Method and apparatus for interference cancellation in full-duplex multi-cell networks

Techniques and architectures for multi-stage cancellation of self-interference (SI) and joint cancellation of mutual-interference (MI) and residual SI in signals received by devices of a full-duplex multi-cell network are disclosed. In various examples, channel estimations and interference cancellation operations are performed utilizing multiple orthogonal training signals transmitted by network devices during a common over-the-air training period. Training signals derived from the orthogonal training signals during transmission are utilized to generate SI estimation information and perform at least a first SI cancellation operation on a received signal that includes at least first and second orthogonal training signals. The received signal and orthogonal training signals are then used to estimate a MI channel impulse response and a (residual) SI channel impulse response for use in joint MI/SI cancellation operations on further received signals. Details regarding the design of the orthogonal training signals and a unique system-level delay calibration procedure are also provided.

BACKGROUND

The advent and advancement of full-duplex (FD) technology in radio transceivers is expected to lead to full-duplex enabled multi-cell networks in Fifth Generation (5G) and beyond wireless communication systems. In a FD mode of operation, a device simultaneously transmits and receives using the same time and frequency resources, leading to a potential doubling of spectral efficiency as compared to half-duplex communications. In view of such potential benefits, and despite a number of design challenges, the capability of operating in a FD mode is considered an important enabling technology in next generation wireless communication devices and networks.

Interference mitigation is a particularly important consideration in the design of FD capable communication systems. For example, a communication device operating in a FD mode can experience relatively high levels of self-interference (SI), in addition to mutual-interference (MI) from other nearby FD-enabled devices.

SUMMARY

The present disclosure describes flexible and scalable techniques and architectures for performing multi-stage self-interference (SI) cancellation and joint cancellation of mutual-interference (MI) and residual SI in a FD multi-cell network. Briefly, SI and MI channel estimations and interference cancellation operations are performed utilizing multiple orthogonal training signals transmitted by transmit/receive points (TRPs) during a shared over-the-air training period. Training signals derived from the orthogonal training signals during transmission are utilized to generate SI estimation information and perform partial SI cancellation operations on a received signal that includes at least first and second orthogonal training signals. The received signal and orthogonal training signals are then used to estimate a MI channel impulse response and a (residual) SI channel impulse response for use in joint MI/SI cancellation operations on further received signals. Details regarding the design of the orthogonal training signals and a unique system-level delay calibration procedure for generating timing and phase synchronization information used in SI and MI cancellation operations are also disclosed.

According to one aspect of the present disclosure, a method is provided for cancelling SI and MI in signals received by a full-duplex transmit/receive point (TRP). The method includes determining a first orthogonal training signal of a plurality of orthogonal training signals, mapping the first orthogonal training signal to a transmit antenna of the TRP, receiving (by a receive antenna of the TRP) a signal including the first orthogonal training signal and a second orthogonal training signal originating from a distinct TRP, performing a first SI cancellation operation on the received signal to generate first SI estimation information and cancel a first SI component of the received signal. The first SI cancellation operation utilizes a training signal derived from the first orthogonal training signal in conjunction with the transmission thereof. The method further includes estimating, based at least in part on the received signal and the first orthogonal training signal, a first channel impulse response (CIR) of a wireless channel between the transmit antenna and the receive antenna, the first CIR relating to a second SI component of the received signal, and estimating, based at least in part on the received signal and transmission information relating to the second orthogonal training signal, a second CIR of a wireless channel between the TRP and the distinct TRP.

In some embodiments of the method according to the above-described aspect of the present disclosure or any other aspects thereof, a number of optional operations and features are employed. One optional feature is the application of the SI estimation information and first and second CIRs in performing SI/MI cancellation operations on a received data signal. In this optional feature, the method further includes receiving, at the receive antenna of the TRP, a received data signal, and performing a SI cancellation operation on the received data signal utilizing the first SI estimation information. The optional feature further includes determining a residual SI component of the received data signal based on the first CIR and data transmitted by the transmit antenna of the TRP, determining a MI component of the received data signal based on the second CIR and data transmitted by the distinct TRP, and cancelling the residual SI component and the MI component of the received data signal.

Another optional feature is the method includes, prior to estimating the first CIR and the second CIR, performing a second SI cancellation operation on the received signal to generate second SI estimation information and cancel a second SI component of the received signal, the second SI cancellation operation utilizing a further training signal derived from the first orthogonal training signal.

Another optional feature includes performing a calibration procedure in which the method includes transmitting, via the transmit antenna, a first calibration training signal during a first calibration period, receiving, via the receive antenna, a second calibration training signal transmitted by the distinct TRP during a second calibration period, and estimating, based on the received second calibration training signal, a propagation delay value relating to the wireless channel between the TRP and the distinct TRP. The propagation delay value is used in another optional feature in estimating the first CIR and the second CIR. In other optional features, the plurality of orthogonal training signals comprises zero correlation zone (ZCZ) sequences or Zadoff-Chu sequences modulated with blocks of random phase rotations. According to the above-described aspect of the present invention, advantages of using these types of sequences include improved CIR estimation accuracy, as well as improved interference cancellation performance and the utilization of a shared training period for transmission of the orthogonal training signals.

According to another aspect of the present disclosure, an apparatus is provided for use in a full-duplex network, the apparatus comprising a transmitter operably coupled to a transmit antenna, the transmitter configured to transmit, via the transmit antenna, a first orthogonal training signal of a plurality of orthogonal training signals, a memory, processing circuitry operatively coupled to the memory, a receiver operatively coupled to a receive antenna and the processing circuitry, the receiver configured to receive, via the receive antenna, a received signal including the first orthogonal training signal and at least a second orthogonal training signal of the plurality of orthogonal training signals, the second orthogonal training signal originating from a distinct apparatus. The processing circuitry is configured to perform a first self interference (SI) cancellation operation on the received signal to generate first SI estimation information, and cancel a first SI component of the received signal utilizing a training signal derived from the first orthogonal training signal by the transmitter. The processing circuitry is further configured to estimate, based at least in part on the received signal and the first orthogonal training signal, a first channel impulse response (CIR) of a wireless channel between the transmit antenna and the receive antenna, the first CIR relating to a second SI component of the received signal, and estimate, based at least in part on the received signal and transmission information relating to the second orthogonal training signal, a second CIR of a wireless channel between the apparatus and the distinct apparatus.

In some embodiments of the apparatus according to this aspect of the present disclosure or any other aspects thereof, the processing circuitry is further configured to perform or utilize optional operations and features described in conjunction with the method of the aspect of the disclosure described above.

According to another aspect of the present disclosure, a method for cancelling self interference (SI) and mutual interference (MI) in signals received by a full-duplex transmit/receive point (TRP), the method comprising determining a plurality of orthogonal training signals including at least a first orthogonal training signal and a second orthogonal training signal, mapping the first orthogonal training signal to a transmit antenna (e.g., one of multiple transmit antennas) of a first TRP, mapping the second orthogonal training signal to a transmit antenna of a second TRP, communicating the first orthogonal training signal and the second orthogonal training signal to the first TRP and the second TRP, respectively, for concurrent transmission, receiving, from the first TRP, a received signal including a non-orthogonal training signal derived from the first orthogonal training signal, receiving, from the first TRP, a received signal including the first orthogonal training signal and the second orthogonal training signal as received by a receive antenna (e.g., one of multiple receive antennas) of the first TRP, and performing a first SI cancellation operation on the received signal. The first SI cancellation operation generates first SI estimation information, and utilizes the non-orthogonal training signal to cancel a first SI component of the received signal. The method further includes estimating, based at least in part on the received signal and the first orthogonal training signal, a first channel impulse response (CIR) of a wireless channel between the transmit antenna and the receive antenna of the first TRP, the first CIR relating to a second SI component of the received signal, and estimating, based at least in part on the received signal and the first orthogonal training signal, a second CIR of a wireless channel between the TRP and the second TRP.

Other aspects and features of the present disclosure will become apparent to those of ordinary skill in the art upon review of the following description of specific implementations of the disclosure in conjunction with the accompanying figures.

DETAILED DESCRIPTION

Wireless networks implemented in accordance with conventional Long Term Evolution (LTE) 4G with Time-Division Duplexing (LTE-TDD) and similar communications standards utilize a given frequency band for both downlink and uplink communications. The frequency band is shared by assigning alternating time slots to transmit and receive operations. Consequently, evolution of such standards to include full-duplex (FD) capabilities can lessen the impact of the temporal division and scheduling aspects of the air-interface (e.g., FD can improve latency and throughput) without greatly affecting the rest of the air-interface design. However, in a FD-capable apparatus such as a full-duplex capable transmit/receive point (TRP) operating in a wireless network, interference mitigation is a significant challenge.

For example, effective cancellation is needed for self-interference (SI) that includes the self-reception of signals transmitted by the TRP, because SI can be shown to be more than 100 dB stronger than the sensitivity level of the receiver of the TRP. Although progress has been made in the last few years in improving SI interference techniques, the impact of FD-enabled devices on a wireless network has been the subject of on-going study. Another issue that may be shown to be particularly detrimental to system gain is the effect of mutual-interference (MI) among FD-enabled TRPs when some or all of the TRPs are operating in a full-duplex mode. The novel architectures and methodologies described below provide for cascaded and joint cancellation of SI and MI within and between FD-enabled TRPs.

Referring now toFIG. 1, an embodiment of a full-duplex enabled multi-cell network100in accordance with the present disclosure is shown, including various types of interference. The illustrated embodiment includes TRP102and TRP104having respective coverage areas108and110. Also illustrated are user equipment (UE)112aand112b,within coverage area108, and UE114aand114bwithin coverage area110. Transmit/receive points, including TRP102and TRP104, can include, for example, mobile-relay stations, base stations, gNodeBs (sometimes called “gigabit” NodeBs), site controllers, microcells, picocells, or femtocells, which can be used in conjunction with remote radio heads (RRHs) in some implementations. A RRH can contain radio frequency circuitry plus analog-to-digital/digital-to-analog converters and up/down converters, and does not communicate independently of a TRP. Each UE112a-114brepresents any suitable end user device, and may also be referred to as a user wireless transmit/receive unit (WTRU), mobile station, fixed or mobile subscriber unit and may include a cellular telephone, personal digital assistant, smartphone, laptop or tablet, for example.

In the example network100ofFIG. 1, the first TRP102and the second TRP104operate in FD mode, and the UE112a,112b,114aand114boperate in a legacy half-duplex mode. The network100can include one or more additional transmit/receive points (represented by third TRP106) and UEs having associated interference, details of which are omitted inFIG. 1for sake of clarity. An example of interference between FD-enabled TRPs is described below in conjunction withFIG. 2.

Using LTE-TDD as an example, inter-cell interference from TRP to TRP (TRP-TRP IF or mutual-interference) is largely avoided due to synchronization on a common UL-DL configuration. In contrast, in a FD-enabled network such as network100where TRPs operate in FD mode, this and other types of interference can be problematic. For example, mutual-interference (MI)116can be simultaneously observed in all neighboring TRPs when shared frequency resources are utilized. In particular, TRP102can experience MI116from TRP104while at the same time TRP104can experience MI116from TRP102. Another type of interference in the network100ofFIG. 1is self-interference (SI)118and120, which may be considered to arise from coupling and reflection of a transmitted signal back to a receiver of the transmitting TRP102or TRP104. This type of interference tends to be the most dominant.

Interference observed at a full-duplex TRP102-106in network100can be (at least partially) cancelled as described herein. In particular, the focus of the disclosed techniques is on cancellation of SI118/120and MI116.

The illustrated network100also includes a central access unit126and fronthaul/backhaul TRP communication links128. In one arrangement, central access unit126can control all or a subset of TRPs102-106via the fronthaul/backhaul TRP communication links128, which can include optical, wireless or other connections. It is understood herein that the capacity of the fronthaul/backhaul TRP communication links128is sufficient and the links are essentially lossless for data transfer. Further, although central access unit126is shown as separate from TRPs102-106, in some embodiments it could alternatively be co-located with one or more TRPs. Also, though not shown, the TRPs102-106could communicate directly with each other (e.g., through an Xn interface). Embodiments and options for centralized and de-centralized control and coordination of interference cancellation procedures in accordance with the present disclosure are described more fully below in conjunction withFIG. 4andFIG. 5.

Although the present disclosure describes example embodiments in the context improvements to existing standards, aspects of the disclosure may be applied to other standards compliant communication systems, as well as non-standards compliant communications systems supporting full-duplex operations.

FIG. 2illustrates an embodiment of a FD network200including a plurality of transmit/receive points in accordance with the present disclosure. In addition to TRP102, the FD network200generally includes Kb TRPs. However, only two additional TRPs, TRP104and TRP106, are shown in the illustrated embodiment.

TRP102includes a first transmitter202-1and a first receiver204-1. The second TRP104includes a second transmitter202-2and a second receiver204-2, and the third TRP106includes a third transmitter202-3and a third receiver204-3. Each of the TRPs102-106further includes processing circuitry206. In the illustrated embodiment, the TRPs of the FD network200are controlled by a central access unit126. In other embodiments, such as described in conjunction withFIG. 4, the TRPs of the FD network200are controlled by a Cloud-RAN (C-RAN).

In the illustrated FD network200, the first TRP102experiences first SI210-1at receiver204-1. Likewise, the second TRP104experiences second SI210-2at receiver204-2, and the third TRP106experiences third SI210-3at receiver204-3. The first TRP102is further shown experiencing MI208-2from the transmitter202-2of the second TRP104, and MI208-3from the transmitter202-3of the third TRP106.

As described more fully below in conjunction with the examples ofFIGS. 3-12, novel architectures and methodologies are introduced for interference cancellation in a full-duplex multi-cell network. Briefly, the described cascaded and joint SI and MI cancellation operations utilize multiple orthogonal training signals, which share a common over-the-air (OTA) training period, as well as training signals derived from the orthogonal training signals transmitted by TRPs during the shared training period, to generate channel estimates (e.g., channel impulse responses (CIRs), MIMO channel estimates, and the like). Details regarding the design of the orthogonal training signals based on system requirements are also provided, as well as system-level delay calibration procedures.

FIG. 3illustrates concurrent transmissions of orthogonal training signals between transmit/receive points to generate channel estimation information in accordance with the present disclosure. In the illustrated timing diagram300, orthogonal training signals (which may also be referred to herein as training signals, orthogonal pilot signals and/or pilot signals) are determined by the central access unit126and communicated to neighboring TRPs102-106for concurrent transmission. In one example, at least a first orthogonal training signal is communicated at302(e.g., via a fronthaul link such as shown inFIG. 4) to TRP102and at least a second orthogonal training signal is communicated at304to a further TRP104/106. Depending on the particular multi-cell network configuration, for example, each of TRP104and TRP106may receive one or more distinct orthogonal training signals for concurrent transmission. In one example, the orthogonal training signals are constructed from a general zero-correlation-zone (ZCZ) sequence, and include cyclic prefixes. Use of ZCZ sequences to generate training signals/sequences allows for improved channel impulse response (CIR) estimation accuracy, as well as improved interference cancellation performance. Selection and generation of appropriate orthogonal training signals in view of communications system requirements are described more fully below.

In some embodiments, the central access unit126may further communicate a mapping (not separately illustrated) of the orthogonal training signals to respective transmit antennas of the TRPs102-106. According to other embodiments (such as the distributed embodiment ofFIG. 5), one or both of the orthogonal training signals and associated transmitter antenna mappings are generated or otherwise determined by processing circuitry of the TRPs102-106.

Concurrent transmission of the orthogonal training signals is advantageously scheduled during a shared training period/frame for concurrent transmissions (at310), an example of which is described below in conjunction withFIG. 7. During the illustrated shared training period, the TRP102transmits (at306) one or more orthogonal training signals for reception by TRPs104/106. Likewise, TRPs104/106transmit (concurrently at308) one or more orthogonal training signals for reception by TRP102and, as applicable, other neighboring TRPs.

Using TRP102as an example, a receiver antenna (e.g., one of a plurality of receiver antennas) receives a signal including the one or more orthogonal training signals transmitted by TRPs104/106during the shared training period. The received signal also includes an SI component resulting from an orthogonal training signal transmitted by a transmit antenna (e.g., one of a plurality of transmit antennas) of TRP102at306. As detailed below, at least a first partial SI cancellation operation (which may also be considered an SI estimation operation because it generates first SI estimation information for use in SI operations on subsequently received data signals) is performed on the received signal. The partial SI cancellation operation utilizes a training signal derived from the orthogonal training signal in conjunction with transmission of the orthogonal training signal at306. For example, the partial SI cancellation operation can utilize a filtered version of the orthogonal training signal from the transmission path of TRP102used to transmit the orthogonal training signal. Various such derived training signals are described in greater detail below with reference toFIGS. 4 and 5.

The received signal is then included in reception information that is provided (at314) to the central access unit126for use in estimating CIRs for wireless channels between the receiver antenna (or multiple receiver antennas) of TRP102and the various transmit antennas used to transmit the orthogonal training signals. In the illustrated embodiment, similar reception information is provided to the central access unit126by TRPs104/106.

In one example, the central access unit126utilizes the estimated CIRs to perform joint SI and MI cancellation operations316on data signals received by TRPs102-106. In an alternate example, such as illustrated byFIG. 5, generation of estimated CIRs and joint SI and MI cancellation operations318/320are performed in a distributed manner by TRPs102-106.

SI channel estimation and cancellation techniques based on orthogonal training signals for complexity reduction and numerical stability improvement were described in U.S. Utility application Ser. No. 14/617,598, filed Feb. 9, 2015 and hereby incorporated herein by reference. Such techniques were extended for use in MI channel estimation and cancellation for FD multi-cell applications in U.S. Utility application Ser. No. 14/879,941, filed Oct. 9, 2015 and hereby incorporated herein by reference. In the present disclosure, novel improvements to and applications of such techniques are described.

Referring more specifically toFIG. 4, a schematic block diagram of an embodiment of full-duplex multi-cell network400in accordance with the present disclosure is shown. The illustrated network400includes TRP102, TRP104and TRP106(collectively referred to herein as TRPs102-106). A network implemented in accordance with this embodiment can include a different number of TRPs than is shown. In general, the TRPs102-106(e.g., remote radio units (RRUs)) receive orthogonal training signals410(or information sufficient to identify, retrieve, construct, or otherwise determine the orthogonal training signals410), relevant downlink (DL) data434and control information from a C-RAN402or other centralized control node, and provide uplink (UL) data436to the C-RAN402. The C-RAN402and TRPs102-106may communicate with each other, for example, utilizing the latest CPRI standard, Coarse or Dense Wavelength Division Multiplexing technology, and/or mmWave to enable transmission of control and baseband signals over a relatively long distance.

The C-RAN402of the disclosed embodiment includes a downlink data modulator404that provides DL data434to the TRPs102-106for transmission. The C-RAN402further includes an uplink data demodulator412for demodulating UL data436received from the TRPs102-106. In this embodiment, a ZCZ sequence set builder406of the C-RAN402generates the orthogonal training signals410-1-410-3(collectively referred to as training signals410) transmitted by the TRPs102-106. The training signals410are individually mapped to respective transmit antennas of the TRPs102-106by ZCZ sequence to Tx antenna mapper408. The resulting mapping information is communicated (e.g., tunneled) to the TRPs102-106in conjunction with the training signals410. An insert pilot module414of each of the TRPs102-106receives the training signals410, which can then be multiplexed with other DL data434for transmission. The training signals410are transmitted during a shared training period and in compliance with a standard air-interface, such as described in conjunction withFIG. 7.

Each of the illustrated TRPs102-106further include a low-pass/emission filter (EMF)416, an upconverter418, a high-power amplifier (HPA)420, transmit/receive antennas422(including multiple transmit antennas and multiple receive antennas), and a downconverter428. Each of these elements operates in a generally conventional manner to form the Tx and Rx chains of the TRPs102-106(other elements of which are not separately illustrated for sake of clarity), and also support the interference mitigation techniques described herein. The TRPs102-106may perform, for example, standard transceiver functionality. As also illustrated, multiple forms of training signals (also referred to herein as derived training signals) are generated at the output nodes (or test points) Fkand Gkof the Tx chains of the TRPs102-106during transmission of orthogonal training signals (which are present at test point Bkduring the transmission process).

As also illustrated, the TRPs102-106of this embodiment include cascaded SI estimation and cancellation stages, shown as a stage-1canceller (S1424) and a stage-2canceller (S2426), and the C-RAN402includes a stage-3canceller (S3430) and a stage-4canceller (S4432). As illustrated in the example ofFIG. 5, other arrangements are possible, and one or both of S3and S4(or the like) could be included in a TRP in a decentralized approach. Although S1-S4are generally referred to as cancellation stages or cancellation operations, channel estimation operations such as those described herein are also performed by S1-S4.

In general, the S1-S4cancellers generate differing SI/MI channel estimations, based in part on (orthogonal and/or non-orthogonal) training signals, and perform SI/MI cancellation operations based on the channel estimations and relevant transmission information (e.g., known transmitted data symbols or information sufficient to effectively recreate such data symbols).

The orthogonal training signals are, primarily, the training signals for MI channel estimation at S4432, and can also be used for residual SI channel estimation and cancellation at S4432. The orthogonal training signals are considered to be wideband signals having a bandwidth that exceeds the normal downlink signal bandwidth (e.g., for a 20 MHz LTE signal, the bandwidth of the orthogonal training signals is approximately 30.72 MHz). A filtered version of the orthogonal training signal, such as an orthogonal training signal filtered by the EMF416(at test point Fk) in a TRP102-106, can serve as a derived training signal for SI channel estimation for S2426and S3430. The filtered training signal is, in general, a narrowband signal that is no longer orthogonal with respect to the wideband orthogonal training signals. The training signal tapped at test point Gkis mainly utilized for SI channel initial estimation (tuning) and continuous tracking for S1424, and can optionally be used at S3430as well. Note that training signal routing ofFIGS. 4 and 5are for illustrative purposes and necessary upconverters and downconverters, including digital-to-analog converters (DACs) and analog-to-digital converters (ADCs), may be required (but are not shown) to fulfill the functionality of a particular S1-S4canceller.

The various interference cancellation stages of a receiver of the illustrated TRPs102-106/C-RAN402have different objectives and different implementations, and as noted utilize differing forms of training signals. S1424, for example, targets the strongest SI, which arises due to leakage and coupling between antennas of a TRP. S1424is implemented in the analog RF domain and operates before the frontend operations of a receiver, functioning to reduce the dynamic performance requirements of the receiver frontend. S1424may utilize different algorithms based on the nature of the training signal at the output of HPA420, and typically requires a dedicated training period for tuning and tracking. Although the algorithms and implementations of S1424are generally beyond the scope of this disclosure, the disclosed architectures provide a framework, including a derived training signal and a shared training period for S1424, to tune and track an SI channel.

S2426targets the relatively strong SI due to close-in signal reflections around antennas, and may be implemented in the analog domain or in mixed analog and digital domains to improve the dynamic range of ADCs used at various stages. S3430targets the SI due to residuals of previous stages and distant reflections, and can occur in the digital domain. In the illustrated network400, a received signal that is operated on by S1424and S2426is provided to S3430/C-RAN402at test point Hk. Both S2426and S3430may utilize a narrowband training signal (e.g., from test point Fk) in a Tx path to take advantage of complexity reduction due to a reduced canceller order. S3430may also utilize a derived training signal tapped at test point Gkto achieve improved cancellation of HPA nonlinearities and noise floor. The disclosed architecture enables SI cancellation operations that are compatible, in both cascaded and joint manners, with the MI cancellation operations at S4432.

The S4432canceller mainly targets the MI between TRPs in a full-duplex multi-cell (FD-MC) network, and is preferably implemented in the digital domain. Residual SI components of a received signal can be cancelled jointly with MI at S4432as described below. S4432can require orthogonal training signals due to the potential of a large number of transmit antennas to be addressed in a FD-MC network, and the least-square based channel estimation used in certain embodiments of S2426and S3430may not be tractable if non-orthogonal training signals are employed. The interference-mitigated signal at the output of S4432(test point Mk) is provided to uplink data demodulator412for further processing.

A C-RAN based FD-MC network can implement aspects of the present invention in a centralized manner because transmission information relating to received signals can be readily available at, for example, a central access unit CU, and interference cancellation can be performed digitally, for example, in centralized baseband circuitry. Aspects of the present application can be implemented at TRPs simultaneously for hybrid analog and digital interference cancellation in TRPs where the required transmission and channel information, or reconstructed copies of the interference, can be passed from the C-RAN to the TRPs with proper timing advances. One of the advantages of two-stage hybrid analog/digital and digital cancellation in TRPs and a C-RAN is that an improved dynamic range can be achieved for both the interference residual and uplink signal.

FIG. 5is a schematic block diagram of another embodiment of a full-duplex multi-cell network500in accordance with the present disclosure. In this embodiment, certain of the operations and functionality occurring in the C-RAN402ofFIG. 4are instead performed in a distributed manner within TRP502, TRP504and TRP506(collectively referred to herein as TRPs502-506). A network implemented in accordance with this embodiment can include a different number of TRPs than is shown inFIG. 5.

In particular, the illustrated TRPs502-506include a downlink data modulator508for modulating data received from a fronthaul link538, as well as an uplink data demodulator534for demodulating received data for provision to a backhaul link540. The TRPs502-506further include a ZCZ sequence set builder and antenna mapper module512, which operates to generate or otherwise determine training signals510-1,510-2, etc. (collectively referred to as training signals510) and associated antenna mappings. In addition, each of the illustrated TRPs502-506include a TRP-TRP calibration module536that generates respective group delay (GD) and time-of-arrival (TOA) estimations for use as detailed below. The remaining elements of a TRP502-506, including insert pilot module514, EMF516, upconverter518, HPA520, antennas522, S1524, S2526, downconverter528, S3530and S4532operate in like manner to the corresponding elements ofFIG. 4. The interference-mitigated signal at the output of S4532(test point Mk) is provided to uplink data demodulator534for further processing.

In the network500, orthogonal training signals510and/or transmission information relating to the transmission of orthogonal training signals510by distinct TRPs is shared between TRPs502-506for use in channel estimation and interference cancellation operations. Sharing of such information can be performed via fronthaul/backhaul link538/540or any other suitable communication link available to the TRPs502-506, examples of which can include links in unlicensed spectrum (LTE-U), license assisted access (LAA) links, IEEE 802.11 links, etc.

A TRP102-106or TRP502-506can, in certain embodiments, save a received training signal (or multiple such signals) locally or remotely. For example, an orthogonal training signal410/510(or information sufficient to recreate or duplicate an orthogonal training signal410/510) may be saved to a local memory, a remote memory, a local database, a remote database, a local server, a remoter server, or the like.

FIG. 6is a logic diagram600illustrating an example of interference cancellation in a full-duplex enabled network in accordance with the present disclosure. In this example, channel estimation/CIR information is generated and utilized to cancel SI and MI in a received data signal. Various of the illustrated operations may be performed, for example, by one or more FD TRPs operating in a distributed manner or in combination with one or more central access units or a C-RAN, such as illustrated inFIGS. 4 and 5.

In the illustrated example, channel estimation operations involving mutually orthogonal training signals are preceded by a calibration procedure (block602) that estimates time-of-arrival (TOA)/propagation delay information for participating TRPs. The calibration procedure, an example of which is described in conjunction withFIGS. 9 and 10, may be performed once in a system-level calibration procedure or, alternatively, on a periodic, scheduled or as-needed basis. Next, a plurality of mutually orthogonal training signals (also referred to as OTSs) are generated (block604) for use in generating estimations of CIRs for communication channels between various transmit and receive antennas of the participating TRPs. Each of the training signals could be a ZCZ signal or any other type of signal satisfying equation (3) below and otherwise meeting system requirements such as described herein. As noted, the mutually orthogonal training signals can be generated by one or more individual TRPs, a central access unit or C-RAN, or combinations thereof. Further, the calibration procedure of block602may utilize orthogonal training signals, such as those generated at block604, or other appropriate training signals.

The OTSs are then mapped to individual transmit antennas of the TRPs (block606), and transmitted at block608. Transmission of the OTSs occurs, for example, during a shared training portion of a subframe such as shown inFIG. 7. The transmitted OTS are received by receive antennas of the participating TRPs. In an example (block610), a receive antenna of a first TRP receives a signal that includes a first OTS transmitted by a transmit antenna of the first TRP and a second OTS transmitted by a transmit antenna of a second or distinct TRP. Additional OTSs of the plurality of OTSs may likewise be concurrently received by the receive antenna of the first TRP. A TRP may contemporaneously receive other data in addition to the transmitted OTSs (at another receive antenna, in other frames/sub-frames, etc.).

The first TRP of the illustrated example then utilizes a training signal derived from the first OTS during transmission thereof to perform at least a first, or partial, SI cancellation operation (block612) on the received signal (which includes the first and second transmitted OTSs). Although not separately illustrated, multiple such partial SI cancellation operations can be performed on signals received at other receive antennas of the first TRP and/or other participating TRPs, and can include cancellation operations utilizing one or more additional training signals derived from an associated OTS (e.g., narrowband signals, non-orthogonal signals, analog or digital domain signals, etc.).

Based at least in part on the received signal and the first OTS, a first CIR is then estimated (block614) for a wireless channel between the transmit antenna and the receive antenna of the first TRP. In addition, the received signal, as well as the second OTS or (transmission) information relating to the second OTS, is utilized to estimate a second CIR (block616) for a wireless channel between the first TRP (e.g., one or more receive antennas of the first TRP that receives the second OTS) and the second TRP (e.g., one or more transmit antennas of the second TRP used to transmit the second OTS).

Upon receiving a data signal (e.g., from a wireless data stream during normal network communications, control signaling, etc.) at a receive antenna of the first TRP (block618), a SI cancellation operation is performed on the (block620). This SI cancellation operation utilizes the first SI estimation information. As discussed above, multiple such SI cancellation operations, including cascaded SI cancellation operations, can be performed utilizing SI estimation information determined using training signals derived from transmitted OTSs.

A residual SI component of the received data signal is then determined (block622) based on the first CIR and data transmitted by the transmit antenna of the first TRP. An MI component of the received data signal is also determined (block624) based on the second CIR and data transmitted by the TRP. Once determined, the residual SI component and the MI component of the received data signal are cancelled (block626), and the resulting interference-free or interference mitigated version of the received data signal is passed to a (centralized) demodulator for further processing. An example of SI/MI interference reconstruction and cancellation is described in conjunction withFIG. 11.

The following are non-limiting examples of orthogonal training signal generation and training period configurations in a FD-enabled multi-cell system in accordance with the present disclosure. In this example, the TRPs could take the form of microcells or picocells, and the cell size (radius) is considered to be in the range of [5, 100] meters. The relative frequency and phase synchronization requirements for microcells/picocells has been specified in Technical Specifications promulgated by the 3GPP as follows:

The absolute time synchronization between a C-RAN and backhaul links is beyond the scope of this disclosure. In this example, the frequency and phase synchronization between TRPs is expected to be achieved, for example, by GPS links for each TRP or CPRI interfaces between the C-RAN servers of each TRP or by measurements during the calibration procedure. Accordingly, the frequency synchronization can be assumed to be near perfect for purposes of this disclosure and phase synchronization is subject to the uncertainties mentioned above.

The phase uncertainty of ±1.5 μs (or 3 μs in total) can be addressed by extending the propagation delay between TRPs, which is a factor in training signal design and SI/MI channel estimation. A calibration procedure for estimating the propagation delays between TRPs is described below in conjunction withFIGS. 9 and 10. Assuming stationary TRPs, this procedure only needs to be done once in the system calibration procedure, but may also be performed on a periodic, scheduled or as-needed basis.

Assuming an LTE baseband sampling rate of 30.72 MHz (Ts=1/30.72 μs), combining the worst case phase uncertainty (3 μs, TeTs) and physical propagation delay (0.34 μs, TdTs) between two TRPs yields the total “equivalent” propagation delay:

Based on the modeling of filters in TRP transmit and receive paths and the delay spread of multipath channel, the number of taps (at the 30.72 MHz sampling rate) required for the channel estimation of TRP-TRP MI is in the range of [100, 300], which suggests that TRP-TRP MI channel maximum delay spread (LmTs)=9.8 μs.

The cyclic prefix (CP) length required by the orthogonal training signals is the sum of the two delay values above, or 13.14 μs, which corresponds to 403 LTE baseband samples. For margin, the CP length for the training signals of this example is set at 450 samples at the LTE baseband sampling rate. The total length of the training signal (with CP) is chosen to be 2560 samples, which is the length of an extended OFDM symbol in LTE. It follows that the length of the ZCZ sequence that performs cross-correlations for channel estimation is 2110 samples (2560—CP length). The parameters for an exemplified design of a training signal and the training period are summarized in Table 1 below.

FIG. 7illustrates an example frame structure including a sequence of subframes in accordance with the present disclosure. The illustrated frame structure of this example is applicable to an air-interface based on an LTE-TDD frame structure. A detailed discussion presenting various frame structures supporting full-duplex operation, an LTE-compatible air-interface design providing flexible allocation of resource configuration, and dedicated training periods for FD operation in multi-cell applications is presented in U.S. Utility application Ser. No. 14/617,679, filed Feb. 9, 2015 and hereby incorporated herein by reference. The methodologies of the present disclosure are applicable to other types of frame structures, standardized communication protocols, and extensions thereof.

In the example ofFIG. 7, a radio frame n is included in a sequence of radio frames n−1, n, n+1, etc. Each radio frame is composed of a sequence of subframes 0-9. In the illustrated configuration, the sequence of subframes includes a pair of special subframes (e.g., subframes1and6of a 3GPP LTE TDD compliant communications system), but can also include other numbers of special subframes. According to an example embodiment, a full-duplex device makes use of the special subframes (e.g., subframes complying with the parameters of Table 1) for use in transmitting the orthogonal training signals during a shared training period. In general, each of the special subframes includes downlink pilot time slot (DwPTS), a training portion (or training period/guard period), and an uplink pilot time slot (UpPTS). The remaining subframes are utilized for normal uplink/downlink communications. The representative training signal of this example includes a ZCZ sequence of length NTsand a cyclic prefix (CP) of length NCPTs, such that the total length of the training signal is (N+NCP)Ts. It is noted that the length of the CP need not necessarily match the normal or extended CP length specified in an LTE standard. Further, the length, as well as the periodicity, of the shared training period may be dependent upon environmental and/or communication system factors.

The reuse of existing subframe configurations, such as shown inFIG. 7, helps maintain compatibility with legacy devices and minimize changes to existing technical standards. These considerations may help improve adoption of full-duplex TRPs and minimize expenditures in implementing full-duplex communications systems. In general, the training period can be reserved in any of the downlink portions of a radio frame (e.g., the DwPTS of a special subframe or any slot of a downlink subframe).

ZCZ sequences suitable for use as orthogonal training signals can be generated, for example, based on a Zadoff-Chu sequence and its cyclic shifts. A base Zadoff-Chu sequence can be defined by

s⁡(n)=sZ⁡(n)={ej⁢⁢π⁢⁢un2/N,N⁢⁢is⁢⁢evenej⁢⁢π⁢⁢un⁡(n+1)/N,N⁢⁢is⁢⁢odd,n=0,1,…⁢,N-1.(1)
where N=2110 and u=1053 for this example. The value of u is selected to minimize the peak-to-average power ratio (PAPR) of the sequence following the low-pass filter/EMF stages of the TRPs. In general, u is chosen to be close to but less than [N/2], and u and N are relatively prime.

An alternative to ZCZ sequences is the generalized chirp-like (GCL) sequence, which can be built by modulating any Zadoff-Chu sequence with a block of random phase rotations
s(n)=sz(n)·exp{φ(nmodmg)}.n=0,1, . . . , N−1.   (2)

In this further example, N=2116, mg=46 and φ(k) can be a random variable that is uniformly distributed over [0, 2π], k=0, . . . mg−1. For purposes of maintaining the same training signal length as in Table 1, the CP length for the alternative ZCZ is 444. The random phase modulation results in the GCL sequence behaving similarly, in terms of PAPR, to surrounding DL data (e.g., random QAM symbols) when passing through a low-pass filter such as the EMF stage. In the following, s(n) is used to represent any type of ZCZ sequence that fulfills the relevant PAPR requirement following a low-pass filter stage of a TRP.

For the maximum delay spread of Lm=300 samples evaluated above, a total number of six (6) ZCZ sequences can be generated from the cyclic shifts of the base Zadoff-Chu sequence s(n). Each of the sequences can be assigned to a distinct transmit antenna in a FD network. For a 2×2 MIMO configuration (Nk=2, q=1,2, . . . , Nk) in a TRP such as TRP102, this number of ZCZ sequences is sufficient for a FD network with up to three (Kb=2, k=0, . . . Kb) TRPs. Specifically, the ZCZ sequence for each of the antennas in each of the TRPs can be derived by
sq,k(n)=s((n+(kNk+q−1)Ncyc)modN),n=0,1,. . . , N−1.   (3)
where k is the index of the TRP in the FD network (k=0, . . . Kb, Kb=2) and q is the transmit (or Tx) antenna port number on a TRP (q=1,2, . . . , Nk, Nk=2 for 2T2R or 2×2 MIMO);

Ncyc=⌊N(Kb+1)⁢Nk⌋=351,
which is the number of cyclic shifts and is no less than Lm. The training signal transmitted from the q-th antenna on the k-th TRP would be a (N+NCP)×1 vector

Turning now to SI estimation and cancellation in a TRP in accordance with the present disclosure, one or more training signals derived from an orthogonal training signal (as transmitted by the TRP) are utilized to perform partial SI cancellations. As generally described above with reference toFIGS. 4 and 5, derived (from orthogonal training signals) training signals and data samples used in cascaded channel estimation and SI reconstruction stages are taken following an EMF stage. It is noted that a derived training signal following the EMF stage is no longer a ZCZ sequence, and does not possess orthogonality between antennas and TRPs. The conventional least-square (LS) based channel estimation algorithm can be used with this type of non-orthogonal training signal, provided the involved matrix inversion and the required storage for pre-computed matrix inversions are manageable.

An advantage of deriving training signals that follow an EMF stage is that it permits satisfactory cancellation performance utilizing a channel estimator and canceller having a smaller number of taps than that of the delay spread of the overall channel impulse response. One design consideration is that this arrangement generally uses matrix inversion in the LS-based channel estimation, and the associated numerical issues for a large matrix may present practical limitations. In the examples illustrated inFIGS. 4 and 5, this type of channel estimation is used for SI cancellation in a local TRP. Another potential consideration is that in the presence of TRP-TRP MI, the SI cancellation performance of a TRP could be limited by the level of MI present. A benefit of the described architectures is that the residual of SI cancellations in previous stages can be jointly cancelled with the MI as elaborated below, and there is no resulting performance penalty due to the coexistence of SI (residual) and MI in the joint cancellation operation.

By virtue of the orthogonality of the training signals between any pair of Tx antennas and the (effectively) perfect autocorrelation and cross-correlations over the entire delay spread of the SI and MI channels, the estimation of the multipath channel between the p-th receive (or Rx) antenna on TRP0and q-th Tx antenna on TRPkcan be reduced to a number of cross-correlations

In the above, ypis an N×1 vector of Rx samples from the p-th Rx antenna on TRP0, which may have been processed by a first stage or multiple stages of SI cancellation. It has the form of
yp=[yp(np,0),yp(np,0+1), . . . ,yp(np,0+N−1)]T,   (6)
where np,0is an offset for the received samples such that the samples are aligned with the intended training signals Xp,q,kfor channel estimation. The offset is a result of timing synchronization or calibration procedure such as described in conjunction withFIGS. 9 and 10.

The matrix Xp,q,kis an N×Lp,q,kchannel convolution matrix with the training signal from the q-th antenna on the TRPk

⁢(7)Xp,q,k=[xq,k⁡(NCP-mp,q,k)xp,k⁡(NCP-mp,q,k-1)…xq,k⁡(NCP-mp,q,k-Lp,q,k+1)xp,k⁡(NCP-mp,q,k+1)xq,k⁡(NCP-mp,q,k)…xq,k⁡(NCP-mp,q,k-Lp,q,k+2)…………xq,k⁡(NCP-mp,q,k+N-1)xq,k⁡(NCP-mp,q,k+N-2)…xq,k⁡(NCP-mp,q,k+N-Lp,q,k)],
where Lp,q,kis the number of taps of the channel and mp,q,kis an offset of the training (reference) signals for channel estimation between Rx antenna p on TRP0and the Tx antenna q on TRPk, and is another output of the calibration procedure described below. The formation of the matrix can take advantage of the cyclic prefix in the transmitted training signal Xq,kof equation (4), and it can be conveniently constructed by a series of circular shifts of the base training sequence sq,k(n) of equation (3). That is,

From equations (5)-(7) above, it can be seen that the number of channel taps for each of the Tx-Rx antenna pairs can be set individually and differently from each other, although it is practically convenient to assume that the number of taps is the same for any pair of Tx-Rx antennas between two TRPs.

Turning now to joint MI/SI cancellation in a FD system, the channel estimation of (5) is an Lp,q,k×1 vector
ĥp,q,k=[hp,q,k(0),hp,q,k(1), . . . ,hp,q,k(Lp,q,k−1)]T.   (10)

The reconstructed canceling signal can be written as

Finally, the interference is canceled (see, e.g.,FIG. 11) from the received samples by
zp(n)=yp(n)−ŷp(n),   (12)
where zp(n) is an interference free or mitigated version of the received data signal that is passed to an uplink demodulator for further processing.

FIG. 8illustrates training periods having a common channel estimation window that accommodates propagation delays and multipath spread in accordance with the present disclosure. Due to the dynamics of phase synchronization errors and propagation delays between TRPs, timing synchronization for the signals received from each transmitting TRP is employed for MI and SI channel estimation and reconstruction in accordance with embodiments of the disclosure. The results of the timing synchronization, or calibration procedure, are the proper timing offsets for the received samples in TRP0and the corresponding offset for the training signal of a transmitting TRPkthat is targeting TRP0.

The time-of-arrivals (TOAs) of received signals at an Rx TRP (TRP0) from surrounding Tx TRPs are illustrated inFIG. 8by an example where MI signals from TRPk1and TRPk2arrive earlier and later, respectively, than an SI signal arrives at TRP0. This situation is possible due to potential phase synchronization errors between TRPs, which could cause a TRPkto transmit earlier or later than TRP0by as much as 1.5 μs, which corresponds to a propagation distance of 450 meters. When the actual distance between TRP0and TRPkis less than 450 meters, early arrival of MI signals from TRPkmay occur at TRP0.

The phase uncertainties, total propagation delays and multipath spread of an example FD system can be handled by the design of training signals, provided the maximum delay difference between the earliest and latest arriving signals is less than the length of the cyclic prefix of the training signal. InFIG. 8, it is assumed that signals from TRPk1arrive the earliest and signals from TRPk2arrive the latest.

To find the common window for channel estimation/cross-correlation determination as shown inFIG. 8, it is necessary to estimate the TOA for the transmitting antennas of each participating TRP. One method for performing the TOA estimation includes finding the timing offset of the peak of cross-correlation between training signal sq,kof equation (9) and the received samples yp(n). A Rx timing synchronization search window of [NCP−Te/2, NCP+Te/2] is expected to cover the maximum range of uncertainty with respect to the start of transmit boundaries at TRP0, which serves as a timing reference for the TOA estimation. Teis the maximum phase error between TRPs. Specifically,

rp,q,k⁡(τ)=∑n=0N-1⁢⁢sq,k*⁡(n)⁢yp⁡(n+τ),⁢τ=NCP-Te/2,…⁢,NCP+Te/2,(13)
represents the cross-correlations between received samples and Tx antenna q on TRPk; and

τp,q,k=arg⁢maxτ⁢{rp,q,k⁡(τ)}-Te2-τg(14)
represents the timing offsets of the correlation peaks.

The timing offset of the received MI with the earliest TOA is then given by

τp,0=minq,k⁢{τp,q,k}.(15)
where τgis the maximum of the overall group delays of filters between any Tx-Rx TRP pair. τgis a static parameter that can be measured in a calibration procedure such as described below in conjunction withFIGS. 9 and 10. It is noted that the estimate of a timing offset includes the propagation delay and group delay of the filters in the transmitting and receiving TRPs. In order for the channel estimation to cover the response of those filters, the maximum of the overall group delays of the filters is removed from the estimated timing offset. The offset for the received samples in (6) used in channel estimation is a result of
np,0=τp,0+NCP.   (16)

Another offset utilized for shifting the training signal and composing the channel convolution matrix in equations (7) or (8) is given by
mp,q,k=τp,q,k−τp,0. tm (17)

FIG. 9illustrates transmission of calibration training signals during an example calibration procedure to generate time-of-arrival (TOA) estimation information in accordance with the present disclosure. In the illustrated timing diagram900, TRPs are assumed to be stationary and TRP-TRP TOA estimations can be performed in a calibration procedure such as described below in conjunction withFIG. 10. A group of FD TRPs102-106are arranged by a central controller (e.g., a C-RAN or central access unit126) to participate in the calibration procedure.

In general, a unique calibration period in time and frequency is reserved for each of the participating TRPs102-106. An example of such a calibration period can be the training period shown inFIG. 7, wherein the calibration period utilizes the whole channel bandwidth and lasts an extended LTE OFDM symbol period in time. In each calibration period, only the TRP for which the calibration period is reserved is allowed to transmit (preferably in a single-input-single-output (SISO) mode), while all other TRPs and the transmitting TRP itself are in a receiving mode. An example of the transmitted calibration training signal can be a training signal designed in accordance withFIG. 7.

In the illustrated timing diagram900, calibration training signals are determined by the central access unit126and communicated (at902/904) to TRPs102-106(e.g., via a fronthaul link such as shown inFIG. 4) for transmission during respective distinct calibration periods910. In some embodiments, the central access unit126may further communicate a mapping of the calibration training signals to respective transmit antennas of the TRPs102-106. According to other embodiments (such as the distributed embodiment ofFIG. 5), one or both of the calibration training signals and associated transmitter antenna mappings are generated by processing circuitry of the TRPs102-106.

In the illustrated example, the TRP102transmits a calibration training signal (at906) during a reserved calibration period, and TRPs104and106transmit calibration training signals (at908) during respective reserved calibration periods. In this example, TOA and group delay information for use in determining training signal parameters and channel estimations is communicated (at912) to the central access unit126for use as described herein.

During each calibration period, the transmitting TRP receives its own “leaked” signal (e.g., at914for TRP102), which is considered self-interference (SI). This signal is used to measure the total group delay in its transmitter and receiver via, e.g., cross-correlations from the beginning of the calibration period:

These equations are based on the example orthogonal training signal described in equations (1)-(4). Here, τgis the total delay between the test points Bkand Hkshown inFIGS. 4and5, and is used in the TOA estimation in equation (14). Note that estimated include the group delays associated with the filters in Tx and Rx chains, plus the processing latency associated with real-time pipelining in the hardware/firmware/software (e.g., FPGAs and DSPs) of the Tx and Rx chains. For purposes of the described TOA estimation procedure, the Tx and Rx chains are considered together. Processing latency can be addressed, for example, by the channel truncation approach described below.

During each calibration period, the non-transmitting TRPs receive the calibration training signal from the only transmitting TRP. This signal is used to measure the propagation delay between test points Bk1and Hk2(k1≠k2) shown inFIGS. 4 and 5, and by equations (13) and (14), for example.

FIG. 10is a logic diagram illustrating an example calibration procedure (e.g., block602ofFIG. 6) to estimate time-of-arrival (TOA) information in accordance with the present disclosure. In particular, the operating states of a participating TRP are shown (at blocks1006-1010). Upon initiation of a calibration procedure602to estimate TOA information for signals communicated between different pairings of transmit antennas and receive antennas of the participating TRPs (block1000), a plurality of (orthogonal or non-orthogonal) calibration training signals are generated (block1002) or otherwise determined as described above and made available to the participating TRPs. Next, at block1004, coordinated transmission of the calibration training signals by the TRPs is scheduled for distinct calibration periods. Some or all of the foregoing operations can be performed by a centralized controller, the TRPs, or a combination thereof.

With reference to an individual one of the participating TRPs, the TRP operates to determine (block1006) if an assigned calibration period of the calibration procedure has been reached (i.e., when the TRP is scheduled to transmit a calibration training signal). If not, the TRP operates in a receive only mode to receive calibration training signals (block1008) from other participating TRPs for use in TRP-TRP TOA measurements and generating a propagation delay value relating to a wireless channel between the TRP and the transmitting TRP. If the assigned calibration period has been reached, the TRP operates in a simultaneous transmit/receive mode and transmits a calibration training signal (block1010). The TRP also receives the transmitted calibration training signal at a receive antenna for use in performing group delay measurements such as described above.

The presence of multipath reflections between TRPs may affect the accuracy of the TOA estimates when using a peak finding approach with, e.g., the cross-correlations in equations (13) and (14). The peak of the overall CIR, which is the convolution of the CIRs of the filters in a Tx-Rx chain and the CIR of the multipath channel, may shift due to the random profile of the multipath CIR. For example, the peak of the overall CIR of a channel could be shifted (delayed) by a few samples relative to that of the CIR of the filters. Because the estimation of TOA in equation (14) is based on the peak position of the overall CIR, retreated by the group delay of the filters, the shift of the CIR peak may cause an error in the TOA estimate, which in turn may cause a miss of some of taps in channel estimation, as shown by equations (5), (6), (7), (16) and (17).

To rectify the potential effect on channel estimation, equation (14) can be modified by

τp,q,k=arg⁢maxτ⁢{rp,q,k⁡(τ)}-Te2-τg-bp,q,k,(20)
where bp,q,kis an empirical number which can be observed in a multipath environment for a given multi-cell configuration.

As noted above, processing delays of the transmit and receive circuitry of a TRP can be included in group delay estimations. A given processing delay can be treated, for example, as a CIR extension with zero taps. In this case, the zero taps do not need to be included as part of channel estimation. In other cases, some of the CIR taps may be small enough that they can be ignored without incurring a meaningful performance penalty. Accordingly, truncated channel estimation can be desirable. The number of truncated taps on each end of a CIR estimation can be calculated by

mcet=τg-Lp,q,k-Lmpc2,(21)
where Lp,q,kis the actual number of overall channel taps to be estimated in equation (7), and τgis the group delay estimate by equation (19). is the actual number of taps of the multipath channel, which can be estimated, for example, by offline channel sounding and observation. mcetcan be combined with the cyclic shifts of an orthogonal training signal for channel estimation by modifying equation (17) as
mp,q,k=τp,q,k−τp,0+mcet.   (22)

FIG. 11illustrates an example interference cancellation module1100in accordance with the present disclosure. The interference cancellation module1100can be included, for example, in canceller S4432ofFIG. 4or canceller S4532ofFIG. 5, and can operate in a multi-phase mode. In a first phase, the interference cancellation module1100generates channel estimation information, such as CIR estimations and/or SI estimation information, using channel estimation module1102. Channel estimation may be performed by the channel estimation module1102, for example, utilizing the transmitted orthogonal training signals xq,k(n) and TOA/group delay information determined during the calibration procedure described in conjunction withFIGS. 9 and 10(e.g., rp,q,k(τ)). Although not separately illustrated, the channel estimation module can include, for example, a cyclic prefix module, a matrix multiplier, a scaling module, a sequence selector, a convolution matrix module, etc.

In a second phase, an interference reconstruction module1104reconstructs the SI and MI based on known transmitted data symbols, channel estimates (e.g., ĥp,q,k) produced by channel estimation module1102, and the TOA/group delay information rp,q,k(τ). A combiner1106operates generally to combine (i.e., subtract) the interference replica ŷp(n) (as generated by interference reconstruction module1104) with the received signal yp(n) to produce an interference free or mitigated version zp(n) of the received data signal. The information contained in zp(n) can then be processed by signal processing circuitry (e.g., demodulation and decoding circuitry).

FIG. 12is a block diagram representation of a transmit/receive point (TRP)1200in accordance with an embodiment of the present disclosure. The TRP1200may be, for example, an eNB or other type of base station, or an UE capable of full-duplex operation. The TRP1200generally includes one or more network interfaces1202, processing circuitry1204, transmitter circuitry1206, receiver circuitry1208, a control system1212, and memory1214. The processing circuitry1204can include, for example, baseband processing circuitry, interference cancellation module1100, a training signal generation module, a frame generation module, a signal mapping module, etc. The memory1214may be any type of memory capable of storing software and data.

The TRP1200of the illustrated embodiment further includes a plurality of antennas1210and1212configurable for use with the transmitter circuitry1206and the receiver circuitry1208(e.g., one or more transmit antennas and one or more receive antennas). In one example, at least one antenna1210is configured as a transmit antenna and at least one antenna1212is configured as a receive antenna. When the antenna1210is relatively close to, collocated with, or shared with antenna1212, signals transmitted via antenna1210may appear at antenna1212at significantly higher power levels than transmissions made by remotely located TRPs that are transmitting to the full-duplex TRP1200. Although TRP1200is shown inFIG. 12as having collocated or shared antennas1210and1212, alternative implementations of TRP1200may utilize one or more remotely located transmit and/or receive antennas.

The receiver circuitry1208receives radio frequency signals bearing information from one or more remote TRPs/UEs. Preferably, a low noise amplifier and a filter (not shown) cooperate to amplify and remove broadband interference from received signals for processing. Down-conversion and digitization circuitry (also not shown) will then downconvert the filtered, received signals to intermediate or baseband frequency signals, which are then digitized into one or more digital streams.

The processing circuitry1204processes the digitized received signals to extract information or data bits conveyed in the received signals. This processing typically includes demodulation, decoding, and error correction operations. Accordingly, the processing circuitry1204is generally implemented in one or more DSPs or application-specific integrated circuits (ASICs). The received information is then sent to an associated network via the network interface(s)1202, or transmitted to another device or terminal serviced by the TRP1200.

On the transmit side, the processing circuitry1204receives digitized data, which may represent training signals, voice, data, or control information, from the network interface(s)1202(e.g., an Xn interface) under the control of the control system1212, and encodes the data for transmission. The encoded data is output to the transmitter circuitry1206, where it is modulated by a carrier signal having a desired transmit frequency or frequencies. A power amplifier (not shown) will amplify the modulated carrier signal to a level appropriate for transmission, and deliver the modulated carrier signal to the antennas1210/1212through a matching network (also not shown). The TRP1200may concurrently transmit and receive signals using multiple antennas1210/1212.

With respect to the central access unit described earlier, a similar architecture to that ofFIG. 12could be used, but with different interfaces and transmit/receive circuitry. For example, the transmit/receive circuitry may be for optical, DSL or any other communication scheme. If collocated with a transmit/receive point, a central access unit could reuse the processing circuitry and other components of the transmit/receive point as necessary (e.g., interfaces for communicating with other transmit/receive points, such as an Xn interface).