Description of the invention

Description of the Figures

Figure 1 shows the architecture of the CBRS ("Citizens Broadband Radio Service") according to the state of the art.

Figure 2 shows an example of deployment of CBSDs in the vicinity of DPAs ("Dynamic Protection Areas").

Figure 3 shows the exchanges between a CBSD and a SAS to obtain an authorization to transmit, or grant, in accordance with the state of the art.

Figure 4 describes the steps for ordering a list of channels according to the risk of interference, in accordance with the present invention.

Figure 5 describes the procedure in Figure 4 in more detail, including a possible channel assessment. Figure 6 shows a notification by the SAS of the available channels, ordered according to the risk of interference, in accordance with the present invention.

Figure 7 shows an example in which a channel is considered to be unusable (in accordance with step 4.A of the procedure described in Figure 4), for a given CBSD, when DPAs are activated.

Figure 8 shows an example where the risk of interference on a given channel and a given CBSD is minimal.

Figure 9 describes the exchanges allowing the SAS to provide alternative channels in response to a grant request.

State of the art on the CBRS (Citizens Broadband Radio Service)

The CBRS is a service defined by the FCC ("Federal Communications Commission", document 15-47) allowing frequency sharing on the 3.5 GFIz band between three priority levels:

- primary users (or priority users),

- PAL secondary users (Priority Access Licensees),

- third-party GAA (General Authorized Access) users.

The Wireless Innovation Forum defines the architecture and protocols for the CBRS service, in accordance with FCC requirements.

Figure 1 shows the architecture of the CBRS service. A centralized Spectrum Access System (SAS) server controls the "access to" and the "use of" the spectrum of fixed radio base stations called CBSDs (Citizens Broadband Radio Service Devices) connected to this SAS server.

These CBSDs can be connected directly to the SAS server (as illustrated by "CBSD 4" in Figure 1), or indirectly through a Proxy Server (SP) connected to CBSDs.

As part of the CBRS service, the Proxy Server (SP) is called Domain Proxy (DP).

The protocol between the SAS server and CBSDs or between the SAS server and an SP makes it possible for the SAS to control the channels for transmitting and receiving over the air, as well as to set the maximum authorized transmit power used by the CBSDs.

Before starting to transmit on a contiguous set of channels, a CBSD must ask the SAS for an authorization to transmit or grant, using the message "grant request", indicating a requested maximum transmission power "maxEIRP". If a CBSD is located in the vicinity of a primary user, and the requested grant overlap with this primary user, the SAS cannot authorize this grant without first ensuring that the aggregation of the existing grants associated with other CBSDs, and this new grant does not exceed the protection threshold of this primary user.

Dynamic Protection Areas (DPAs) are an example of geographic areas associated with a primary user. DPAs have the particularity of being able to be activated or deactivated according to the presence or not of the primary user.

The presence of a primary user can be detected using radio sensors near these DPAs; the presence of a primary user is then transmitted by the sensor to the SAS. A DPA can also be activated directly by a primary user, for example via a web portal.

When a DPA is active, the aggregation of CBSD interference must not exceed a predefined protection threshold associated with this DPA.

In the event that a SAS does not have the information on the status of a DPA, the SAS considers that the DPA is active.

Figure 2 shows a deployment of CBSDs, as an example, in the vicinity of DPAs. When a DPA is activated on a given channel, all of the interference caused by the CBSD grants operating on that channel must not exceed a predefined protection threshold, at any point in this DPA.

At regular time intervals (for example daily), the SAS determines, at each point of a DPA (with a predefined granularity, for example every 50 meters), the list of grants, called "move list", which must be suspended so that the aggregation of the remaining grants (not present in the "move list") does not exceed the predefined protection threshold associated with this DPA.

This process, carried out at regular time intervals, is called CPAS ("Coordinated Periodic Activities among SASes").

Other operations are carried out during the CPAS, which are not relevant to the description of the present invention.

The "move list" associated with a DPA consists of the union of all the "move list" calculated at each point of this DPA.

The procedure for calculating a move list is described in requirement "R2-SGN-24" of document "WINNF-TS-0112- CBRS Operational and Functional Requirements" published by the Wireless Innovation Forum.

For example, in figure 2, the DPA 3 consists of two points PI and P2 to be protected.

It is assumed that all the CBSDs in the figure have a grant on the same channel (for example 3550-3560 M Hz).

Each CBSD is connected to a SAS. There may be several SASes controlling different CBSDs deployed in the same geographic area. In this case the SASes must exchange, during the CPAS, the grants that they control, so that each SAS is able to calculate the aggregated interference of all the CBSDs at a given point.

The "move list"(s) at each point of the DPA 3 consist of the CBSDs indicated in bold in figure 2.

In PI, the "move list" associated with the 3550-3560 MHz channel consists of CBSD 2 and CBSD 3.

In P2, the "move list" associated with the 3550-3560 M Hz channel consists of CBSD 5, CBSD 4 and CBSD 2.

The "move list" associated with DPA 3 on the 3550-3560 MHz channel is the union of the "move list"(s) calculated for each point of this DPA and therefore consists of the following CBSDs: CBSD 5, CBSD 4, CBSD 3, CBSD 2.

When DPA 3 is activated, only CBSD 1, which is not present in the "move list", will be authorized to transmit on channel 3550-3560 M Hz. Figure 3 shows the exchanges between a CBSD and a SAS to obtain a grant in accordance with the state of the art.

It is assumed that the CBSD 2 is located in the vicinity of DPA 1. It is assumed that DPA 1 is activated on the channel "3640-3650 M Hz".

Sending the "registration request" message allows the CBSD to provide the SAS with its installation parameters (such as its location and maximum transmit power). This information will be used by the SAS to determine the interference of this CBSD at any given point in the territory.

Sending the "grant request" message allows the CBSD to indicate to the SAS the channels on which it requests authorization to operate, as well as the requested transmit power.

In this example, the SAS sends SUCCESS response, indicating that the grant is accepted. The CBSD cannot however transmit on this grant yet. Indeed, a CBSD is only authorized to transmit on receipt of a "hearbeat response" message containing the response "SUCCESS".

For each grant, the CBSD then sends a "hearbeat request" message at regular time intervals.

According to the state of the art, the SAS automatically adds the grant to the "move list" of each DPA channel impacted by the grant, while waiting the next CPAS. Thus, in this example, since DPA 1 is activated on one of the channels associated with the grant, the SAS sends the response SUSPENDED_GRANT, indicating that the CBSD is not authorized to transmit.

During the next CPAS, the SAS recalculates the "move list" associated with DPA 1. If the grant requested by CBSD 2 is not included in the move list, CBSD 2 will be authorized to transmit. Otherwise, CBSD 2 will continue to be suspended as long as the DPA is active.

This mechanism, although protecting correctly the DPAs from interference, implies in a sub-optimal use of the spectrum. In particular, when a DPA is activated all the time (or is considered to be activated, for example when a SAS is unable to know the status of a DPA), CBSD 2 will be suspended the next day (and the following days) if CBSD 2 is in the "move list" of that DPA at the end of the next CPAS (and the following CPASs), while other channels may be available for this CBSD.

Description of the present invention

The present invention describes a method to determine the risk, for a CBSD, of being unauthorized to transmit on a given channel, when a DPA located in the vicinity of this CBSD is active.

The risk, for a given channel, of being unauthorized can be used:

- by a CBSD, to choose the channels to be included in a grant request,

- by the SAS, to indicate to a CBSD the recommended channels (for example when responding to a "grant request" message or when responding to a "heartbeat request" message).

The figure 4 describes the steps to determine the risk, for a CBSD, of being unauthorized to transmit on a given channel, when a DPA located in the vicinity of this CBSD is active.

Description of steps 1 to 4 to determine the risk for a CBSD of being unauthorized to transmit on a given channel

In step 1, the list of the points of a DPA impacted by the CBSD is identified. A CBSD impacts a point of a DPA if it is located at a distance from this point less than the neighborhood distance associated with this DPA.

Steps 2 to 4 are then carried out for each channel "c" on which a CBSD is likely to transmit (in the case of CBRS, channels are 10 MHz wide, and go from 3550 MHz to 3700 M Hz).

In step 2, the variable "lnterference_Risk (c)" is initialized to 0. At the end of the procedure described in the figure 4, the higher the value of this variable, the more the risk for this CBSD of being suspended on channel "c" is high.

Steps 3 and 4 are carried out on each point "p" of the DPA to be protected on channel "c". In step 3, the interference generated by the CBSD at point "p" and for channel "c" (l_CBSD_p_c) is calculated using a radio propagation model (e.g., Irregular Terrain Model (ITM)). This calculation takes into account:

- the location of the CBSD,

- the maximum transmit power of the CBSD ("maxEIRP"),

- its antenna height,

- the antenna gain of the CBSD in the direction of the point to be protected,

- the terrain between the CBSD and the point considered.

This step 3 can be carried out only once for all the channels since the radio propagation varies little from one channel to the other.

During step 4, the condition for excluding channel (c), based on the value l_CBSD_p_c calculated in step 3, is evaluated. An example of such condition is described in figure 5.

If the condition for excluding channel (c) is satisfied, the channel is considered as not usable by the CBSD (case 4.A).

Otherwise, the "lnterference_Risk (c)" variable, whose value expresses the risk for a CBSD of being suspended when a DPA located near this CBSD is activated, is increased according to the value l_CBSD_p_c. An example of updating this variable is described in figure 5.

At the end of this procedure, the higher the value of the variable "lnterference_Risk (c)" for a given channel "c", the more the risk, for the CBSD during this procedure, of being not authorized to transmit on this "c" channel is high.

In an alternative, steps 3 and 4 are carried out on a predefined subset of points N (for example the N points of DPA closest to the CBSD).

In an alternative, the variable "lnterference_Risk (c)" is increased or weighted in proportion to the following value: [l_CBSD_p_c -DPA_Threshold] This allows to take into account the fact that closer a grant is to exceed the authorized threshold on a DPA, the higher the risk for this grant of being unauthorized when this DPA is active.

In an alternative, the variable "lnterference_Risk (c)" is increased or weighted inversely proportional to the number of grants not present in the "move list" associated with point "p" and channel "c", and whose interference at point "p" is greater than the value l_CBSD_p_c. This makes it possible to take into account the fact that these grants will be the first to be included in the "move list" in the event of interference being exceeded during a next CPAS and thus free an additional margin of interference for the other grants.

Example of condition for evaluating channels and updating the variable "lnterference_Risk (c)"

Figure 5 describes in more detail the procedure of figure 4 including:

- an example of a possible condition for evaluating channels,

- an example of updating the variable "lnterference_Risk (c)".

In this example, the condition for excluding channel (c), evaluated in step 4, is as follows: if the calculated value l_CBSD_p_c is greater than the interference threshold authorized for this DPA ("DPA_Threshold") for the DPA associated with point "p", this channel is considered to be unusable (step 4.A) when this DPA is active. This is based on the fact that whatever the activity of the other CBSDs located nearby, this CBSD cannot be authorized to transmit on this channel "c", assuming that it uses its maximum power "maxEIRP" transmission.

In this example, the update of the variable "lnterference_Risk (c)" is as follows: If the value l_CBSD_p_c is greater than the value "Max_Current_lnterference_p_c" (previously initialized as described below), the variable "lnterference_Risk (c)" is incremented.

Initialization of the "Max_Current_lnterference_p_c" parameter The "Max_Current_lnterference_p_c" parameter, used during step 4.B of figure 5, is initialized during each CPAS, at each point "p" of DPA and for each channel "c".

This parameter contains the value of the interference at point "p" of the grant satisfying the following conditions:

- this grant is not present in the "move list" associated with this point "p" and this channel "c", calculated during the CPAS, and,

- this grant is the one that interferes the most on this point "p", among all the grants both operating on channel c and not present in this "move list".

If during the CPAS, after calculating a "move list", there is no grant satisfying these conditions, the parameter "Max_Current_lnterference_p_c" is initialized to the value "DPA_Threshold".

Sorting of available channels according to the risk of interference, when processing a "spectrum inquiry request" message

Figure 6 shows an example of indication by the SAS of the available channels, ordered according to the risk of interference, in accordance with the present invention.

The CBSD provides, in the message "spectrum inquiry request", the minimum frequency and the maximum frequency of the channels for which the information on the risk of interference is requested (respectively 3600 M Hz and 3650 M Hz in this example).

The SAS performs the procedure described in figure 4, for each requested channel, using the maximum transmit power "maxEIRP" of the CBSD.

The SAS then provides, using the "spectrum inquiry response" message, the list of channels sorted by order of risk of interference, omitting the channels excluded during the procedure described in figure 4.

In this example, channels [3600-3610] and [3610-3620] were excluded during the procedure described in figure 4. The channel on which the CBSD has the least risk of being suspended when activating DPA is channel [3620-3630], followed by channel [3640-3650] and finally by channel [3630-3640]

In an alternative, the procedure described in figure 4 is executed several times, by varying the parameter "maxEIRP" over a set of values less than or equal to the maximum value supported by the CBSD.

For each channel, the most optimal "maxEIRP" value among all these "maxEIRP" values (for example the highest "maxEIRP" value for which the channel is not excluded during the procedure of figure 4) is then included in a new parameter of the "spectrum inquiry request" message.

Example of exceeding the authorized interference threshold

Figure 7 shows an example in which a channel is excluded (in accordance with step 4.A of the procedure described in figure 4), for a given CBSD.

In this example, the CBSD impacts DPA 3 and DPA 4. The black dots indicate the locations where the interference caused by the CBSD is above the admissible threshold.

It is assumed that the maximum EIRP value for the CBSD is 47 dBm / 10 M Hz.

The interference in PI is calculated as follows: CBSD_ EIRP - CBSD_Pathloss = 47 - 104 = -57 dBm / 10 M Hz where "CBSD_Pathloss" is the path loss between the CBSD and point PI. Since -57 dBm is greater than -144 dBm, the interference threshold is considered by the SAS to be exceeded by the CBSD.

Example of CBSD with minimal risk of interference

Figure 8 shows an example where the risk of interference on a given channel "c" and a given CBSD is minimal according to the evaluation criteria described in figure 5.

In this example, CBSD 6 is located in the vicinity of DPA 3 and DPA 4.

To simplify, we only consider the interference at two points PI and P2.

We suppose that:

- CBSDs 1 to 5 obtained a grant on channel "c", - the CBSDs causing interference on point PI are the following, in decreasing order of interference: CBSD 1, CBSD 4, CBSD 3, CBSD 2,

- The CBSDs causing interference on point P2 are the following, in decreasing order of interference: CBSD 4, CBSD 1, CBSD 3, CBSD 5.

The "move list" in PI calculated during the last CPAS contains CBSD 1 and CBSD 4. The CBSD authorized to transmit on channel "c" and generating the most interference in PI is CBSD 3 (among all the CBSDs authorized to transmit on this channel).

The value of "Max_Current_lnterference_p_c" described in figure 5 for point PI is therefore the interference of the grant from CBSD 3 at point PI.

The "move list" in P2 calculated during the last CPAS contains CBSD 4. The CBSD authorized to transmit on channel "c" and generating the most interference in P2 is CBSD 1 (among all the CBSDs authorized to transmit on this channel).

The value of "Max_Current_lnterference_p_c" described in figure 5 for point P2 is therefore the interference of the grant from CBSD 1 at point P2.

It is assumed that the interference in PI of CBSD 6 is less than that of CBSD 3.

It is assumed that the interference in P2 of CBSD 6 is less than that of CBSD 1.

Consequently, during the execution of the procedure described in figure 5, the value of the variable "lnterference_Risk (c)" will be zero (step 4. B will not be executed for any point of DPA).

The risk for CBSD 6 of being unauthorized to transmit on channel "c" when DPA 3 or DPA 4 is active is therefore minimal.

Identification of alternative channels when processing a "grant request" message

Figure 9 describes the exchanges allowing the SAS to provide alternative channels in response to a "grant request".

Upon receipt of a grant request message, the SAS evaluates the channels in accordance with the procedure described in figure 4, with the following details: the value "maxEIRP" is the value requested by the CBSD for this grant and supplied in the message "grant request".

In this example, one of the channels [3630-3640] is excluded during the procedure described in figure 4. This grant cannot therefore be accepted.

The value of the "lnterference_Risk" variable associated with each channel can be used by the SAS as a criterion to select alternative channels, in addition to other criteria, such as the channels used by other CBSDs deployed in the same geographic area, or the presence of priority users in the vicinity of the CBSD.

In this example, the SAS identifies channels [3650-3660] and [3660-3670] as having a lower risk of being suspended, and provides these channels in the "grant response" message.

Application of the invention in case several grants are requested between two CPAS.

If a grant is accepted for a given CBSD, this grant is added temporarily (until the next CPAS), for each grant channel, to the "move list" of each point of the DPA impacted by this grant.

If another CBSD sends a "grant request" message or a "spectrum inquiry request" message before the next CPAS is executed, each "move list" thus updated is used in the procedure described in figure 4 to evaluate the channels.