Apparatus and method for forming a forward link transmission beam of a smart antenna in a mobile communication system

[0001] This application claims priority under 35 U.S.C. § 119 to an application entitled “Apparatus and Method for Forming Forward Link Transmission Beam of Smart Antenna in a Mobile Communication System” filed in the Korean Intellectual Property Office on May 17, 2002 and assigned Serial No. 2002-27324, the contents of which are incorporated herein by reference.

[0002] 1. Field of the Invention

[0003] The present invention relates generally to a smart antenna apparatus and method, and in particular, to an apparatus and method for forming forward link transmission beams of a smart antenna in a mobile communication system.

[0004] 2. Description of the Related Art

[0005] In order to meet a rapidly increasing demand for mobile communication and provide various multimedia services to users, there is a great necessity to increase capacity of a forward link. Typically, frequency division multiple access (FDMA) and time division multiple access (TDMA) are used to secure as large a subscriber capacity as possible with the limited frequency bandwidth available. In FDMA technology, a given frequency bandwidth is divided into a plurality of frequency channels necessary for communication, so that subscribers each use unique frequency channels. However, in TDMA technology, each subscriber uses a single frequency channel only for a predetermined time slot assigned thereto. Also, code division multiple access (CDMA) has been proposed which uses the same frequency band but distinguishes subscribers by assigning different codes to the subscribers.

[0006] However, the method of increasing efficiency of a limited frequency band by these multiple access technologies has a limitation in accommodating many subscribers. In order to overcome the limitation, cellular technology has been proposed. Cellular technology refers to a mobile communication technology that divides a service area into a plurality of small regions, or cells, and uses the same frequency band at two cells sufficiently distanced from each other to increase the number of spatially distributed channels, thereby securing sufficient subscriber capacity. Moreover, it is possible to further increase base station capacity by sectoring a base station antenna. For example, a base station antenna is sectored by changing an omnidirectional antenna with a 360° radiation pattern into three directional sector antennas with a 120° radiation pattern. Particularly, in a CDMA system, if a base station antenna is sectored, noises from subscribers of other sectors are reduced, contributing to an increase in call capacity of the base station.

[0007] Such conventional omnidirectional antenna or sector antenna transmits both a common channel signal and a transmission channel signal to a mobile station through a single common beam. The common channel signal includes a pilot channel signal, a synchronization channel signal and a paging channel signal, all of which must be transmitted from a base station to all mobile stations in a corresponding cell. The transmission channel signal refers to a traffic channel signal that must be transmitted to a particular mobile station. A considerable amount of radiation energy is wasted except the radiation energy transmitted to a particular mobile station, e.g., when a particular signal such as the transmission channel signal is transmitted to the particular mobile station in the same manner as the common channel, rather than when a predetermined signal such as the common channel signal is transmitted from a base station's transmission antenna to all mobile stations. In addition, such radiation energy acts as an interference signal to other mobile stations except the corresponding mobile station.

[0008] Therefore, if it is possible to transmit a transmission channel signal in a direction of a particular mobile station by certain means, it is possible to maintain high call quality while maintaining low transmission power and reducing interference signals to other mobile stations, thereby contributing to an increase in call capacity. An antenna based on such a concept is an adaptive array antenna, also known as an intelligent antenna or smart antenna.

[0009] A smart antenna system refers to an intelligent antenna system which can automatically change its radiation beam pattern in response to a predetermined signal environment. The smart antenna system adopts a technology for arranging a plurality of antenna elements in a specific form and multiplying an output of each antenna element by a complex weight, thereby forming an antenna beam in a direction of a desired mobile station.

[0010] Such a smart antenna system is a technology that can be widely used in a mobile communication field. Herein, however, the smart antenna system will be described with reference to a CDMA cellular mobile communication system. In addition, the smart antenna system is a technology in which a base station receives only a signal transmitted from a desired mobile station, in a reverse link, and concentrates transmission power only to a desired mobile station, on a forward link. Herein, the smart antenna system will be described on the assumption that a forward link transmission beam is formed.

[0011] A method for forming a forward link transmission beam of a smart antenna in a CDMA mobile communication system is disclosed in U.S. Pat. No. 6,108,565, which is incorporated herein by reference. The disclosed method calculates forward link transmission beam forming information by estimating an angle of arrival (AOA) and a time of arrival (TOA) from signals received at an antenna array of a base station from mobile stations. In addition, the patent discloses a method for forming a forward link transmission beam for each mobile station according to the calculated AOA and TOA of a received signal. That is, a common channel signal is transmitted through a wide beam, i.e., common beam, while a transmission channel signal for each mobile station is transmitted through a narrow beam, i.e., transmission beam, according to the calculated forward link transmission beam forming information. A beamwidth of the narrow beam is determined according to the distance between a mobile station and a base station. As the distance becomes shorter, the beamwidth becomes wider, while as the distance becomes longer, the beamwidth becomes narrower. In addition, a beamwidth of the narrow beam for the transmission channel signal is controlled according to a frame error rate (FER) reported over a reverse link.

[0012] A description of a method for forming a forward link transmission beam can be separately made with reference to one case where only a common pilot channel is provided to all mobile stations in a cell and another case where a dedicated pilot channel is provided to each mobile station so that each mobile station can easily perform coherent detection. In the latter case where the dedicated pilot channel is provided, since the dedicated pilot channel and the transmission channel use the same transmission beam, phase matching between both channels is guaranteed. However, in the former case where only the common pilot channel is provided, since the common pilot channel and the transmission channel use different forward link transmission beams, phase mismatching occurs between both channels. The phase mismatching has a different aftereffect according to a modulation scheme. Generally, a mobile communication system uses a modulation scheme of BPSK (Binary Phase Shift Keying) or QPSK (Quadrature Phase Shift Keying), commonly called “MPSK (Multiple Phase Shift Keying).” When the MPSK is used as a modulation scheme, a phase difference between a common channel signal and a transmission channel signal must be minimized in order to minimize a bit error rate (BER). That is, the phase mismatching must be minimized to obtain desired call quality. Therefore, in a general mobile communication system, there is a necessity to minimize the phase mismatching.

[0013] In the CDMA mobile communication system, a signal from one user acts as an interference signal to another user, so the interference must be well controlled in order to increase channel capacity. In particular, as demand for data communication having higher power than voice communication has increased recently, the inference problem becomes more significant. The smart antenna system has been proposed to drastically reduce the interference signals by forming a forward link transmission beam so that a transmission channel signal is transmitted in a direction of a desired particular mobile station. Actually, however, a part of the transmission channel signal transmitted to the desired mobile station is provided to the other mobile stations inside and outside a cell, causing undesired interference. However, such inference is not considered in the forward link transmission beam forming method disclosed in the U.S. Pat. No. 6,108,565.

[0014] Meanwhile, if a beamwidth of a transmission beam is increased to be as wide as a beamwidth of a common beam in order to minimize phase mismatching, interference to other mobile stations is increased. In contrast, if the bandwidth of the transmission beam is decreased in order to minimize interference, the phase mismatching is increased. That is, since the two conditions have a trade-off relation, it is necessary to consider the two conditions together in order to form an optimal transmission beam.

[0015] It is, therefore, an object of the present invention to provide an apparatus and method for optimizing a forward link transmission beam by simultaneously considering an interference problem and a phase mismatching problem in a mobile communication system using a smart antenna.

[0016] It is another object of the present invention to provide an apparatus and method for calculating a direction of a weight vector of a transmission beam in order to maximize in-phase component power for a common channel signal in a transmission channel signal for a mobile station and to minimize the sum of quadrature-phase component power and interference power for other mobile stations inside and outside a cell, caused by the transmission channel signal for the mobile station, in a base station apparatus including an antenna array.

[0017] It is further another object of the present invention to provide an apparatus and method for calculating a direction of a weight vector of a transmission beam so as to minimize interference power caused because a part of a transmission channel signal for a desired particular mobile station is flowed out to other mobile stations inside and outside a cell when a dedicated pilot channel is provided, in a base station apparatus including an antenna array.

[0018] It is yet another object of the present invention to provide an apparatus and method for independently calculating direction and magnitude of a weight vector for a transmission beam for transmitting a transmission channel signal for a particular mobile station in a base station apparatus including an antenna array.

[0019] It is still another object of the present invention to provide an apparatus and method for independently calculating weight vectors of transmission beams for a plurality of mobile stations serviced by a base station apparatus including an antenna array.

[0020] To achieve the above and other objects, there is provided a transmission beam forming control apparatus of a base station, for forming a transmission beam for a transmission channel signal to be transmitted to a mobile station from an antenna array including a plurality of antenna elements. A transmission beam weight vector calculator estimates a direction of the mobile station by using a reception signal and a reverse link power control bit received from the mobile station, and calculates a transmission beam weight vector in the estimated direction. A transmission beam former applies the calculated transmission beam weight vector to the transmission channel signal and providing the applied transmission channel signal to the antenna array.

[0021] To achieve the above and other objects, there is provided a transmission beam forming control method of a base station, for forming a transmission beam for a transmission channel signal to be transmitted to a mobile station from an antenna array including a plurality of antenna elements. The method comprises the steps of estimating a direction of the mobile station by using a reception signal and a reverse link power control bit received from the mobile station, and calculating a transmission beam weight vector in the estimated direction; and applying the calculated transmission beam weight vector to the transmission channel signal; and providing the applied transmission channel signal to the antenna array.

[0030] Several embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In the drawings, the same or similar elements are denoted by the same reference numerals. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for conciseness.

[0031] The embodiments of the present invention will be described with reference to two different cases. The first case provides a method for calculating an optimal transmission weight vector when there is no dedicated pilot channel and only a common pilot channel exists. The second case provides a method for calculating an optimal transmission weight vector when a dedicated pilot channel exists, e.g., the special case described in conjunction with the prior art. Embodiments of the present invention can be generally applied to a current mobile communication system, such as FDMA, TDMA and CDMA. However, for convenience, the embodiments of the present invention will be described with reference to a CDMA system, especially a 3^rd generation CDMA mobile communication system such as a CDMA2000 system and a WCDMA (Wideband CDMA) system. Meanwhile, it will be assumed herein that direction and magnitude of a weight vector for a common beam were previously calculated by known means.

[0032] FIG. 1 is a system diagram illustrating an example of a channel model of a forward link smart antenna according to an embodiment of the present invention. Specifically, FIG. 1 illustrates a channel model of a forward link smart antenna on the assumption that M mobile stations MS_i (i=1,2, . . . ,M) in a cell are communicating with a base station BS. In FIG. 1, s_m (t) represents a forward link traffic channel signal for an m^th mobile station MS from a base station BS, h_m represents a forward link channel response vector from the base station BS to an m^th mobile station MS_m, and w_m represents a forward link weight vector from the base station BS to an m^th mobile station MS_m. In addition, channel response vectors from the base station BS to a plurality of mobile stations located in other cells are represented by h_oc.

[0033] If a forward link signal transmitted from the base station BS to mobile stations via an antenna array comprised of a plurality of antenna elements is defined as s(t), the s(t) becomes a linear combination of a common channel signal s_p(t) with a common weight vector w_p and a traffic channel signal s_i(t)(i=1,2, . . . , M) with an individual transmission weight vector w_i(i=1,2, . . . , M). That is, the s(t) can be represented by

[0034] $\begin{array}{cc}\underset{\_}{s}\left(t\right)={\underset{\_}{w}}_p{\underset{\_}{s}}_p\left(t\right)+\sum _{i=1}^M{\underset{\_}{w}}_i{s}_i\left(t\right)& \mathrm{Equation}\left(1\right)\end{array}$

[0035] Although the common channel signal includes a pilot channel signal, a synchronization channel signal and a paging channel signal, only the pilot channel signal will be designated herein as the common channel signal, for the convenience of explanation. The pilot channel signal s_p(t) is a signal transmitted over an omidirectional beam or a sector beam in order to provide time and phase criteria for coherent demodulation to mobile stations in a cell. Therefore, a weight vector w_p for the pilot channel signal should be able to entirely cover the inside of a cell or the inside of a sector. In the present examples, it is assumed that the w_p is previously calculated through known means. What is provided is a method for calculating transmission weight vectors w_i(i=1,2, . . . , M) for traffic channel signals s_i(t)(i=1,2, . . . , M) transmitted to each of the mobile stations in the cell, by an optimal criterion. For convenience, the description will be limited to a method for calculating a transmission weight vector w_m for an m^th mobile station MS_m among M mobile stations. Of course, transmission weight vectors for the other mobile stations can also be calculated in the same method. Therefore, it is possible to independently calculate transmission weight vectors for the respective mobile stations.

[0036] When a base station transmits a signal s(t) over a radio channel, a signal r_m(t) received at an m^th mobile station MS_m can be represented by

[0037] $\begin{array}{cc}\begin{array}{c}{r}_m\left(t\right)={\underset{\_}{s}\left(t\right)}^H{\underset{\_}{h}}_m\\ ={\underset{\_}{w}}_p^H{\underset{\_}{h}}_m{s}_p^{*}\left(t\right)+\sum _{i=1}^M{\underset{\_}{w}}_i^H{\underset{\_}{h}}_m{s}_i^{*}\left(t\right)\end{array}& \mathrm{Equation}\left(2\right)\end{array}$

[0038] In Equation (2), * denotes a conjugate operator, and H denotes a Hermitian operator.

[0039] The signal r_m(t) is divided into a pilot channel signal r_p(t), a traffic channel signal r_d(t) for an m^th mobile station MS_m, and an interference signal r_imp(t) indicating a signal transmitted to other mobile stations but flowed into or provided to an m^th mobile station MS_m, as illustrated in Equation (3).

[0040] $\begin{array}{cc}\begin{array}{c}{r}_p\left(t\right)={\underset{\_}{w}}_p^H{\underset{\_}{h}}_m{s}_p^{*}\left(t\right)\\ r_d\left(t\right)={\underset{\_}{w}}_m^H{\underset{\_}{h}}_m{s}_m^{*}\left(t\right)\\ r_{\mathrm{imp}}\left(t\right)=\underset{i\ne m}{\sum _{i=1}^M}{\underset{\_}{w}}_i^H{\underset{\_}{h}}_m{s}_i^{*}\left(t\right)\end{array}& \mathrm{Equation}\left(3\right)\end{array}$

[0041] Power of each signal received at an m^th mobile station MS_m shown in Equation (3) can be expressed by Equation (4) below. In Equation (4), P_p represents power of a pilot channel signal, P_d represents power of a traffic channel signal, and P_imp represents power of an interference signal.

[0042] $\begin{array}{cc}\begin{array}{c}{P}_p={\underset{\_}{w}}_p^H{R}_m{S}_p{\underset{\_}{w}}_p\\ P_d={\underset{\_}{w}}_m^H{R}_m{S}_m{\underset{\_}{w}}_m\\ P_{\mathrm{imp}}=\underset{i\ne m}{\sum _{i=1}^M}{\underset{\_}{w}}_i^H{R}_m{S}_i{\underset{\_}{w}}_i\end{array}& \mathrm{Equation}\left(4\right)\end{array}$

[0043] In Equation (4), R_m represents a forward link transmission covariance matrix for an m^th mobile station MS_m and is R_m=E[h_mh_m^H], and S_p represents power of a pilot channel signal transmitted from a base station and is S_p=E[|s_p(t)|²]. In addition, S_i represents power of a forward link traffic channel signal transmitted from a base station to an i^th mobile station MS_i and is S_i=E[|s_i(t)|²], and S_m represents power of a forward link traffic channel signal transmitted to an m^th mobile station MS_m.

[0044] When there is no dedicated pilot channel and only a common pilot channel exists, a weight vector w_p applied to a common pilot channel is generally different from a weight vector w_m applied to a traffic channel, thus causing phase mismatching between a pilot channel signal and a traffic channel signal received at an m^th mobile station MS_m, illustrated in Equation (3). Here, phase variation due to s_p(t) and s_m(t) of the signal is excluded.

[0045] However, since a traffic channel is synchronized by a pilot channel, a common pilot channel signal becomes a phase criterion in a mobile station. Therefore, of the traffic channel signal of Equation (3), a component phase-matched to the pilot channel signal acts as a signal component, while a component phase-mismatched to the pilot channel signal acts as an interference component. Herein, the component phase-matched to the pilot channel signal is referred to as “in-phase component,” and the component phase-mismatched to the pilot channel signal is referred to as “quadrature-phase component.” Considering this, the traffic signal power P_d of Equation (4) can be divided into in-phase power P_i and quadrature-phase power P_q, as shown in Equation (5) below.

[0046] $\begin{array}{cc}\begin{array}{c}{P}_i={{\underset{\_}{w}}_m^H{R}_m{S}_m{\underset{\_}{w}}_p^{\prime }}^2\\ P_q={\underset{\_}{w}}_m^H{R}_m{S}_m{\underset{\_}{w}}_p^{-}{{\underset{\_}{w}}_m^H{R}_m{S}_m{\underset{\_}{w}}_p^{\prime }}^2\end{array}& \mathrm{Equation}\left(5\right)\end{array}$

[0047] In Equation (5), w′_p is

[0048] ${\underset{\_}{w}}_p^{\prime }={\underset{\_}{w}}_p/\sqrt{{\underset{\_}{w}}_p^H{R}_m{S}_m{\underset{\_}{w}}_p}$
and represents a normalized weight vector of a pilot channel.

[0049] Meanwhile, of the s(t) transmitted from the base station BS, an interference signal r_exp(t) indicating a traffic channel signal

[0050] ${\underset{\_}{w}}_m^H{s}_m^{*}\left(t\right)$
to be transmitted only to an mobile station MS_m but flowed out to other mobile stations MS_i (i=1,2, . . . ,M) in a cell, and an interference signal r_oc(t) indicating the traffic channel signal flowed out to mobile stations belonging to other cells can be represented by

[0051] $\begin{array}{cc}\begin{array}{c}{r}_{\exp }\left(t\right)=\underset{i\ne m}{\sum _{i=1}^M}{\underset{\_}{w}}_m^H{\underset{\_}{h}}_i{s}_m^{*}\left(t\right)\\ r_{\mathrm{oc}}\left(t\right)={\underset{\_}{w}}_m^H{\underset{\_}{h}}_{\mathrm{oc}}{s}_m^{*}\left(t\right)\end{array}& \mathrm{Equation}\left(6\right)\end{array}$

[0052] Power of the inference signals due to outflow of a signal to other mobile stations inside and outside the cell can be defined as

[0053] $\begin{array}{cc}\begin{array}{c}{P}_{\exp }=\underset{i\ne m}{\sum _{i=1}^M}{\underset{\_}{w}}_m^H{R}_i{S}_m{\underset{\_}{w}}_m\\ P_{\mathrm{oc}}={\underset{\_}{w}}_m^H{R}_{\mathrm{oc}}{S}_m{\underset{\_}{w}}_m\end{array}& \mathrm{Equation}\left(7\right)\end{array}$

[0054] In Equation (7), R_oc, denotes a forward link transmission covariance matrix for mobile stations within other cells, and can be represented by

[0055] $R_{\mathrm{oc}}=E\left[{\underset{\_}{h}}_{\mathrm{oc}}{\underset{\_}{h}}_{\mathrm{oc}}^H\right].$

[0056] In addition, a thermal noise generated within a mobile station can be considered together with interference signals from other mobile stations, and the thermal noise can be given by
P_th=σ_th²  Equation (8)

[0057] A weight vector for a transmission beam is calculated using the enumerated signal and interference power. First, in order to calculate a direction of a weight vector for a transmission beam for an m^th mobile station MS_m, a signal-to-inference plus noise ratio (SINR) for forward link beam forming (FLBF) is defined as

[0058] $\begin{array}{cc}{\mathrm{SINR}}_m^{\mathrm{FLBF}}=\frac{P_i}{P_q+P_{\exp }+P_{\mathrm{oc}}}=\frac{|{\underset{\_}{w}}_m^H{R}_m{S}_m{\underset{\_}{w}}_p^{\prime }{|}^2}{\left({\underset{\_}{w}}_m^H{R}_m{S}_m{\underset{\_}{w}}_{m^{-}}|{\underset{\_}{w}}_m^H{R}_m{S}_m{\underset{\_}{w}}_p^{\prime }{|}^2\right)+\underset{i\ne m}{\sum _{i=1}^M}{\underset{\_}{w}}_m^H{R}_i{S}_m{\underset{\_}{w}}_m+{\underset{\_}{w}}_m^H{R}_{\mathrm{oc}}{S}_m{\underset{\_}{w}}_m}& \mathrm{Equation}\left(9\right)\end{array}$

[0059] The

[0060] ${\mathrm{SINR}}_m^{\mathrm{FLBF}}$
is not a value that can be actually measured in a corresponding mobile station MS_m. However, in order to maximize the

[0061] ${\mathrm{SINR}}_m^{\mathrm{FLBF}}$
in Equation (9), it is necessary to minimize the sum of a quadrature-phase component power P_q and interference powers P_exp and P_oc indicating a signal transmitted to a corresponding mobile station MS_m but flowed out to other mobile stations inside and outside a cell while maximizing in-phase component power P_i, for a phase of a pilot channel signal serving as a phase criterion during coherent detection. As a result, from the viewpoint of the overall system, actual

[0062] ${\mathrm{SINR}}_m^{\mathrm{FLBF}}$
is increased in a mobile station MS_m. Therefore, it is possible to define the SINR of Equation (9) instead of using the actual

[0063] ${\mathrm{SINR}}_m^{\mathrm{FLBF}}.$

[0064] If a transmission beam weight vector w_m for maximizing

[0065] ${\mathrm{SINR}}_m^{\mathrm{FLBF}}$
is calculated through the definition, the calculated value becomes an optimal weight vector

[0066] ${\underset{\_}{w}}_m^{\mathrm{opt}}$
for minimizing phase mismatching between a pilot channel signal and a traffic channel signal and also minimizing power of an interference signal for other mobile stations, thereby achieving objects of the present invention.

[0067] In addition, the

[0068] ${\mathrm{SINR}}_m^{\mathrm{FLBF}}$
becomes a function of only a transmission beam weight vector w_m for an m^th mobile station MS_m when a common beam weight vector w_p is given. As a result, it is possible to independently optimize the transmission beam weight vector according to mobile stations. Particularly, it can be understood from Equation (9) that

[0069] ${\mathrm{SINR}}_m^{\mathrm{FLBF}}$
depends upon only a direction of a forward link channel response vector w_m regardless of magnitude of a forward link channel response vector w_m from a base station to a corresponding mobile station and a forward link traffic channel signal S_m for the corresponding mobile station. From this, it can be noted that it is possible to independently calculate direction and magnitude of the forward link channel response vector w_m.

[0070] In Equation (9), a calculation result obtained by selecting the optimal forward link weight vector

[0071] ${\underset{\_}{w}}_m^{\mathrm{opt}}$
as a value for maximizing

[0072] ${\mathrm{SINR}}_m^{\mathrm{FLBF}}$
becomes

[0073] $\begin{array}{cc}\begin{array}{c}{\underset{\_}{w}}_m^{\prime }=\underset{{\underset{\_}{w}}_m}{\mathrm{Maximize}}{\mathrm{SINR}}_m^{\mathrm{FLBP}}\\ ={\left(\sum _{i=1}^M{R}_i+R_{\mathrm{oc}}\right)}^{-1}{R}_m{\underset{\_}{w}}_p\\ {\gamma }_m={\underset{\_}{w}}_m^{\prime H}{R}_m{\underset{\_}{w}}_p\\ {\underset{\_}{w}}_m^{\mathrm{opt}}={\gamma }_m{\underset{\_}{w}}_m^{\prime }\end{array}& \mathrm{Equation}\left(10\right)\end{array}$

[0074] For covariance matrixes used in Equation (10), other means, for example, a forward link covariance matrix fed back from a mobile station MS_m to a base station BS or its equivalent information can be used. When there is no information fed back from the mobile station to the base station, the base station can estimate signals received from mobile stations. A detailed description of this will be made later. A signal γ_m received from an m^th mobile station MS_m was introduced to calculate w_m for maximizing

[0075] ${\mathrm{SINR}}_m^{\mathrm{FLBF}}$
and then additionally control a phase between a common beam and a transmission beam.

[0076] A description has been made of a method for calculating an optimal weight vector when there is no dedicated pilot channel and only a common pilot channel exists. However, when a dedicated pilot channel exists, a weight vector for the dedicated pilot channel serving as a phase criterion during coherent detection is identical in phase to a weight vector applied to a traffic channel. Therefore, if a dedicated pilot channel exists, a phase mismatching problem between the dedicated pilot channel and the traffic channel does no occur. Thus, Equation (5) can be written as
P_i=P_d=w_m^HR_mS_mw_m  Equation (11)
P_q=0

[0077] Therefore,

[0078] ${\mathrm{SINR}}_m^{\mathrm{FLBF}}$
for forward link beam forming is defined as

[0079] $\begin{array}{cc}{\mathrm{SINR}}_m^{\mathrm{FLBF}}=\frac{{\underset{\_}{w}}_m^H{R}_m{S}_m{\underset{\_}{w}}_m}{\sum _{\underset{i\ne m}{i=1}}^M{\underset{\_}{w}}_m^H{R}_i{S}_m{\underset{\_}{w}}_m+{\underset{\_}{w}}_m^H{R}_{\mathrm{oc}}{S}_m{\underset{\_}{w}}_m}& \text{Equation (12)}\end{array}$

[0080] When the dedicated pilot channel exists, an optimal weight vector for a traffic channel represents a value for maximizing

[0081] $\begin{array}{cc}{\underset{\_}{w}}_m^{\mathrm{opt}}=\mathrm{Principal}\mathrm{Eigenvector}{\mathrm{of}\left(\sum _{i=1}^M{R}_i+R_{\mathrm{oc}}\right)}^{-1}{R}_m& \text{Equation (13)}\end{array}$
Of Equation (12) and is calculated by

[0082] ${\mathrm{SINR}}_m^{\mathrm{FLBF}}$

[0083] As described above, for forward link beam forming according to presence/absence of the dedicated pilot channel, forward link transmission covariance matrixes R_m(m=1,2, . . . ,M) and R_oc, i.e.

[0084] $R_m^{\mathrm{FL}}\left(m=1,2,\dots ,M\right)$
and

[0085] $R_{\mathrm{oc}}^{\mathrm{FL}},$
are required as illustrated in Equation (13). So far, a reverse link (RL) related signal and a forward link (FL) related signal have not been distinguished for purposes of simplicity, since they may not be confused. However, henceforth, a superscription ‘FL’ will be used for a forward link and a superscription ‘RL’ will be used for a reverse link, for signal distinguishment. As stated above, if such information is provided from a mobile station, the information can be used as it is. In contrast, if the information is not provided, a base station must estimate a forward link covariance matrix from a signal received from a mobile station. However, since a reverse link covariance matrix

[0086] $R_m^{\mathrm{RL}}{S}_m^{\mathrm{RL}}$
is obtained from the received reverse link signal, the base station is required to first estimate transmission power

[0087] $S_m^{\mathrm{RL}}$
of a mobile station MS_m and to eliminate the estimated transmission power.

[0088] In the current mobile communication standard, a base station BS has no way to directly receive transmission power

[0089] $S_m^{\mathrm{RL}}$
of a mobile station MS_m. Instead, the base station BS can indirectly estimate transmission power

[0090] $S_m^{\mathrm{RL}}$
of a mobile station MS_m, using a reverse power control bit transmitted to the mobile station MS_m every slot for reverse link power control.

[0091] FIG. 2 is a diagram illustrating an example of estimating reverse link transmission power according to an embodiment of the present invention. Specifically, FIG. 2 illustrates an example of estimating transmission power

[0092] $S_m^{\mathrm{RL}}$
of a mobile station MS_m from a reverse link power control bit. In FIG. 2,

[0093] ${\mathrm{PCB}}_m^{\mathrm{RL}}\left(t_k\right)$
means a reverse link power control bit that a base station BS transmits to an m^th mobile station MS_m every slot f_k(k=1,2, . . . ) As illustrated in FIG. 2, transmission power of a mobile station is increased or decreased in a predetermined ratio according to

[0094] ${\mathrm{PCB}}_m^{\mathrm{RL}}\left(t_k\right).$
The transmission power

[0095] $S_m^{\mathrm{RL}}\left(t_k\right)$
of a mobile station is calculated by

[0096] $\begin{array}{cc}{S}_m^{\mathrm{RL}}\left(t_k\right)=S_0·{10}^{\frac{\mathrm{Increment}\times \sum _{j=1}^k{\mathrm{PCB}}_m^{\mathrm{RL}}\left(t_j\right)}{10}}& \text{Equation (14)}\end{array}$

[0097] In Equation (14), ‘Increment’ represents a transmission power ratio (dB) that increases or decreases according to a reverse link power control bit, and S_o represents initial transmission power. Equation (14) provides a value that the base station BS can calculate, and through this, the base station can estimate transmission power of a mobile station. Equation (14) is given on the assumption that no error occurs during transmission and demodulation of the reverse link power control bit. However, even when an error occurs during transmission of the reverse link power control bit, an actual value can be immediately recovered through feedback of the reverse link power control bit.

[0098] When power

[0099] $S_m^{\mathrm{RL}}$
of a reverse link traffic channel received from an m^th mobile station is estimated, a transmission covariance matrix

[0100] $R_m^{\mathrm{RL}}$
for a reverse link channel from the m^th mobile station can be calculated from the estimated power. In addition, AOA and beamwidth are estimated from the transmission covariance matrix

[0101] $R_m^{\mathrm{RL}}$
for a reverse link channel received from the m^th mobile station. Further, a transmission covariance matrix

[0102] $R_m^{\mathrm{FL}}$
for a forward link channel to an m^th mobile station can be estimated by synthesizing a covariance matrix considering a difference between transmission and reception frequency bands from the estimated AOA and beamwidth.

[0103] Even for a transmission covariance matrix R_oc for mobile stations in other cells, a similar method can be used. However, it can be difficult to individually detect transmission power of mobile stations inside and outside a cell. In this case, it is necessary to previously determine expected average values of a transmission covariance matrix R_oc for mobile stations inside and outside other cells. Generally, since it is assumed that interference from other cells is spatially uniform, the expected average value can be applied without any problem.

[0104] A description has been made of a method of simultaneously considering a phase mismatching problem between a common pilot channel signal and a traffic channel signal and an interference problem for other mobile stations when a dedicated pilot channel is not provided. Another method of calculating a direction of an optimal transmission beam weight vector through forward link beam forming considering an interference problem when the dedicated pilot channel is provided. From now on, a description will be made of a process of calculating magnitude of a transmission beam weight vector using forward link power control (FLPC).

[0105] Equation (15) shows an example of

[0106] ${\mathrm{SINR}}_m^{\mathrm{FLPC}}$
for forward link power control on an m^th mobile station MS_m in a cell.

[0107] $\begin{array}{cc}{\mathrm{SINR}}_m^{\mathrm{FLPC}}=\frac{P_i}{P_q+P_{\mathrm{imp}}+P_p+P_{\mathrm{th}}}& \text{Equation }\left(15\right)\end{array}$

[0108] The

[0109] ${\mathrm{SINR}}_m^{\mathrm{FLPC}}$
is a value that can directly measured by an m^th mobile station MS_m. The

[0110] ${\mathrm{SINR}}_m^{\mathrm{FLPC}}$
for forward link power control can be defined differently according to systems. In the present example, since a forward link power control function provided in an existing CDMA system will be applied as is, a detailed definition of

[0111] ${\mathrm{SINR}}_m^{\mathrm{FLPC}}$
is not important.

[0112] An m^th mobile station MS_m compares a target value of the

[0113] ${\mathrm{SINR}}_m^{\mathrm{FLPC}}$
with its current bit measured value, and determines a forward link power control bit

[0114] ${\mathrm{PCB}}_m^{\mathrm{FL}}$
according to the comparison result. A base station BS receives the forward link power control bit

[0115] ${\mathrm{PCB}}_m^{\mathrm{FL}}$
over a reverse link channel, and determines signal power

[0116] $S_m^{\mathrm{FL}}$
of a forward traffic channel, i.e., magnitude of a weight vector for a transmission beam, according to a value of the received forward link power control bit.

[0117] By calculating a direction of an optimal weight vector of each mobile station through forward link beam forming and independently calculating desired base station transmission power through forward link power control, a smart antenna adopting a forward link beam forming algorithm proposed herein can achieve a desired SINR with minimum base station transmission power, as compared with an omnidirectional antenna or a sector antenna. Thus, it is possible to increase the number of available mobile stations in a cell, contributing to an increase in subscriber capacity, an object of the smart antenna.

[0118] A detailed description will now be made of an embodiment of the present invention with reference to the accompanying drawings.

[0119] FIG. 3 is a detailed block diagram illustrating an example of components of a base station transmission apparatus with an antenna array according to an embodiment of the present invention. As illustrated, the transmission apparatus for a base station includes an antenna array 300, an RF (Radio Frequency) part 310, a transmission beam former 320, a transmission beam controller 330, a common channel signal generator 340, a transmission channel signal generator 350, a reception beam former 360, and a base station modem receiver 370. It is assumed that the base station is currently communicating with M mobile stations in the cell.

[0120] It is assumed that the antenna array 300 is comprised of N identical antenna elements. An antenna array can be classified into a transmission antenna array and a reception antenna array. Here, a description will be made with reference to the transmission antenna array. However, a hardware structure of the antenna array is commonly designed such that it can be jointly used for both transmission and reception by use of a duplexer. The antenna array 300 transmits transmission beams formed by the transmission beam former 320, and provides RF signals received from several mobile stations inside and outside a cell to the RF part 310.

[0121] The RF part 310 is comprised of N RF units corresponding to the N antenna elements of the antenna array 300, and each RF unit is connected to its associated antenna element. Each RF unit, though not illustrated, includes a low-noise amplifier, a frequency down converter, and an analog-to-digital (A/D) converter. The RF part 310 converts RF signals received from mobile stations via the antenna array 300 into a baseband digital reception signal x.

[0122] The reception beam former 360 converts the baseband digital reception signal x output from the RF part 310 into beams z_i(i=1,2, . . . ,M) formed as to mobile stations, and provides the output beams to the base station modem receiver 370. The reception beam former 360 serves as a spatial filter capable of amplifying or eliminating a signal based on a direction of a signal received from each mobile station via the antenna array 300. When a RAKE receiver, not shown for purposes of simplicity, is used to eliminate an interference signal due to multipath fading, the reception beam former 360 can be positioned before or after a demodulator in each finger of the RAKE receiver.

[0123] The base station modem receiver 370 modulates the beams z_i(i=1,2, . . . ,M) output from the reception beam former 360 into voice or data signals of corresponding mobile stations. In addition, the base station modem receiver 370 restores forward link power control bits

[0124] ${\mathrm{PCB}}_i^{\mathrm{FL}}\left(i=1,2,\dots ,M\right)$
transmitted from corresponding mobile stations and measures SINR for the corresponding mobile stations, thereby determining reverse link power control bits

[0125] ${\mathrm{PCB}}_i^{\mathrm{RL}}\left(i=1,2,\dots ,M\right).$
Moreover, the base station modem receiver 370 restores a forward link FER transmitted from a corresponding mobile station by help of an upper layer.

[0126] The transmission beam controller 330 calculates weight vectors for controlling forming of transmission beams, and includes a common beam weight vector calculator 331, a transmission beam weight vector calculator 333, a common beam power calculator 335 and a transmission beam power calculator 337. In embodiments of the present invention, the calculators can be optionally implemented by hardware or software.

[0127] The transmission beam former 320 includes a common beam former 323 for forming a common beam, M transmission beam formers 325 for forming transmission beams for M mobile stations, and N adders 321 for forming M forward transmission beams by adding the common beam to the M transmission beams, and then providing the formed forward transmission beams to the N RF units corresponding thereto.

[0128] A detailed description will now be made of an operation of forming transmission beams by a base station having the structure stated above.

[0129] RF signals received from several mobile stations inside and outside the cell through N antenna elements of the antenna array 300 are converted into baseband digital reception signal x by the RF part 310, and then provided to the reception beam former 360 and the transmission beam weight vector calculator 333. The transmission beam weight vector calculator 333 calculates forward link transmission covariance matrixes

[0130] $R_i^{\mathrm{FL}}\left(i=1,2,\dots ,M\right)$
in the above-stated method by receiving the baseband digital reception signal x from the RF part 310 and the reverse link power control bits

[0131] ${\mathrm{PCB}}_i^{\mathrm{RL}}\left(i=1,2,\dots ,M\right)$
from the base station modem receiver 370. When a dedicated pilot channel is not provided, the forward link transmission covariance matrix is calculated using Equation (10) above. In contrast, when the dedicated pilot channel is provided, the forward link transmission covariance matrix is calculated using Equation (13) above. Therefore, the transmission beam weight vector calculator 333 can be designed to include both of the two calculation methods so that it can optionally use one of the two calculation methods. In an embodiment of the present invention, the transmission beam weight vector calculator 333 can be designed to include only a calculation method corresponding to a particular system. In this method, the transmission beam weight vector calculator 333 calculates optimal transmission beam weight vectors w_i(i=1,2, . . . ,M) for the mobile stations on a real-time basis, and provides the optimal transmission beam weight vectors to the corresponding transmission beam formers 325.

[0132] The transmission beam power calculator 337 calculates transmission beam powers S_i(i=1,2, . . . ,M) for the mobile stations by receiving the forward link power control bits

[0133] ${\mathrm{PCB}}_i^{\mathrm{FL}}\left(i=1,2,\dots ,M\right)$
from the base station modem receiver 370, and provides the calculated transmission beam powers to the transmission channel signal generators 350. Each transmission channel signal generator 350 generates transmission channel signals s_i(i=1,2, . . . ,M) by multiplying the transmission channel signals having a unit magnitude by square roots of the transmission beam powers S_i(i=1,2, . . . ,M), and provides the generated transmission channel signals to the corresponding transmission beam former 325.

[0134] The transmission beam formers 325 form transmission beams by multiplying optimal transmission beam vectors w_i(i=1,2, . . . ,M) for the mobile stations by the transmission channel signals s_i(i=1,2, . . . ,M). The transmission beams formed in this manner are provided to the adders 321 associated with the N antenna elements of the antenna array 300.

[0135] A weight vector for a common beam is determined by the common beam weight vector calculator 331 and the common beam power calculator 335. The common beam weight vector calculator 331 previously calculates a common beam weight vector w_p capable of covering a cell or a sector, and provides the calculated common beam weight vector to the common beam former 323. Meanwhile, the common beam power calculator 335 previously calculates common beam power S_p, and provides the calculated common beam power to the common channel signal generator 340. The common channel signal generator 340 generates a common channel signal s_p by multiplying a common channel signal having a unit magnitude by a square root of the common beam power S_p, and provides the generated common channel signal to the common beam former 323.

[0136] The common beam former 323 multiplies the common channel signal s_p by the common beam weight vector w_p, and provides its output to the adders 321 associated with the N antenna elements of the antenna array 300.

[0137] The adders 321 form base station transmission signal vectors s(t) by summing up outputs of the common beam former 323 and the transmission beam formers 325, and provide their outputs to the corresponding RF units of the RF part 310. The transmission signal vectors s(t) from the base station to the mobile stations are converted into RF signals by the RF units 310 after being power-amplified through a D/A converter, a frequency up converter and a power amplifier, and then transmitted to the mobile stations in the cell over forward link channels through the antenna array 300.

[0138] FIG. 4 is a detailed block diagram illustrating an example of components of the transmission beam weight vector calculator 333 shown in FIG. 3. Specifically, FIG. 4 illustrates an apparatus and method for estimating forward link covariance matrixes for M mobile stations MS_i(i=1,2, . . . ,M) from a reverse link base station reception signal vector x. Although the forward link covariance matrix can be estimated from the reverse link base station reception signal vector as mentioned above, it can also be directly fed back from the mobile stations or can be calculated using other methods. In addition, it should be appreciated by those skilled in the art that although the transmission beam weight vector calculator 333 is realized by hardware in FIG. 4, it can also be implemented by software without departing from the scope of the present invention.

[0139] Referring to FIG. 4, the transmission beam weight vector calculator 333 includes M forward link covariance matrix calculators 400 associated with M mobile stations MS_i(i=1,2, . . . ,M), M optimal weight vector calculators 420, and an other cell covariance matrix calculator 410. Each forward link covariance matrix calculator 400 is comprised of a reverse link covariance matrix estimator 401, an AOA estimator 403, a beamwidth estimator 405, and a forward link covariance matrix synthesizer 407.

[0140] The reverse link covariance matrix estimator 401 first calculates a covariance matrix

[0141] $R_i^{\mathrm{RL}}{S}_i^{\mathrm{RL}}$
for an i^th mobile station MS_i by receiving the baseband digital reception signal vector x. Further, the reverse link covariance matrix estimator 401 estimates mobile station transmission power

[0142] $S_i^{\mathrm{RL}}$
using a reverse link power control bit

[0143] ${\mathrm{PCB}}_i^{\mathrm{RL}},$
and then calculates a reverse link transmission covariance matrix

[0144] $R_i^{\mathrm{RL}}$
of an i^th mobile station from the estimated mobile station transmission power. The reverse link covariance matrix estimator 401 provides the calculated reverse link covariance matrix to the AOA estimator 403 and the beamwidth estimator 405. The AOA estimator 403 and the beamwidth estimator 405 calculate an AOA estimation value AOA_i and a beamwidth estimation value BW_i for a corresponding mobile station from the reverse link covariance matrix estimation value

[0145] $R_i^{\mathrm{RL}},$
and provide the calculated AOA_i and BW_i to the forward link covariance matrix synthesizer 407. In an abnormal state where FER is increased abruptly for some reason, the beamwidth estimator 405 detects the abrupt increase in the FER from a forward link

[0146] ${\mathrm{FER}}_i^{\mathrm{FL}}$
received from a mobile station and then increases or decreases the beamwidth by a predetermined value, thereby appropriately coping with the abnormal state.

[0147] The forward link covariance matrix synthesizer 407 synthesizes a forward link covariance matrix estimation value

[0148] $R_i^{\mathrm{FL}}$
from the AOA estimation value AOA_i and the beamwidth estimation value BW_i, and provides the synthesized forward link covariance matrix estimation value to the optimal weight vector calculator 420. In an FDD (Frequency Division Duplexing) system where a transmission frequency band is different from a reception frequency band, a difference between the transmission frequency and the reception frequency is compensated for in the forward link covariance matrix synthesizer 407.

[0149] The other cell covariance matrix calculator 410 calculates a covariance matrix estimation value R_oc for interference to mobile stations in other cells due to a transmission channel signal for an i^th mobile station MS_i, by receiving the baseband digital reception signal vector x, and providing the calculated covariance matrix estimation value to the optimal weight vector calculator 420. Estimation of the reverse link covariance matrix and the other cell covariance matrix can be performed using a known technique. For example, a method disclosed in “Performance Analysis of CDMA Mobile Communication Systems using Antenna Arrays”, B. Suard, A. Naguib, G, Xu, A. Paulraj, Proc. ICASSP, 1993, which is incorporated herein by reference, can be used.

[0150] The optimal weight vector calculator 420 calculates an optimal weight vector w_i in accordance with Equation (10) or Equation (11) by receiving a forward link covariance matrix estimation value R_i for an i^th mobile station MS_i from the forward link covariance matrix calculator 400, an estimation value R_oc from the other cell covariance matrix calculator 410, and a common beam weight vector w_p from the common beam weight vector calculator 331, and then provides the calculated optimal weight vector to the transmission beam former 325.

[0151] FIG. 5 is a diagram illustrating an example of the transmission beam former 325 for an m^th mobile station MS_m according to an embodiment of the present invention. If it is assumed that the antenna array of the base station includes N antenna elements, the transmission beam former 325 includes N complex multipliers 510 associated with the antenna elements. A transmission beam weight vector w_m for an m^th mobile station MS_m is divided into N elements associated with the antenna elements, and then applied to the corresponding complex multipliers 510. It is well known that the w_m can be represented by w_m=[w_m,1w_m,2 . . . w_m,N−1w_m,N]^T. The complex multipliers 510 complex-multiply a traffic channel signal s_m(f) for an m^th mobile station MS_m by the elements of the weight vector w_m, and provide their outputs to the corresponding adders 321.

[0152] Although the transmission beam former 325 for an m^th mobile station MS_m is illustrated in FIG. 5, transmission beam formers for other mobile stations also have the same structure. Also, common beam formers for other mobile stations have the same structure as the common beam former 323.

[0153] FIG. 6 is a graph illustrating a forward link transmission beam pattern according to the prior art in terms of magnitude and degree, and FIG. 7 is a graph illustrating a forward link transmission beam pattern according to an embodiment of the present invention in terms of magnitude and degree. FIGS. 6 and 7 are based on the assumption that the number of forward link transmission beams is 2 in a base station's linear antenna array that has 4 antenna elements and a gap between the antenna elements is half a wavelength. Specifically, FIG. 6 illustrates a forward link transmission beam pattern formed by considering only AOA and beamwidth from a signal received at the base station from the mobile station, and FIG. 7 illustrates a forward link transmission beam pattern formed according to an embodiment of the present invention. It is further assumed in FIGS. 6 and 7 that forward link transmission beams 610 and 710 shown by solid lines are for AOA=−40° and beamwidth=20°, and forward link transmission beams 620 and 720 also shown by solid lines are for AOA=−0° and beamwidth=20°.

[0154] It can be understood that compared with the forward link transmission beam pattern illustrated in FIG. 6, the forward link transmission beam pattern illustrated in FIG. 7 has interference reduced by about 3 to 4 dB within a non-transmission angle range. That is, when a forward link transmission beam is formed according to the disclosed embodiments of the present invention, it is possible to increase the amount of subscriber capacity by the same amount as the reduction in the interference signal.

[0155] As described above, the disclosed embodiments of the present invention can form an optimal transmission beam for minimizing phase mismatching between a common beam and a transmission beam and also minimize interference to other mobile stations due to the transmission beam. That is, the invention can achieve high performance forward link transmission, and contribute to an increase in bandwidth capacity of a mobile communication system, improvement in call quality, and a reduction in transmission power of a mobile station.

[0156] While the invention has been shown and described with reference to certain embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.