METHOD AND APPARATUS FOR PER-ANTENNA PILOT TRANSMISSION SUPPORTING

MULTI-USER MIMO TRANSMISSION

BACKGROUND Technical Field

[0001] The present invention generally relates to wireless communications, and particularly relates to a method and apparatus for per-antenna pilot transmission by a user terminal to facilitate Multi-User Multiple-lnput-Multiple-Output (MU-MIMO) transmissions by a supporting base station Background

[0002] Multi-user (MU) Multiple-lnput-Multiple-Output (MIMO) precoding techniques yield extremely high spectral efficiency, at least under some circumstances These techniques represent an area of increasing interest in developing wireless communication standards targeting the IMT-Advanced requirements to be set by the International Telecommunication Union (ITU) Examples of MU-MIMO precoding techniques include Successive Minimum Mean Square Error (SMMSE), such as taught in V Stankovic and M Haardt, "Multi-user MIMO Downlink Precoding for Users with Multiple Antennas," Proc of the 12th Meeting of the Wireless World Research Forum (WWRF), Toronto, ON, Canada, Nov 2004 Regularized Block Diagonalization (RBD) represents another example of MU-MIMO precoding, such as taught in V Stankovic and M Haardt, "Novel Linear And Non-Linear Multi-User MIMO Downlink Precoding With Improved Diversity And Capacity," WWRF#16, April 2006

[0003] As a general proposition, MU-MIMO precoding techniques involve the use of a precoding matrix to produce weighted information streams for transmission from different transmit antennas The precoding weights must accurately reflect the channel conditions between respective pairings of transmit and receive antennas in order to eliminate or minimize multi-user interference Thus, MU-MIMO precoding techniques depend on having accurate, timely channel state information (CSI) for use by the transmitter in precoding weight generation

[0004] Consequently, there is particular interest in MU-MIMO precoding in low mobility scenarios, where channel conditions generally are not rapidly changing and it is therefore more feasible to track channel conditions accurately for precoding weight generation Such scenarios include (but are not limited to) short range communications (e g in homes, hotspot areas, and office buildings), and low-mobility modes in microcellular deployments in city centers Rather large channel coherence times can be expected in those scenarios, meaning that the CSI will not vary too rapidly

[0005] Furthermore, in Time Division Duplex (TDD) systems, where the same frequencies are used at different times for the uplink and the downlink, a downlink transmitter, e g , a base station, can directly acquire the CSI needed for precoding its MIMO transmissions by estimating the involved channels from pilot symbols received from each user equipment (UE) of interest In cases where one or more of the UEs are equipped with more than one receive antenna, which is expected to become more commonplace as higher performance systems are deployed, each multi-antenna UE must transmit per-antenna pilots on the uplink The transmission of pilot symbols on the uplink from each UE antenna is necessary because MU-MIMO precoding at the base station is done over individual receive antennas, not users

[0006] One approach to providing per-antenna pilot symbols from a UE is to implement a radio transmitter chain per antenna, thereby allowing the transmission of different pilot signals from each UE antenna Of course, having one radio transmitter chain per antenna is beneficial in other regards, too For example, that arrangement allows implementation of MIMO transmission techniques in the uplink With a radio transmitter chain per antenna, the UE could implement uplink transmit diversity, e g in the form of Alamouti coding For reference, see, e g , S M Alamouti, "A simple transmit diversity technique for wireless communications," IEEE Journal on Selected Areas in Comm , vol 16, no 8, pp 1451-1458, Oct 1998

[0007] However, the performance gams that such techniques might provide do not necessarily support the inclusion of multiple radio transmitter chains In particular, for low

cost, multi-antenna terminals, it may not be economically feasible to include more than one transmitter radio chain Thus, it cannot be taken for granted that UEs with multiple receive antennas also will have multiple radio transmitter chains A more likely scenario is an operational mix of multi-antenna UEs That mix will include UEs with a radio transmitter chain per antenna, which permits each one to transmit antenna-specific pilots from a respective UE antenna and thereby directly support acquisition of antenna-specific CSI by the base station, and UEs with a single radio transmitter chain, thereby lacking the capability to provide antenna-specific pilots in the manner of the UEs with multiple radio transmitter chains

SUMMARY

[0008] According to the teachings presented herein, a wireless communication terminal includes one radio transmitter chain, but time-wise switches that transmitter between two or more of its antennas This switching between antennas may be referred to as "antenna hopping" and the terminal is configured to perform such hopping to thereby provide a supporting base station with antenna-specific pilot information for use in Multi-User Multiple- Input-Multiple-Output (MU-MIMO) precoding of the base station's downlink transmissions [0009] In one or more embodiments, such a method comprises switching a transmitter radio circuit of a wireless communication terminal between individual ones of two or more antennas of the wireless communication terminal according to a desired per-antenna pilot transmission timing, and transmitting per-antenna pilot information from each of the two or more antennas according to the desired per-antenna pilot transmission timing The desired per-antenna pilot transmission timing may be determined, for example, according to a Time Division Duplex (TDD) frame timing governing downlink and uplink transmissions between the supporting base station and the wireless communication terminal In one embodiment of this method, then, the terminal transmits pilot information in the uplink portion of one TDD frame using one of its antennas, transmits pilot information from another one of its antennas in the uplink portion of the next TDD frame, and so on

[0010] To carry out antenna hopping as taught herein, the terminal comprises, in one or more embodiments, two or more antennas, a transmitter radio circuit for transmitting pilot information, a coupling circuit operative to switch the transmitter radio circuit between individual ones of the two or more antennas, and a control circuit The control circuit is configured to transmit per-antenna pilot information from each of the two or more antennas by controlling the coupling circuit to selectively switch the transmitter circuit between individual ones of the two or more antennas according to the desired per-antenna pilot transmission timing In at least one embodiment, the control circuit comprises hardware, software, or any combination thereof For example, the terminal may include a baseband processing circuit, which itself may incorporate selected transmission processing functions, and which may be configured to perform the antenna-hopping as part of overall transmit/receive operations management in the TDD context [0011] Of course, the present invention is not limited to the above features and advantages Indeed, those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Fig 1 is a partial diagram of a wireless communication network, including one embodiment of a base station configured for Multi-User Multiple-lnput-Multiple-Output (MU- MIMO) precoding of its downlink transmissions, and at least one associated wireless communication terminal that is configured to using antenna-hopping for providing per- antenna pilot information on the uplink in support of that downlink precoding [0013] Fig 2 is a diagram of one embodiment of a wireless communication terminal configured for per-antenna pilot transmission via antenna hopping

[0014] Fig 3 is a logic flow diagram of one embodiment of antenna-hopping processing at a wireless communication terminal

[0015] Fig 4 is a block diagram of one embodiment of functional circuit details for implementing antenna hopping at a wireless communication terminal [0016] Fig 5 is a diagram of antenna hopping for per-antenna pilot transmission in accordance with a defined TDD frame timing

DETAILED DESCRIPTION

[0017] As an example embodiment, Fig 1 partially illustrates a wireless communication network that includes a base station 10 that uses Multi-User Multiple-lnput-Multiple-Output (MU-MIMO) precoding to serve a number of wireless communication terminals 12 Terminals 12-1 , 12-2, and 12-3 are illustrated as an example As non-limiting examples, the terminals 12 — which also may be referred to as user equipments (UEs) or mobile stations (MSs) — comprise essentially type of wireless communication device, such as cellular radiotelephones, pagers, radio network cards or modules, etc

[0018] The base station 10 and terminals 12 may be configured, for example, according to a desired communications standard, such as IEEE 802 16 (WiMAX) Of course, that is a non-limiting example Regardless of the standards/protocols adopted, of particular interest is the use of MU-MIMO precoding at least on the transmission downlink To that end, the base station 10 includes a precoding processor 20 for producing weighted (precoded) streams, combining circuits 22 for combining the weighted streams, and a set of transmit antennas 24 for transmitting the weighed streams

[0019] On the terminal side, the representative terminals 12 are illustrated as each comprising a set of receive antennas 30 for receiving the base station's precoded downlink transmissions and which also may be used for transmission on the uplink, and receiver circuits 32 for obtaining information streams of interest from the received downlink transmissions The base station 10 requires full Channel State Information (CSI) for the propagation channels between each of its transmitting antennas and each of the terminal's receiving antennas, and the terminals 12 therefore provide independent pilot information

from each of their antennas (or at least those antennas involved in MU-MIMO downlink reception)

[0020] Advantageously, at least one of the terminals 12, e g , terminal 12-1 , includes a single transmitter radio circuit but nonetheless provides per-antenna pilot information to the base station 10 for supporting the base station's MU-MIMO precoding by time-wise switching that single transmitter radio circuit between the terminal's antennas In this manner, the terminal 12-1 provides pilot information from each of the terminal's antennas that is fully resolvable by the base station 10 with respect to the pilot information sent from the terminal's other antennas

[0021] However, before detailing the advantageous, "antenna-hopping" operation of the example terminal 12-1 for facilitating MU-MIMO precoding downlink transmission by the base station 10, it may be helpful to present a system model of downlink MU-MIMO precoding, with reference to Fig 1 As shown, the base station (BS) 10 is equipped with M ₁ antennas, and each terminal 12 is equipped with M _{R k} antennas (where "A-" equals 2 in

the diagram Further, there are K users in total out of which K _u are served simultaneously within one resource block via Spatial Division Multiple Access (SDMA) Consequently, for the example case of two receive antennas per terminal 12, the total number of receive antennas to be considered in MU-MIMO precoding by the base station 10 is equal to

M _R = ∑ λ/ _{R A} = 2 - K cc Each terminal 12 ("user") receives >\ = 1 data streams, and the total

A =] number of data streams is therefore equal to

/ ^• = £>i = λ' _tt Eq (1)

A = I

[0022] The data model can be summarized as y = D (H F x + n) Eq (2)

where D e C ^A/R IS a block-diagonal matrix containing the receiver processing filter D, e C'' ^α/ " , the matrix H e C ^{Uβ vλ/τ} represents the MIMO channel matrix,

F = [F _v , F _κ ] e C ^λ ' ^T ' denotes the overall precoding matrix and the vectors x , y and n represent the vectors of sent symbols, received symbols, and additive noise at the receive antennas, respectively

[0023] The precoding matrices F _k e C ^{A// /(} should be calculated such that the multi-user interference is minimized while balancing it with noise enhancement As previously noted, SMMSE and RBD are two precoding techniques that can achieve that desired balance Further explanatory details, including performance results, can be found, for example, IST-4- 027756 WINNER II, "D3 4 1 The WINNER Il Air Interface Further Refinement Spatial- Temporal Processing Solutions," November 2006 Significantly, however, the base station processing relies on the assumption that the BS 10 has accurate CSI available for computation of the precoding matrices

[0024] In a Frequency Division Duplex (FDD) system the CSI must be fed back from the receiving terminals, because uplink and downlink channels are not correlated That sort of feedback generally represents significant feedback overhead However because of uplink/downlink channel reciprocity in a TDD system, the BS 10 can acquire the needed CSI based on making channel estimates from the uplink signals received from the terminals 12 However, as noted, the BS 10 must be able to estimate propagation channels relative to individual ones of the antennas 30 at each terminal 12

[0025] A basic mechanism for enabling this estimation is the transmission of pilot information, e g , reference symbols, pilot symbols, training sequences, or other predefined information, from each antenna for which downlink channel estimation is to be performed To this end, one or more given ones of the terminals 12 may include a radio transmitter circuit for each antenna, such that each radio transmitter circuit can be used to transmit pilot information from its respective antenna However, as noted, it may be too expensive or otherwise undesirable to implement multiple transmitter radio chains in given ones of the terminals

[0026] Thus, as an example, at least one of the terminals 12 is advantageously configured to provide per-antenna pilot information, without requiring more than one transmitter radio chain That is, at least one of the terminals 12, e g , terminal 12-1 , includes a single transmitter radio circuit — not shown in Fig 1 — but nonetheless provides per-antenna pilot information to the base station 10 for supporting the base station's MU-MIMO precoding by time-wise switching that single transmitter radio circuit between the terminal's antennas 30 In this manner, the terminal 12-1 provides pilot information from each of the terminal's antennas 30 that is fully resolvable by the base station 10 with respect to the pilot information sent from the terminal's other antennas

[0027] Fig 2 illustrates the terminal 12-1 in more detail, and in particular illustrates the inclusion of an antenna-hopping transmit processor 40, which may be implemented in hardware, software, or any combination thereof In at least one embodiment, the antenna- hopping transmit processor comprises a microprocessor-based circuit that is provisioned or otherwise configured through the execution of stored program instructions to carry out the desired antenna-hopping transmission processing An example of such processing appears in the logic flow diagram of Fig 3

[0028] According to Fig 3, the antenna-hopping transmit processor 40 of the terminal 12-1 is configured to implement a method of transmitting per-antenna pilot information to enable antenna-specific downlink transmission precoding by the BS 10 Particularly, in the illustrated embodiment, the method includes switching a (single) transmitter radio circuit of the terminal 12-1 between individual ones of two or more antennas 30 of the terminal 12-1 according to a desired per-antenna pilot transmission timing (Block 100) The method further includes transmitting per-antenna pilot information from each of the two or more antennas 30 according to the desired per-antenna pilot transmission timing (Block 102) [0029] Of course, the terminal 12-1 also may transmit uplink data from different ones of the antennas 30, as part of its antenna hopping For example, it may switch to a first one of the antennas 30 and transmit pilot information, along with uplink data Then, at a later transmission interval, it may transmit pilot information from a second one of the transmit

antennas 30, along with further uplink data In general, the terminal 12-1 thus may "cycle" through pilot information from each one in the set of antennas 30, or in a subset thereof [0030] As a non-limiting example, Fig 4 illustrates one embodiment of a functional circuit arrangement of the terminal 12-1 , configured to implement antenna-hopping for per-antenna pilot transmission according to the teachings presented herein In the illustration, the terminal 12-1 includes first and second receiver radio circuits 40-1 and 40-2 that are respectively coupled to first and second antennas 30-1 and 30-2 through a coupling circuit 42 As such, the terminal 12-1 has an equal number of receiver radio circuits ("chains") and receiving antennas 30 However, the terminal 12-1 includes only a single transmit radio circuit 44, which also is coupled to the antennas 30 through the coupling circuit 42 [0031] More particularly, a control circuit 46 controls the coupling circuit 42 such that the transmitter radio circuit 44 is disconnected from both of the antennas 30, or coupled to a selected one of the antennas 30, i e , coupled either to antenna 30-1 or antenna 30-2 With this control arrangement, the terminal 12-1 implements TDD reception/transmission [0032] For example, for reception, switches SW1 and SW2 are switched to their illustrated upper positions, thereby connecting the receiver radio circuit 40-1 to the antenna 30-1 and the receiver radio circuit 40-2 to the antenna 30-2 With switches SW1 and SW2 thus configured, the transmitter radio circuit 44 is disconnected from the antennas 30 regardless of the position of the switch SW3 Contrastingly, for transmission from the antenna 30-1 , the switch SW3 is placed in its upper position, and the switch SW1 is placed in its lower position For transmission from the antenna 30-2, the switch SW3 is placed in its lower position, and the switch SW2 is placed in its lower position [0033] Those skilled in the art will appreciate that the coupling circuit 42 may include elements not illustrated, such as impedance matching circuits, etc , and will further appreciated that the switches may be implemented, for example, using MEMS technology, or other robust switching arrangements Moreover, those skilled in the art will recognize that the illustrated coupling circuit 42 is a non-limiting example

[0034] Different switching arrangements may be adopted, and different "switching" elements may be used, as needed or desired For example, the transmitter radio circuit 44 may not literally be disconnected and connected, but rather different signal paths may be provided with signal isolation, path-enabling, etc Functionally, it is necessary only that the terminal 12-1 have some mechanism for separately selecting which antenna it transmits from, so that per-antenna pilot information may be sent from different antennas at different times

[0035] In any case, the control circuit 46 thus may control the coupling circuit 42 for TDD-based reception and transmission, and may change which one of the antennas 30 is used in each transmit interval More particularly, this arrangement permits the terminal 12-1 to "hop" from the antenna 30-1 to the antenna 30-2 (and back) as needed or desired, to transmit pilot information from each antenna according to a desired pilot information transmission timing Thus, the control circuit 46, or some other functional element of the terminal 12-1 may be configured to determine the desired per-antenna pilot transmission timing according to a TDD frame timing governing downlink and uplink transmissions between the supporting base station 10 and the terminal 12-1 [0036] In that case, the terminal 12-1 switches between the individual ones of the transmit antennas 30 to the TDD frame timing That switching is illustrated, for example, in Fig 5, where a series of TDD frames is illustrated, and the cyclic hopping between antennas 30-1 and 30-2 (with reference to Fig 4) is illustrated Note, too, that the terminal 12-1 may be configured to transmit the same pilot information from each of the antennas 30 This approach is permissible because the time separation of the pilot transmissions from different ones of the antennas 30 allows the BS 10 to fully distinguish pilots from one of the antennas 30 versus those from another one of them

[0037] Of course, it is also contemplated in at least one embodiment to send different pilot information from different ones of the antennas 30 Further, while a general case might be a balanced or otherwise equal split of pilot transmissions across the antennas 30, it is contemplated that more pilots may be sent from one antenna versus another, such as by

selecting one antenna more frequently than another, or selecting it for longer durations (if the system timing permits) In other variations, the terminal 12-1 may operate in modes, such as one mode where it uses antenna hopping and another mode where it does not Signaling between it and the BS 10 may be used for mode control Or, in some cases, signaling may be used by the terminal 12-1 simply to report its "class," or otherwise to indicate that it will use antenna-hopping to effect per-antenna pilot transmissions in support of the BS's MU- MIMO precoding That information is useful at the BS 10 for understanding the rates and/or times at which per-antenna pilot information will be supplied by the terminal 12-1 Of course, in other embodiments, the timings and other information can be preconfigured, such that BS 10 knows by default that the terminal 12-1 will use antenna-hopping [0038] Broadly, the antenna-hopping teachings presented herein facilitate uplink dedicated pilot transmissions per antenna, from properly configured terminals, without requiring multiple radio transmitter chains in those terminals As explained, this result is achieved by using antenna hopping/switching for uplink transmissions by each such terminal That is, a given terminal has one radio transmitter chain that switches between the different antennas according to a certain time interval, say each frame That allows, for example, the supporting base station to acquire CSI from a first antenna in a first frame from a second antenna in a second frame, and so on

[0039] With the above approach, and assuming two antennas at the terminal, the supporting base station would obtain per-antenna pilots at half the rate as it could if the terminal had a dedicated transmitter per antenna However, because target scenarios for MU-MIMO precoding generally are low mobility scenarios (in-home, hotspot, in-office, etc ), it is reasonable to assume that the channel coherence times are rather large (ι e , the channel does not change too rapidly), which means that the reduction in CSI update rate should not be problematic Further, the use of antenna hopping offers the advantage of reducing pilot overhead, in at least some embodiments contemplated herein [0040] Pilot overhead reduction arises from the ability to reuse the same pilot information, e g , the same pilot symbol(s), across the terminal's antennas, because only

one antenna at a time transmits pilot information In contrast, terminals that use multiple transmitter radio chains to send pilots from each antenna in the same transmission interval, generally must send different pilot information from each antenna, e g , different pilot symbol/coding values, to allow the base station to fully resolve the per-antenna channels The time-separated transmission of pilots from one antenna at a time obviates the need to use different pilots Of course, one or more embodiments contemplated herein may, in fact, not send the same pilot information from each antenna

[0041] However, whether the same or different pilot information is sent using antenna hopping, the benefit of reduce terminal expense and, potentially, reduced terminal power consumption, is obtained by switching a single transmitter radio circuit between selected ones in a set of terminal antennas, so that the same transmitter radio circuit can be used to send pilot information from each one of the antennas In this manner, a low-cost terminal can still participate in and facilitate MU-MIMO downlink precoding Notably, all participating terminals may be configured to use antenna-hopping for dedicated, per-antenna pilot transmission on the uplink, or there may be any mix of multi-transmitter terminals not using antenna hopping and single-transmitter terminals using antenna hopping [0042] Thus, the foregoing description and the accompanying drawings represent non- limiting examples of the methods and apparatus taught herein As such, the present invention is not limited by the foregoing description and accompanying drawings Instead, the present invention is limited only by the following claims and their legal equivalents