WO2014183802A1

WO2014183802A1 - Methods and nodes in a wireless communication network

Info

Publication number: WO2014183802A1
Application number: PCT/EP2013/060291
Authority: WO
Inventors: Bjorn Sihlbom
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2013-05-17
Filing date: 2013-05-17
Publication date: 2014-11-20

Abstract

Radio network node (110), and method (500) in a radio network node (110), for calibration of wireless signals communicated in antenna streams with a user equipment (120) in a wireless communication system (100). The radio network node (110) comprises a multiple antenna array (210) configured for MIMO. The method (500) comp-rises receiving (501) anuplink wireless signal, assuming (502) the received (501) signal to comprise a calibration error, calculating (503) a compensation factor, compensating the received (501) signal for the assumed (502) calibration error, and compensating (504) the received (501) signal, based on the calculated (503) compensation factor.

Description

METHODS AND NODES IN A WIRELESS COMMUNICATION NETWORK

FIELD OF INVENTION

Implementations described herein relate generally to a radio network node and a method in a radio network node. In particu- lar is herein described a mechanism for calibration of wireless signals communicated in antenna streams with a user equipment in a multiple antenna environment.

BACKGROUND OF INVENTION A User Equipment (UE) , also known as a mobile station, wire^¬ less terminal and/ or mobile terminal is enabled to communi^¬ cate wirelessly in a wireless communication network, sometimes also referred to as a cellular radio system. The communication may be made, e.g., between UEs, between a UE and a wire con- nected telephone and/ or between a UE and a server via a Radio Access Network (RAN) and possibly one or more core net-works.

The wireless communication may comprise various communication services such as voice, messaging, packet data, video, broad^¬ cast, etc. The UE may further be referred to as mobile telephone, cellu^¬ lar telephone, computer tablet or laptop with wireless capa^¬ bility, etc. The UE in the present context may be, for exam^¬ ple, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/ or data, via the radio access network, with another entity, such as another UE or a server.

The wireless communication network covers a geographical area which is divided into cell areas, with each cell area being served by a radio network node, or base station, e.g., a Radio Base Station (RBS) , which in some networks may be referred to as "eNB", "eNodeB", "NodeB" or "B node", depending on the technology and/ or terminology used. Sometimes, the expression "cell" may be used for denoting the radio network node itself. However, the cell may also in nor^¬ mal terminology be used for the geographical area where radio coverage is provided by the radio network node at a base sta^¬ tion site. One radio network node, situated on the base sta- tion site, may serve one or several cells. The radio network nodes may communicate over the air interface operating on ra^¬ dio frequencies with any UE within range of the respective ra^¬ dio network node.

In some radio access networks, several radio network nodes may be connected, e.g., by landlines or microwave, to a Radio Net^¬ work Controller (RNC) , e.g., in Universal Mobile Telecommunications System (UMTS) . The RNC, also sometimes termed Base Station Controller (BSC), e.g., in GSM, may supervise and co^¬ ordinate various activities of the plural radio network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Special Mobile) .

In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) radio network nodes, which may be referred to as eNodeBs or eNBs, may be connected to a gateway, e.g., a radio access gateway, to one or more core networks.

In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the radio network node to the UE . The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the UE to the radio network node. Beyond 3G mobile communication systems, such as e.g., 3GPP LTE, offer high data rate in the downlink by employing multiple antenna systems utilising Multiple-Input and Multiple- Output (MIMO) . Massive MIMO is a recently emerged technology that uses large Antenna Arrays Systems (AAS) with a plurality of individual transceivers to improve throughput of wireless communication systems. The benefit of these large arrays is the ability to spatially re-solve and separate received and transmitted sig- nals with high resolution.

The resolution is determined by the number of antenna ele^¬ ments, and their spacing. Typically the number of transceivers may be as high as lOx the maximum rank of the system. The rank is defined as the total number of parallel (same time and fre- quency) transmissions, including both wanted and unwanted sig^¬ nals (i.e. interference). Massive MIMO is sometimes loosely defined as a system using comprising 100 or more transceivers. Various investigations in this community have shown Massive MIMO systems that benefit from several hundred' s of transceiv- ers .

In theory it may be enough having the same number of antennas as the rank in the massive MIMO system, if these antennas were ideally adapted/ designed for each specific scenario. In prac^¬ tice this is impossible. The physical number of antennas is typically lOx the number of spatial layers, which sometimes also may be referred to as ranks, or logical antennas. The complexity of baseband receive and transmit MIMO algorithms scales exponentially with number of antennas, leading to high requirements for computational ability, which may require additional dedicated hardware in form of very high capacity processing platforms. Further, com- putational complexity adds processing time, delaying the transmission/ reception, and consume power, leading to high energy costs and additional heating.

Further it is required that the antenna array, including transceivers, is coherent and reciprocal. Coherent means that the phase and amplitude offsets between elements are known, and compensated. Reciprocal is that receive and transmit di^¬ rections experience the same inter-element phase and amplitude offsets. To achieve this various calibration methods are ap- plied.

Known methods use a dedicated reference signal source (trans^¬ mitter) to calibrate the multitude of receivers in the large antenna array. Insufficient long term stability forces the system to go offline and perform calibration in regular inter- vals.

The transmitter is calibrated in a similar way using a dedi^¬ cated reference receiver. In a Time Division Duplex (TDD) sys^¬ tem where transmit and receive directions operate on the same frequency band; the reference transceiver can be one of the regular elements in the antenna array. In a Frequency Division Duplex (FDD) system where transmit and receive operate on dif^¬ ferent frequency bands, a dedicated reference element is re^¬ quired .

The main limiting factors of prior art massive MIMO comprises: 1. The overwhelming computational complexity.

2: Calibration requirements.

3: The Channel State Information (CSI) . The latter in particular for downlink, where it is deemed not feasibility to have mobile terminals report channel measurements for base station antennas. Further, prior art schemes requires TDD to assess the downlink channel based on uplink measurements since the radio channel in TDD is reciprocal, i.e. the same in both uplink and downlink since they operate on the same frequen- cies.

It appears that massive MIMO and AAS require further develop^¬ ment for becoming feasible for practical implementation.

SUMMARY OF INVENTION It is therefore an object to obviate at least some of the above mentioned disadvantages and to improve the performance in a wireless communication network.

According to a first aspect, the object is achieved by a method in a radio network node, for calibration of wireless signals communicated in antenna streams with a user equipment in a wireless communication system. The radio network node comprises a multiple antenna array configured for MIMO. The method comprises receiving an uplink wireless signal. Also, the method comprises assuming the received signal to comprise a calibration error. Furthermore, the method also moreover comprises calculating a compensation factor, compensating the received signal for the assumed calibration error. In addi^¬ tion, the method also comprises compensating the received sig^¬ nal, based on the calculated compensation factor. According to a second aspect, the object is achieved by a ra^¬ dio network node, configured for calibration of wireless sig^¬ nals communicated in antenna streams with a user equipment in a wireless communication system. The radio network node comprises a multiple antenna array configured for MIMO. The radio network node comprises a receiver, configured for receiving wireless signals from the user equipment. In addition, the ra^¬ dio network node also comprises a processing circuit, config^¬ ured for assuming the received signal to comprise a calibra^¬ tion error. The processing circuit is also configured for cal- culating a compensation factor, compensating the received signal for the assumed calibration error. Furthermore, the proc^¬ essing circuit additionally is moreover configured for compen^¬ sating the received signal, based on the calculated compensa^¬ tion factor. Embodiments of the herein described invention are directed to^¬ wards avoiding dedicated reference transceivers, startup and intermittent off-line calibration, which have been required to meet coherency and reciprocity requirements according to prior art. Furthermore continuous calibration has the potential to significantly reduce the residual calibration errors and thus increase the performance of AAS and massive MIMO.

Thereby an improved performance within the wireless communica^¬ tion network is provided.

Other objects, advantages and novel features of the embodi- ments of the invention will become apparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described in more detail with reference to attached drawings illustrating examples of em^¬ bodiments in which:

Figure 1 is a block diagram illustrating a wireless communica^¬ tion network according to some embodiments. Figure 2 is a block diagram illustrating a wireless communica^¬ tion network according to some embodiments.

Figure 3 is a block diagram illustrating a wireless communica^¬ tion network according to some embodiments. Figure 4A is a block diagram illustrating a radio network node architecture according to some embodiments.

Figure 4B is a block diagram illustrating a radio network node architecture according to some embodiments.

Figure 4C is a block diagram illustrating receiver calibration according to some embodiments.

Figure 5 is a flow chart illustrating a method in a radio net^¬ work node according to an embodiment of the invention.

Figure 6 is a block diagram illustrating a radio network node according to an embodiment of the invention.

DETAILED DESCRIPTION OF INVENTION

Embodiments of the invention described herein are defined as a radio network node and a method in a radio network node, which may be put into practice in the embodiments described below. These embodiments may, however, be exemplified and realised in many different forms and are not to be considered as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and com^¬ plete . Still other objects and features may become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illus^¬ tration and not as a definition of the limits of the herein disclosed embodiments, for which reference is to be made to the appended claims. Further, the drawings are not necessarily drawn to scale and, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.

Figure 1 is a schematic illustration over a wireless communi^¬ cation network 100 comprising a radio network node 110 and a User Equipment (UE) 120.

The wireless communication network 100 may at least partly be based on radio access technologies such as, e.g., 3GPP LTE, LTE-Advanced, Evolved Universal Terrestrial Radio Access Net^¬ work (E-UTRAN) , Universal Mobile Telecommunications System (UMTS) , Global System for Mobile Communications (originally: Groupe Special Mobile) (GSM)/ Enhanced Data rate for GSM Evo^¬ lution (GSM/EDGE) , Wideband Code Division Multiple Access (WCDMA) , Time Division Multiple Access (TDMA) networks, Fre^¬ quency Division Multiple Access (FDMA) networks, Orthogonal FDMA (OFDMA) networks, Single-Carrier FDMA (SC-FDMA) networks, World-wide Interoperability for Microwave Access (WiMax) , or Ultra Mobile Broadband (UMB) , High Speed Packet Access (HSPA) Evolved Universal Terrestrial Radio Access (E-UTRA) , Universal Terrestrial Radio Access (UTRA) , GSM EDGE Radio Access Network (GERAN) , 3GPP2 CDMA technologies, e.g., CDMA2000 lx RTT and High Rate Packet Data (HRPD) , just to mention some few op^¬ tions. The expressions "wireless communication network" and "wireless communication system" may within the technological context of this disclosure sometimes be utilised interchangea- bly. The wireless communication network 100 may be configured to operate according to the Time Division Duplex (TDD) and/ or the Frequency Division Duplex (FDD) principle, according to different embodiments. TDD is an application of time-division multiplexing to separate uplink and downlink signals in time, possibly with a Guard Period (GP) situated in the time domain between the up^¬ link and downlink signalling. FDD means that the transmitter and receiver operate at different carrier frequencies. Further, the wireless communication network 100 may be configured for massive MIMO and AAS, according to some embodiments.

The purpose of the illustration in Figure 1 is to provide a simplified, general overview of the wireless communication network 100 and the involved methods and nodes, such as the radio network node 110 and UE 120 herein described, and the functionalities involved. The methods, radio network node 110 and UE 120 will subsequently, as a non-limiting example, be described in a 3GPP LTE/ LTE-Advanced environment, but the em^¬ bodiments of the disclosed methods, radio network node 110 and UE 120 may operate in a wireless communication network 100 based on another access technology such as, e.g., any of the above already enumerated. Thus, although the embodiments of the invention are described based on, and using the lingo of, 3GPP LTE systems, it is by no means limited to 3GPP LTE. The illustrated wireless communication network 100 comprises the radio network node 110, which may send radio signals to be received by the UE 120.

It is to be noted that the illustrated network setting of one radio network node 110 and one UE 120 in Figure 1 is to be re- garded as a non-limiting example of an embodiment only. The wireless communication network 100 may comprise any other number and/ or combination of radio network nodes 110 and/ or UEs 120. A plurality of UEs 120 and another configuration of radio network nodes 110 may thus be involved in some embodiments of the disclosed invention.

Thus whenever "one" or "a/ an" UE 120 and/ or radio network node 110 is referred to in the present context, a plurality of UEs 120 and/ or radio network nodes 110 may be involved, ac^¬ cording to some embodiments. The radio network node 110 may according to some embodiments be configured for downlink transmission and may be referred to, respectively, as e.g., a base station, NodeB, evolved Node Bs (eNB, or eNode B) , base transceiver station, Access Point Base Station, base station router, Radio Base Station (RBS), micro base station, pico base station, femto base station, Home eNodeB, sensor, beacon device, relay node, repeater or any other network node configured for communication with the UE 120 over a wireless interface, depending, e.g., of the ra^¬ dio access technology and/ or terminology used. The UE 120 may correspondingly be represented by, e.g. a wire^¬ less communication terminal, a mobile cellular phone, a Per^¬ sonal Digital Assistant (PDA) , a wireless platform, a mobile station, a tablet computer, a portable communication device, a laptop, a computer, a wireless terminal acting as a relay, a relay node, a mobile relay, a Customer Premises Equipment (CPE) , a Fixed Wireless Access (FWA) nodes or any other kind of device configured to communicate wirelessly with the radio network node 110, according to different embodiments and dif^¬ ferent vocabulary. Embodiments herein concerns on-line calibration without dedi^¬ cated transceivers, and without special reference signals. Further, according to some embodiments, a distributed daisy chain implementation is presented.

Figure 2 is a schematic illustration over the wireless commu^¬ nication network 100, as illustrated in Figure 1, comprising a radio network node 110 and three embodiments of UEs 120-1, 120-2, 120-3. The radio network node 110 comprises, or is con^¬ nected to, a multiple antenna array 210, comprising a plural^¬ ity of transceivers labelled 210-1, 210-2, 210-n. The mul^¬ tiple antenna array 210 may sometimes be referred to as a large Antenna Arrays System (AAS) . The multitude of transceiv^¬ ers 210-1, 210-2, 210-n may sometimes also be referred to as antenna elements, or Antenna Array Modules (AAM) . The radio network node 110 may be configured for massive MIMO.

When an uplink radio signal is received at the radio network node 110, a spatial profile is established for the signal. In analogy with auto-focus on cameras, the spatial contrast may be used to align and focus the multiple antenna array 210 in order to achieve inter AAM alignment. Concerning the downlink calibration, each transceiver 210-1, 210-2, 210-n may be individually calibrated by knowledge of the phase offset be^¬ tween the transceivers.

The multiple antenna array 210 may in some embodiments be daisy chained, wherein each transceiver 210-1, 210-2, 210-n may access the receiver signal from a neighbour transceiver 210-1, 210-2, 210-n and calculate a compensation factor.

The individual transmitter-receiver calibration may be solved internally in each respective the transceiver 210-1, 210-2, 210-n according to some embodiments.

Figure 3 is a schematic illustration over the wireless commu- nication network 100, as illustrated in Figure 1 and/ or Fig^¬ ure 2, comprising a radio network node 110 and a plurality of signal sources 310-1, 310-2, 310-3, 310-4. Each such signal source 310-1, 310-2, 310-3, 310-4 may comprise e.g. a UE 120, or any other arbitrary device configured for wireless trans^¬ mission. Further, a virtual reference 320 is comprised in the illustrated example.

The transceivers 210-1, 210-2, 210-n may receive a seem^¬ ingly random signal.

If there is a spatial profile of the received signal, there may effectively be a systematic non-random phase and amplitude offset between the received signals of the transceivers 210-1, 210-2, 210-n. These systematic offsets may be estimated and removed according to some embodiments. By carefully weighting samples, the offsets estimated will be the same as if using a reference signal in a certain Angle of Arrival (AoA) and dis- tance to the respective transceivers 210-1, 210-2, 210-n.

The signal seemingly received from the virtual reference 320 may effectively be the "Centre of Power" of all the received signal (s) . Calibration may be made during operation, which is an advantage as the system 100 does not have to be taken down. Absolute AoA and distance to the virtual reference 320 may not be required in some embodiments. Instead, all other AoA and distances to other signal sources 310-1, 310-2, 310-3, 310-4 may be measured with reference to the virtual reference 320.

To enable spatial multiplexing in the downlink the multiple antennas 210 may be coherent and reciprocal. The above de^¬ scribed receiver-receiver calibration may create a coherent multiple antenna array 210 in the uplink. Each transceiver 210-1, 210-2, 210-n may be individually calibrated to be reciprocal, being considered to have the same or similar off- set for the multiple antenna array 210. In FDD the local os^¬ cillators of reception and transmission may lock in a random phase versus each other. Component and temperature variation over the multiple antenna array 210 may cause additional off^¬ sets. Thus calibration may be made during operation.

According to some embodiments, a received complex baseband signal x (n, k) with calibration error may be assumed. This may be expressed as: x(n,k)=rx(n,k) · e ( k) , where n is the antenna element index, and k is the time sample index. The signal rx (n, k) is the actual received signal, while e(k) is the complex calibration error (assume time invariant, or at least short term constant) .

The compensated received signal, y, may be expressed as: y(n,k) = rx (n, k) · residual ( k) , where the residual error may be further divided into 3 fac- tors, residual (k) = harmless + harmful =

R0 -Rl (k) -R2 (k) +harmful :

1. R0 : Constant phase and/ or Amplitude offset (constant over all antenna element index k) .

2. Linear phase ramp angle (Rl (k) ) = pO+k-pl. And normalized amplitude: abs(Rl)=l.

3. Parabolic phase ramp: angle (R2 (k) ) = k2-p3. And normal^¬ ised amplitude: abs(R2)=l.

To reach y(n,k), a compensation factor z may be calculated and applied. In some embodiments, this may be done in the follow- ing 4 steps:

Step 1: Pk ½0 -10)

Calibration of each antenna element k, only depends on input data x of element k, and output data y and the equations may be reorganised:

Step 1: Pk = ∑„ ½0 fe-i0

Step 3: afePfe

Step 4: y_fe (n) = z_fe x_fe (n)

In some embodiments implementing the herein described method, the antenna elements 210 may be connected in a daisy-chain fashion; where the output of the preceding antenna element 210 may serve as reference input.

Figure 4A is illustrating a radio network node architecture ac^¬ cording to some embodiments.

In the illustrated example, the radio network node architec^¬ ture is configured for online calibration in a TDD environ- ment . Figure 4B is illustrating a radio network node architecture ac^¬ cording to some embodiments.

In the illustrated example, the radio network node architec^¬ ture is configured for online calibration in a FDD environ- ment .

Figure 4C is illustrating receiver calibration according to some embodiments.

Assume antenna element 210-n receiver has the initial error/ offset RXn - The error may be defined as the offset from the an- tenna reference point to the virtual reference 320, or compen^¬ sation point.

The offset from the arc to the antenna reference point is X_n , and is the same for the receiver (Rx) and the transmitter (Tx) . The offset (phase/amplitude/delay) R may be the same for re^¬ ceiver, transmitter and all antenna elements 210-1, 210-2, 210-n; from the virtual reference 320 to the arc in some em^¬ bodiments .

After a calculated compensation of all paths/ elements may have the same offset to/ from a point perpendicular to the AAS, at infinite distance. In reality, a small residual cali^¬ bration error may remain; which however may be neglected.

The ^xfirst' antenna element 210-1 in the multiple antenna ar^¬ ray 210 serve as reference element in the illustrated example, i.e. no compensation is made for this antenna element 210-1 according to some embodiments. The successively neighbouring antenna elements 210-2, 210-3, 210-n may be calibrated to have the same phase/ amplitude and delay. Due to range and angle off centre the residual calibration phase offsets of the multiple antenna array 210 may be an off^¬ set circle fragment.

The calculation of transmitter pre-compensation factors may comprise:

B_N = RX_N ^■ TX_N ,

where B_n is the offset for antenna element n, n is an antenna element index and X is another offset for antenna element n.

Further, in some embodiments, the transmitter pre-compensation factor may be calculated as:

CTX_N = A_N/B_N/C% = (X_Q/RXQ)/(X_N ^■ TX_N) .

However, in some embodiments, the transmitter pre-compensation factor may be calculated as: CTX_N = A_N/B_N/C_N = RX₀/TX_N.

The transmitter factors at centre of gravity of received sig^¬ nals may in some embodiments be calculated as:

CTX_N ^■ TX_N ^■ X_N ^■ R = R^■ X_Q/RX_Q .

However, in some embodiments, the transmitter factors at infi- nite distance and perpendicular to the AAS may be calculated as :

The receiver compensation factors may in some embodiments calculated as: CRX_n = 1/ A_n = (¾^■ RX₀)/(X_n ^■ RX_n) .

The receiver compensation factors may however in some embodiments be calculated as:

CRX_n = C_n/ A_n = RX₀/RX_n .

The reciver factors at centre of gravity of received signals may in some embodiments be calculated as:

R ' RX i ' Χγι ' C RX i ⁼ R ' RXo ¹ -^o

The reciver factors at infinite distance and perpendicular to the AAS may be calculated as:

Thus, the offset between receiver (RX) and transmitter (TX) becomes, merely a constant:

Receive / Transmit = RX₀ · RX₀ .

Without Cn, an uplink/ downlink pointing error may be intro^¬ duced :

Receive / Transmit = (X_n/ XQ)² _t according to some em^¬ bodiments .

Figure 5 is a flow chart illustrating embodiments of a method 500 in a radio network node 110, for calibration of wireless signals communicated in antenna streams with a User Equipment (UE) 120 in a wireless communication system 100. The radio network node 110 comprises a multiple antenna array 210 con^¬ figured for Multiple Input Multiple Output (MIMO) . The wire^¬ less communication system 100 may be configured for massive MIMO, according to some embodiments. The multiple antenna ar^¬ ray 210 may in some embodiments comprise a multitude of an- tenna elements 210-1, 210-2, 210-n mounted at a distance from each other such that some, a subset, or all of the an^¬ tenna elements may be able to receive the same signal from the UE 120. The wireless communication network 100 may be based on 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) . Further, the wireless communication system 100 may be based on Frequency Division Duplex (FDD) . The radio network node 110 may comprise an evolved NodeB (eNodeB) according to some em- bodiments.

To appropriately perform the calibration, the method 500 may comprise a number of actions 501-508.

It is however to be noted that any, some or all of the de^¬ scribed actions 501-508, may be performed in a somewhat dif- ferent chronological order than the enumeration indicates, be performed simultaneously or even be performed in reversed or^¬ der. Further, it is to be noted that some actions may be per^¬ formed in a plurality of alternative manners according to dif^¬ ferent embodiments. The method 500 may comprise the following actions:

Action 501

An uplink wireless signal is received.

The wireless signal may be received directly from the UE 120, or may be received indirectly from the UE 120 via scattering reflections. Further, the received wireless signals may be re^¬ ceived from interferers, i.e. other UEs in some embodiments.

The wireless signal may not be a dedicated reference signal according to some embodiments. Action 502

The received 501 wireless signal is assumed to comprise a calibration error.

Action 503

A compensation factor, compensating the received 501 signal for the assumed 502 calibration error is calculated.

Action 504

The received 501 wireless signal is compensated, based on the calculated 503 compensation factor.

The compensation may be performed during operation of the radio network node 110, based on the received 501 signal.

The calculation 503 of the compensation factor and the compensation of the signal to be transmitted may be performed by:

Pk =

n

zk — ^zk-l

ak Pk

where x is the received 501 signal, n is an antenna element index, k is a time sample index, z is the compensation factor and y is the received 501 signal, compensated for the assumed 502 calibration error.

However, in some embodiments, the calculation 503 of the com^¬ pensation factor and the compensation of the signal to be transmitted may be performed by:

¾^" J∑n ly_fe-i(n) l ²

Yk 00 = ^zk ^xk O) where x is the received 501 signal, n is an antenna element index, k is a time sample index, z is the compensation factor and y is the received 501 signal, compensated for the assumed 502 calibration error.

Action 505

This action may be performed in some, but not of necessity all embodiments wherein pre-compensation of a signal to be trans^¬ mitted is performed, based on the calculated 503 compensation factor, and individual receiver-transmitter (RX-TX) calibration.

The offset between receiver and transmitter of each individual antenna element 210-1, 210-2, 210-n in the multiple antenna array 210 may be estimated.

The estimation of the offset may be performed by calculating: B_n = RX_n ^■ TX_n where B is the offset for antenna element n, n is an antenna element index.

Action 506 A global phase reference may be estimated. Action 507

This action may be performed in some, but not of necessity all embodiments .

Transmitter pre-compensation factors may be calculated. The calculation of transmitter pre-compensation factors may comprise :

B_n = RX_n ^■ TX_n,

where B is the offset for antenna element n, n is an antenna element index and X is another an offset for antenna element n .

Transmitter pre-compensation factor = CTX_n = A_n/B_n/C^ = (X_Q/RX_Q)/ (X_n'TX_n), in some embodiments. However, transmitter pre-compensation factor = CTX_n = A_n/B_n/C_n = l/RX₀/TX_nr in some embodiments.

Action 508

This action may be performed in some, but not of necessity all embodiments . The signal to be transmitted may be pre-compensated, based on the calculated 506 transmitter pre-compensation factors.

Figure 6 is a block diagram illustrating a radio network node 110 in a wireless communication network 100. The radio network node 110 is configured for performing the above mentioned method 500 according to any, some or all of the actions 501- 508 for calibration of wireless signals communicated in an^¬ tenna streams with a UE 120 in a wireless communication system 100. The radio network node 110 comprises a multiple antenna array 210 configured for MIMO.

The wireless communication system 100 may furthermore be con^¬ figured for massive MIMO, according to some embodiments. The multiple antenna array 210 may comprise a multitude of antenna elements 210-1, 210-2, 210-n which may be mounted, in some embodiments, at a distance from each other such that at least some, a subset, or all of the antenna elements 210-1, 210-2, 210-n may be able to receive the same signal from the UE 120.

The wireless communication network 100 may be based on 3GPP LTE . Further, the wireless communication system 100 may be based on FDD. The radio network node 110 may comprise an eNodeB according to some embodiments.

For enhanced clarity, any internal electronics or other compo^¬ nents of the radio network node 110, not completely indispen- sable for understanding the herein described embodiments have been omitted from Figure 6.

The radio network node 110 comprises a receiver 610, config^¬ ured for receiving wireless signals. The signals may be re^¬ ceived from the UE 120 directly or indirectly via scattering/ reflections. Further, the signals may be received from inter- ferers, i.e. other UEs, directly or indirectly via scattering/ reflections .

Also, the radio network node 110 comprises a processing cir^¬ cuit 620. The processing circuit 620 is configured for assum- ing the received signal to comprise a calibration error. Fur^¬ ther, the processing circuit 620 is also configured for calcu^¬ lating a compensation factor, compensating the received signal for the assumed calibration error. In addition the processing circuit 620 is further configured for compensating the re- ceived signal, based on the calculated compensation factor.

According to some embodiments, the processing circuit 620 may be further configured for pre-compensating a signal to be transmitted, based on the calculated compensation factor, and individual receiver-transmitter calibration. Furthermore, the processing circuit 620 may be additionally configured for es^¬ timating the offset between receiver and transmitter of each individual antenna element 210-1, 210-2, 210-n. The proc^¬ essing circuit 620 may also be configured for calculating transmitter pre-compensation factors. In addition, according to some embodiments the processing circuit 620 may also be configured for pre-compensating the signal to be transmitted based on the calculated transmitter pre-compensation factors.

The processing circuit 620 may comprise, e.g., one or more in^¬ stances of a Central Processing Unit (CPU) , a processing unit, a processing circuit, a processor, an Application Specific Integrated Circuit (ASIC) , a microprocessor, or other processing logic that may interpret and execute instructions. The herein utilised expression "processing circuit" may thus represent a processing circuitry comprising a plurality of processing cir- cuits, such as, e.g., any, some or all of the ones enumerated above . The processing circuit 620 may further perform data processing functions for inputting, outputting, and processing of data comprising data buffering and device control functions, such as call processing control, user interface control, or the like.

Also, the radio network node 110 may further in some embodi^¬ ments comprise a transmitter 630, configured for transmitting wireless signals, to be received by the UE 120.

The antenna elements 210-1, 210-2, 210-n comprised in the multiple antenna array 210 may be mounted at a distance from each other such that some, a subset or all of the antenna ele^¬ ments 210-1, 210-2, 210-n may be able to receiving the same signal from the UE 120 in some embodiments.

Furthermore, the radio network node 110 may comprise at least one memory 625, according to some embodiments. The memory 625 may comprise a physical device utilised to store data or pro^¬ grams, i.e., sequences of instructions, on a temporary or per^¬ manent basis. According to some embodiments, the memory 625 may comprise integrated circuits comprising silicon-based transistors. Further, the memory 625 may be volatile or non^¬ volatile.

The previously described actions 501-508 to be performed in the radio network node 110 may be implemented through the one or more processing circuits 620 in the radio network node 110, together with computer program code for performing the functions of at least some of the actions 501-508. Thus a computer program product, comprising instructions for performing the actions 501-508 in the radio network node 110 may perform the method 500 for calibration of wireless signals communicated in antenna streams with a UE 120 in a wireless communication sys- tern 100, when the computer program product is loaded in a processing circuit 620 of the radio network node 110.

The computer program product mentioned above may be provided for instance in the form of a data carrier carrying computer program code for performing any, at least some, or all of the actions 501-508 according to some embodiments when being loaded into the processing circuit 620. The data carrier may be, e.g., a hard disk, a CD ROM disc, a memory stick, an opti^¬ cal storage device, a magnetic storage device or any other ap- propriate medium such as a disk or tape that may hold machine readable data in a non transitory manner. The computer program product may furthermore be provided as computer program code on a server and downloaded to the radio network node 110 re^¬ motely, e.g., over an Internet or an intranet connection. The terminology used in the detailed description of the inven^¬ tion as illustrated in the accompanying drawings is not in^¬ tended to be limiting of the described method 500 and/ or ra^¬ dio network node 110, which instead are limited by the en^¬ closed claims. As used herein, the term "and/ or" comprises any and all com^¬ binations of one or more of the associated listed items. In addition, the singular forms "a", "an" and "the" are to be in^¬ terpreted as "at least one", thus also possibly comprising a plurality of entities of the same kind, unless expressly stated otherwise. It will be further understood that the terms "includes", "comprises", "including" and/ or "comprising", specifies the presence of stated features, actions, integers, steps, operations, elements, and/ or components, but do not preclude the presence or addition of one or more other fea- tures, actions, integers, steps, operations, elements, compo^¬ nents, and/ or groups thereof.

Claims

1. A method (500) in a radio network node (110), for cali^¬ bration of wireless signals communicated in antenna streams with a user equipment (120) in a wireless communication system (100), which radio network node (110) comprises a multiple an^¬ tenna array (210) configured for Multiple Input Multiple Out^¬ put, MIMO, the method (500) comprising: receiving (501) an uplink wireless signal, assuming (502) the received (501) signal to comprise a calibration error, calculating (503) a compensation factor, compensating the received (501) signal for the assumed (502) calibration error, and compensating (504) the received (501) signal, based on the calculated (503) compensation factor.

2. The method (500) according to claim 1, further comprising pre-compensating a signal to be transmitted, based on the cal^¬ culated (503) compensation factor, and individual receiver- transmitter calibration by the further actions of: estimating (505) an offset between receiver and transmitter of each individual element (210-1, 210-2, 210-n) com^¬ prised in the multiple antenna array (210), estimating (506) a global phase reference, calculating (507) transmitter pre-compensation factors, and pre-compensating (508) the signal to be transmitted based on the calculated (507) transmitter pre-compensation factors.

3. The method (500) according to claim 1, wherein the signal is not a dedicated reference signal.

4. The method (500) according to claim 1, wherein the calibration (504) is performed during operation of the radio network node (110), based on the received (501) signal.

5. The method (500) according to claim 1, wherein the calcu^¬ lation (503) of the compensation factor and the compensation (504) of the received (501) signal is performed by: p_k =nx_fc(n)X_fc_₁(n)

Yk fa) = z_k x_k (n), where x is the received (501) signal, n is an antenna element index, k is a time sample index, z is the compensation factor and y is the received (501) signal, compensated (504) for the assumed (502) calibration error.

6. The method (500) according to claim 1, wherein the calcu^¬ lation (503) of the compensation factor and the compensation (504) of the received (501) signal is performed by:

Zv =

k k

Yk 00 = ^zk ^xk (n)

where x is the received (501) signal, n is an antenna element index, k is a time sample index, z is the compensation factor and y is the received (501) signal, compensated (504) for the assumed (502) calibration error.

7. The method (500) according to claim 2, wherein the estimation (505) of the offset is performed by calculating:

B_n = RX_n ^■ TX_n where B is the offset for antenna element n, n is an antenna element index.

8. The method (500) according to claim 2, wherein the calcu^¬ lation (507) of transmitter pre-compensation factors comprises :

B_n = RX_n ^■ TX_n ,

transmitter pre-compensation factor = CTX_n = A_n/B_n/C^ = (X₀/RX₀)/ (Χ_η·ΤΧ_η~), where B is the offset for antenna element n, n is an antenna element index and X is another an offset for antenna element n.

9. The method (500) according to claim 2, wherein the calcu^¬ lation (507) of transmitter pre-compensation factors comprises : transmitter pre-compensation factor = CTX_n = A_n/B_n/C_n = 1/RX₀/ where B is the offset for antenna element n, n is an an^¬ tenna element index and X is another an offset for antenna element n.

10. The method (500) according to claim 1, wherein the wire^¬ less communication system (100) is based on Frequency Division Duplex, FDD.

11. The method (500) according to claim 1, wherein the antenna elements (210-1, 210-2, 210-n) comprised in the mul^¬ tiple antenna array (210) are mounted at a distance from each other such that at least some of the antenna elements (210-1, 210-2, 210-n) are able to receive the same signal from the user equipment (120) .

12. The method (400) according to claim 1, wherein the radio network node (110) comprises an evolved NodeB, eNodeB; and wherein the wireless communication network (100) is based on 3rd Generation Partnership Project Long Term Evolution, 3GPP LTE .

13. A radio network node (110), configured for calibration of wireless signals communicated in antenna streams with a user equipment (120) in a wireless communication system (100), which radio network node (110) comprises multiple antennas (210) configured for Multiple Input Multiple Output, MIMO, further comprising: a receiver (610), configured for receiving wireless sig^¬ nals from the user equipment (120); a processing circuit (620), configured for assuming the received signal to comprise a calibration error, and config- ured for calculating a compensation factor, compensating the received signal for the assumed calibration error and in addi^¬ tion configured for compensating the received signal, based on the calculated compensation factor.

14. The radio network node (110) according to claim 13, fur- ther configured for pre-compensating a signal to be transmit^¬ ted, based on the calculated compensation factor, and individ^¬ ual receiver-transmitter calibration, wherein the processing circuit (620) also is configured for estimating the offset be^¬ tween receiver and transmitter of each individual antenna ele- ment (210-1, 210-2, 210-n) , and also configured for calcu^¬ lating transmitter pre-compensation factors, and additionally configured for pre-compensating the signal to be transmitted based on the calculated transmitter pre-compensation factors.

15. The radio network node (110) according to claim 13, wherein the antenna elements (210-1, 210-2, 210-n) com^¬ prised in the multiple antenna array (210) are mounted at a distance from each other such that at least some of the an^¬ tenna elements (210-1, 210-2, 210-n) are able to receive the same signal from the user equipment (120) .

16. A computer program product in a radio network node (110), according to claim 13, configured for performing the method (500) according to claim 1, for calibration of wireless signals communicated in antenna streams with a user equipment (120) in a wireless communication system (100), when the com- puter program product is loaded in a processing circuit (620) of the radio network node (110) .