Not Applicable
Not Applicable
The present invention relates to communications networks. More particularly, and not by way of limitation, the present invention is directed to a system and method implementing interference suppression for channel parameter estimation utilizing irregular reference signal patterns in a telecommunications network. One of the channel parameters that may be estimated is the delay of the first path of a multipath channel. The estimation of this parameter enables the determination of the time of arrival of the received signal. Estimation of the time of arrival of the signal is key to the estimation of the position of mobile stations in a wireless communication system which has become an important component of the services provided by a wireless operator. In the United States, operators are required to provide positions of mobile users when the user makes an emergency call. Requirements on the accuracy of position estimates are quite stringent. For solutions that are based in the mobile station, 67% of all positions must have accuracy better than 50 meters while 95% of all positions must have accuracy better than 150 meters.
These requirements, as well as the heightened interest in commercial location based services, has made the provision of a mechanism to compute the position of the mobile station an important part of a wireless communication standard. The Evolved Universal Terrestrial Radio Access standard (E-UTRA) is a standard being developed by the third generation partnership project (3GPP). It is also known as long term evolution (LTE) and provides very high peak data rates and spectral efficiency. Part of the work being done in the development of this standard relates to the estimation of a position of a mobile station.
The LTE standard is based on orthogonal division frequency multiplexing (OFDM) in the downlink. In the development of positioning solutions for LTE, methods based on an estimation of the time of arrival (TOA) of a signal transmitted by a base station (e.g., eNodeB 108) at a mobile station are being considered. Oftentimes, the terms base station and cell may be interchangeably used to denote a single sector in a site that may have many sectors. For example, a single eNodeB 108 may have three sectors spanning an angle of 120 degrees each. TOA estimation based methods are widely considered to be the most robust and accurate among the many different options for position estimation using a terrestrial wireless communication system. The most popular method overall is TOA estimation based positioning using global positioning system (GPS) satellites. The TOA estimates from various cell sites are combined with knowledge of the positions of the eNodeB sites, to compute the unknown position of the mobile station. The fact that the difference between the TOA of a signal at the UE 110 and time of its transmission by the eNodeB 108 is related to the distance between the two nodes is used in the computation of the position from the TOA measurements.
Position computation as discussed above requires the estimation of the TOA from at least three cell sites in geographically different locations. A problem that often arises when measuring the TOA at a UE is that the signal-to-interference ratio (SINR) for some of the sites used to compute the UE's position is very poor. Cellular systems are typically designed to optimize the SINR to the serving cell, but not to the neighboring cell sites. This problem presents a significant challenge in the design of a positioning solution for LTE as well.
One of the solutions that may be used to increase SINR to neighboring cell sites is to increase the reuse of time and frequency resources so that different base stations use orthogonal resources as far as possible. In order to do this, one of the solutions that is being considered in the standardization of LTE is the definition of low-interference subframes (LIS). The LIS are designed to only carry reference signals that may be used for making TOA measurements, but no user data. Ideally, the LIS is aligned across the system, i.e., all cells in the system transmit their low-interference subframes at the same time. The performance of the system is then determined by the design of the reference signals being carried in these subframes.
The reference signals generally occupy resource elements (RE) in the time frequency plane. In LTE, the time frequency plane is split up into units called resource blocks (RB) where each resource block consists of 14 OFDM symbols and 12 subcarriers. A number of resource blocks stacked in the frequency dimension form a subframe with the number of blocks dependent on the bandwidth of the signal. It is desirable for the design of the reference signals to be the same across resource blocks in frequency for ease of implementation at the eNodeB and the UE.
The mixed reference signals B and C are depicted at 204 in
The use of the approach based on irregular patterns has an advantage in terms of not requiring the use of any careful planning in the assignment of sequences to cells. However, depending on the parameters of the resource block dimensions, it may result in poorer SINR to neighboring cell sites. With both approaches, SINR may be improved with interference suppression at the UE. Specifically, some of the interfering signals are suppressed using signal processing at the UE.
The existing solutions for suppressing interference require the estimation of the parameters of the channel between the interfering eNodeB and the UE before suppressing or cancelling the interference. Other sub-optimal solutions with lower complexity may be used, but typically result in large performance degradation and/or a high degree of complexity. Thus, existing solutions do not sufficiently address the problems discussed above.
The present invention provides a novel interference suppression scheme that may be used with irregular reference signal patterns. Due to the small number of overlapping resource elements, two patterns being used by different cell sites interfere only in a few of the OFDM symbols of the resource blocks while they are orthogonal along the remaining OFDM symbols. This property is used to avoid OFDM symbols that exhibit the maximum interference while using the other resource elements to determine the time of arrival measurement.
In one aspect, the present invention is directed at a method of implementing interference suppression for estimating a parameter of a signal transmitted by a base station in a telecommunications network. The method assigns irregular reference signal patterns to a plurality of base stations. From these base stations, a list is compiled of base stations for which a parameter of the transmitted signal is to be estimated and of OFDM symbols and subcarriers to avoid for each of the base stations that is to be measured. The compiled list is sent as measurement information to a User Equipment. The UE receives the information and performs measurements for each base station in the compiled list based on the received measurement information. Simple interference avoidance is performed by utilizing OFDM symbols and subcarriers that are not on the list of symbols and subcarriers to avoid and the measurements are sent to the network.
In another aspect, the present invention is directed at a system for implementing interference suppression to estimate a parameter of a signal transmitted by a base station in a telecommunications network. Irregular reference signal patterns are assigned to a plurality of base stations. A UE requests information for estimating a parameter of the signal and a node in the network compiles a list of base stations that will have a parameter of its transmitted signal estimated. A list of OFDM symbols and subcarriers to avoid for each base station to be measured is provided and the node sends the compiled list to the UE. The UE performs measurements for each base station on the compiled list and simple interference avoidance is performed by utilizing OFDM symbols and subcarriers that are not on the avoidance list provided. The measurements are then sent to the network.
In a further aspect, the present invention is directed at a User Equipment (UE) implementing interference suppression for estimating a parameter of a signal transmitted by a base station in a telecommunications network. The UE requests information for estimating the parameter of the signal and receives a compiled list of base stations to measure and a list of OFDM and subcarriers to avoid for each base station to be measured. The UE performs the measurements using simple interference avoidance by using OFDM symbols and subcarriers that are not on the avoidance list provided. The measurements are then sent to the network.
In still another aspect, the present invention is directed at a node for implementing interference suppression for estimating a parameter of a signal transmitted by a base station in a telecommunications network. The node assigns irregular reference signal patterns to base stations and from those base stations a list is complied for determining an estimate of the parameter of the signal. Then a list of Orthogonal Division Frequency Multiplexing symbols and subcarriers to avoid for each base station is compiled and sent to the UE.
In the following section, the invention will be described with reference to exemplary embodiments illustrated in the figures, in which:
In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components and circuits have not been described in detail so as not to obscure the present invention.
The present invention is a suppression scheme for use when reference signals based on irregular patterns are utilized. Due to the small number of overlapping resource elements, two patterns being used by different cell sites interfere only in a few of the OFDM symbols of the resource blocks while the remaining OFDM symbols are orthogonal. This property is used to avoid the OFDM symbols that exhibit the maximum interference while using the other resource elements to determine time of arrival measurements. The present invention provides various embodiments for the determination of the OFDM symbols to avoid.
In one embodiment, let sm[n,k], wherein n={1,2, . . . , N},k={1, 2, . . . , K}, represent the sampled version of the transmitted reference signal in OFDM symbol n and subcarrier k, where N is the number of OFDM symbols used to transmit the reference signal and K is the number of subcarriers in an OFDM symbol. Then, the received signal from the Mth cell may be represented by:
r[n,k
p
D(n)]=cD[n,kpD(n)]·sD[n,kpD(n)]+cI[n,kpD(n)]·si[n,kpD(n)]+z[n,kpD(n)],n={1,2, . . . , N}
where kPD(n) is the subcarrier occupied by the pth reference symbol in OFDM symbol n for the desired reference signal, sI[n, kpD(n)] is the interfering symbol in the subcarrier occupied by the pth symbol in OFDM symbol n for the desired reference signal, cD[n,k9D(n)] and cI[n, kPD(n)] are the channels for the desired and interfering signals and z[n, kp(n)] is the noise in the associated subcarrier.
In the scenario where the approach with regular patterns with a standard reuse is used as discussed in
The most common method to measure time of arrival is through the use of a correlator whose output may be described as follows:
b[t]=Σ
t
s*[q]·r[q+t],t={−T,−T+1, . . . , T},
where 2T is the TOA time uncertainty window. The time of arrival is then estimated as the peak of the function |b[t]|2. The signal s[q] is then defined purely in time, with q={1, 2, . . . , NK} where N is the number of OFDM symbols and K is the number of samples per symbol. In the first embodiment, only the subcarriers p and the OFDM symbols n for which kpD(n) ≠kpI(n) are utilized to perform time of arrival estimation. This may be achieved by modifying the signal s[q] prior to performing the correlation operation as discussed above. For example, if some OFDM symbols are not to be used, this may be achieved by simply setting the corresponding parts of the signal s[q] to zero. Specifically, if symbol v is to be avoided, s[q]=0 for q={(v-1)K+1, (v-1)K+2, . . . , vK}.
The choice of which symbols and subcarriers to use and which to leave out may be made in several ways. In one embodiment, this information is provided to the UE by the eNodeB or some other node in the cellular network. The telecommunications network is aware of the reference signals patterns that have been assigned to all the neighboring sites and may use this information to compute which symbols and subcarriers should be avoided. Information on which reference signals to use for measurements is typically provided as assistance data to the UE when measurements for position estimation are to be made. This information is supplemented with the information on which subcarriers and symbols should not be used when measuring the TOA of each reference signal.
In another embodiment of the present invention, the UE determines which symbols to avoid by performing partial correlations with the received signal and leaving out correlation outputs corresponding to the symbols that have the maximum energy. The symbols having the maximum energy are most likely to have interference in addition to the desired symbols. This signal can be split up as the sum of a number of individual parts with each part spanning one OFDM symbol period. Thus:
s[q]=s
1
[q]+s
2
[q]+ . . . +s
N[q], where
si[q]=0 everywhere except when q={(i-1)·K+1, (i-1)·K+2, . . . , (i-1).K+K}, i={1, 2, . . . , N}
The correlator output above can then be expressed as:
b[t]=Σ
i(Σtsi*[q]·r[q+t])=Σibi[t],t={−T,−T+1, . . . , T},i={1,2, . . . , N}
In another embodiment of the present invention, N correlator outputs bi[t]=Σt si[q+t]·r[q], i=1, 2, . . . , N are formed with the number of operations in each significantly reduced since most of the signal, except for one OFDM symbol, is spanned by zeros. Subsequently, the maximum energy in each of the correlators, bi[t], is measured and the U symbols with the highest correlator energy are not used in measuring the TOA where U is an adjustable parameter. This is achieved by setting bi[q]=0, when the ith symbol is one of the U symbols corresponding to the highest correlator energy, before summing the correlator outputs to obtain b[t].
The present invention utilizes the suppression of interference for making TOA measurements by avoiding certain OFDM symbols and subcarriers. Two embodiments have been discussed for determining which symbols and subcarriers should be avoided. However, in the present invention, any variation of the methodology discussed above may be used to determine specific OFDM symbols and/or subcarriers to avoid in order to suppress interference.
In another embodiment of the present invention, the UE selects fewer symbols to perform correlations with when operating in interference limited environment while selecting more or all symbols when operating in a noise limited environment. When cell sizes are small, the impairment tends to be dominated by signals from other cells. In such a scenario, the UE may avoid some symbols and correlate with fewer symbols to achieve interference suppression and boost SINR. When cell sizes are large, the impairment tends to be dominated by white noise. In this case, the UE may utilize all the symbols since this provides gain against noise.
The present invention provides many advantages over existing telecommunications systems. The complexity of the interference suppression scheme associated with irregular patterns with minimal overlapping information elements is quite low compared to interference suppression when a substantial number of the resource elements of the desired and interfering signals overlap. In addition, the use of irregular patterns with minimal overlaps as reference signals transmitted from cells in conjunction with the low-complexity interference suppression scheme provides an improvement in the SINR at the UE and consequently in position estimation accuracy.
As will be recognized by those skilled in the art, the innovative concepts described in the present application can be modified and varied over a wide range of applications. Accordingly, the scope of patented subject matter should not be limited to any of the specific exemplary teachings discussed above, but is instead defined by the following claims.
This application claims the benefit of U.S. Provisional Application No. 61/175,278, filed May 4, 2009, the disclosure of which is incorporated herein by reference.
Number | Date | Country | |
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61175278 | May 2009 | US |