Technique for the base station to measure the internal uplink and downlink delays of the node without pre-calibration

Systems and methods for measuring end-to-end data path delays between a Radio Equipment Controller (REC) and a Radio Equipment (RE) of a base station are disclosed. In one embodiment, a system includes a RE configured to transmit a chirped sine wave signal from the RE to a REC on an uplink data path from the RE to the REC. The REC is configured to correlate a reference chirped sine wave signal and a received signal from the RE, where the received signal includes the chirped sine wave signal. The REC is further configured to determine an uplink data path delay from the RE to the REC based on results of the correlation of the reference chirped sine wave signal and the received signal from the RE on the uplink data path. In another embodiment, a downlink data path delay is measured in a similar manner.

FIELD OF THE DISCLOSURE

The present disclosure relates to a base station in a cellular communication network and more particularly relates to compensation of radio equipment processing delays.

BACKGROUND

In wireless, or cellular, communication networks, it is important that the downlink and uplink frame timing be synchronized between a base station and mobile devices served by the base station. The mobile devices connected to the base station use the same transmit and receive frequencies.

To ensure that there is no interference between the mobile devices, the mobile devices are assigned time slots or sub-channel frequencies depending on the type of multiplexing (e.g., Time Division Duplex (TDD) or Frequency Division Duplex (FDD)). In either case, frame timing must be precisely maintained over radio links between the base station and the mobile devices.

As shown inFIG. 1, timing must be aligned between a Radio Equipment Controller (REC)10and a Radio Equipment (RE)12such that the first sample of the Downlink (DL) radio frame is transmitted into the air, i.e., reaches an Antenna Reference Point (ARP)14, at the same time as the REC's transmit Basic Frame Transmit Reference Point (BFN@TRP)16. Specifically, the following events all happen simultaneously: the REC's downlink and uplink internal frame timing reference (BFN@TRP), downlink frame (e.g., CPRI downlink frame) starts from the REC10, the uplink frame (e.g., CPRI uplink frame) arrives at the REC10, the downlink radio frame leaves the ARP, and the uplink radio frame arrives at the ARP. The allowed timing error in the radio is typically 20 nanoseconds (ns). This means that the first sample may reach the ARP14at the BFN@TRP16with a ±20 ns delay.

On the Uplink (UL) the first sample of the UL radio frame is the one received at the ARP14at the BFN@TRP. The allowed timing error in the radio is also 20 ns. This means that the sample marked by the radio as the first in the UL radio frame must have entered the ARP14at the BFN@TRP±20 ns.

For DL path delay compensation, the REC10advances the DL baseband data such that it arrives at the radio's ARP14point precisely when it starts out at the REC's transmit reference point16(BFN@TRP). The REC10computes the compensation using the measured DL delay to the radio and the radio DL processing delay the REC10receives from the radio during Common Public Radio Interface (CPRI) path setup.

For the UL path delay compensation, the radio uses path delay information received from the REC10and the radio's internal UL processing delay to advance the CPRI data such that the arrival time of this UL data is aligned with the outbound data. It is up to the radio to provide further internal timing compensation for each carrier and account for variations due to frequency, operating temperature, and component age on both the UL and DL data paths.

During radio production both the DL and UL data paths must be precisely calibrated for timing alignment. The in-equipment delay, or TOFFSET, obtained at production and stored at each radio is then used for the synchronization process. For this to work, a large amount of delay calibration data must be stored in non-volatile memory. A radio must be re-calibrated after factory repairs and this process is complex and time consuming. In addition, with change in frequency, temperature, and component aging, the stored in-equipment delay can change, which results in timing errors. Although the equipment is designed to allow certain timing errors, wide variations can still occur. If such large variations occur, further calibration is required in the field, which is expensive, time consuming, and introduces maintenance problems.

As such, there is a need for an automatic delay calibration technique which eliminates the need to store calibration data with each radio.

SUMMARY

Systems and methods for measuring end-to-end data path delays between a Radio Equipment Controller (REC) and a Radio Equipment (RE) of a base station are disclosed. In one embodiment, a system includes an RE configured to transmit a chirped sine wave signal from the RE to an REC on an uplink data path from the RE to the REC. The REC is configured to correlate a reference chirped sine wave signal and a received signal from the RE on the uplink data path, where the received signal from the RE includes the chirped sine wave signal. The REC is further configured to determine an uplink data path delay from the RE to the REC based on results of the correlation of the reference chirped sine wave signal and the received signal from the RE on the uplink data path. In this manner, a single measurement of the uplink data path delay is made across the uplink data path. This is particularly beneficial for complex base station topologies (e.g., a cascaded topology) where a single measurement of the uplink data path delay avoids build up in tolerance errors across multiple sections of the uplink data path.

In one embodiment, the uplink data path delay is a delay from a radio frequency receive port of the RE to a receive reference point in the REC.

In one embodiment, the uplink data path traverses one or more nodes between the RE and the REC in a cascade arrangement. The one or more nodes include one or more additional REs and/or one or more additional RECs.

In one embodiment, in order to transmit the chirped sine wave signal, the RE is further configured to mix the chirped sine wave signal and a predetermined uplink carrier frequency to provide an upconverted signal, inject the upconverted signal into a radio frequency receive port of the RE such that the upconverted signal passes through a radio frequency interface of the RE to provide samples of the chirped sine wave signal at an output of the radio frequency interface, and transmit the samples of the chirped sine wave signal on the uplink data path from the RE to the REC. Further, in order to correlate the reference chirped sine wave signal and the received signal from the RE on the uplink data path and determine the uplink data path delay from the RE to the REC based on the results of the correlation, the REC is further configured to sample the received signal on the uplink data path to thereby provide samples of the received signal, where the received signal includes the samples of the chirped sine wave signal transmitted from the RE to the REC on the uplink data path. The REC correlates the samples of the received signal and the reference chirped sine wave signal and determines the uplink data path delay from the RE to the REC based on results of the correlation of the samples of the received signal and the reference chirped sine wave signal.

In one embodiment, the RE is further configured to transmit the chirped sine wave signal from the RE to the REC on the uplink data path from the RE to the REC and the REC is further configured to correlate the reference chirped sine wave signal and the received signal from the RE and determine the uplink data path delay from the RE to the REC in response to a link between the REC and the RE becoming operational.

In another embodiment, the RE is further configured to transmit the chirped sine wave signal from the RE to the REC on the uplink data path from the RE to the REC and the REC is further configured to correlate the reference chirped sine wave signal and the received signal from the RE and determine the uplink data path delay from the RE to the REC in response to activation of a carrier in the RE.

In one embodiment, the REC is further configured to transmit a chirped sine wave signal from the REC to the RE on a downlink data path from the REC to the RE. The RE is further configured to receive a signal on the downlink data path from the REC to the RE, where the signal includes the chirped sine wave signal transmitted by the REC on the downlink data path from the REC to the RE. The RE is further configured to pass the signal received on the downlink data path through a radio frequency interface of the RE to provide a radio frequency output signal at a radio frequency transmit port of the RE, sample the radio frequency output signal to provide samples of the radio frequency output signal, and correlate the samples of the radio frequency output signal and the reference chirped sine wave signal. A downlink data path delay from the REC to the RE is determined based on results of the correlation of the samples of the radio frequency output signal and the reference chirped sine wave signal.

In one embodiment, the RE is further configured to determine the downlink data path delay from the REC to the RE based on the results of the correlation of the samples of the radio frequency output signal and the reference chirped sine wave signal.

In another embodiment, the RE is further configured to provide, to the REC, the results of the correlation of the samples of the radio frequency output signal and the reference chirped sine wave signal. The REC is further configured to determine the downlink data path delay from the REC to the RE based on the results of the correlation of the samples of the radio frequency output signal and the reference chirped sine wave signal.

In one embodiment, the downlink data path delay is a delay from a transmit reference point in the REC to the radio frequency transmit port of the RE.

In one embodiment, the downlink data path traverses one or more nodes between the REC and the RE in a cascade arrangement, the one or more nodes comprising at least one of a group consisting of: one or more additional REs and one or more additional RECs.

In one embodiment, the REC is further configured to transmit the chirped sine wave signal from the REC to the RE on the downlink data path from the REC to the RE and the RE is further configured to determine the downlink data path delay from the REC to the RE in response to a link between the REC and the RE becoming operational.

In another embodiment, the REC is further configured to transmit the chirped sine wave signal from the REC to the RE on the downlink data path from the REC to the RE and the RE is further configured to determine the downlink data path delay from the REC to the RE in response to activation of a carrier in the RE.

In another embodiment, a system includes an REC and an RE, where the REC is configured to transmit a chirped sine wave signal from the REC to the RE on a downlink data path from the REC to the RE. The RE is configured to receive a signal on the downlink data path from the REC to the RE, where the signal includes the chirped sine wave signal transmitted by the REC on the downlink data path from the REC to the RE. The RE is further configured to pass the signal received on the downlink data path through a radio frequency interface of the RE to provide a radio frequency output signal at a radio frequency transmit port of the RE, sample the radio frequency output signal to provide samples of the radio frequency output signal, and correlate the samples of the radio frequency output signal and a reference chirped sine wave signal. A downlink data path delay from the REC to the RE is determined based on results of the correlation of the samples of the radio frequency output signal and the reference chirped sine wave signal. In this manner, a single measurement of the downlink data path delay is made across the downlink data path. This is particularly beneficial for complex base station topologies (e.g., a cascaded topology) where a single measurement of the downlink data path delay avoids build up in tolerance errors across multiple sections of the downlink data path.

In one embodiment, the RE is further configured to determine the downlink data path delay from the REC to the RE based on the results of the correlation of the samples of the radio frequency output signal and the reference chirped sine wave signal.

In another embodiment, the RE is further configured to provide, to the REC, the results of the correlation of the samples of the radio frequency output signal and the reference chirped sine wave signal. The REC is further configured to determine the downlink data path delay from the REC to the RE based on the results of the correlation of the samples of the radio frequency output signal and the reference chirped sine wave signal.

In one embodiment, the downlink data path delay is a delay from a transmit reference point in the REC to the radio frequency transmit port of the RE.

In one embodiment, the downlink data path traverses one or more nodes between the REC and the RE in a cascade arrangement. The one or more nodes include one or more additional REs and/or one or more additional RECs.

In one embodiment, the REC is further configured to transmit the chirped sine wave signal from the REC to the RE on the downlink data path from the REC to the RE and the RE is further configured to determine the downlink data path delay from the REC to the RE in response to a link between the REC and the RE becoming operational.

In another embodiment, the REC is further configured to transmit the chirped sine wave signal from the REC to the RE on the downlink data path from the REC to the RE and the RE is further configured to determine the downlink data path delay from the REC to the RE in response to activation of a carrier in the RE.

In one embodiment, an RE is provided. The RE includes a radio frequency interface having a radio frequency transmit port and a radio frequency receive port, a communication interface configured to communicatively couple the RE to an REC, and circuitry configured to mix a chirped sine wave signal and a predetermined uplink carrier frequency to provide an upconverted signal, inject the upconverted signal into the radio frequency receive port of the radio frequency interface such that the upconverted signal passes through the radio frequency interface of the RE thereby provide samples of the chirped sine wave signal at an output of the radio frequency interface, and transmit the samples of the chirped sine wave signal on an uplink data path from the RE to the REC via the communication interface.

In one embodiment, the circuitry is further configured to receive a signal on a downlink data path from the REC to the RE, where the signal includes a chirped sine wave signal transmitted by the REC on the downlink data path from the REC to the RE. The circuitry is further configured to pass the signal received on the downlink data path through the radio frequency interface of the RE to provide a radio frequency output signal at the radio frequency transmit port of the RE, sample the radio frequency output signal to provide samples of the radio frequency output signal, and correlate the samples of the radio frequency output signal and a reference chirped sine wave signal.

In one embodiment, the circuitry is further configured to determine a downlink data path delay from the REC to the RE based on results of the correlation of the samples of the radio frequency output signal and the reference chirped sine wave signal.

In another embodiment, the RE is further configured to provide, to the REC, results of the correlation of the samples of the radio frequency output signal and the reference chirped sine wave signal.

In one embodiment, an REC is provided. In one embodiment, the REC includes a communication interface configured to communicatively couple the REC to an RE and circuitry configured to correlate a reference chirped sine wave signal and a received signal from the RE on an uplink data path, where the received signal includes a chirped sine wave signal. The circuitry is further configured to determine an uplink data path delay from the RE to the REC based on results of the correlation of the reference chirped sine wave signal and the received signal from the RE on the uplink data path.

In one embodiment, the circuitry is further configured to transmit a chirped sine wave signal from the REC to the RE on a downlink data path from the REC to the RE via the communication interface.

In one embodiment, a method of operation of a system including an RE connected to an REC for measuring end-to-end processing delay is provided. In one embodiment, the method includes transmitting, by the RE, a chirped sine wave signal from the RE to the REC on an uplink data path from the RE to the REC and correlating, by the REC, a reference chirped sine wave signal and a received signal from the RE on the uplink data path. The received signal includes the chirped sine wave signal. The method further includes determining an uplink data path delay from the RE to the REC based on results of the correlation of the reference chirped sine wave signal and the received signal from the RE on the uplink data path.

In one embodiment, a method of operation of a system including an RE connected to an REC for measuring end-to-end processing delay is provided. In one embodiment, the method includes transmitting, by the REC, a chirped sine wave signal from the REC to the RE on a downlink data path from the REC to the RE and receiving, by the RE, a signal on the downlink data path from the REC to the RE. The signal includes the chirped sine wave signal transmitted by the REC on the downlink data path from the REC to the RE. The method further includes passing, by the RE, the signal received on the downlink data path through a radio frequency interface of the RE to provide a radio frequency output signal at a radio frequency transmit port of the RE, sampling, by the RE, the radio frequency output signal to provide samples of the radio frequency output signal, correlating, by the RE, the samples of the radio frequency output signal and a reference chirped sine wave signal, and determining a downlink data path delay from the REC to the RE based on results of the correlation of the samples of the radio frequency output signal and the reference chirped sine wave signal.

DETAILED DESCRIPTION

Before describing embodiments of the present disclosure, definitions of a few terms used throughout this description are beneficial. As used herein, a “chirped sine wave” or “chirp sine wave” is a signal which varies from a low frequency to a higher frequency. For example, in one embodiment, a chirped sine wave varies (e.g., linearly or exponentially) from, for example, 100 kilohertz (kHz) to 1 Megahertz (MHz).

“Cross correlation” is a signal processing technique used to measure similarity between two wave forms as a function of a time-lag applied to one of them. For example, in some embodiments, cross-correlation is used to measure a delay between a chirped sine wave signal and a reference chirped sine wave signal with a resolution of, e.g., ±1 nanoseconds (ns).

As indicated previously with reference toFIG. 1, in order to maintain precise frame timing, a number of delays are determined. The particular delays can be described as follows in relation toFIG. 2. According to the Common Public Radio Interface (CPRI) specification, a base station20includes a Radio Equipment Controller (REC)22and a Radio Equipment (RE)24. The REC22is typically connected to the RE24via one or more cables, such as one or more fiber cables. The REC22includes radio functions in the digital baseband domain, whereas the RE24includes analog radio frequency functions. A generic interface between the REC22and the RE24, which is referred to herein as a CPRI interface, enables communication between the REC22and the RE24.

In order to maintain precise frame timing, a number of delays are determined. Specifically, as shown inFIG. 2, these delays include:T12: Cable delay (T12) is a delay between an output interface (R1) of the REC22and an input interface (R2) of the RE24due to a cable connecting the output interface (R1) of the REC22to the input interface (R2) of the RE24,TOFFSET: In-equipment delay (TOFFSET) is an internal delay of the RE24from the input interface (R2) of the RE24to an output interface (R3) of the RE24,T34: Cable delay (T34) is a delay from the output interface (R3) of the RE24to an input interface (R4) of the REC22due to a cable connecting the output interface (R3) of the RE24to the input interface (R4) of the REC22,T14: Total round-trip delay (T14) that is the sum of T12, TOFFSET, and T34,T2a: In-equipment downlink delay (T2a) is an internal delay of the RE24from the input interface (R2) of the RE24to a Transmit Port (TX) or antenna of the RE24, andTa3: In-equipment uplink delay (Ta3) is an internal delay of the RE24from a Receive Port (RX) or antenna of the RE24to the output interface (R3) of the RE24.In operation, the RE24obtains the in-equipment delay (TOFFSET), the in-equipment downlink delay (T2a), and the in-equipment uplink delay (Ta3) and reports those estimates to the REC22. The cable delays (T12and T34) are then determined by the REC22using a synchronization process.

For the synchronization process, the REC22transmits a sync byte, which is referred to as a K28.5 sync byte in the CPRI specification, from the output interface (R1) of the REC22to the input interface (R2) of the RE24. The RE24then passes the sync byte from the input interface (R2) of the RE24to the output interface (R3) of the RE24such that the sync byte is looped-back to the REC22. Using the sync byte, the REC22measures the total round-trip delay (T14), which is the amount of time between a time at which the sync byte was sent from the output interface (R1) of the REC22and a time at which the sync byte was received at the input interface (R4) of the REC22. Then, the REC22computes the cable delays (T12and T34) as:

T12=T34=(T14-TOFFSET)2
where T14is the total round-trip delay measured by the REC22using the sync byte.

With reference toFIG. 3, the base station delays and required corrections are shown. As illustrated above, the base station20computes the connection cable delay using (T14−TOFFSET)/2. The reported in-equipment delay and connection cable delay values are then used to compute the buffering to align the uplink and downlink frames over the CPRI link. The base station20corrects for the uplink and downlink mismatch. The Y (uplink in-equipment delay) and X (downlink in-equipment delay) are aligned using buffers inside the base station20.

With reference toFIG. 4, a block diagram of the radio equipment for measuring actual in-equipment delays in the downlink and uplink directions is illustrated according to one embodiment of the present disclosure. With the embodiments of the present disclosure, there are disclosed systems and methods for measuring end-to-end processing delay in an RE connected to an REC between an input interface and a transmit port in the downlink direction and a receive port and an output interface on the uplink direction. Once a downlink radio channel is activated at a predetermined carrier frequency, a chirp sine wave is added at the input interface such that it becomes mixed with the carrier frequency. Sampling of a received downlink mixed signal is then done at the transmit port once converted to an analog Radio Frequency (RF) signal. The sampled mixed signal is then cross-correlated with a reference chirp sine wave to obtain a signal phase shift and the signal phase shift is then converted to a time delay associated with the downlink processing delay of said RE. Similarly, once the uplink radio channel is activated at a predetermined carrier frequency, the received carrier frequency is mixed at the receive port with a chirp sine wave. Sampling of a received uplink mixed carrier frequency signal is then done at the output interface once the mixed signal is converted to a digital signal. The sampled mixed signal is then cross-correlated with the reference chirp sine wave to obtain a signal phase shift and then converted to a time delay associated with the uplink processing delay of the RE. The downlink and uplink processing delays are then added to obtain the time delay associated with the end-to-end processing delay of the RE.

One advantage of using the system and method of the present disclosure is that the radio re-computes its processing delay after each restart. This avoids having to store component age and operating frequency and temperature calibration data. As indicated above, this data is necessary to re-adjust the factory recorded delay data. Each radio has analog power amplifiers and filter components whose transit delay is affected by age and operating frequency and temperature. The digital components are not as affected by age but their transit delay is still affected by channel frequency and operating temperature.

Another advantage is that the radio can compute its processing delay for each new carrier setup request, such that the base station is provided with the most current radio processing delay information. When there are cellular network timing issues trouble shooters can use this precision measured data to determine root cause of timing failures. As this data has been measured by the radio for the current operating conditions, the technician does not have to repeat these measurements in the field.

With reference toFIGS. 4, 5A, and 5B, a block and flow diagrams illustrate the radio auto calibration of processing delay according to an embodiment of the present disclosure. In an embodiment of the present disclosure, measurement of the end-to-end processing delay is done at start-up as shown inFIG. 5Aand upon activation of a carrier as shown inFIG. 5B.

InFIG. 4, the in-equipment delay of an RE40is measured in the downlink41and uplink42directions. The radio is comprised of a CPRI interface43for interfacing with an REC (not shown) via an input interface44and an output interface45. The radio also has an RF interface46for interfacing with radio antennas (not shown) via transmit port48and receive port49. The radio also has a Digital-to-Analog (D/A) converter and an Analog-to-Digital (A/D) converter50at the RF interface46and a Digital Signal Processor (DSP)51to manage and control the automatic delay calibration functionalities of the RE40.

In order to measure the in-equipment delay of the RE40, on the downlink path41, a low frequency chirped sine wave52is injected into the CPRI interface43at the input interface thereof44. A chirped sine wave is a signal which varies from a low to a higher frequency. In one embodiment, the chirped sine wave varies from 100 kHz to 1 MHz. The chirped sine wave can be a digital signal either stored in memory53or generated on demand using simple trigonometric functions running in the DSP51. At the CPRI interface43, the chirped sine wave is added to the baseband section as data. The mixed downlink signal is passed through the radio converted to an analog signal at the D/A and A/D converter50at the RF interface46, and exits at the transmit port48as a downlink RF signal. The downlink RF signal is then sampled at the transmit port48by means of an RF microwave diode54. The RF microwave diode54is used to sample the RF output power of the RF signal on the downlink channel. In one embodiment, the RF microwave diode54samples the Root Mean Square (RMS) output power at a rate at least double the baseband channel rate for 10 Milliseconds (ms).

The output of the RF microwave diode54is fed to a cross-correlating function55of the DSP51. Cross-correlation is a signal processing technique used to measure the similarity between two waveforms as a function of a time-lag applied to one of the waveforms. In the present embodiment, the added chirped sine wave signal is delayed as it travels the downlink data path of the radio from the input interface44to the transmit port48or the uplink data path from the receive port49to the output interface45. The cross-correlation measures the delay to a resolution of +/−1 ns.

In the present embodiment, the cross-correlation is done against the original chirped sine wave to derive a phase shift between the original chirped sine wave and the recovered sine wave at the output of the RF microwave diode54. Computation of the signal path delay from the cross-correlating function55is a common mathematical technique and need not be described further.

In order to measure the in-equipment delay of the RE40, on the uplink path42, the low frequency chirped sine wave52is mixed with the RF carrier frequency by means of a diode mixer56and then injected into the RF interface46at the receive port input thereof.

In the present embodiment, the diode mixer56mixes a 92.16 MHz chirped sine wave with an RF frequency corresponding to the carrier frequency of the uplink channel.

The mixed uplink signal is passed through the radio RF interface46, converted to a digital signal, and is recovered at the transmit port48of the CPRI interface43by sampling 10 ms of the received baseband signal.

The sampled output of the CPRI interface43is fed to the cross-correlating function55of the DSP51. As for the downlink path, in the present embodiment, the cross-correlation is done against the original chirped sine wave to derive a phase shift between the original chirped sine wave and the recovered sine wave at the output interface45of the CPRI interface43. Computation of the signal path delay is then done as before from the cross-correlating function55. The downlink and uplink path delays are then combined to obtain the end-to-end processing delay of the RE40or TOFFSET. These computed path delays are also used to provide precise time alignment adjustment on the uplink and downlink paths of the RE40.

The embodiments described above relate to determining the in-equipment delay of an RE for the downlink and uplink paths (i.e., T2aand Ta3illustrated inFIG. 2). The end-to-end delays for the uplink and downlink directions are then determined using conventional techniques. Specifically, the cable delays (T12and T34) are measured as

T14-TOFFSET2.
The end-to-end downlink delay is then T12+T2a, and the end-to-end uplink delay is T34+Ta3. Compensation is then applied to synchronize the uplink and downlink radio frames.

For downlink delay compensation, the REC advances the downlink baseband data such that the first sample of the downlink radio frame is transmitted into the air, i.e., reaches the Antenna Reference Point (ARP), at the REC's transmit reference point (Basic Frame Transmit Reference Point (BFN@TRP)). In order to do so, the REC transmits a corresponding CPRI frame starting at a time equal to

BFN@TRP+T14-TOFFSET2+T2⁢a+DLcorrection,
where DLcorrection is a downlink correction. In one embodiment, the downlink correction (DLcorrection) is selected such that

T14-TOFFSET2+T2⁢a+DLcorrection
is equal to a predefined maximum delay. Due to the cable delay

(T14-TOFFSET2),
the CPRI frame arrives at the RE at a time equal to BFN@TRP+T2a+DLcorrection The RE applies a delay equal to the downlink correction (DLcorrection). As a result of the delay for the downlink correction (DLcorrection) and the actual in-equipment downlink delay (T2a) of the RE, the downlink radio frame arrives at the ARP at a time that is equal to the BFN@TRP, within some predefined tolerance (e.g., ±20 ns).

Similarly, for uplink delay compensation, the first sample of the uplink radio frame is the sample received at the ARP of the RE at BFN@TRP. The RE applies a delay equal to an uplink correction (ULcorrection) such that, due to the delay for the uplink correction (ULcorrection), the actual in-equipment uplink delay (Ta3), and the cable delay

(T14-TOFFSET2),
the corresponding CPRI frame arrives at the REC at a time equal to

BFN@TRP+T14-TOFFSET2+Ta⁢⁢3+ULcorrection.
In one embodiment, the uplink correction (ULcorrection) is selected such that

T14-TOFFSET2+Ta⁢⁢3+ULcorrection
is equal to the predefined maximum delay. By selecting DLcorrection and ULcorrection such that both

T14-TOFFSET2+T2⁢a+DLcorrection⁢⁢andT14-TOFFSET2+Ta⁢⁢3+ULcorrection
are both equal to the predefined maximum delay, the uplink and downlink frame timing is aligned.

In one example implementation, the allowed time error tolerance in the base station is summarized as:

ToleranceNode element+/−10 nsREC interface+/−15 nsRE interface+/−35 nsPath delay compensation+/−30 nsPath regulation and timing
In this example, the REC interface tolerance is a factor delay measurement tolerance between the BFN@TRP to the REC's CPRI port, the RE interface tolerance is the factory delay measurement tolerance between the RE's CPRI port and the RE's ARP, the path delay compensation tolerance is the base station's end-to-end delay compensation tolerance for each cascaded path between a REC and the destination RE, and the path regulation and timing tolerance is the maximum allowable variation in timing compensation due to jitter on the CPRI interfaces between a RCE and a particular RE over cascaded paths.

In some implementations, the nodal topology of a base station can be quite complex. For example, the base station may include multiple RECs and multiple REs connected in a cascaded arrangement, star arrangement, or some other arrangement. Depending on the arrangement, time alignment errors build up across sections of the uplink/downlink path. For example, in a cascaded arrangement, the time alignment errors of the RECs and REs build up along the sections of the uplink/downlink path between the nodes in the uplink/downlink path because each section in the overall path between the REC and the RE has its own measurement tolerances. This build-up of time alignment errors along the uplink/downlink path limits the number of RECs and REs that can be cascaded. Further, determining the delays between different RECs and REs in a complex topology is difficult and time consuming (e.g., a large amount of delay calibration data must be stored in non-volatile memory). Also, using the conventional approach, the REC and the RE must be re-calibrated after factory repair. Another issue is that REC and RE component aging and temperature delay compensation is measured only for a small representative sample of devices. Therefore, there is a need for systems and methods for determining end-to-end delays between an REC and an RE, particularly in a complex base station topology.

Systems and methods for measuring end-to-end uplink and downlink path, or processing, delays between an REC and an RE of a base station are disclosed. These systems and methods may be combined with those described above to provide complete delay compensation. For instance, the systems and methods described below may be used to measure end-to-end path delay (uplink and/or downlink) including the internal RE delays, and the systems and methods described above may be further used to fine tune delay adjustments at the RE. For example, the RE may advance the IQ data on the uplink based on the end-to-end uplink path delay measurement and also provide an additional fine delay adjustment using the systems and methods described above.

While these systems and methods described herein can be used for any nodal topology of the base station, they are particularly beneficial for complex nodal topologies such as, for example, a cascade topology where the base station includes multiple RECs and multiple REs connected in a cascade arrangement. One example of a base station58including multiple RECs60-1through60-4and multiple REs62-1through62-12connected in a cascade arrangement is illustrated inFIG. 6. The RECs60-1through60-4are generally referred to herein collectively as RECs60and individually as REC60. Likewise, the REs62-1through62-12are generally referred to herein collectively as REs62and individually as RE62. The RECs60and the REs62are connected via cables (e.g., optical cables) and communicate according to, in the exemplary embodiments described herein, the CPRI protocol.

In this cascade arrangement, the end-to-end uplink and downlink path delays between each REC60and each RE62are needed in order to provide proper time alignment of uplink and downlink radio frames. As discussed below in detail, in order to measure the end-to-end uplink path delay between, for example, the RE62-3and the REC60-1, the RE62-3upconverts a low frequency chirped sine wave signal to the appropriate Rand injects the upconverted chirped sine wave signal into a RF interface of the RE62-3. The upconverted chirped sine wave signal is then passed through the RF interface of the RE62-3such that the chirped sine wave signal is recovered at baseband and then sent to the REC60-1via a CPRI interface of the RE62-3as In-phase and Quadrature (IQ) data. The REC60-1cross correlates a signal received from the RE62-3including the chirped sine wave signal and a reference chirped sine wave signal. Based on the results of the correlation, a phase shift or difference between the two chirped sine wave signals is determined. This phase shift is converted into a time delay, which is the end-to-end uplink path delay between the RE62-3and the REC60-1.

In order to measure the end-to-end downlink path delay between, for example, the REC60-1and the RE62-3, the REC60-1injects a low frequency chirped sine wave signal into a CPRI interface of the REC60-1for transmission to the RE62-3. The RE62-3receives a signal including the chirped sine wave signal from the REC60-1via its CPRI interface and passes the received signal through the RF interface of the RE62-3to provide an RF output signal at an RF transmit port of the RF interface. The RE62-3samples an RF output signal at the RF transmit port of the RE62-3using, e.g., an RF diode. The RE62-3cross correlates the samples of the RF output signal with a reference chirped sine wave signal either at RF or at baseband. A phase shift, or difference, between the two chirped sine wave signals is determined based on the results of the correlation. This phase shift is converted into a time delay, which is the downlink processing delay between the REC60-1and the RE62-3.

The end-to-end uplink and downlink path delays between each REC60and each RE62can be measured in the same manner. The measurements may be made, for example, when a CPRI link to an RE62becomes operational and/or when an REC60activates a carrier for an RE62. This measurement scheme enables measurement of the end-to-end uplink and downlink path, or processing, delays for the current operating temperature, carrier frequency, and component age. This enables an REC60to apply delay compensation on the downlink path to the ARP of an RE62and to provide the RE62with at least part of the uplink path delay information that the RE62uses to compute uplink delay compensation data correction.

Note that the RE62may still use one of the embodiments described above to measure and compensate for its own in-equipment uplink and downlink delays and to compute the delay adjustment that the RE62applies on the uplink. On the uplink path the RE62advances the IQ data based on the internal processing delay of the RE62(measured using, for example one of the embodiments described above with respect toFIGS. 1-5B) and the end-to-end uplink path delay measured using one of the embodiments described with respect toFIGS. 6-10). In this manner, the REC60can precisely compute the end-to-end uplink delay which does include the uplink processing delay of the RE62. The RE60can then compensate fine delay for the uplink path. The same can be done for the downlink, where fine compensation of the internal radio processing delay of the RE62can be performed using, for example, one of the embodiments described above with respect toFIGS. 1-5Band the end-to-end downlink path delay can be measured using one of the embodiments ofFIGS. 6-10.

FIG. 7is a block diagram of one of the RECs60and one of the REs62according to one embodiment of the present disclosure. In this example, the REC60and the RE62are connected through one or more cascaded RECs60and/or REs62. However, the present disclosure is not limited thereto. For example, the REC60and the RE62may alternatively be directly connected (e.g., in a star topology). As illustrated, the REC60includes a DSP64and a CPRI interface66. In this embodiment, the DSP64includes a Cross-Correlation (C-C) function68and memory70. The RE62includes a DSP72, a CPRI interface74, and an RF interface76. The DSP72includes a cross-correlation function78and memory80. The RF interface76includes a D/A converter82and an A/D converter84. While not illustrated, the RF interface76further includes analog transmitter and analog receiver components (e.g., mixers, filters, amplifiers, etc.). The RE62also includes a mixer86(e.g., a diode mixer) and an RF diode88(e.g., a microwave RF diode).

The operation of the REC60and the RE62ofFIG. 7to provide measurements of the uplink and downlink path delays is illustrated inFIGS. 8A and 8B, respectively. In particular,FIG. 8Aillustrates the operation of the REC60and the RE62to measure the uplink path delay between the RE62and the REC60. As illustrated, the DSP72provides a low frequency chirped sine wave signal to the mixer86. At the mixer86, the chirped sine wave signal is upconverted to a desired carrier frequency (fc). The resulting upconverted chirped sine wave signal is then injected into the RF receive port of the RF interface76. The upconverted chirped sine wave signal passes through a receive path of the RF interface76(e.g., amplification, downconversion, filtering, etc.) such that samples of the chirped sine wave signal are recovered and output by the A/D converter84of the RF interface76. At this point, the chirped sine wave signal has experienced a processing delay from the RF receive port to the output port of the RF interface76. The samples of the chirped sine wave signal are then transmitted to the REC60via the CPRI interface74of the RE62in a CPRI frame. At the REC60, the cross-correlation function68of the DSP64cross-correlates the signal received from the RE62in the CPRI frame (which includes the chirped sine wave signal) and a reference chirped sine wave signal (e.g., stored in the memory70) to thereby determine a phase difference between the chirped sine wave signal received from the RE62and the reference chirped sine wave signal. This phase difference is then converted to a time delay, and this time delay is the end-to-end uplink path delay from the receive port of the RE62(i.e., the ARP of the RE62) to the REC60(specifically the BFN@TRP of the RE60).

FIG. 8Billustrates the operation of the REC60and the RE62to measure the downlink path delay between the REC60and the RE62. As illustrated, the DSP64of the REC60injects a chirped sine wave signal into the CPRI interface66to be transmitted to the RE62. At the RE62, a signal including the chirped sine wave signal is received from the REC60via the CPRI interface74of the RE62. The DSP72passes the signal to the input port of the RF interface76where the signal is D/A converted by the D/A converter82and then passed through a transmit path of the RF interface76. The resulting RF output signal at the transmit port of the RF interface76is sampled by the RF diode88. The samples of the RF output signal are provided to the DSP72where the DSP72cross-correlates the samples of the RF output signal and a reference chirped sine wave signal (either at baseband or at RF). Based on the results of the correlation, a phase shift between the two chirped sine wave signals is determined. This phase difference is then converted to a time delay, where this time delay is the end-to-end downlink path delay from the REC60(specifically the BFN@TRP of the REC60) to the transmit port of the RE62(specifically the ARP of the RE62). Notably, in one embodiment, the RE62determines the end-to-end downlink path delay and returns this delay to the REC60via the CPRI interface74. In another embodiment, the RE62returns results of the cross-correlation or the phase difference to the REC60via the CPRI interface74, where the REC60then uses this information to determine the end-to-end downlink path delay.

FIG. 9is a flow chart that illustrates the operation of the REC60and the RE62to obtain end-to-end uplink and downlink delay measurements according to one embodiment of the present disclosure. In this embodiment, the measurements are made when a CPRI link between the REC60and the RE62becomes operational (i.e., in response to a CPRI link between the REC60and the RE62becoming operational). For the end-to-end downlink delay path measurement, the REC60transmits a chirped sine wave signal over a downlink data path to the RE62(step100). More specifically, in one embodiment, the REC60activates a carrier at a mid-band center frequency for the RE62and sets a power output to a minimum value. The REC60then injects the chirped sine wave signal onto a CPRI link from the REC60to the RE62with a sampling rate corresponding to, in one embodiment, a maximum channel bandwidth over the CPRI link.

The RE62receives a signal from the REC60on the downlink data path (step102). More specifically, the RE62receives a signal from the REC60over the CPRI link, where the signal includes the chirped sine wave signal. The RE62passes the signal received from the REC60through the RF interface76to provide an RF output signal at the RF transmit port of the RE62(step104). The RE62samples the RF output signal at the RF transmit port (step106). In one embodiment, the RE62samples the RMS power of the RF output signal via the RF diode88. In one embodiment, the sampling rate is two times the maximum channel rate. The RF output signal is sampled for an appropriate amount of time. In one embodiment, the RF output signal is sampled for 10 ms. The RE62, and in particular the cross-correlation function78, correlates the samples of the RF output signal and a reference chirped sine wave signal (step108). In this embodiment, the RE62then determines the end-to-end downlink path delay based on results of the correlation and returns the end-to-end downlink path delay to the REC60over the CPRI link (steps110and112). Note, however, that in an alternative embodiment, the RE62returns the results of the correlation or a phase difference determined based on the results of the correlation to the REC60, where the REC60then uses this information to determine the end-to-end downlink delay.

For the end-to-end uplink path delay, the RE62transmits a chirped sine wave signal over an uplink data path from the RE62to the REC60(step200). More specifically, in one embodiment, the REC60activates a radio uplink channel at a mid-band center frequency. The RE62uses the mixer86to mix the chirped sine wave signal (e.g., a 92.12 MHz chirped sine wave signal) with the receive carrier frequency (fc) to provide an upconverted, or RF, chirped sine wave signal. The RE62injects the upconverted chirped sine wave signal into the RF receive port of the RE62(i.e., the RF receive port of the RF interface76of the RE62). Using the A/D converter84, the RE62samples the resulting baseband received signal. The RE62then transmits the baseband receive signal to the REC60over the CPRI link. In one embodiment, the RE62sends a 10 ms segment of the received baseband signal that includes the chirped sine wave signal. The REC60correlates the received signal from the RE62with a reference chirped sine wave signal (step202). The REC60then determines the end-to-end uplink path delay based on results of the correlation (step204). More specifically, in one embodiment, the REC60determines a phase shift between the two chirped sine wave signals based on the results of the correlation and then converts the phase shift, or phase difference, to a time delay. This time delay is the end-to-end uplink path delay from the ARP of the RE62and the BFN@TRP of the REC60.

FIG. 10is a flow chart that illustrates the operation of the REC60and the RE62to obtain end-to-end uplink and downlink delay measurements according to another embodiment of the present disclosure. In this embodiment, the measurements are made when the REC60activates a carrier in the RE62(i.e., in response to the REC60activating a carrier in the RE62). For the end-to-end downlink delay path measurement, the REC60transmits a chirped sine wave signal over a downlink data path to the RE62(step300). More specifically, in one embodiment, the REC60activates the carrier at the RE62and sets a power output to a minimum value. The REC60then injects the chirped sine wave signal onto a CPRI link from the REC60to the RE62with a sampling rate corresponding to, in one embodiment, a maximum channel bandwidth over the CPRI link.

The RE62receives a signal from the REC60on the downlink data path (step302). More specifically, the RE62receives a signal from the REC60over the CPRI link, where the signal includes the chirped sine wave signal. The RE62passes the signal received from the REC60through the RF interface76to provide an RF output signal at the RF transmit port of the RE62(step304). The RE62samples the RF output signal at the RF transmit port (step306). In one embodiment, the RE62samples the RMS power of the RF output signal via the RF diode88. In one embodiment, the sampling rate is two times the maximum channel rate. The RF output signal is sampled for an appropriate amount of time. In one embodiment, the RF output signal is sampled for 10 ms. The RE62, and in particular the cross-correlation function78, correlates the samples of the RF output signal and a reference chirped sine wave signal (step308). In this embodiment, the RE62then determines the end-to-end downlink path delay based on results of the correlation and returns the end-to-end downlink path delay to the REC60over the CPRI link (steps310and312). Note, however, that in an alternative embodiment, the RE62returns the results of the correlation or a phase difference determined based on the results of the correlation to the REC60, where the REC60then uses this information to determine the end-to-end downlink delay.

For the end-to-end uplink path delay, the RE62transmits a chirped sine wave signal over an uplink data path from the RE62to the REC60(step400). More specifically, in one embodiment, the REC60activates the radio uplink channel at the carrier frequency. The RE62uses the mixer86to mix the chirped sine wave signal (e.g., a 92.12 MHz chirped sine wave signal) with the receive carrier frequency (fc) to provide an upconverted, or RF, chirped sine wave signal. The RE62injects the upconverted chirped sine wave signal into the RF receive port of the RE62(i.e., the RF receive port of the RF interface76of the RE62). Using the A/D converter84, the RE62samples the resulting baseband received signal. The RE62then transmits the baseband receive signal to the REC60over the CPRI link. In one embodiment, the RE62sends a 10 ms segment of the received baseband signal that includes the chirped sine wave signal. The REC60correlates the received signal from the RE62with a reference chirped sine wave signal (step402). The REC60then determines the end-to-end uplink path delay based on results of the correlation (step404). More specifically, in one embodiment, the REC60determines a phase shift between the two chirped sine wave signals based on the results of the correlation and then converts the phase shift, or phase difference, to a time delay. This time delay is the end-to-end uplink path delay from the ARP of the RE62and the BFN@TRP of the REC60.

The measurement techniques described above provide an automated process by which the base station58measures end-to-end uplink and/or downlink path delay to an accuracy of, for example, +/−20 ns. The disclosed measurement techniques can be used regardless of the topology of the base station58. For instance, the measurement techniques disclosed herein can be used if the base station58has a cascaded REC or RE topology, a star REC or RE topology, or any other suitable topology. As there is a single measurement across the uplink or downlink end-to-end paths there is no build up in tolerance errors when each section of the uplink/downlink path is measured separately and there is no need for factory pre-calibration of delay parameters. Further, the need to store component age and operating frequency and temperature calibration data in the REC60is avoided. The measured processing delay information would be for current operating temperature, component age of devices over the links, and the carrier frequency. This is not possible with the current state of the art which requires factory calibration and storage of typical component age and frequency and temperature compensation data on the base station and the radio. Also, when there are cellular network timing issues, technicians can use the uplink and downlink path delay measurements to determine root cause of timing failures. As this data has been measured by the base station58for the current operating conditions, the technician does not have to repeat these measurements in the field.

The following acronyms are used throughout this disclosure.A/D Analog-to-DigitalARP Antenna Reference PointBFN@TRP Basic Frame Transmit Reference PointC-C Cross-CorrelationCPRI Common Public Radio InterfaceD/A Digital-to-AnalogDL DownlinkDSP Digital Signal ProcessorFDD Frequency Division DuplexIQ In-phase and QuadraturekHz KilohertzMHz Megahertzms Millisecondns NanosecondRE Radio EquipmentREC Radio Equipment ControllerRF Radio FrequencyRMS Root Mean SquareRX Receive PortTDD Time Division DuplexTX Transmit PortUL Uplink