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(12) EUROPEAN PATENT APPLICATION

(43) Date of publication: 06.01.2016 Bulletin 2016/01

(21) Application number: 14306103.4

(22) Date of filing: 04.07.2014

(51) Int Cl.:H04W 76/02 (2009.01) H04W 16/14 (2009.01)

(84) Designated Contracting States: AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TRDesignated Extension States: BA ME

(71) Applicant: Sequans Communications S.A.92073 Paris (FR)

(72) Inventors: • Bertorelle, Jerome

92200 Neuilly-sur-Seine (FR)• Vivier, Guillaume

75015 Paris (FR)

(74) Representative: Morrall, Jonathan Ian McLachlan et alKilburn & Strode LLP 20 Red Lion StreetLondon WC1R 4PJ (GB)

(54) LTE transmission in unlicensed bands

(57) A method of receiving LTE data by a user device,the data being transmitted on a channel of an unlicensedband comprising the steps of receiving a cell ID from a

primary LTE cell, receiving system information from theprimary LTE cell for access to an unlicensed channel,receiving LTE data on the channel of an unlicensed band.

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Description

[0001] This invention relates to augmenting LTE trans-mission in unlicensed bands. It is particularly suitable for,but by no means limited to, use with WIFI.

Background

[0002] LTE is a cellular system operating in licensedbands, where an operator has the exclusive usage of theallocated frequencies. The rising demand for mobile datacreates the need for the use of more spectrum. A com-mon way to increase the available spectrum is to useWiFi offloading, where instead of using the cellular net-work, data connectivity is provided through WiFi in unli-censed bands, and in particular the 5 GHz unlicensedspectrum.[0003] However using WiFi for the unlicensed spec-trum has several drawbacks: WiFi is not as spectrallyefficient as LTE, and using WiFi requires integrating twodifferent technologies. Some estimations put LTE as be-ing twice as spectrally efficient as WiFi, therefore usingLTE could double the available bandwidth using thesame amount of spectrum resources. For this reasonthere is a growing interest in using LTE in unlicensedbands, which may be considered as part of LTE release13 (the currently deployed LTE release being 9, or 10 inthe most advanced deployments). For this LTE shouldbe modified to share the unlicensed spectrum with othertechnologies, mainly WiFi, in a fair manner.[0004] LTE is designed for licensed bands, so there isan assumption that an LTE channel is fully dedicated toLTE. As such, there is no current mechanism to sharethe spectrum with other users, and possibly other tech-nologies. Conversely, in unlicensed bands there is a re-quirement to share the available capacity with other usersand technologies in a fair manner.[0005] The key principle behind this fair coexistenceon an unlicensed channel is "Listen Before Talk" (LBT).With LBT, in order for a device to transmit, it will listen tothe channel and only start transmission if no other trans-mission is on-going. However, two (or more) such devic-es could listen at the same time to an unused channeland decide to start transmission at the same time, leadingto a collision and a failed transmission. LBT cannot avoidthis issue, but is still an important mechanism to avoidcollisions and share the spectrum between uncoordinat-ed users and technologies.[0006] Although not required, a number of technolo-gies using unlicensed bands can start transmitting at anytime, in order to quickly use the spectrum as soon as itis available.[0007] LTE uses a completely different scheme: thechannel usage is split into 1 millisecond subframes, withfixed time synchronization, and the eNodeB base station(eNB) centrally schedules the spectrum usage by explic-itly allocating part of each subframe subcarriers to differ-ent devices (UE, user equipment) as would be under-

stood by the skilled person. Typically, the eNB sendsallocation order to a device over the PDCCH (PhysicalDownlink Control CHannel) to describe an allocation ina given subframe PDSCH or PUSCH (Physical Downlink/ Physical Uplink Shared CHannels). An allocation pro-vided by the eNB has to be used.[0008] The LTE allocation scheme is inflexible and, asis, is not suitable to share unlicensed spectrum in a fairmanner. A recently developed mechanism, carrier ag-gregation (CA), provides a little more flexibility in that asecondary channel can be activated/deactivated withina few milliseconds (up to 8), and so does not need to beused all of the time. A primary cell (PCell) in a licensedband combined with a secondary cell (SCell) in an unli-censed band would bring more flexibility than the regular,’always on’ LTE used on the primary cell. But once asecondary cell is activated on the license-exempt band,the regular LTE framing and eNB scheduling is used, andis still not flexible enough. It is noted that the secondarycell could be downlink (DL) only as denoted for instancein 3GPP as supplementary DL. In the context of LTE-U(refering to LTE that has been modified and extended towork in unlicensed bands), it is assumed that the sec-ondary cells used in the license-exempt spectrum is DLonly.[0009] There is a need for a scheme to use the LTEwaveform in unlicensed bands, with minimal modificationon the device side to help implementation and adoption.There is also a need to make the LTE allocation moreflexible when an unlicensed band is used, while preserv-ing most of the LTE waveform and its high bandwidthefficiency.

Summary

[0010] According to a first aspect there is provided amethod of receiving LTE data by a user device, the databeing transmitted on a channel of an unlicensed band asdefined in Claim 1 of the appended claims. Thus thereis provided a method of receiving LTE data by a userdevice, the data being transmitted on a channel of anunlicensed band comprising the steps of receiving a cellID from a primary LTE cell, receiving system informationfrom the primary LTE cell for access to an unlicensedchannel, receiving LTE data on the channel of an unli-censed band.[0011] Optionally, the method wherein the cell ID andsystem information is received as unicast data.[0012] Optionally, the method wherein the unlicensedchannel is served by a secondary LTE cell of a differenteNodeB than the primary LTE cell.[0013] Optionally, the method further comprises deter-mining from PDCCH whether an LTE transmission sub-frame on the unlicensed channel is one of used, partiallyused or not used.[0014] Optionally, the method wherein the user devicedetermines one or more transmission measurements ofthe unlicensed channel in relation to transmission on that

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channel based on used and/or partially used LTE trans-mission subframes on the unlicensed channel.[0015] Optionally, the method wherein the user devicedetermines one or more channel measurements of theunlicensed channel in relation to interference and/ornoise on that channel based on unused LTE transmissionsubframes.[0016] Optionally, the method of claim 5 or 6 whereinthe user device sends data indicative of the one or moretransmission and/or channel measurements to the pri-mary cell.[0017] Optionally, the method wherein the user devicesends its geographical position to the primary LTE cell.[0018] Optionally, the method wherein the user devicecalculates a timing error based on at least one of cellspecific RS, user device specific RS and positioning RS.[0019] Optionally, the method further comprises usingPDCCH to compensate for the timing error.[0020] Optionally, the method further comprises re-ceiving a synchronization signal through RRC from theprimary cell.[0021] Optionally, the method further comprises re-ceiving a synchronization signal through MAC from theprimary cell.[0022] Optionally, the method wherein the synchroni-zation signal comprises one or more LTE OFDM sym-bols.[0023] Optionally, the method wherein a determinationis made as to whether a synchronization signal is requiredand optionally as to the length of the synchronization sig-nal, when the user device registers to the primary cell.[0024] Optionally, the method wherein the user devicemeasures a power level of Wifi signals on the channel ofthe unlicensed band and compares the power level to apredetermined threshold; wherein the user device holdsoff transmitting any signals on the channel if the meas-ured power level is above the predetermined threshold.[0025] Optionally, the method further comprises theuser device decoding a WiFi header from the channel ofthe unlicensed band and wherein the user device holdsoff transmitting any signals on the channel for a durationspecified in the WiFi header.[0026] Optionally, the method further comprises theuser device decoding a WiFi header from the channel ofthe unlicensed band and wherein the user device turnsoff its WiFi receiver if the measured power level is abovethe predetermined threshold.[0027] Optionally, the method further comprises theuser device decoding a WiFi header from the channel ofthe unlicensed band and wherein the user device turnsoff its LTE-U WiFi capable receiver for a duration spec-ified in the header if the header indicates that the WiFiheader has not originated from the secondary LTE cell.[0028] Optionally, the method further comprises deter-mining from received LTE DCI whether a detected WiFiheader has originated from the secondary LTE cell, andif it has not originated from the secondary LTE cell, turn-ing off its LTE-U WiFi capable receiver for the rest of the

duration specified in the header.[0029] According to a second aspect there is provideda computer readable medium as defined in claim 16 com-prising instructions that when executed on a processorcause the processor to carry out the method.[0030] According to a third aspect there is provided auser device as defined in claim 17 arranged to carry outthe method.[0031] According to a fourth aspect there is provideda method of transmitting LTE data in a channel of anunlicensed band comprising the steps of determiningwhether a channel of the unlicensed band is free to ac-cept transmissions, determining an LTE subframe onwhich the transmission can commence, transmitting inthe channel of the unlicensed band an LTE transmissionframe comprising the LTE data starting at the determinedsubframe.[0032] Optionally, the method further comprises deter-mining whether the channel of interest comprises a fre-quency band utilized by WiFi.[0033] Optionally, wherein determining whether achannel of the unlicensed band is free to accept trans-missions comprises measuring a power level of trans-mission on a channel of the unlicensed band and com-paring to a predetermined threshold.[0034] Optionally, wherein determining whether achannel of the unlicensed band is free to accept trans-missions comprises detecting a WiFi header on a chan-nel of the unlicensed band.[0035] Optionally, wherein if a channel of the unli-censed band is free to accept transmissions, formulatinga pseudo WiFi header comprising a signal field wherethe signal field defines a duration longer than the LTEtransmission frame to be transmitted.[0036] Optionally, the method wherein the LTE trans-mission frame comprises a padding frame to keep theunlicensed channel busy until the determined subframe.[0037] Optionally, the method wherein a plurality ofpseudo WiFi headers are transmitted on adjacent WiFichannels of the unlicensed band.[0038] Optionally, the method further comprises deter-mining a reception offset between a primary channelcomprising standard LTE transmission and a secondarychannel comprising the LTE transmission in a channelof an unlicensed band.[0039] Optionally, the method further comprises add-ing a synchronization signal to the LTE transmissionframe, the synchronization signal comprising LTE OFDMsymbols.[0040] Optionally, the method wherein determining theLTE subframe upon which the transmission can com-mence comprises determing the first LTE subframe sub-sequent to the channel becoming free.[0041] Optionally, the method wherein determining thefirst LTE subframe subsequent to the channel becomingfree comprises determining the first LTE subframce sub-sequent to the channel becoming free and the transmis-sion of any pseudo WiFi header and/or LTE synchroni-

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zation field.[0042] Optionally, the method wherein the paddingframe comprises an integer number of LTE OFDM sym-bols transmitted after the time when the channel becamefree and after the transmission of any pseudo WiFi head-er and/or LTE synchronization field[0043] Optionally, the method wherein the paddingframe further comprises additional padding signals to fillthe gap between the last LTE OFDM padding symboland the determined subframe.[0044] According to a fifth aspect there is provided acomputer readable medium comprising instructions thatwhen executed on a processor cause the processor tocarry out the method of the fourth aspect and optionallyany optional steps.[0045] According to a sixth aspect there is provided anLTE eNodeB arranged to carry out the method of thefourth aspect and optionally any optional steps.[0046] With all the aspects, preferable and optionalfeatures are defined in the dependent claims.[0047] The term LTE-U as used herein refers to LTEthat has been modified and extended to operate in unli-censed bands.

Brief Description of the Drawings

[0048] Embodiments will now be described, by way ofexample only, and with reference to the drawings inwhich:

Figure 1 illustrates a structure of an LTE-U transmis-sion frame according to an embodiment;Figure 2 illustrates a fake IEEE 802.11a header ac-cording to an embodiment;Figure 3 illustrates a Wifi signal field;Figure 4 illustrates the positions of a primary cell, asecondary cell and a UE device and the correspond-ing propagation delays;Figure 5 illustrates LTE regular transmission portion;Figure 6 illustrates a timing diagram of a WiFi headerand synchronization period;Figure 7 illustrates a method of initiating LTE usagein an unlicensed band;Figure 8 illustrates formulation of the optional WiFiProtection Header;Figure 9 illustrates formulation of the optional syn-chronization signal;Figure 10 illustrates a method of receiving LTE datain an unlicensed band at a UE;Figure 11 illustrates steps carried out at a UE in re-lation to channel measurement and reporting;Figure 12 illustrates steps carried out at a UE in re-lation to Wifi transmission;andFigure 13 illustrates steps carried out at a UE in re-lation to optional synchronisation.

[0049] In the figures, like elements are indicated by like

reference numerals throughout.

Overview

[0050] Disclosed herein is a scheme to support an LTEextension to unlicensed bands using carrier aggregation,where the primary carrier is operating in a licensed bandand one or several secondary carriers may be in unli-censed bands. A focus is the modification and extensionsto apply to the LTE standard to support such secondarycarriers in unlicensed bands. In addition to the LTE stand-ard mechanisms, modifications are described herein.[0051] The unlicensed spectrum may be used for a DLonly LTE channel, where the traffic only goes from theeNB to the UE. The UL traffic uses the primary carrieroperated in a licensed band in the known LTE manner.Modifications are applied at the eNB and at the UE com-pared to legacy LTE equipment.[0052] An LTE-U capable eNB is responsible for lis-tening to the unlicensed channel and performing listen-before-talk (LBT) before transmitting, as required for thefair use of the channel.[0053] In order not to interfere with other users, all theusual LTE periodic transmissions are disabled in an un-licensed band outside of LTE transmissions (see below):

• The Primary Synchronization Signal (PSS) and theSecondary Synchronization Signal (SSS) are thesynchronization sequences used in legacy LTE tosupport UE synchronization to the cell. Moreover,from PSS and SSS, the UE is able to derive the phys-ical cell identifier (cellID). In order to minimize inter-ference with other users in the unlicensed band,PSSS/SSS are not transmitted. Synchronization ona secondary unlicensed channel is performed as de-scribed below instead; the cellID is provided to a UEthrough unicast data on the primary channel (andreceived at step 100 of Figure 10 which illustrates amethod of receiving LTE data in an unlicensed bandat a UE);

• The Physical Broadcast Channel (PBCH) in legacyLTE is the channel which carries the information forinitial access to the cell: the LTE System Information(SI). In order not to interfere with other users, it isnot transmitted. As the unlicensed channel is a sec-ondary cell, the SI is provided to a UE as unicastdata on the primary channel (and received at step102 of Figure 10);

• Reference signals (RS), also called pilots, are nottransmitted in unused frames, except possibly forfine time synchronization, as described below.

[0054] The end result is that when no user data trans-mission occurs on the unlicensed channel, no transmis-sion is performed from the LTE-U eNB. This allows thetypical LTE periodic transmissions on the unlicensedchannel to be kept to a minimum, or at least reduced andhence LBT is more likely to return a result which is indic-

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ative of an eNB being allowed to transmit to a UE on theunlicensed channel.[0055] The above schemes provide protection in unli-censed bands that WiFi and other technologies may use.LTE-U could be used in any unlicensed bands, not onlythe unlicensed bands also used by WiFi. The schemedescribed herein provides additional protection againstWiFi specifically, which is justified by the widespread useof WiFi. WiFi use is common enough such that improvedprotection against Wifi interference in unlicensed bandsis worth the added complexity of the LTE-U system.

Detailed Description

[0056] The structure of an LTE-U transmission frame10 from an eNB in an unlicensed band is shown in figure1 (not to scale). The structure 10 comprises an optionalWiFi protection header portion 12, an optional LTE syn-chronization portion 14, and an LTE regular transmissionportion 16.[0057] Each portion will be described in turn.

Optional WiFi Protection Header 12

[0058] The regular LBT process comprises comparingthe measured power level on the channel at issue to apredetermined threshold T, and uses the channel if thispower is below T.[0059] The main technology used today in the 5 GHzband is WiFi. In addition to T, WiFi has a second powerthreshold W (higher than T), above which the receivercan demodulate a valid WiFi header. When the powerlevel is between T and W, a WiFi device cannot demod-ulate the header but will also consider the channel asused. This provides extra protection to a device trans-mitting a WiFi compatible header at the start of its trans-mission against other WiFi users in the band.[0060] Per the IEEE 802.11-2012 specification, a WiFidevice must consider the channel busy if detected poweron a channel is above -62 dBm for a 20 MHz channel. Ifa Wifi preamble is detected, the threshold W is reducedto -82 dBm (although in practice implementations aremuch better than this and can detect a further 5 to 10 dBbelow).[0061] Figure 7 shows a method of initiating LTE usagein an unlicensed band by an eNB. At step 70, an eNBoptionally determines whether an unlicensed channel ofinterest is in a frequency band utilized by WiFi, and hencewhether to add an optional Wifi protection header 12. Ifthere is no Wifi utilization, the method continues to step72, otherwise, the method continues to step 80 of Figure8 where the power level on the unlicensed channel ismeasured as described in the previous paragraph.[0062] Another benefit for WiFi devices is that the WiFiheader indicates the length of transmission. Thereforeafter decoding a valid WiFi header, a WiFi device maystop reception (and optionally turn off its WiFi receiver)for the indicated transmission duration, as it knows that

the channel will be used. This can be used to reduce theWiFi device power consumption.[0063] As an example for the 5 GHz band, a fake IEEE802.11a header 20 as shown in figure 2 (not to scale).The header may comprise at least one of a regular pre-amble field 22 and a signal field 24 and may be addedat the start of the LTE-U eNB transmission portion 16(step 82 of Figure 8) to offer increased protection againstWiFi interference. The time durations of the preambleand signal field as defined by the 802.11-2012 standardare included in the figure for completeness.[0064] The WiFi signal field 24 may optionally be fol-lowed by padding field 26 (step 84 of Figure 8) used tokeep the channel busy as further described in the LTERegular Transmission Section of this specification.[0065] The WiFi signal field 24 is typically used to in-dicate the modulation and coding scheme (MCS) and thelength of the associated WiFi transmission as shown infigure 3 (not to scale). The bit size for each portion asdefined by the 802.11-2012 specification is shown forcompleteness.[0066] The "rate" field portion 30 provides the WiFiMCS, and "length" portion 32 provides an amount oftransmitted data in bytes. From these two pieces of in-formation, a WiFi receiver can compute a transmissionlength corresponding to the duration required to transmita well formed WiFi frame at the indicated MCS containingthe indicated amount of data bytes. A WiFi receiver suc-cessfully decoding such a preamble will consider thechannel as busy, and will not try to transmit, over thisduration. Decoding this limited WiFi header is sufficientfor any WiFi device to set its network access vector(NAV), as per the IEEE 802.11 standard. This standardbehaviour of a WiFi device will, in effect, protect an un-licensed channel for LTE use.[0067] Any transmission will provide protection againstany technology that supports LBT, including WiFi. Theabove provides additional protection against WiFi use inan unlicensed channel.[0068] An LTE-U eNB can take advantage of this byusing rate and length values corresponding to a durationthat is typically just larger than the coming LTE transmis-sion. These values are unrelated to any WiFi transmis-sion, so a WiFi device will fail to decode any field afterthe "signal" header field 24. This has no unwanted con-sequence, as this partial header decoding is sufficient tomark the channel as used for the indicated duration andprovide protection to the LTE-U transmission.[0069] For a given LTE-U transmission of duration X,several combinations of rate and length can provide asufficient protection during this period. The actual com-bination used is of no importance, and the eNB may sim-ply pick a combination giving duration of protection higherthan but as close as possible to X.[0070] A WiFi protection has been shown for an802.11a header for the 5 GHz band. The same schememay be used with an 802.11b or 802.11g header in the2.4 GHz band, or any other WiFi version header format.

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The main interest of LTE-U is in using the oldest com-monly used WiFi header format for the used unlicensedband, in order to obtain unlicensed WiFi protection fromthe largest portion of WiFi devices.[0071] When the LTE eNB occupies more than 20MHz,by using carrier aggregation in unlicensed bands for ex-ample, the LTE-U eNB, at step 86, can send several suchheaders 20 on adjacent WiFi channels to cover the fullLTE-U bandwidth. This is similar to what is done to handlelarger 802.11n or 802.11ac WiFi channels. For example,in 802.11ac the minimum bandwidth is 20 MHz. The WiFipreamble is transmitted on 20 MHz, and larger channels.For example, 40, 80 and up to 160 MHz are handled bysending several headers in adjacent frequencies. ForLTE the maximum bandwidth is 20 MHz. Therefore, typ-ically, the 20Mhz WiFi bandwidth is sufficient for the LTE-U signal bandwidth. If more LTE-U BW is needed, adja-cent WiFi channels may be used.[0072] The fake WiFi header is typically short (20 msfor the 802.11a header above and as shown in figure 2,compared to over 100 ms for a single LTE OFDM symbol)and offers higher protection to LTE, and so it typicallymakes a good usage of the channel, however its trans-mission is optional. The LTE-U eNB may decide dynam-ically to use such a header or not based on the observedactual WiFi interference in the used channel(s), for ex-ample, based on a power level measured in the unli-censed channel of interest as described in relation to step80 of figure 8, or dependent on whether WiFi signals aredetected by the eNB.[0073] When the header has been formulated, methodflow at the eNB returns to step 72.[0074] Corresponding actions may be taken by a UEin relation to protection against Wifi interference andchannel usage in the unlicensed band. The UE maymeasure power level on the channel at issue and com-pare to a predetermined threshold (step 120 of figure 12).The user device may then hold off transmitting any sig-nals on the WiFi channel if the measured power level isabove the predetermined threshold. The UE device maycontinue to hold off transmitting any signals on the WiFichannel while the measured power level remains abovethe predetermined threshold. The UE may turn-off itsWiFi receiver for the same duration. The UE may meas-ure the power level at any time. The UE may decode aWiFi header on the channel at issue and may hold offtransmitting any signals on the channel for the (transmis-sion) duration specified in the header (step 122 of Figure12). The UE may turn-off its WiFi receiver for the sameduration. The signals on the WiFi channel referred to maybe unrelated to LTE transmission.[0075] In the following, the term LTE-U WiFi capablereceiver is used to describe a receiver that is capable ofreceiving LTE-type transmissions in the WiFi band. Thiscould comprise at least one of a standard WiFi receiver,a hybrid LTE receiver with WiFi band capability or a spe-cific LTE-U capable receiver that can operate in the WiFiband.

[0076] In order that the UE does not turn off its WiFireceiver when a WiFi based LTE-U transmission on thechannel is meant for the UE, the UE may determinewhether the WiFi header has originated from the LTE-UeNB or another WiFi device (a station, STA or accesspoint, AP as would be understood) in the following man-ner:

1. The UE may wait for the expected PDCCH signalsaccording to standard LTE transmission (see appro-priate section later on in this specification). If afterthe PDCCH there is no LTE allocation (no valid DCI),the LTE-U UE can determine that the WiFi transmis-sion is not from the LTE-U eNB in question and turnoff its LTE-U WiFi capable receiver for the rest of theduration specified in the header.

[0077] Approach 1 may be used based on heuristics(when a reception level is perceived to be good enoughfor example) because if the DCI deocindg should fail, theUE may miss all other LTE subframes in the particulartransmission.

2. In a true WiFi header, more information exists re-garding the desired destination of the coming WiFitransmission (WiFi MAC addresses etc as would beunderstod). If a UE detects such information, it isknown that the WiFi transmission is not originatingfrom an LTE-U eNB and therefore, the UE may turnoff its LTE-U WiFi capable receiver.

[0078] The two approaches may be carried out in par-allel, with the first successful ’match’ causing a powerdown of the WiFi receiver in the UE.[0079] With approach 1, additional LTE-U eNBs shar-ing a single unlicensed channel may be put into low powermode for the duration that the LTE-U eNB serving thecurrent transmission to the UE is transmitting. Approach2 protects against true WiFi transmissions only.

Optional LTE synchronization 14

[0080] Preferably, the eNB should not transmitPSS/SSS synchronization signals in the unlicensedbands. Such signals are normally sent every 5 ms at aprecise time and could collide with other users of thechannel transmissions in the unlicensed bands. As de-scribed herein, the synchronization on the unlicensedchannel may use a different scheme.[0081] The synchronization on the secondary (unli-censed) channel may be carried out by taking the primarychannel as reference. If, as would be common, the sameeNB supports the unlicensed channel and the primarychannel, then there is no timing difference between theunlicensed channel and the primary (as the communica-tion path is almost identical) and hence the primary chan-nel may provide an accurate synchronization reference.At step 72 of figure 7, the eNB optionally determines

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whether the primary channel and the secondary channelare situated at the same eNB. If they are, the differencein timing coming from using different frequencies is lim-ited and can be addressed as part of the regular LTEchannel estimation using reference signals (RS), andhence optional LTE synchronization is not required. Theflow of the method therefore continues to step 74. Forexample, the reference signals, or pilots, are signalswhere the position in time and frequency, and originalamplitude, is known. Therefore, a device such as device48 to measure any offset in time, frequency and ampli-tude to estimate channel attenuation and frequency andtiming offsets, and correct or compensate for those off-sets accordingly. This is the standard process of channelestimation and compensation, and allows, in particular,the device 48 to track and correct small timing errors.[0082] If the primary channel is on a different eNB thanthe unlicensed channel and they are located at differentsites, there will be a time offset related to the differentpropagation times between the two sites and the UE aswould be understood. In this event, the flow of the methodat the eNB therefore continues to step 90 of Figure 9.The steps of figure 9 may be carried out again at anytime as desired by the eNB, for example at a rate that isslower than every LTE transmission from the eNB. Thesteps of figure 9 may be carried out as an update to syn-chronization if channel parameters are detected as hav-ing changed since the last determination was made. Anexample of the relationship between positions of a pri-mary cell, a secondary cell and a UE device 48 and thecorresponding propagation delays are shown in figure 4.Primary cell 40 is at a different physical location than theLTE-U cell 42. Accordingly, propagation time 44 to fromprimary cell 40 to device 48 is different to propagationtime 46 from LTE-U cell 42 to the same device 48. Bothcells (40, 42) are synchronized, as per the LTE standardrequirement, but the signals travel different distances tothe device and so they are not perfectly aligned at thereceiver device 48. Moreover the reception offset of sucha device 48 can vary over time as the device moves,although the expected LTE-U usage is mostly of interestin high frequency bands and at low mobility. If the recep-tion offset is large enough, typically higher than half ofan LTE cyclic prefix (CP) duration such that the normalRS based time estimation and compensation is not suf-ficient, the LTE-U eNB can schedule a dedicated LTE-Usynchronization signal 14 just before the actual data de-coding starts as shown in figure 1. At step 90 the receptionoffset is determined. The maximum offset can be derivedfrom the known positions of the primary and LTE-U cellsand based on the maximum available distance from theprimary or LTE-U cell of a UE. The LTE-U cell will typicallyhave a lower maximum distance to the UE due to thecommunication scheme used (lower-power WiFi or otherlocal communication scheme rather than the telephonynetwork of the primary cell). The actual offset may bederived from the actual UE position, if available with highenough accuracy (the resolution of the UE location infor-

mation, for example GPS, may be too large for the rela-tively short distance of the UE from the LTE-U cell).[0083] Corresponding optional steps may be taken bythe UE to aid determination of any synchronization signalby the eNB. At step 130 of figure 13, the UE may provideits geographical position to the primary LTE cell.[0084] As the skilled person would understand thereare two CP durations in the LTE standard, therefore theduration of the reception offset can be defined relative tothe CP as the CP is the relevant reference for timingerrors. The possible CP durations are defined by the LTEstandard.[0085] By way of background explanation, there areseveral types of reference signals or pilots defined by theLTE standard:

• Cell specific RS are transmitted by the eNB at alltimes, and have characteristics that are independentof any UE (they are global to the cell of the eNB).

• UE specific RS are used with beam-forming and arebeam-formed towards a given UE. As such, they areonly usable by the targeted UE.

• Positioning RS are global in the same manner ascell specific RS, but have a specific pattern that al-lows a finer / higher precision timing error computa-tion. Positioning RS are typically used for positioningapplications, where the timing offset is used to derivea distance from the eNB. By using several cells, aposition of a UE can be triangulated.

[0086] All the above types of RS may be used by theUE to compute a timing error based on at least one ofcell specific RS, user device specific RS and positioningRS at step 132 of figure 13.[0087] Even when the timing offset is compensated foroptimal data reception, there may not be a need for asynchronization signal 14. The coming LTE subframe willinclude a PDCCH area before the PDSCH area, and itmay be sufficient to use its RS to have a sufficient de-modulation quality over the PDSCH area. It is up to theLTE-U network to consider this before determiningwhether a synchronization area 14 is needed, and ifneeded what its size should be. At step 92, a determina-tion is made by the eNB as to whether a synchronizationsignal 14 is required, or whether the RS of the next LTEsubframe will be sufficient for compensating for a recep-tion timing offset. The LTE DL transmission is made of asequence of subframes of 1 ms duration. The subframestarts with an area called PDCCH (Physical DownlinkControl Channel) of 1 to 3 OFDM symbols, followed bythe PDSCH (Physical DL Shared Channel). The PDCCHcontains allocation information, but may not be used withcarrier aggregation: the control information may be pro-vided on the primary channel PDCCH instead. In thiscase the UE doesn’t need to decode anything on thePDCCH, so its timing need not be perfect there. And thePDCCH contains RS too. So the UE can measure andcompensate its timing error on the PDCCH (step 134 of

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figure 13) and be compensated enough for receiving onthe PDSCH, if the residual timing error is small enoughto be compensated by RS measurement from the PD-CCH OFDM symbols. The supported residual timing er-ror also depends on the encoding used on the PDSCH:a more robust modulation and coding scheme supportsa higher residual error.[0088] If it is determined that a synchronization signal14 is required at step 92, the synchronization signal 14may comprise a configurable minimum number of LTEOFDM symbols containing reference signals. The refer-ence signals may be a mix of cell specific and UE specific,depending on the transmission mode used as would beunderstood. The chosen (minimum duration of the) syn-chronization signal 14 can be provided to the UE throughRadio Resource Control (RRC) signaling from its primaryeNB at step 94 of Figure 9 and correspondingly receivedby the UE at step 136 of figure 13, and could be adjustedas needed. Further, any synchrronization signal may beprovided to the UE at the MAC level as would be under-stood. Depending on parameters collected or known bythe eNB (for example, any one or more of cell size, mo-bility of the UE, past link quality on secondary channelsof UE, nature of the environment (indoor, outdoor, rural,urban...), the eNB may decide to allocate zero, one ormore than one LTE OFDM symbols containing referencesignals on the LTE-U transmission to support fine syn-chronization of the UE. Preferably, the chosen minimumduration of the synchronization signal 14 is typically de-cided by the eNB based on the site context (whether theprimary and secondary are at the same or different sites),performance targets, channel conditions and/or potentialerror.[0089] The synchronization period (the length of thesynchronization signal 14) may be extended for synchro-nization purposes as explained in the LTE Regular Trans-mission Section of this specification. The device can thenuse the regular RS channel estimation to measure andcompensate the timing offset with respect to the primarychannel. Alternatively a specific RS pattern may be usedin the synchronization process to improve the time esti-mation, for example by reusing the LTE positioning RSpattern (see 3GPP TS 36.211 R9 or later, section 6.10.4)or any new RS pattern that could be defined for the pur-pose of synchronization. Legacy LTE considers PSS,SSS and pilots (for example cell specific, UE specific,pilot for positioning) for - among other uses - the purposeof synchronization. The LTE-U scheme described hereinmay utilize those legacy pilot schemes to support syn-chronization in the unlicensed channel but does not pre-clude the definition of new pilot schemes explicitly dedi-cated for synchronization.[0090] When the synchronization signal 14 has beenformulated, method flow at the eNB returns to step 74.

LTE Regular Transmission 16

[0091] The LTE regular transmission portion 16 is

made of a standard succession of LTE subframes 50 asshown in figure 5.[0092] The start of each subframe is aligned based onstandard LTE synchronization which requires alignmentbetween primary and secondary cell transmissions. Dur-ing the regular transmission interval the LTE-U 42 sendsa normal LTE signal, except possibly for small extensionsas described below.[0093] Because of the LTE frame alignment constraint(a frame on primary and secondary cells must bealigned), the first subframe cannot start at an arbitrarytime, and is constrained by the LTE framing. When theunlicensed channel becomes free (determined at step74, preferably with LBT and optionally, by way of detect-ing a WiFi header, for further protection of WiFi, as pre-viously discussed), the LTE-U eNB 42 will compute whichLTE subframe will start the regular transmission periodbased on the minimum WiFi header and synchronizationperiod at step 76. Figure 6 shows a timing diagram of aWiFi header and synchronization period. H denotes theoptional LTE-U header duration 60 comprising the op-tional minimum WiFi protection header (without padding)and optional minimum LTE-U synchronization duration.The time axis of the timing diagram of figure 6 is shownrelative to arbitrary 1ms subframes 61 of regular LTEtransmission. From the time T 62 when the unlicensedchannel becomes free and usable by the LTE-U eNB 42,the regular transmission can start with the first subframestarting at or after the time T+H (64).[0094] Because both the WiFi pseudo header and LTEsynchronization period are optional, duration H (60) maybe equal to zero. This does not change the process tofind the first usable subframe start 68.[0095] Let F (68) be the starting time of this first usablesubframe. Typically F will be subsequent to time T+H(64), meaning there would be a gap between the end ofa minimum header (if used) and the beginning of the firstusable subframe. If the channel is left unused during thistime, another device may start transmitting. Therefore,the LTE-U eNB 42 optionally fills this gap with paddingsignal sent at a similar power level as the LTE signal atstep 78. Similar may be defined as the same averagepower spectral density as the LTE signal, or slightly larg-er. The aim is not to saturate a close-by UE (so not muchmore power), but to keep the channel busy as perceivedby non-LTE far devices i.e. any device that may desireto transmit on the unlicensed channel of interest (so pref-erably should not be less power).[0096] This padding may be made for example of ad-ditional synchronization symbols, and additional WiFipadding, or any waveform occupying the used channelat an appropriate power level to keep it busy as seenfrom other devices. Let D (66) be this duration (D = F -T-H) and O the duration of an LTE OFDM symbol, then forexample:

• N = floor(D / O) synchronization LTE OFDM symbolsmay be added to the LTE synchronization area, or

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if no synchronization area was required, then a syn-chronization area of N OFDM symbols can be cre-ated; (floor(x) being the function giving the first inte-ger below x)

• A WiFi padding signal of duration D - NxO can beused or, if no WiFi header is used, a padding signalof this duration can be transmitted starting at time T.

[0097] As can be seen, it is preferable to fill durationD as much as possible with OFDM symbols(N=floor(D/O)), as the RS they contain helps the UE per-form channel estimation and compensation to improvereception performance. If duration D is not an integernumber of OFDM symbols, the remaining gap (after pad-ding with N OFDM symbols) may be filled with "useless"padding, such as a WiFi padding signal. The uselesspadding does not contribute to aiding LTE reception qual-ity.[0098] The LTE transmission portion 16 together withany optional WiFi Header portion 12 and synchronizationportion 14 is transmitted on the unlicensed band of inter-est at step 79 and correspondingly received by the userdevice at step 104 of figure 10. Once the transmissionhas taken place, a further LTE transmission in an unli-censed band can be initiated in the same manner by re-turning to step 70.[0099] The example above uses the duration D for syn-chronization as required, and fills the rest of the durationwith redundant padding transmission. Any other split ispossible as long as the channel is kept busy before theregular LTE transmission starts.[0100] An important property of this transmissionscheme for unlicensed bands is that LTE transmissionsare not continuous. Whereas in a licensed band the UEcan rely on the cell specific RS to be transmitted at alltimes, in an unlicensed channel the RS are only trans-mitted as part of the synchronization and regular trans-mission period. This has an impact on the UE measure-ments (typically the Reference Signal Receive Power(RSRP), Reference Signal Received Quality (RSRQ),Channel Quality indicator (CQI), Precoding Matrix indi-cator (PMI) Rank indicator (RI)) normally based on RSand pertaining to transmission type measurements of theunlicensed channel (referred to at step 112 of figure 11below). Moreover, using the regular LTE signaling, a de-vice would only be aware of unicast transmission target-ed at it. This would further reduce the period of time thata device could use to measure the channel, especiallyfor terminals having no data scheduled over the second-ary carrier at a given time but which needs to performthe measurements, in case of future allocation. Theknowledge of an on-going allocation on the secondarycarrier operated in the unlicensed spectrum helps theuser device to perform measurement at the right time.[0101] In order to remove this limitation, and let a UEperform measurements during all the subframes wherethe LTE-U eNB is transmitting, a new scheme is intro-duced (figure 11) where the PDCCH is used to indicate

the used subframes to all active devices. By either intro-ducing a new Radio Network Temporary Identifier(RNTI ) dedicated to this signaling, or adding this functionto existing broadcasted Downlink Control Information(DCI), the device could receive DCI based informationto indicate the following LTE-U events on the unlicensedchannel:

1. The unlicensed transmission starts in the associ-ated subframe;2. The unlicensed transmission stops in the associ-ated subframe;3. The associated subframe is used on the associ-ated subframe.

[0102] The subframe associated to the DCI is definedas per the LTE standard (in the DL it is the same subframecontaining the DCI for example). The scheduling of suchDCI information can be done using cross-scheduling asper the standard LTE carrier aggregation scheme if de-sired.[0103] The latter option is more efficient when a singlesubframe is used. Alternatively, only events 1 and 2 couldbe used (the subframe is partially used). Or only event 3could be used (the subframe is used or not used). In theend, the goal is to provide very low latency reliable broad-cast information on the actual use of the unlicensed chan-nel. At step 110 of figure 11, the UE may determine fromPDCCH whether an LTE subframe is used, partially usedor not used.[0104] Based on this information, the UE may leverageall the used LTE subframes for measurements (step 112of figure 11). Based on information in the header, it mayalso deduce time periods when the LTE-U eNB is inactiveand perform channel measurement to assess interfer-ence from other devices and technologies and noise levelin the channel (step 114 of Figure 11).[0105] With standard periodic reporting, the UE meas-ures LTE subframes and regularly sends a measurementreport to the eNB. Aperiodic reporting can be used inunlicensed bands, but periodic reporting is not well suiteddue to the intermittent use of the channel. A modifiedperiodic reporting can be carried out on the primary chan-nel, based on the transmitted LTE subframes only. WithLTE-U, the channel may not be used by the eNB for ar-bitrary durations. With periodic reporting the reports overa time comprising no LTE use ("LTE empty") durationsor where part of the period the report is concerned withcomprises an "LTE empty" situation, a modified periodicreporting can be utilised at step 116 of figure 11. Forexample, a report may only be computed over LTE sub-frames that are used (known to the eNB). There wouldbe no need to send a report over fully unused durations(when the LTE subframes are empty).[0106] LTE-U brings several advantages and benefits:

• LTE has a higher spectral efficiency than WiFi;• Unlicensed bands can be leveraged using a single

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technology, potentially simplifying the client devicesand avoiding the complexity of inter-RAT (radio ac-cess technology) integration on the network side.

• Minimising change required in both the eNB and UEimplementations.

[0107] Further, for client devices, the scheme de-scribed herein reduces cost by opening the possibility toremove WiFi capability. Removing WiFi would reducecomponent count and cost as well as PCB real-estaterequirements. Further, users obtaining WiFi from an op-erator box that would also support LTE-U could buy acheaper UE (for example a smartphone) with LTE-U ca-pability instead of WiFi.[0108] The scheme also lowers power consumption aspresent schemes where LTE and WiFi can be used to-gether require extra power to cope with added interfer-ence and data integrity. This is not required with an LTE-U system that is able to share unlicensed channels (withother technologies such as WiFi) in a fair manner.[0109] The order that the various steps are executedmay be swapped or changed in any manner, for example,determining whether the unlicensed band is in a WiFispectrum (step 70) and whether synchronization is re-quired (step 72) may be swapped. Whether or how muchsynchronization is required may not need to be assessedfor each transmission. It could be done once when a UEregisters to the cell for example. Further, optional steps120 and 122 may be carried out by a UE before step 100of figure 10 and optional steps 130 to 136 may be carriedout before or after steps 120 and 122, and before step100.[0110] The various methods described above may beimplemented by a computer program. The computer pro-gram may include computer code arranged to instruct aprocessor to perform the functions of one or more of thevarious methods described above. The computer pro-gram and/or the code for performing such methods maybe provided to an apparatus, such as a processor, on acomputer readable medium or computer program prod-uct. The computer readable medium could be, for exam-ple, an electronic, magnetic, optical, electromagnetic, in-frared, or semiconductor system, or a propagation me-dium for data transmission, for example for downloadingthe code over the Internet. Alternatively, the computerreadable medium could take the form of a physical com-puter readable medium such as semiconductor or solidstate memory, magnetic tape, a removable computer dis-kette, a random access memory (RAM), a read-onlymemory (ROM), a rigid magnetic disc, and an opticaldisk, such as a CD-ROM, CD-R/W or DVD.[0111] An apparatus such as a processor may be con-figured in accordance with such code to perform one ormore processes in accordance with the various methodsdiscussed herein. Such an apparatus may take the formof a data processing system. Such a data processingsystem may be a distributed system. For example, sucha data processing system may be distributed across a

network.

Claims

1. A method of receiving LTE data by a user device,the data being transmitted on a channel of an unli-censed band comprising the steps of:

receiving a cell ID from a primary LTE cell;receiving system information from the primaryLTE cell for access to an unlicensed channel;receiving LTE data on the channel of an unli-censed band.

2. The method of claim 1 wherein the cell ID and systeminformation is received as unicast data.

3. The method of claim 1 or 2 wherein the unlicensedchannel is served by a secondary LTE cell of a dif-ferent eNodeB than the primary LTE cell.

4. The method of any preceding claim further compris-ingdetermining from PDCCH whether an LTE transmis-sion subframe on the unlicensed channel is one ofused, partially used or not used.

5. The method of any preceding claim wherein the userdevice determines one or more transmission meas-urements of the unlicensed channel in relation totransmission on that channel based on used and/orpartially used LTE transmission subframes on theunlicensed channel.

6. The method of any preceding claim wherein the userdevice determines one or more channel measure-ments of the unlicensed channel in relation to inter-ference and/or noise on that channel based on un-used LTE transmission subframes.

7. The method of claim 5 or 6 wherein the user devicesends data indicative of the one or more transmis-sion and/or channel measurements to the primarycell.

8. The method of any preceding claim wherein the userdevice sends its geographical position to the primaryLTE cell.

9. The method of any preceding claim wherein the userdevice calculates a timing error based on at leastone of cell specific RS, user device specific RS andpositioning RS and optionally the method furthercomprising using PDCCH to compensate for the tim-ing error.

10. The method of any preceding claim further compris-

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ing receiving a synchronization signal through RRCor MAC from the primary cell; and optionally whereinthe synchronization signal comprises one or moreLTE OFDM symbols.

11. The method of claim 9 or 10 wherein a determinationis made as to whether a synchronization signal isrequired and optionally as to the length of the syn-chronization signal, when the user device registersto the primary cell.

12. The method of any preceding claim further compris-ing the user device decoding a WiFi header from thechannel of the unlicensed band and wherein the userdevice turns off its LTE-U WiFi capable receiver fora duration specified in the header if the header indi-cates that the WiFi header has not originated fromthe secondary LTE cell.

13. The method of any preceding claim further compris-ing the user device determining from LTE DCI wheth-er a detected WiFi header has originated from thesecondary LTE cell, and if it has not originated fromthe secondary LTE cell, turning off its LTE-U WiFicapable receiver for the rest of the duration specifiedin the header.

14. A computer readable medium comprising instruc-tions that when executed on a processor cause theprocessor to carry out the method of any precedingclaim.

15. A user device arranged to carry out the method ofany of claims 1 to 13.

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