part of the interpolated correlation peak range bin can be used to determine the actual sub-clock time delay by dividing the fractional part with the chip rate, e.g. 0.345/2 GChips/s=0.1725 ns. That time delay can be compensated by the hardware described below. 
     Each exemplary chip contains the HW shown in  FIG. 9  to support the fine synchronization process. The pulse-swallow block can be controlled to skip one 8 GHz pulse. When a pulse is skipped, the div-32 block state does not advance, therefore, the 250 MHz clock edges will be delayed by 125 ps. The glitchless flip circuit allows for a delay of 62.5 ps to be realized. The pulse-swallow and flip blocks provide a mechanism through which the state of the div-32 blocks on each chip of the multi-chip system can be independently controlled (delayed) in 62.5 ps increments. 
     The clock gating block, which can stop and restart the 8 GHz clock going to all 12 TX and 8 RX dividers, is used for intra-chip synchronization. Since the 8 GHz clock distribution is using transmission lines, the first rising edge of the restarted clock arrives at the input of all 20 dividers at substantially the same time (&lt;30 ps error). Intra-chip synchronization is accomplished by stopping the 8 GHz clock, asynchronously resetting all 20 dividers, and restarting the clock. 
     Even though the command to restart the TX/RX clocks can be issued at any time, asynchronously, the clock gating block will release the TX and RX clocks only when the rising edge of the 250 MHz GTCLK occurs. This extra synchronization clock was put in place to support TDM operation, which requires the repeated synchronization of the RX dividers only. 
     The actual number of clock pulses to be swallowed for the fine alignment can be computed by diving the sub clock time, e.g. 0.1725 ns/62.5 ps=2.76≈3. That means we need to swallow 3 elks (@16 GHz), which is the same as 1 clk 8 GHz together with the inverter. In case the sub-clock time is larger than ½*1/(2 GChips/s)=0.25 ns. 
       FIG. 10  illustrates the steps to an exemplary method for synchronizing the chips of a multi-chip MIMO radar system. In step  1002  of  FIG. 10 , the radar system is powered up. In powering the radar system, each chip of a plurality of chips are powered up and clocks on each chip are started. In step  1004  of  FIG. 10 , intra chip synchronization is performed. Such intra chip synchronization includes the use of an internal synchronization signal (e.g., a START signal) on each chip that synchronizes all transmitter dividers and all receiver dividers to transition on a same edge of an LO-I 16 GHz input clock. By using such a START signal, all transmitters and receivers of a chip will be synchronized and the synchronization should not change between a new START signal initiation assuming no clocks were stopped. 
     In step  1006  of  FIG. 10 , a rough inter chip synchronization is performed. Such rough inter chip synching uses a signal between each of the chips to roughly synchronize an internal timer (e.g., 2 GHz), such that the chips will be synchronized to within 10-100 ns. 
     In step  1008  of  FIG. 10 , a 2 GHz chip synchronization is performed. This 2 GHz chip synchronization is performed on a master chip of the plurality of chips. A small scan (e.g., one pulse repetition interval) can be transmitted by the master chip using a known pattern. The transmitted pattern is correlated by the master Chip as well as on each slave chip of the plurality of chips. A correlation output can then be used to adjust internal timer offsets (e.g., 2 GHz) of each slave chip, such that subsequent scans will start at a proper clock boundary and range bin (RB)  0  will be aligned across all chips. 
     In step  1010  of  FIG. 10 , fine-tuned inter-chip synchronization is performed. Such fine-tuned inter-chip synchronization includes the use of inter-rangebin interpolation to compute sub-chip misalignments between the master chip and the slave chips. The determined misalignment can be corrected via “pulse swallowing” (in 62.5 ps increments) the required number of clock pulses to ensure that the subsequent scans are aligned to the desired tolerance level. 
     In step  1012  of  FIG. 10 , the inter-chip synchronization is validated to ensure that the scans are properly synchronized on a sub-chip accuracy. In step  1014  of  FIG. 10 , if the validation fails, the process returns to step  1010  to repeat the fine-tuned inter-chip synchronization, after which, the synchronization validation in step  1012  is repeated. 
     Thus, embodiments of the present invention provide methods and a system for improving performance of a radar system through the synchronization of multiple radar system ASCIs that make up a MIMO radar system. As described herein, after each chip of a multi-chip radar system is individually synchronized, a single scan from a master chip of a plurality of chips of the radar system is used to synch slave chips of the plurality of chips to the master chip. Further synchronization is performed through inter-rangebin interpolation to compute a sub-chip misalignment that may be adjusted by removing (“pulse swallowing”) a desired number of clock pulses to align subsequent scans. Thus, all the transmitters and receivers of a given chip of the plurality of chips will be synchronized with all the transmitters and receivers of another chip of the plurality of chips to within a desired tolerance level. By synchronizing the different chips of the radar system, and therefore ensuring that all transmitters of the radar system can transmit exactly at the same time and all receivers of the radar system can receive exactly at the same time, the overall performance of the radar system can be improved. 
     Changes and modifications in the specifically described embodiments can be carried out without departing from the principles of the present invention which is intended to be limited only by the scope of the appended claims, as interpreted according to the principles of patent law including the doctrine of equivalents.