METHOD FOR AVOIDING DOWNLINK INTERFERENCE BETWEEN INDOOR DAS SYSTEM AND SMALL BASE STATION

Abstract

Provided in the present invention is a method for avoiding downlink interference between an indoor DAS system and a small base station, the steps comprising: step 1: determining an initial access RU, and establishing a signal strength table; step 2: the DAS system, by means of the chosen RU, attempts to establish a downlink with UE 1, and detects the signal to interference plus noise ratio (SINR) value of the downlink signal of UE 1, and comparing same with a preset threshold value γ.sub.1; step 3: UE 1 maintains one or a plurality of RU downlinks assigned thereto by the DAS system, during the communication process continuously detects an SINR value at a set time interval, and on the basis of whether same is greater than γ.sub.1, ensures a corresponding service quality.

Claims

1. A method for avoiding downlink interference between an indoor DAS and a small base station, comprising: step 1: determining an initial access RU, and establishing a signal strength table; wherein when UE 1 needs to access the DAS, an access request is sent to the DAS; the DAS establishes a signal strength table related to UE 1 in an MU according to signal powers at which various RUs receive the request signal; the signal strength table comprises IDs of the RUs and corresponding normalized received signal strengths; the normalized received signal strengths are ratios of practical signal strengths of the RUs to a signal strength of an RU having a strongest received signal, and the RU having the strongest received signal is assigned to UE 1 as an initial access RU of UE 1; and the DAS constantly update the signal strength table during entire communication with UE 1; and wherein the MU is a main unit of the indoor DAS, the RU is a remote unit and an antenna connected to the remote unit; and UE 1 is a terminal accessing the DAS; step 2: attempting to establish, by the DAS, a downlink with UE 1 via the RU, detecting a signal to interference plus noise ratio (SINR) value of a downlink signal of UE 1, and comparing the SINR value with a predetermined threshold γ.sub.1 as follows: (1) if the SINR value is greater than γ.sub.1, UE 1 performs the following step: establishing a downlink with the RU assigned to UE 1, and starting sending downlink data; (2) if the SINR value is less than γ.sub.1, UE 1 performs the following steps: step 201b: UE 1 feeds back the SINR value to the DAS via the control channel; step 202b: the DAS select an appropriate number of RUs from candidate RUs according to the SINR value for collaboratively sending data to UE 1; step 203b: the DAS inquires, via the control channel, a terminal UE 2 accessing the small base station using the same frequency point as UE 1, wherein UE 2 is a terminal accessing the small base station; step 204b: after confirming that UE 2 uses the same frequency point as UE 1, UE 2 reports this information to the DAS; step 205b: the DAS subsequently assigns an idle timeslot to UE 2 to instruct UE 2 to send a training sequence; step 206b: UE 2 sends the training sequence in the assigned timeslot, and the DAS subsequently estimates channels from the selected RUs to UE 2; step 207: the DAS collaboratively sends precoded data to UE 1 via the selected RUs, such that an SINR value of a received signal of UE 1 is greater than γ.sub.1, and meanwhile, a precoding method is enabled to prevent the signal from causing interference to a received signal of UE 2; and step 3: maintaining, by UE 1, a downlink with one or a plurality of RUs assigned by the DAS, and constantly detecting the SINR value at a specific time interval, and performs the following steps based on whether the SINR value is greater than γ.sub.1: (1) if the SINR value is less than γ.sub.1, operations in step 201b-207b are performed; and (2) if the SINR value is greater than γ.sub.1, UE 1 continues to compare the SINR value with a predetermined threshold γ.sub.2, and perform the following steps: if the SINR value is less than γ.sub.2, UE 1 continuously maintains communication with the plurality of RUs assigned by the DAS, and constantly detects the SINR value at a specific time interval; and if the SINR value is greater than γ.sub.2, UE 1 reports the SINR value, and sequentially removes the RUs with reference to signal strengths of the RUs participating collaborative data sending and according to a strength sequence of the received signals, until an SINR value predicted by the DAS is greater than γ.sub.1 or only one RU remains.

2. The method for avoiding downlink interference between an indoor DAS and a small base station according to claim 1, wherein the RUs are selected as follows: if a current SINR value is γ, excluding an RU with a normalized signal strength of 1 and sequentially selecting the RUs based on a strength sequence of the normalized signal strengths according to the signal strength table, until the following formula is satisfied: $\begin{array}{cc}\gamma \times \left(1+\underset{i=2}{\overset{N}{.Math.}}.Math.P_i\right)>{\gamma }_1& \left(1\right)\end{array}$ wherein P.sub.i indicates a normalized signal strength of an RU ranking the i.sup.th in a sequence of the normalized signal strengths in the signal strength table, and parameter N indicates the number of desired RUs satisfying a condition of formula (1).

3. The method for avoiding downlink interference between an indoor DAS and a small base station according to claim 1, wherein the precoding method is as follows: assuming that N RUs participate in collaboratively sending data to UE 1, via channel estimation, channel attenuation coefficients from the N RUs to UE 2 are acquired, respectively represented by h.sub.n2(n=1, . . . , N); h.sub.2 indicates a row vector formed of h.sub.n2(n=1, . . . , N), that is, h.sub.2=[h.sub.1,2, h.sub.2,2, . . . , h.sub.N,2]; likewise, h.sub.1=[h.sub.1,1, h.sub.2,1, . . . , h.sub.N,1] which indicates a row vector formed of the channel attenuation coefficients of the N RUs participating in collaborative sending data to UE 1; and a precoding matrix is w=[w.sub.1, w.sub.2, . . . , w.sub.N], which is a N-lengthed row vector; then the received signal of UE 1 is:
y.sub.1=h.sub.1w.sup.Hx.sub.1+J+W  (2) wherein J indicates interference caused by a downlink signal from the small base station to UE 2, and W indicates noise; the n.sup.th RU participating in collaborating data sending in the DAS has a sending signal w.sub.nx.sub.1, that is, data sent by the RU is data x.sub.1 weighted by w.sub.n; and the signal power is increased by adding an RU, to thus improve the SINR value, such that UE 1 is capable of successfully demodulating data, and in this case, the received signal of UE 2 is:
y.sub.2=hx.sub.2+h.sub.2w.sup.Hx.sub.1+W  (3) wherein scalar h indicates state information of a channel from the small base station to UE 2, and x.sub.2 indicates data sent by the small base station to UE 2; the precoding matrix w is a feature vector corresponding to a zero feature value of h.sub.2.sup.Hh.sub.2, and assuming that v is a feature vector corresponding to the zero feature value of the matrix h.sub.2.sup.Hh.sub.2, then the precoding matrix is: $w=\frac{.Math.h_1.Math..Math.v}{.Math.h_1.Math.v.Math.}$ wherein calculator |•| indicates that |y|=√{square root over (yy.sup.He)}, wherein y is a row vector.

4. The method for avoiding downlink interference between an indoor DAS and a small base station according to claim 1, wherein the predetermined threshold γ.sub.1 has a value of −3 dB to 3 dB, and the predetermined threshold γ.sub.2 has a value of 5 dB to 15 dB.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0041] FIG. 1 is a schematic diagram of a scenario of avoiding downlink interference between a DAS and a small base station;

[0042] FIG. 2 is a schematic diagram of a DAS according to the present invention;

[0043] FIG. 3 is a flowchart of avoiding downlink interference between an indoor DAS and a small base station;

[0044] FIG. 4 illustrates a specific embodiment 1 according to the present invention;

[0045] FIG. 5 illustrates a specific embodiment 2 according to the present invention; and

[0046] FIG. 6 is a flowchart of collaboratively sending data by a plurality of RUs in a DAS.

DETAILED DESCRIPTION

[0047] The specific embodiments of the present invention are described in detail hereinafter with reference to the accompanying drawings.

[0048] FIG. 1 is a schematic diagram of a scenario of a specific application of the present invention, wherein the scenario involves a DAS and a small base station. In general, the DAS improves indoor signal coverage by means of multiple of RUs distributed in buildings, but cannot enhance network capacity. The small base station is used to cover hot spot regions, which may effectively improve capacity. However, with respect to large buildings, a universal coverage similar to the DAS cannot be implemented. Therefore, in a long period of time, the DAS and the small base station system are hybridly deployed. As such, the universal signal coverage of the buildings is satisfied, and capacity requirements of the hot spot regions are accommodated. The present invention, rightly in this scenario, proposes a solution of avoiding downlink interference between a DAS and a small base station, such that co-frequency deployment is feasible. As illustrated in FIG. 1, UE 1 is a terminal accessing the DAS, and UE 2 is a terminal accessing the small base station. In a case of downlink transmission, downlink transmit signals of the small base station may cause interference to UE 1; and meanwhile, downlink transmit signals of the DAS may also cause interference to UE 2. However, typically, the transmit power of the small base station is 25 dBm, whereas the transmit power of a single RU is only 15 dBm. Therefore, the present invention mainly considers the problem that the small base station causes interference to UE 1; and meanwhile, in the specific implementation process, the multi-antenna feature of the DAS is also fully considered, and the interference caused by the DAS to UE 2 is avoided using the precoding technology.

[0049] FIG. 2 is a schematic diagram of an indoor DAS according to the present invention. The indoor DAS mainly comprises an MU and a plurality of RU modules. The functions of the MU related to the present invention are as follows:

[0050] 1) establishing and maintaining a normalized received signal strength table of UE 1 according to received signal powers fed back by the RUs with respect to UE 1;

[0051] 2) performing channel estimation according to training sequence signals received by the RUs; and

[0052] 3) calculating a precoding matrix according to the estimated channel, and assigning weighting coefficients to the RUs.

[0053] The functions of the RU related to the present invention are as follows:

[0054] 1) feeding back powers of received signals to the MU; and

[0055] 2) weighting the sent data according to the weighting coefficient assigned by the MU.

[0056] FIG. 3 is a normalized received signal strength table established by an MU. The MU establishes a normalized received signal strength table as illustrated in FIG. 3 for each UE accessing the DAS. The Table name corresponding to the table for each UE is an ID of the UE. Each table comprises two fields, respectively an ID of an RU and a normalized received signal power. The normalized received signal power is a ratio of the receive power of the RU to the receive power of an RU with the greatest receive power among all the RUs. Therefore, with respect to the RU with the greatest receive power, the normalized received signal power thereof is 1, and the normalized received signal powers of the other RUs are all less than 1.

[0057] FIG. 4 is a flowchart of avoiding downlink interference between an indoor DAS and a small base station. With reference to the scenarios as illustrated in FIG. 4 and FIG. 6, specific embodiments are described for the problem solving flowchart illustrated in FIG. 4.

[0058] In the embodiment illustrated in FIG. 5, the DAS totally has four RUs, and a small base station is deployed in the DAS. The distance from RU 1 to UE 1 is 14 meters, the distance from RU 2 to UE 1 is 10 meters, the distance from RU 3 to UE 1 is 40 meters, the distance from RU 4 to UE 1 is 35 meters, and the distance from the small base station to UE 1 is 17 meters. In this embodiment, assuming that the transmit power of each RU is 15 dBm, and the transmit power of the small base station is 25 dBm, then in this embodiment, γ.sub.1 has a value of 1 (that is, 0 dB), γ.sub.2 has a value of 4 (that is, 6 dB), the path loss is inversely proportional to the fourth power of the distance, and the signal-to-noise ratio of the received signal is 10 dB. In the entire communication time period between UE 1 and the DAS, UE 2 also maintains communication with the small base station.

[0059] In step 1, UE 1 sends an access request to a DAS via a control channel. Upon receiving the request signal, the RUs in the DAS sends the received signal power to the MU in addition to sending the access request to the MU. The MU establishes a normalized received signal strength table according to the received signal power.

TABLE-US-00001 Normalized received signal ID of RU strength RU1 0.2603 RU2 1 RU3 0.004 RU4 0.007

[0060] Therefore, the system selects RU 2 as an initial access RU of UE 1.

[0061] In step 2, RU 2 attempts to establish a downlink with UE 1, RU 2 sends data to UE 1, and UE 1 calculates an SINR of the received signal to be 0.83 (that is, −8.63 dB) which is less than a predetermined threshold γ.sub.1. Subsequently, the following steps are performed:

[0062] Step 201b: UE 1 feeds back the SINR value to the DAS via the control channel.

[0063] Step 202b: The DAS selects, according to the SINR value and based on the normalized received signal strength table, an appropriate number of RUs for collaboratively sending data to UE 1.

[0064] In this embodiment, the RUs are sequentially selected according to a sequence of the powers from the normalized received signal strength table. Therefore, RU 1 is firstly selected, and whether the selected RU 1 satisfies formula (1) is determined. In this embodiment, the selected RU 1 satisfies formula (1), that is, 0.83×(1+0.2603)=1.046>1.

[0065] Step 203b: The DAS inquires, via a control channel, UE 2 accessing the small base station using the same frequency point as UE 1.

[0066] Step 204b: After confirming that UE 2 uses the same frequency point as UE 1, UE 2 reports this information to the DAS.

[0067] Step 205b: the DAS subsequently assigns an idle timeslot to UE 2 to instruct UE 2 to send a training sequence;

[0068] Step 206b: UE 2 sends the training sequence in the assigned timeslot, and the DAS subsequently estimates channels from the selected RUs to UE 2.

[0069] Step 207: The DAS collaboratively sends precoded data to UE 1 via the selected RUs, such that an SINR value of a received signal of UE 1 is greater than γ.sub.1, and a corresponding quality of service is ensured. Meanwhile, a precoding method is enabled to prevent the signal from causing interference to a received signal of UE 2.

[0070] In this embodiment, only RU 1 and RU 2 are involved in collaborative data sending. Therefore, h.sub.1=[h.sub.1,1, h.sub.2,1] and h.sub.2=[h.sub.1,2, h.sub.2,2]. The matrix h.sub.2.sup.Hh.sub.2 is a 2×2 matrix, and a non-zero feature value and a zero feature value are present. Assuming that v is a feature vector corresponding to the zero feature value, then h.sub.2v.sup.H=0. To ensure that the power upon precoding is unchanged, the precoding matrix is assumed to w=|h.sub.1|v/|h.sub.1v| which is a 1×2 vector. Therefore, the received signal of UE 1 is y.sub.1=h.sub.1w.sup.Hx.sub.1+J+W, and since an RU is added for collaborative data sending, the SINR is increased from 0.83 in independent data sending to 1.046 in collaborative data sending. In addition, the received signal of UE 2 is y.sub.2=hx.sub.2+h.sub.2w.sup.Hx.sub.1+W=hx.sub.2+W, and the DAS may not cause additional interference to the small base station.

[0071] In step 3, in this embodiment, in the communication between UE 1 and the DAS, UE 2 constantly maintains communication with the small base station. Therefore, the SINR is constantly 1.046, which satisfying γ.sub.1 and less than γ.sub.2, and thus the system only detects the SINR at a specific time interval. This time interval may be defined in the system configuration file according to the actual needs.

[0072] In the embodiment illustrated in FIG. 6, considering dense deployment of the RUs, the DAS totally has six RUs, and a small base station is deployed in the DAS. The distance from RU 1 to UE 1 is 6 meters, the distance from RU 2 to UE 1 is 5 meters, the distance from RU 3 to UE 1 is 10 meters, the distance from RU 4 to UE 1 is 6 meters, the distance from RU 5 to UE 1 is 15 meters, the distance from RU 6 to UE 1 is 14 meters, and the distance from the small base station to UE 1 is 9 meters. In this embodiment, assuming that the transmit power of each RU is 15 dBm, and the transmit power of the small base station is 25 dBm, then γ.sub.1 has a value of 1 (that is, 0 dB), γ.sub.2 has a value of 2.5 (that is, 6 dB), the path loss is inversely proportional to the fourth power of the distance, and the signal-to-noise ratio of the received signal is 10 dB. In the entire communication time period between UE 1 and the DAS, the communication between UE 2 and the small base station involves three stages. At the first stage, the small base station does not communicate with UE 2; at the second stage, the small base station communicates with UE 2; and at the third stage, the communication between the small base station and UE 1 is complete.

[0073] According to step 1, UE 1 sends an access request to a DAS via a control channel. Upon receiving the request signal, the RUs in the DAS sends the received signal power to the MU in addition to sending the access request to the MU. The MU establishes a normalized received signal strength table according to the received signal power.

TABLE-US-00002 Normalized received signal ID of RU strength RU1 0.6339 RU2 1 RU3 0.1768 RU4 0.6339 RU5 0.0642 RU6 0.0762

[0074] Therefore, the system selects RU 2 as an initial access RU of UE 1.

[0075] In step 2, RU 2 attempts to establish a downlink with UE 1, and RU 2 sends data to UE 1. Since in this case, the small base station causes no downlink interference, the SINR is a signal-to-noise ratio of the system, which is 10 dB and is greater than γ.sub.1. Then step 201a is performed.

[0076] In step 3, UE 1 maintains a downlink with RU 2 assigned by the DAS, and constantly detects the SINR value in the communication at a specific time interval.

[0077] In this embodiment, if UE 2 does not access the small base station, the SINR remains unchanged, the system does not perform any operation, and UE 1 only constantly detects the SINR value based on a predetermined time interval. Once UE 2 accesses the small base station, as illustrated at stage 2 in FIG. 6, UE 1 may detect changes of the SINR within an SINR detection period. According to assumptions of the parameters in this embodiment, the SINR in this case is lowered to about 0.43 (that is, −3.7 dB). In this case, the SINR is less than γ.sub.1, and thus the system performs steps 201b-207b.

[0078] Step 201b: UE 1 feeds back the SINR=0.43 to the DAS via the control channel.

[0079] Step 202b: The DAS selects, according to the SINR value and based on the normalized received signal strength table, an appropriate number of RUs for collaboratively sending data to UE 1.

[0080] In this embodiment, the RUs are sequentially selected according to a sequence of the powers from the normalized received signal strength table. Therefore, RU 1 (or RU 4, random selection) is firstly selected, and whether the selected RU 1 satisfies formula (1) is determined. If the selected RU 1 does not satisfy formula (1), another RU is selected. In this embodiment, RU 1, RU 4 and RU 3 need to be selected to satisfy formula (1), that is, 0.43×(1+0.6339+0.6339+0.1768)=1.051>1.

[0081] Step 203b: The DAS inquires, via a control channel, UE 2 accessing the small base station using the same frequency point as UE 1.

[0082] Step 204b: After confirming that UE 2 uses the same frequency point as UE 1, UE 2 reports this information to the DAS.

[0083] step 205b: the DAS subsequently assigns an idle timeslot to UE 2 to instruct UE 2 to send a training sequence;

[0084] Step 206b: UE 2 sends the training sequence in the assigned timeslot, and the DAS subsequently estimates channels from the selected RUs to UE 2.

[0085] Step 207: The DAS collaboratively sends precoded data to UE 1 via the selected RUs, such that an SINR value of a received signal of UE 1 is greater than γ.sub.1, and a corresponding quality of service is ensured. Meanwhile, a precoding method is enabled to prevent the signal from causing interference to a received signal of UE 2.

[0086] In this embodiment, only RU 1 and RU 2 are involved in collaborative data sending. Therefore, h.sub.1=[h.sub.1,1, h.sub.2,1, h.sub.3,1, h.sub.4,1] and h.sub.2=[h.sub.1,2, h.sub.2,2, h.sub.3,2, h.sub.4,2]. The matrix h.sub.2.sup.H h.sub.2 is a 4×4 matrix, and a non-zero feature value and a zero feature value are present. The feature vector corresponding to a zero feature value is randomly selected, and then h.sub.2v.sup.H=0. To ensure that the power upon precoding is unchanged, the precoding matrix is assumed to w=|h.sub.1|v/|h.sub.1v| which is a 1×4 vector. Therefore, the received signal of UE 1 is y.sub.1=h.sub.1w.sup.Hx.sub.1+J+W, and since an RU is added for collaborative data sending, the SINR is increased from 0.43 in independent data sending to 1.051 in collaborative data sending. In addition, the received signal of UE 2 is y.sub.2=hx.sub.2+h.sub.2w.sup.Hx.sub.1+W=hx.sub.2+W, and the DAS may not cause additional interference to the small base station.

[0087] If the precoding technology is not used and UE 2 is at the edge of a coverage range of the small base station, since the DAS employs multi-RU collaborative data sending, the SINR is lowered. In this embodiment, considering the position relationship between UE 2 and RU 2, between UE 2 and RU 4 and between UE 2 and the small base station (assuming that the distances from UE 2 to RU 2, from UE 2 to RU 4 and from UE 2 to the small base station are all 6 meters), the SINR of UE 2 may be lowered by 3 dB.

[0088] After step 207b is performed, step 3 is performed.

[0089] In step 3, UE 1 maintains downlinks with RU 1, RU2, RU3 and RU4 RU 2 assigned by the DAS, and constantly detects the SINR value in the communication at a specific time interval. In this embodiment, if UE 2 does not quit the small base station, the SINR remains unchanged, the system does not perform any operation, and UE 1 only constantly detects the SINR value based on a predetermined time interval. If the system enters the third stage as illustrated in FIG. 6, that is, UE 2 quits the communication prior to UE 1, then due to elimination of interference, the SINR is far greater than the signal-to-noise ratio of the system, which is 10 dB in this embodiment, in this case, the following operations are performed.

[0090] Firstly according to the normalized signal strength table, the RU with the weakest received signal strength is removed out of collaboratively data sending. In this embodiment, the firstly removed RU is RU 3. Then the SINR is calculated. If the calculated SINR is still greater than γ.sub.2, which is 4 (6 dB) in this embodiment, the RU with the weakest received signal strength is continuously removed. This removal operation is performed until the SINR is grater than γ.sub.1 but less than γ.sub.2, or the SINR is greater than γ.sub.1 but the system has only one RU which maintains communication with UE 1. In this embodiment, since the signal-to-noise ratio is 10 dB, finally only RU 2 maintains downlink communication with UE 1. Afterwards, the system enters step 3.

[0091] In step 3, UE 1 maintains downlink communication with RU 2, and continuously detects the SINR at a specific time interval and performs a corresponding operation according to the detected SINR.

[0092] The above embodiments are merely exemplary embodiments of the present invention, but are not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, or improvement made without departing from the spirit and principle of the present invention should fall within the protection scope of the present invention.