Patent Description:
The present invention further relates to a system comprising at least two different wireless networks, a first wireless network, 'FWN', and a second wireless network, 'SWN', each network comprising one or more stations, 'FWN-S', 'SWN-S', connected to each other within the respective network forming said respective network, wherein transmission between said SWN-S within the SWN is performed via channels, wherein after each successful transmission a channel is free of transmissions for a predetermined time period, 'PTP', and wherein said FWN and said SWN operate at least in part in overlapping frequency bands.

Although applicable to wireless networks in general, the present invention will be described with regard to LTE-based networks and WLAN IEEE <NUM>-based networks respectively.

Although applicable in general to any kind of frequency bands, the present invention will be described with regard to LTE-unlicensed bands.

Wireless communications have shifted from bit rates of few Mbps to Gbps in order to accommodate the increasing demand of bandwidth during the last ten years. Such increase in data rates has been achieved by means of using higher modulations schemes, improved channel codes, MIMO transmissions, etc. Nevertheless, the use of larger parts of the overall spectrum remains still the most effective and simple way to increase the network throughput.

In the case of cellular networks, operators have started to use the unlicensed band as a means of decongesting the scarce and expensive licensed spectrum. For instance, 3GPP Rel. <NUM> allows mobile devices to do offloading using an IEEE <NUM> network.

Coexistence with <NUM> WLANs has already been studied for Bluetooth, Zigbee and WiMaX. In the non-patent literature of <NPL>, a coexistence mechanism is proposed between LTE and <NUM> WLANs based on Almost Blank Subframes (ABS), i.e., almost silent periods in LTE transmissions, which is later extensively evaluated with simulations in the non-patent literature of <NPL>, and analytically in the non-patent literature of <NPL>.

In the non-patent literature of <NPL>, and in the non-patent literature of <NPL>, coexistence mechanisms and methods are proposed giving a certain degree of fairness.

All the aforementioned methods show that the throughput of LTE can be improved, but none of them are compliant with Listen-Before-Talk, 'LBT' mechanisms as specified in the non-patent literature of <NPL>". Therefore, they can only be deployed in some regions.

In the non-patent literature of 3rd Generation Partnership Project, <NPL>" a range of LBT-compliant mechanisms and respective evaluations are presented. The results show that in some scenarios LTE-U can be configured to not degrade an <NUM> station more than if another station were added to the network. Nevertheless, the conventional configurations are implementation-dependent and some of the parameters are unlikely to be known in real networks. Further, none of the conventional methods determines how much additional throughput an LTE-U can obtain.

A further conventional method is Carrier Sense Adaptive Transmissions, 'CSAT', which comprises adapting the time a channel is used by an LTE station in order to provide coexistence/fairness to the network. CSAT however does not have any control on the access mechanism to the channel, i.e., it does not take into account if the channel is being used by a WiFi station.

In the US Patent Application <CIT>, a base station, 'BS', is disclosed a) performing a Clear Channel Assessment, 'CCA', and then b) transmitting a first waveform to a set of UEs over unlicensed spectrum when channel is sensed as idle. This is used for channel reservation. Then, one or more user equipment UE reply back with another waveform to indicate nearby WiFi devices that a BS has channel access, and another waveform to provide the BS with information for data transmission.

Other conventional methods are disclosed in the United States Patent Application <CIT> and in the United States Patent Application <CIT> for downlink and uplink unlicensed access respectively. In the United States Patent Application <CIT> power control for LTE is proposed to operate in unlicensed bands with minimum impact to WiFi networks while maximizing channel utilization. A conventional LBT scheme is disclosed in the <CIT> for carrier aggregation in the uplink using CCA.

<CIT> addresses that multiple devices utilize radio resources of the same shared channel by competing for the channel access and resources first, such as U-LTE devices and Wi-Fi devices.

The present invention is defined by independent claims, to which reference should now be made. Further enhancements are characterized by the dependent claims.

Embodiments of the invention therefore address the problem of enabling a fair coexistence in the sense that access time for a station in a first network within the second network's channel is maximized while reducing collisions with existing transmissions within a second network.

In an embodiment the present invention provides a method for operating a plurality of wireless networks, comprising at least two different wireless networks, a first wireless network, 'FWN', and a second wireless network, 'SWN', each network comprising one or more stations, 'FWN-S', 'SWN-S', connected to each other within the respective network and forming said respective network, wherein transmission between said SWN-S within the SWN is performed via channels, wherein after each successful transmission a channel is free of transmissions for a predetermined time period, 'PTP', and wherein said FWN and said SWN operate at least in part in overlapping frequency bands, comprising the steps of.

In a further embodiment the present invention provides to a system comprising at least two different wireless networks, a first wireless network, 'FWN', and a second wireless network, 'SWN', each network comprising one or more stations, 'FWN-S', 'SWN-S', connected to each other within the respective network and forming said respective network, wherein transmission between said SWN-S within the SWN is performed via channels, wherein after each successful transmission a channel is free of transmissions for a predetermined time period, 'PTP', and wherein said FWN and said SWN operate at least in part in overlapping frequency bands, wherein said FWN-S is adapted.

The terms "airtime" is to be understood preferably in the claims, in particular in the description as channel time successfully used to transmit data. The term "transmission performance" is to be understood preferably in the claims, in particular in the description as the amount of said "airtime".

At least one embodiment of the present invention may have at least one of the following advantages:.

Throughout the description the following abbreviations are used unless otherwise indicated:.

Further features, advantages and further embodiments are described or may become apparent in the following:
Blocking of said channel comprises checking if said channel is free of transmissions. This ensures that collisions are reduced and thus performance within the SWN is decreasing.

Blocking of said channel comprises sending said information indicating the occupancy time of said channel. This allows that stations in the SWN do not transmit when said FWN-S is transmitting in the SWN.

Said checking is performed at the beginning of a PTP. This ensures that the channel is always sensed idle.

When said channel is sensed busy another checking may be performed based on a Listen-Before-Talk operation mode. For instance this enhances the flexibility since this allows using for example the frame-based equipment FBE or the load based-equipment LBE operation mode which is described in the non-patent literature of <NPL>"and in the non-patent literature of <NPL>. The FBE operates as follows: Before a transmission a station fixes the time T that it aims to occupy the channel. If the channel is detected idle during the CCA the station transmits and waits <NUM>% of the time T before performing another CCA. In case the channel is declared busy the station will have to wait a time T before performing another CCA. That is, the penalty for finding the channel busy is proportional to the amount of time the station would have used it in case it was sensed idle. Time T must be selected between <NUM> and <NUM> and control messages (e.g., acknowledgements) which are triggered by the receiver can be transmitted without requirement of another CCA as long as time T is not exceeded.

A time period is specified indicating a period with a number of PTPs wherein said FWN-S then transmits after every PTP. This enables to maximize the performance for transmission within said period.

Said number of PTPs is dependent on the fraction of idle slots for transmission in said SWN that would have to change to non-idle slots for an expected transmission duration. This enables a so-called duty cycle-based operation: A number Δ is defined; this is a period of PTP opportunities (AIFS opportunities in the context of IEEE <NUM>). Then, an FWN-S, e.g. an LBT-node, transmits after every AIFS opportunity during a total number of <MAT> opportunities (This is an "on" period). Then, it skips the next <MAT> opportunities, wherein the quantity <MAT> captures the fraction of idle slots in the network that would have to change to successful slots in order to accommodate transmission of expected duration. ρ is a configuration parameter and <MAT> is the fraction of idle slots in a SWN with n SWN-Ss.

Said FWN-S may transmit after every PTP with a probability indicating the fraction of idle slots for transmission in said SWN that would have to change to non-idle slots for expected transmission duration. This allows a so-called Aloha-based operation. If for example a Listen-Before-Talk node is used as station then said FWN-S transmits for every AIFS opportunity with a probability <MAT>.

Said FWN-S may transmit only after a certain number of PTPs. This enables a DCF-based operation.

A backoff counter may be decremented to count said number of PTPs. This allows an implementation of a DCF-based operation.

Traffic within the SWN may be analysed to estimate the number of active SWN-Ss and if said active SWN-S are not saturated and one or more FWN-S do not have enough traffic to send in order to cause saturation of said SWN-Ss, then the total airtime is set to infinity. An FWN-S can increase the load until SWN channel is saturated, though SWN-Ss are using the amount of channel time each of them need. This makes it possible to estimate an approximation to <MAT> easily. However, if the SWN-S are not saturated and the FWN-S do not have enough traffic to cause saturation of said SWN-S then there is no coexistence issue because all nodes would be transmitting at the rate they want.

The total airtime may be recomputed periodically. The frequency of the recomputation may be application- or implementation-dependent. Recomputing the total airtime periodically enables to include changing traffic characteristics or load within the SWN.

Said FWN-S may always transmit after a predetermined number of sequential idle transmission slots in said SWN. This enables to consider low load within the SWN: If the load for example of WiFi stations is low then the rate of transmission opportunities for the FWN-S is low as well. If a threshold α for the number of sequential idle WiFi slots is set then the FWN-S restarts for example a backoff counter to said number for every busy slot. This counter may be decremented for every idle WiFi slot. If it reaches zero then the FWN-S will attempt to transmit as if they were a transmission opportunity in that slot and restarts the backoff counter to said threshold.

Said threshold may be set dynamically. For instance: α is set to αmax every WiFi transmission and decremented by a number q every time the backoff counter reaches zero until a value α = αmin is reached.

Said FWN may be a LTE-based wireless network and said SWN may be an <NUM>-based wireless network and said checking may be performed in form of a clear channel assessment, 'CCA' and said information may be provided in form of a CTS-to-self message. This allows an easy implementation when the first network is a LTE-network and the second network is a WiFi network.

There are several ways how to design and further develop the teaching of the present invention in an advantageous way. To this end it is to be referred to the patent claims subordinate to independent patent claims on the one hand and to the following explanation of further embodiments of the invention by way of example, illustrated by the figure on the other hand. In connection with the explanation of the further embodiments of the invention by the aid of the figure, generally further embodiments and further developments of the teaching will be explained.

<FIG> shows part of steps of a method according to an embodiment of the present invention.

In <FIG> a schematic illustration of the coexistence mechanism/procedure is shown using a <NUM> CCA as an example.

In the non-patent literature of <NPL>", it is specified that before a transmission a station must perform a Clear Channel Assessment (CCA) using energy detection for at least <NUM>. Namely, depending on the energy detected during a time equal or greater <NUM> the channel is declared idle or busy. In case the channel is declared idle the station can start a transmission immediately, otherwise it will need to perform another CCA. When to perform another CCA after the channel is declared busy or idle depends on the LBT operation mode, which can be Frame Based Equipment (FBE) and Load Based Equipment (LBE) -- both specified in the non-patent literature of ETSI. EN <NUM><NUM> v1. <NUM>: "Broadband Radio Access Networks (BRAN); <NUM> high performance RLAN; Harmonized EN covering essential requirements of article <NUM> of the R&TTE Directive", and already mentioned above.

Further in the following the SWN is a IEEE <NUM> network (with WiFi or <NUM> nodes) and the FWN is an LBT-based LTE network (with LBT-nodes), said IEEE <NUM> network being divided in MAC slots and a station in said network transmits after observing Ym idle slots, where Ym is a random variable selected uniformly at random from {<NUM>,<NUM>,. , <NUM>mCWmin, - <NUM>} where m = <NUM>,<NUM>,<NUM>,. is the number of successive collisions experience by the station. After a successful transmission m is set to <NUM>. According to IEEE <NUM> a parameter CWmax is defined that limits the expected number of idle slots a station has to wait after m successive collisions, i.e., <NUM>mCWmin = CWmax for m ≥ m.

In IEEE <NUM> WLANs the following procedures are implemented:.

In one or more of the embodiments of the invention the minimum duration of an AIFS (<NUM>) is longer than the CCA minimum time (<NUM>) specified in the regulation ETSI <NUM><NUM> v1. Thus the CCA (<NUM>) is smaller than <NUM> of the AIFS, i.e. in general any CCA that is smaller than an AIFS can be used. Then, if an LBT-node performs a CCA at the beginning of an AIFS period, the channel will be sensed idle and the LBT-node (with FBE) will transmit before any <NUM> station does. The latter is always be true if there is no interference that makes the LBT-node sense the channel busy, which is assumed to be the case here. An LBT-node can determine when an AIFS period starts by scanning the network traffic with an IEEE <NUM> interface. Any minimum CCA time allowed in the above-mentioned regulation being lower than <NUM> is valid for future modifications of the same.

The first point enables that the channel is always sensed idle and the second point enables that the <NUM> stations do not transmit while the LBT-node is transmitting. In effect, said procedure can be informally regarded as having an <NUM> station with higher AC that only transmits after successful transmissions.

An LBT-node thus does not affect the transmission probability attempts of the stations in the WLAN, and therefore, the airtime in the system is divided into two orthogonal airtimes. It is assumed here that embodiments may need <NUM> transmissions in order to work; however, when there are no sufficient <NUM> transmission load in the network there is no problem with coexistence.

An embodiment of the present invention is schematically illustrated in <FIG> with a network with one <NUM> station and one LBT-node: The LBT-node is able to transmit before the <NUM> station does and that the next AIFS period starts when the LBT-node has finished its transmission.

So far it was specified how the LBT-node should transmit, but not how much airtime an LBT-node can use in order to be compliant with a coexistence criterion: not to degrade the throughput of the WLAN more than if an <NUM> station were added to the network. In the following, the maximum airtime an LBT-node can use to meet said requirements/criterion is computed.

Based on WLAN with ideal channel conditions and n saturated stations, i.e., each station always has a packet ready for transmission. The conditional transmission attempt probability of a station in a MAC slot (which depends on the number of stations and the BEB configuration) can be expressed as the probability of transmitting in each MAC slot with a fix probability as for example described in the non-patent literature of <NPL>. That is, a station i ∈ {<NUM>,. , n} transmits in a MAC slot with probability <MAT>. For simplicity of exposure it is assumed that the stations in the WLAN are homogeneous and therefore <MAT> for all i ∈ {<NUM>,.

Then, the probability that a MAC slot is idle is given by the probability that none of the stations in the network transmit, <MAT>; the probability that it is occupied by a successful transmission is <MAT>, where <MAT> τ(n))n-<NUM> is the probability that a single station transmits in a MAC slot. Finally, the probability that a slot is occupied by a collision is given by <MAT> and the probability of a slot being busy is <MAT>. The throughput of a station is given by <MAT> where σ, B and T are, respectively, the duration of a MAC slot, the expected number of bits in a transmission and the expected duration of a transmission (successful or collision).

As described above the transmission of the LBT-nodes are orthogonal to <NUM> transmissions and the throughput of a station in a WLAN is non-increasing with the number of stations, i.e., s(n) ≥ s(n+<NUM>) for every n = <NUM>,<NUM>,. For simplicity of exposure only the case is considered here where the LBT-node captures the impact of one additional <NUM> station in the network, however, the extension to multiple stations is described below.

There always exists an airtime A ≥ <NUM> such that the following holds: <MAT> where s(n+LBT) is the throughput that an <NUM> station would experience if an LBT-network used A airtime. Since the stations are saturated and the LBT-network is orthogonal, the transmission attempts probabilities of the <NUM> stations in (<NUM>) does not change for any A ≥ <NUM>.

To work in terms of fractions of MAC slots rather than airtime, i.e., the airtime is written as <MAT> where ρ ∈ [<NUM>,<NUM>] and T' ≥ <NUM>. The quantity <MAT> captures the fraction of idle slots in the system that would have to change to successful slots in order to accommodate transmissions of expected duration (T' - σ).

The LBT-node acts like an <NUM> station that knows in which MAC slots to transmit in order to not collide with the other stations in the WLAN. With this change of variable the RHS in (<NUM>) can be written as follows <MAT>.

Then the expression <MAT> such that (<NUM>) holds.

When considering a WLAN with n homogeneous stations in saturated conditions and with T, T' > σ. Then, for every ρ ∈ [<NUM>, ρ] with <MAT> equation (<NUM>) holds, i.e. <MAT> is the maximum fraction of orthogonal transmissions an LBT network should do to meet or satisfy coexistence criterion, i.e. shows the fraction of idle slots that could be occupied with successful transmissions of expected duration T' - σ while being compliant with the coexistence criterion. The bound in equation (<NUM>) depends on <MAT> and <MAT>, but in saturation conditions a good approximation of these values can be easily obtained as shown in the non-patent literature of <NPL>.

Alternatively, if some throughput degradation is accepted for WiFi stations, LBT-nodes can transmit with <MAT>.

To implement said coexistence procedure that guarantees zero impact to WiFi stations the following options may be applicable:.

In case of multiple LBT-nodes can happen. To handle multiple LBT-nodes, collisions in some cases, it can be assumed that nodes can be efficiently coordinated in a centralised manner. For instance, by means of the duty cycle-based scheme proposed previously interleaving "on" periods of different LBT-nodes so they do not overlap.

If coordination is not possible, Aloha-based or DCF-based schemes could be implemented to allow multiple LBT-nodes to coexist. In the first case, each LBT-node will transmit for every AIFS opportunity with <MAT>, where k is the estimated number of LBT-nodes contending. In the second case (DCF-based) the contention window shall be set to <MAT>.

Since LBT and <NUM> airtimes are orthogonal, any increase of the LBT airtime is equivalent to increasing the load of the stations in the network. An LBT-node thus affects a non-saturated <NUM> station in a WLAN either by leaving it non-saturated or saturating it. If the stations do not get saturated, coexistence is then irrelevant since all traffic can be served; and when the stations change to saturation, the optimal airtime can be computed so that the LBT-node does not affect more than an <NUM> station. Since the LBT-node does not collide with the <NUM> stations, the traffic in the network can be analysed to determine the number of contending station in the network as disclosed in the non-patent literature of<NPL>. Further, under regularity conditions it is possible to determine which fraction of stations in the network are actually saturated.

The parameter ρn,k ∈ [<NUM>, ρn,k] depends on the traffic characteristics/load in the network, and so it can be periodically recomputed. The frequency ρn,k is recomputed is a design parameter.

If the load of WiFi stations is low, the rate of transmission opportunities for LBT-nodes will be overly low as well. LBT-nodes can then set a threshold α of number of sequential idle WiFi slots. LBT-nodes may then restart a backoff counter to α for every busy slot. This counter is decremented for every idle WiFi slot; if it reaches zero, then the LBT-node will attempt to transmit as if it were an AIFS opportunity in that slot and restarts the backoff counter to α.

Said threshold α can be set dynamically, for instance: α is set to αmax every WiFi transmission and decremented by a number q every time the backoff counter reaches zero until a value α = αmin is reached.

<FIG> shows a system according to a further embodiment of the present invention.

In <FIG> a scenario on which embodiments of the invention are based is shown.

A first wireless network comprises a plurality of stations FWN-S and a second wireless network SWN comprises a plurality of stations SWN-S. Said first wireless network FWN and said second wireless network SWN are overlapping not only in frequency than but also in terms of connecting ranges. In the overlapping region OR the first wireless network stations FWN-S may use second wireless network stations SWN-S for orthogonal transmission.

<FIG> shows part of steps of a method according to a further embodiment of the present invention.

In <FIG> steps of the matter for operating a plurality of networks is shown:
In a first step a CAA is performed with the <NUM> limit.

In a second step computation of a number of slots is performed that can be used for a LTE-U transmissions.

In a third step LTE-U transmission is performed according to the level computed.

In summary the present invention enables.

At least one embodiment of the present invention may have one of the following advantages.

Claim 1:
A method for operating a plurality of wireless networks, comprising at least two different wireless networks, a first wireless network, 'FWN', and a second wireless network, 'SWN', each network comprising one or more stations "FWN-S", "SWN-S", connected to each other within the respective first wireless and second wireless network and forming said respective first wireless and second wireless network, wherein transmission between said SWN-S within the SWN is performed via channels, wherein after each successful transmission a channel is free of transmissions for a predetermined time period, 'PTP', and wherein said FWN and said SWN operate at least in part in overlapping frequency bands (OR),
comprising the steps of
Sensing (<NUM>) of a channel in the SWN at the beginning of a PTP if being idle and when being sensed idle blocking of said channel within a first time period by a FWN-S for a transmission by said FWN-S, connected to said FWN and said SWN by an overlapping frequency band, wherein said first time period being smaller than said PTP, comprising indicating part of a total airtime said channel in the SWN will be occupied by said FWN-S for the transmission,
wherein the total airtime is computed as a sum of time slots used for successful transmission in said SWN, their number being computed in order to a transmission performance of said SWN is not degraded more than a level of transmission performance is degraded if said FWN-S would be operating for transmission within said SWN only, wherein the transmission performance being defined as throughput of said SWN, and wherein
a second time period is specified including a number of PTPs and wherein
said FWN-S then transmits after beginning of every PTP of said number of PTPs, said number of PTPs being dependent on a fraction of idle time slots for transmission in said SWN that would have to change to non-idle time slots for an expected transmission duration and wherein
said fraction is calculated based on a probability that none of the SWN-S transmit and a period of PTP opportunities, wherein said probability is calculated based on the probabilities of the SWN-S transmitting in a time slot .