HK1100794B - Method and apparatus for multi-beam antenna system - Google Patents

Description

Method and apparatus for multi-beam antenna system

Background

The present invention relates generally to wireless communication nodes, and more particularly to wireless communication nodes utilizing a multi-beam antenna system.

Adaptive antenna arrays have been successfully used in various cellular communication systems, such as the GSM system. An adaptive antenna array replaces a conventional sector antenna with two or more antenna elements in close spatial proximity. The antenna array directs a narrow beam of radiated energy to a particular mobile user to minimize interference to other users. Adaptive antenna arrays have been shown to greatly improve performance in GSM and TDMA systems, measured in terms of increased system capacity and/or increased range, compared to ordinary sector coverage antennas.

Adaptive antenna systems can be divided into two categories: a fixed beam system in which radiated energy is directed into a plurality of fixed directions; and a steered-beam (stepped-beam) system in which the radiated energy is directed to any desired location. Both types of narrow beam systems are shown generally in fig. 2, and also show a fan beam covering a fan cell. Advantages of the adaptive antenna system include: efficient utilization of spectral resources by using spatial (angular) separation of users, cost efficiency, increased range or capacity and easy integration, i.e. no mobile terminal modifications are required, as would be required in other schemes such as Multiple Input Multiple Output (MIMO) schemes employing multiple antennas both at the terminal and at the base station.

The fixed beam may be generated at baseband frequency or Radio Frequency (RF). Baseband generation requires calibration components that estimate and compensate for any signal distortion present in the signal path from baseband through Intermediate Frequencies (IF) and RF up to each antenna element in the array. The RF method generates a fixed beam at radio frequency using, for example, a Butler (Butler) matrix.

Under some assumptions, such as a uniform linear array in which the antenna elements are separated by a half wavelength, there is a one-to-one correspondence between a certain direction of arrival (DOA) of the incoming wavefront and the phase shift of the signal output at the antenna elements. By appropriately phase shifting the signal prior to transmission (or reception), the adaptive antenna system can direct radiated energy to (or from) a desired mobile user while minimizing interference to other mobile users. Steered beams require calibration to estimate and compensate for any signal distortion present in the signal path from baseband to the radio and vice versa.

Time-varying multipath fading severely degrades the quality of received signals in many wireless communication environments. One way to mitigate the effects of deep fading and provide reliable communication is to introduce redundancy (diversity) in the transmitted signal. The added redundancy may be in the temporal or spatial domain. Time diversity is implemented using channel coding and interleaving. Spatial diversity is achieved by transmitting signals on spatially separated antennas or using antennas of different polarizations. Such a strategy ensures independent fading on each antenna. Spatial transmit diversity can be subdivided into closed-loop or open-loop transmit diversity modes depending on whether feedback information is transmitted from the receiver back to the transmitter.

In adaptive antenna systems, user-specific data signals are transmitted using narrower beams (fixed or steerable). System-specific or common signals are typically transmitted via another antenna, such as a sector antenna, having a wider coverage beam. A typical common signal is a base station (primary) pilot signal. The pilot signal comprises a known data sequence that each mobile radio uses to estimate the radio propagation channel. The radio propagation channel also changes as the mobile station moves. The pilot signal is used as a "phase reference" because good channel estimation is necessary to detect user-specific data. Beam-specific auxiliary pilot signals may be present on each beam and may also be used as phase references. Mobile users transmitting signals using the same beam then use the same secondary pilot signal. Alternatively, the mobile station specific pilot signal may be transmitted using the same beam as the user specific signal and may be used as a phase reference. The network indicates which phase reference the mobile user should use.

There are several drawbacks to current multi-beam architectures. The first drawback is cost. Fixed beam antenna arrays that form narrow beams at radio frequencies may require additional sector coverage antennas to be implemented. Hardware complexity and cost are related to: the number of feeder cables (for a sector coverage antenna) equal to the number of beams plus 1, the physical weight determined by the antenna size, and the height and size of the mast. Different sector and narrow beam antennas add significant cost to the base station.

A second drawback is associated with phase reference mismatch and quality of service (QoS) degradation. The radio channel of the main pilot signal transmitted by the sector coverage antenna and the radio channel of the user-specific data transmitted by the narrow beam do not have to be the same. If the mobile station is instructed to use the primary pilot signal as a phase reference, the mobile station will expect that the user-specific data uses the same radio channel as the primary pilot signal. But those channels are different. Therefore, phase reference errors, detection and decoding errors increase, and quality of service (QoS) decreases.

A third drawback is poor resource utilization. To compensate for phase reference mismatch, the mobile station may be instructed to use beam-specific secondary pilot signals or user-specific dedicated pilot signals as the phase reference. In the former case, all users within the same beam use the same pilot signal, while in the latter case each user utilizes a unique pilot signal. QoS is improved but additional allocated resources (e.g., power, codes, etc.) are required. As a result, less power is available to other mobile users, adversely affecting system capacity and data throughput.

Further drawbacks relate to inflexibility and signalling delays. It is assumed that the mobile station may receive better signals from each beam's alternate secondary pilot. The network must therefore regularly investigate which secondary pilot is most appropriate, i.e. received at maximum power. The network must signal the antenna system and the mobile radio to report back several measurement reports. If the network determines that user-specific data should be transmitted using a new beam, the antenna system is instructed to change beams and the mobile radio will be signaled to start using the alternative secondary pilot channel as a phase reference. Such procedures result in delays and require considerable signaling overhead.

Receiver diversity is widely used in today's wireless infrastructure and it provides many advantages in terms of uplink coverage and capacity. In addition, transmit diversity can be used to improve downlink performance, and it can become an important feature in third generation wireless systems. However, even if the intended mobile user is located in a certain direction, the transmit diversity signal is transmitted throughout the cell, resulting in increased interference to other users. However, transmit diversity combined with narrower directional beams may provide considerable advantages.

The above identified deficiencies of current multi-beam architectures are overcome by an antenna system comprising an antenna array for transmitting common signals in a wider beam covering a sector cell and mobile user specific signals in a narrower beam covering only a portion of the sector cell. The transmit circuit is coupled to the antenna array and the filter circuit. In a first, "mixed beam" embodiment, the filtering circuitry filters the user-specific signal and the common signal to compensate for distortion associated with their conversion from baseband frequency to radio frequency. The filtering circuitry and beam weighting circuitry ensure that the user-specific and common signals are approximately time-aligned and in phase at the antenna array, preferably at the center antenna element. The user-specific signal weights (weights) are designed to radiate a narrower beam (as compared to the wide fan-shaped coverage beam) in the direction of the mobile stations so that each mobile station can use the same common signal as a phase reference for channel estimation and demodulation.

In a second, "steered beam" embodiment, the filtering circuitry filters the user-specific signal and the common signal to compensate for distortion associated with their conversion from baseband frequency to radio frequency. The filtering circuit and beam weighting circuit ensure that the user specific signal and the common signal are time aligned and have a controlled phase difference when received by each mobile user in the cell. Each mobile user may use the common signal as a phase reference for channel estimation and demodulation. The phase difference is preferably controlled to obtain a good compromise between the desired transmit power, radiated interference and quality of service to the user. The beamforming weights are used not only to radiate a narrower beam to the desired mobile user (as in the mixed beam embodiment), but also to direct a wider common signal beam to reach all mobile users in the cell.

In one example steered beam implementation, the wide beam carrying the common signal is transmitted only from the center antenna element in the antenna array. The use of a central antenna element to generate a wide common beam allows the correlation of the controlled phase difference between the common signal received by the mobile users and the user-specific signal to be less than or equal to a target value that ensures the desired quality of service. Alternatively, a wide beam carrying a common signal may be generated using multiple antenna elements in an antenna array. Since the antenna elements are typically fixed at a predetermined "look direction" during antenna array installation, all antenna elements may be used in conjunction with baseband signal processing to form a wide beam with desired characteristics that may change over time according to the cell plan. The beamforming weights applied to the user-specific signals result in a narrower beam being directed from the antenna array to the mobile user. Such beam steering is provided for user-specific signal beams and common signal beams, which allows for more intelligent steering of both signal types in a cell.

In a more detailed non-limiting example of the mixed beam embodiment, the antenna array includes N antenna elements, where N is an odd positive integer greater than 1. A beamforming network is coupled between the antenna array and the transmit circuitry. The beam forming network receives the user-specific signal and the common signal in each beam and generates N signals that are provided to the antenna array. Each signal passes through a beam specific transmit filter circuit before the beam forming network receives the N signals. The beam transmit filter cancels the common signal in all outputs of the beam forming network except the output of the central antenna element. But transmits a common signal on the N beams simultaneously with equal or approximately equal power and phase.

The beam weighting circuit weights the user-specific signals using the beam weights corresponding to each beam and provides the weighted user-specific signals to the corresponding beam transmit filters. Each user-specific beam weight may be a function of the average power of the uplink received in the corresponding beam. One example function is the square root. The user-specific beam weights are selected to direct radiated energy from the antenna array in a relatively narrow beam to a desired mobile user.

The receive circuit is coupled to the beamforming network and the signal processor. The signal processor combines the signals received on the N beams to estimate the received signals and determines an average uplink power for each beam. Those average uplink powers are used to determine user-specific beam weights. The mixed beam embodiment may be implemented in a transmit diversity branch and/or a receive diversity branch.

In a more detailed example of the steered beam embodiment, the antenna array includes N antenna elements, where N is a positive integer-odd or even. The filtering circuit includes N antenna transmit filters, and each antenna transmit filter is associated with a corresponding antenna element. The common signal and the user-specific signal may be transmitted simultaneously from all N antenna elements. The user-specific signals are transmitted using N user-specific beam weights, each user-specific beam weight corresponding to one of the N antenna elements. The beam weights are complex numbers used to phase rotate and amplify the user-specific signals. The common signal is transmitted using N common signal beam weights, each common signal beam weight corresponding to one of the N antenna elements. These beam weights may also be complex numbers used to phase rotate and amplify the common signal. Alternatively, the common signal may be transmitted from only one antenna, such as a central antenna unit. In this case, the beam weights for the other antenna elements may be set to zero.

In the steered beam embodiment, user-specific signal beamforming weights and common signal beamforming weights are determined (1) to produce high antenna gain to reduce generated interference, and (2) to maintain phase differences between the user-specific signals and the common signals at an acceptable level. The common signal is a phase reference signal for all mobile stations in the cell and the controlled phase difference between the common signal and the user-specific signal can be considered random, the distribution of which is affected by the statistics of the channel and the transmitter weights used.

At the receiving end of the steered beam embodiment antenna system, a beam forming network (not required at the transmitting end of the steered beam embodiment) may be coupled to the N antenna elements for generating the N receive beams. The receive circuit is coupled to the beamforming network and the signal processor. The signal processor processes the signals received on the N receive beams to estimate the received signals. The signal processor determines uplink channel statistics for each user and predicts corresponding downlink channel statistics. The steered beam embodiment may also be used in transmit and/or receive diversity branches.

The present invention provides a number of advantages. First, common signals and user-specific signals can be transmitted without separate sector antennas. Second, no auxiliary or dedicated pilot signal is required as a phase reference. Third, the common signal and the user-specific signal are transmitted without distortion due to traversal/processing from the baseband output to the antenna elements. Fourth, the common and user-specific signals are approximately in phase (in the mixed beam case) or affected by some controlled random variation (in the controlled beam case) and time aligned when received by the mobile terminal, i.e., affected by approximately the same channel delay profile. Fifth, interference to spatially separated mobile users is suppressed because the antenna array radiates user-specific channels in a narrower beam directed to the desired mobile user. Sixth, combining beamforming with transmit diversity or transmit/receive diversity provides considerable advantages. A seventh advantage is transparency. The mobile user does not need to be aware of the architecture or implementation of the antenna array. Eighth, backward compatibility allows for rapid system integration. No changes to the radio network controller in the radio network are required. Finally, the present invention may be used in any wireless system that may utilize downlink beamforming.

Brief Description of Drawings

Figure 1 shows an adaptive antenna system transmitting in a sectored cell;

figure 2 shows a cellular network with a base station transmitting a fan beam, a base station transmitting multiple beams and a base station transmitting a steerable beam;

FIG. 3 illustrates a cellular communication system;

fig. 4 shows an antenna system according to a hybrid beam exemplary embodiment;

figures 5A-5D illustrate beam patterns for synthesizing a fan coverage beam and a narrow beam and the relative phase offset between the synthesized fan beam and the narrow beam as a function of direction of arrival;

6A-6B illustrate relative phase offsets between a received common signal and a received user-specific signal as a function of mobile station direction;

fig. 7 shows an antenna system according to an exemplary embodiment of a steered beam;

fig. 8 shows an antenna system according to a special case of a steered beam example embodiment;

9A-9B illustrate the performance of an exemplary embodiment of mixed and steered beams;

figure 10 shows an exemplary hybrid beam diversity embodiment; and

fig. 11 illustrates an exemplary steered beam diversity embodiment.

Detailed Description

The following description is intended to be illustrative rather than restrictive, and specific details are set forth in order to provide an understanding of the present invention. It will be understood by those skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details. In other instances, detailed descriptions of well-known methods, devices, techniques, etc. are omitted so as not to obscure the description with unnecessary detail. Individual functional blocks are shown in one or more of the figures. Those skilled in the art will appreciate that functions may be implemented using discrete components or multi-functional hardware. The processing functions may be implemented using a programmed microprocessor or general purpose computer, using one or more Application Specific Integrated Circuits (ASICs), and/or using one or more Digital Signal Processors (DSPs).

The present invention relates to a multi-beam antenna system. A non-limiting example of a multi-beam antenna system is an adaptive array antenna, such as the array shown in fig. 1, where fig. 1 shows an example narrow antenna beam transmitted from the adaptive antenna, surrounding a relatively narrow region in the sectorized cell where the desired mobile station is located. The narrow beam interferes less with other mobile stations and neighboring cells because the sidelobes are relatively low. In addition, the intended mobile radio is more likely to receive the desired transmission with a higher signal-to-noise ratio using the directional narrow beams shown in fig. 1.

Figure 2 shows a cellular network having a base station transmitting a fan beam in one sector cell, a base station transmitting a fixed multi-beam antenna pattern in another sector cell, and a base station transmitting a steerable beam in a third sector cell. Fig. 1 and 2 show how adaptive antennas spread less interference in the downlink direction and suppress spatial interference in the uplink direction. This improves the signal-to-interference ratio in both the uplink and downlink directions and thus improves overall system performance.

An exemplary cellular system 1 in which the present invention may be employed is shown in fig. 3. A Radio Network Controller (RNC) Base Station Controller (BSC)4 is coupled to a plurality of base stations 8 and other networks represented by cloud 2. Each illustrated base station BS1 and BS2 serves a plurality of sectored cells. Base station BS1 serves sectored cells S1, S2 and S3, and base station BS2 serves sectored cells S4, S5 and S6.

An antenna system according to a mixed beam non-limiting example embodiment will now be described in connection with fig. 4. The antenna system 10 includes an antenna array 12 having a plurality of antenna elements 14. The antenna array 12 includes an odd integer number N of antennas denoted a₁、A₂、......、A_NThe antenna unit of (1). In the example of fig. 4, N ═ 3. A single Beam Forming Network (BFN)16 generates N narrow beams. The same beam is used for both uplink and downlink. The beamforming network is a multiple-input multiple-output port device. Each beamforming network port corresponds to one of the narrow beams of the multi-beam antenna system. The beamforming network may include active or passive components. With passive components, the beam is designed and held stationary during the manufacturing process. For active components, the beams may be adaptively controlled. Operate in the Radio Frequency (RF) range, from uniformityA well-known, suitable passive beam forming network that produces multiple narrow beams by an array of spaced antenna elements is a butler matrix.

The beamforming network in fig. 4 operates in both transmit and receive directions. The signal to be transmitted is connected to one of the input ports of the beam forming network 16, which then directs and transmits the signal over all antenna elements. Each signal assigned to a certain antenna element undergoes a certain phase rotation depending on the selected input port. The overall result is that a main lobe or beam is generated in a certain direction. When using an alternative beam port, the beam appears in the other direction. In short, the output of the antenna elements is a shaped beam.

Each beam input to the beam forming network is coupled to a corresponding duplex filter (Dx) 18. The duplex filter 18 provides a high degree of isolation between the transmitter and receiver and allows one antenna to be used for uplink reception and downlink transmission. Each beam also has a corresponding transmitter (Tx)20 coupled to a corresponding duplex filter 18. The transmitter 20 typically includes a power amplifier, a frequency up-converter, and other well-known elements. Each duplex filter 18 is also coupled to a corresponding receiver (Rx) 22. Each receiver 22 typically includes a low noise amplifier, an intermediate frequency down-converter, a baseband down-converter, an analog-to-digital converter, and other well-known elements. The output from the receiver 22 is provided to a signal processor 32 which decodes the received signal from the mobile user and generates a signal shown as d^ULTo output of (c). The signal processor 32 also generates N beam weights (w) that are applied to the user-specific signals as indicated by weighting block 28_n)。

Is shown as d^DLIs input to a weighting block 28, which includes means for associating user-specific signals with corresponding beam weights w_nN multipliers 30 for multiplication. Common signal c^DLSplit by signal splitter 29 into N copies of the common signal, but not weighted in this example. Each weighted user-specific signal and common signal are summed in a corresponding summer 26, wherein each summer 26 is associated with one of the beams. Each additionThe output of the device 26 is forwarded to a beam filter (F)_n)24, each beam having its own beam filter 24. The output of each beam filter 24 is then provided to its corresponding transmitter 20.

From the central unit a in this example embodiment₂The beam generated by one of the antenna elements will be a wide beam. When two or more antenna elements are used in the antenna array, the generated beam may be narrower. In contrast to conventional fixed beam systems, where a single uplink beam with the strongest average received power is used to transmit user-specific signals in the downlink, user-specific signals are transmitted in the downlink on all beams.

One of the advantages of the mixed beam embodiment is that the user-specific and common signals are (1) approximately in-phase and time-aligned when they are received at (2) the central antenna element in the base station antenna array and at each mobile user. The primary common pilot signal is an example common signal that is typically used for measurement and as a phase reference and therefore is typically transmitted over the entire sector cell. The pilot signal comprises a known data sequence which is used by each mobile station to estimate the radio propagation channel. The radio propagation channel also changes as the mobile station moves. Despite the channel change, the mobile station needs accurate radio channel estimation (determined from the received common signal) to detect and decode the user-specific data transmitted in the narrower beam.

Common signals such as primary common pilots, pages, etc. are transmitted at equal power on all beams simultaneously. The common signal is split by a splitter 29 and applied to each beam path via a corresponding summer 26 to the associated beam specific transmit filter 24. In one example of a mixed beam embodiment, each filter 24 is designed such that the common signal is transmitted only by the center antenna element 14 of the antenna array 12. In one example implementation, except in this case antenna A₂The filter 24 may cancel the common signal in all outputs of the beam shaping network 16 in addition to the output of the central antenna. Each beam specific transmit filter 24 compensation starts from the baseband frequency and continuesDistortion in the radio chain to the output of the beamforming network 16. The transmit filter 24 is designed to ensure that the user-specific and common signals are at the central antenna element a₂In phase and time aligned.

Unlike the common signal transmitted with equal power on all downlink beams in this embodiment, the user-specific beam weights w applied to each downlink beam are used_nThe user-specific signals are weighted. Each user-specific transmission w applied to a downlink beam n_nIs selected to be the uplink average received power p_nAs a function of (c). Examples of such functions are 1, 2, 1When positive real numbers are represented as follows:

equation 1:

here, p₁、p₂And p₃Representing the average uplink power on beams 1, 2 and 3, respectively. The average uplink power depends on the radio channel statistics and antenna array design. It can be assumed that the average downlink power is about the same as the average uplink power. As an example, the beam weights are chosen to be proportional to the square root of the received energy,and β 1/2.

The signals received from all beams in the uplink direction via the beam forming network 16, the duplexer 18 and the receiver 22 are combined in a signal processor 32 to produce a decoded uplink signal d^ULIs estimated. In addition, the average uplink work per beamRate p_nMeasured and used by the signal processor 32 to calculate beam-specific weights w according to the equation above_n. The average uplink beam power provides information about the average angle of arrival and scattering in the radio environment of the desired input signal. The average direction of arrival is approximately equal to the average direction of departure of the desired signal.

This example of a mixed beam embodiment ensures that a common signal is transmitted on the center wide coverage antenna element of the antenna array 12 and that user-specific signals are transmitted from all antenna elements 14 in the antenna array 12. Beam specific weights w_nThe radiated energy is directed toward the desired user via a narrower directional beam, which limits the interference caused by the beam to other mobile users. A separate sector antenna is not required. There is no need to transmit a separate secondary pilot signal on each beam. And no pilot is needed on the dedicated channel.

To illustrate the advantages of the mixed beam embodiment of fig. 4, the graphs in fig. 5A-5D compare the relative antenna gain and phase offset between the fan-shaped coverage beam and one of the fixed narrow beams as a function of direction of arrival. As outlined below, fig. 5A and 5B employ non-optimized random beam weights to transmit common signals: Martinex-Munoz, "Nortel network CDMA Advantage of AABS Smart Antenna Technology" ("Nortel Networks CDMA Advantages of AAB S Smart Antenna Technology", The CDG Technology Forum, 2002, 10/1), The contents of which are incorporated herein by reference. Fig. 5C and 5D employ beam specific transmit filters 24 tuned in accordance with the present invention so that the common signal is transmitted only from the central antenna. The relative phase offset is measured near the antenna array and not at the mobile user location.

For a sector cell, the relative phase offset between the user-specific signal transmitted in the best beam and the common signal is zero over the entire range of angles of arrival. For non-optimized beam weights, the relative phase offset and amplitude vary greatly with angle of arrival. Thus, in the simple case of no angular spread (angular spread), the mixed beam embodiment provides a smooth and stable fan-shaped coverage beam and phase alignment between the common signal and the user-specific signal. With the mixed beam embodiment, the common channel can be used for channel estimation without quality degradation due to phase offset. On the other hand, the embodiment solution random beam weights will suffer from a degradation due to larger phase offset variations.

Fig. 6A and 6B show the mean and standard deviation of the relative phase offsets between the user-specific signal and the common signal for angular spreads of 5 degrees and 10 degrees from the mobile terminal. Signals are transmitted using the mixed beam example embodiment of fig. 4. The beam weights are selected according to equation 1 above,and β 1/2. The average value of the phase offset is zero regardless of the angular spread and the standard deviation is small, resulting in only a moderate performance degradation for all mobile terminals in the sector cell when the common channel is used as the phase reference for channel estimation.

A second, non-limiting exemplary embodiment, referred to below as a steered beam embodiment, is now described in conjunction with the antenna system 40 shown in fig. 7. Like reference numerals refer to like elements throughout. By selecting beam forming weights w₁-w₃(user-specific) and v₁-v₃The (common) is an arbitrary complex number, weighting both the user-specific signal and the common signal, the resulting beam pattern of the user-specific signal and the common signal can be controlled in any direction, which is more flexible than the mixed beam embodiment. The antenna array 12 may include an even or odd number N of antenna elements 14. Thus, the three antenna elements A1-A3 shown are examples only.

In the transmit direction, the beamforming network 16 in steered beam embodiment 40 is not necessary. Accordingly, the beamforming network 16 is disposed between the duplexer 18 and the receiver 22 and is used to form a receive beam B that is processed by the receiver 22 and the signal processor 42₁、B₂And B₃. The signals output by the transmitters 20 are provided to their respective duplexers 18Without the need for beam forming network 16 processing. The beam forming network 16 is optional for receiving mobile user signals in a steered beam embodiment.

Each antenna a is compared to the mixed beam embodiment_nTransmitting filter (F) directly specific to the corresponding antenna_n)24 are associated. The signal to be transmitted on the nth antenna element is assigned to pass through the nth filter (F)_n)24. The antenna-specific transmit filter 24 is designed so that the common signal and the user-specific baseband signal arrive at each antenna without the gain, phase and timing distortions that would otherwise result from baseband-to-RF conversion. The filtering circuit, together with the beamforming weights for the user-specific signals, also ensures that the user-specific signals and the common signal are time-aligned and have a controlled phase difference when received by each mobile user in the cell. This allows each mobile user to use the common signal as a phase reference for channel estimation and demodulation. Keeping in mind that the signals received by the mobile stations in the mixed beam embodiment are approximately in phase. In the steered beam embodiment, the phase error or difference between the user-specific signal received at each mobile station and the common signal is controlled to obtain a good compromise between required transmit power, radiated interference and quality of service to the user.

The phase difference effect in the steered beam embodiment depends on noise and interference in both the channel estimate and the user-specific signal to be demodulated. From a system perspective, it may not be meaningful to minimize the phase difference if the effects of noise and interference have a decisive effect on how the user-specific signal is demodulated and decoded at the mobile terminal. Thus, filter and beamforming weight optimization may take into account the effects of noise and interference and expected operating conditions. One example beam weight optimization scheme selects user-specific beam weights such that the correlation between the resulting channels is real (real), so that its magnitude, which is affected by the norm constraint on the weight vector, is maximized. A more sophisticated approach is to minimize the norm of the beam weight vector while ensuring that the correlation coefficient is equal to (or greater than) some target value. The noise and interference levels may be estimated, set as planning parameters, or considered as variables that are adjustable while operating the system.

A common signal may be transmitted on all antenna elements. In the particular case shown in fig. 8, they may alternatively be transmitted only on the central wireless unit. This may be done, for example, by weighting the common signal beam v₁And v₃Set to zero. In this particular case, via the common signal c^DLTo the central antenna element a by a corresponding adder 26₂Common signal c^DLOnly one of the paths to the antenna elements is provided. In the steered beam implementation of fig. 7 and 8, user-specific signals are transmitted on all antenna elements and corresponding user-specific beam weights w are used_nIt is weighted.

Beamforming weights w_nAnd v_nFor example, may be complex numbers that are used to phase rotate and amplify their respective user-specific or common signals. Each mobile user has its own beam weight w_nAnd (4) collecting. From the signals received in the uplink, the signal processor estimates the direction and channel statistics of the mobile users in the cell and from this information decides a wide beam shape to be used in the downlink to ensure that all mobile users in the cell receive a common signal with satisfactory signal strength. The wider beam shape depends on the beam weights v_n. Various methods for designing the beam shape are well known to those skilled in the art. See, for example, "smart antennas for wireless communications: IS-95 and Third Generation CDMA applications "(Smartantenna for Wireless Communications: IS-95 and Third Generation CDMAapplications, J.C.Liberti and T.S.Rappaport, Rentice Hall PTR, 1999). Finally, the beamforming beam weights w_nAnd v_nUser-specific signals are allowed to be directed specifically to mobile users and a common signal is transmitted to all users in the cell.

These beam weights are preferably optimized so that the antenna array gain is maximized, the interference spread is minimized, and the common signal can be used as a phase reference by all mobile users in the cell. Can be used forSelecting beam weights w_nN ═ 1, 2, · N, N and v_nN-1, 2, 1.... and N, such that the correlation between the channels experienced by the user-specific and common signals is real and the magnitude of the correlation, which is affected by the norm constraint on the weights, is maximized. This exemplary scheme is set forth in equation (9) below.

Another beamforming weight optimization technique is to maximize the gain of the antenna array, which can be viewed as minimizing the generated interference using constraints on the phase difference at the mobile station between the mobile station's received common signal and the user-specific signal. Equation (13) below describes this optimization problem. The signal processor 42 predicts the phase error at the mobile station based on a statistical model of the downlink channel based on the channel covariance matrix given in equation (7) below, the beam weights for the common signals, and possibly other feedback from the mobile station, such as the block error rate (BLER), noise level, and interference level, as determined by the mobile station feedback or base station measurements.

The graphs in fig. 9A and 9B show the performance of the mixed beam and steered beam example embodiments at an angular spread of 5 degrees. In fig. 9A, assuming an antenna array with three antenna elements, the antenna gains are shown for both the mixed and steered beam embodiments relative to a sector antenna. The antenna gain for the steered beam embodiment is nearly constant across the sector cell and is as high or much higher than the gain of the mixed beam embodiment. Fig. 9B shows the relative phase offset between the received common signal and the user-specific signal at the mobile station. The standard deviation of the phase difference is generally smoother and lower than for the mixed beam embodiment. The steered beam embodiment thus provides as good and in most cases better performance than the mixed beam embodiment.

Two detailed example schemes for optimizing beamforming weights for steered beam embodiments will now be described. Of course, other weight optimization schemes may be employed.

Let 2N +1 denote the number of antenna elements in the uniform linear antenna array. For simple liftingSee, odd numbers of antenna elements are considered for ease of labeling, but the scheme and optimization is not limited to this case. Two adjacent cells are separated by a half wavelength denoted as λ/2. Common signal r_cAnd a user-specific signal r_dThe experienced channel is modeled as:

equation 2: r is_c＝v^Hh

Equation 3: r is_d＝w^Hh

Where v and w are column vectors containing transmit antenna weights for the common signal and user-specific signal, respectively. The signal from the multiple transmit antennas to the mobile station is denoted as h. In particular, h is modeled as

Equation 4:

wherein, P, theta_pAnd alpha_pRespectively representing the number of propagation paths, the angle of arrival (or departure) of the p-th path, and the complex path gain of the p-th path. From at theta_pThe antenna array response of an incident wave is given by the following equation

Equation 5:

suppose that: angle of arrival theta_pIs a random variable with an average value theta that is independent and uniformly distributed (i.i.d.)₀Sum variance σ_θ ². Suppose f (θ)_p|θ₀，σ_θ ²) Denotes theta_pProbability density function (pdf). The pdf of θ is generally assumed to be gaussian, single-valued, or laplacian. Complex path gain alpha_pIs a complex Gaussian random variable with zero mean and variance σ_α ². Further, assume that the path gain and angle of arrival are statically independent and their joint distribution is represented as follows:

equation 6:

wherein, CN (x: mu, sigma)²) Denotes x as a complex Gaussian random variable distribution with mean μ and variance σ². Without loss of generality, we assume σ_α ²＝1/P。

The correlation between the dedicated channel and the common channel is expressed as follows:

equation 7:

where R represents a channel covariance matrix, which is expressed as follows:

equation 8: r ═ E { hh^H}＝E{a(θ)a^H(θ)}

The correlation being dependent on theta₀Angle and angular spread. For example only, assume that the common signal is transmitted on a center antenna. That is, v ═ 0_1xN，1，0_1xN]^H。

The transmit antenna weights w may be selected such that the correlation p is real and maximized for a norm constraint on the weights. This results in the following equation:

equation 9: w-kRv

Where k is the positive real value selected to implement the selected norm constraint.

The pdf, f (θ) of the relative phase θ between two related zero-mean gaussian random variables X and Y has been analyzed below: proakis, Digital Communications, 3^rdEd., McGraw-Hill, 1995. Let μ denote the correlation coefficient between X and Y, i.e.:

equation 10:

subsequently, as shown in the Proakis article cited just:

equation 11:

respectively by r_cAnd r_dReplacing X and Y and accounting for noise in channel estimation and noise in demodulation, the correlation coefficient between the dedicated channel and the common channel is expressed as follows:

equation 12:

wherein σ_c ²And σ_d ²Representing channel estimatesAnd noise in the received user-specific signal to be demodulated. The noise level may be estimated or taken as a parameter and updated. Obviously, the standard deviation of the phase offset is determined by the correlation coefficient. Furthermore, for PSK signaling, the coefficients also determine the bit error probability. The possible optimization procedure then minimizes the norm of w affected by the constraint such that the cross-correlation coefficient is real and such that the magnitude is equal to or greater than the target value μ that determines the standard deviation and the bit error probability_target：

Equation 13:

this is straightforward using lagrange multipliers. Other constraints may also be included, such as minimizing interference that is spread in certain directions.

A third exemplary, non-limiting embodiment combines the mixed beam embodiment with transmit and receive diversity, as shown in fig. 10. The mixed beam embodiment may be combined with transmit diversity only or receive diversity only. Diversity may be implemented using different polarizations, spatially separated antennas, or other well-known techniques. Combining transmit diversity with beamforming reduces the interference that would otherwise be generated when the entire cell transmits diversity signals. Thus, it is possible to benefit from both diversity gain and antenna gain.

Like reference numerals refer to like elements already described above with the following exceptions. The left side of fig. 10 comprises a transmit diversity branch 1(TxDB 1) and a receive diversity branch 1(RxDB 1). The right side of fig. 10 shows the second transmit and receive diversity branches TxDB2 and RxDB 2. The common signal distribution block 36 distributes the common signal toTwo transmit diversity branches. Similarly, the user-specific signal distribution block 37 distributes the specific signal to the two transmit diversity branches. Multiplexers 34 and 35 multiplex all received signals into two received signal streams which are processed by signal processor 32 to generate decoded mobile user signals d^ULAnd beam specific beam weights w_n。

Fig. 11 illustrates a fourth non-limiting example embodiment, which is a steered beam embodiment that includes both transmit diversity and receive diversity. The steered beam embodiment may be combined with transmit diversity only or receive diversity only. Diversity may be implemented using different polarizations, spatially separated antennas, or other well-known techniques. Various diversity branches are labeled in fig. 11.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims (50)

1. Apparatus comprising an antenna array (12) and transmit circuitry (20) coupled to the antenna array, the antenna array comprising a plurality of antenna elements (14) for transmitting a wide beam covering a majority of a sector cell and comprising a common signal and at least one narrow beam covering only a portion of the sector cell and comprising a mobile user-specific signal, the apparatus further characterized by:

circuitry (24, 26, 28, 29) coupled to the transmit circuitry (20) for ensuring that the user-specific signal and the common signal are substantially in phase and substantially time aligned at the antenna array.

2. The apparatus of claim 1, wherein the circuitry (24, 26, 28, 29) coupled to the transmit circuitry (20) comprises filtering circuitry (24) configured such that the common signal is transmitted only from a center antenna element in the antenna array.

3. The apparatus of claim 1, characterized in that the circuitry (24, 26, 28, 29) coupled to the transmit circuitry (20) is configured to ensure that the user-specific signal is in phase and time-aligned with a central antenna element of the common signal in the antenna array (12).

4. The apparatus of claim 1, wherein the circuitry coupled to the transmit circuitry (20) includes filtering circuitry (24) configured to compensate for distortion in the common signal and the user-specific signal associated with conversion of the common signal and the user-specific signal from baseband frequency to radio frequency.

5. The apparatus of claim 1, wherein said antenna array (12) includes an odd number N of antenna elements (14), where N is a positive integer greater than 1, said apparatus further comprising:

a beam forming network (16) coupled between the antenna array (12) and transmit circuitry (20) for receiving the user-specific signals and the common signal and generating N narrow beams to be provided to the antenna array (12).

6. The apparatus of claim 5, characterized in that the beam forming network (16) is configured to transmit the common signal on the N narrow beams simultaneously with equal power.

7. The device of claim 6, characterized in that the beam forming network (16) is configured to transmit the user-specific signals simultaneously on the N narrow beams at a power determined using N user-specific beam weights (W), each user-specific beam weight corresponding to one of the N narrow beams, such that a narrower beam than a wide beam carrying the common signal radiates in the direction of the user.

8. The apparatus of claim 7, wherein each user-specific beam weight is proportional to a function of the average signal power received on the corresponding beam.

9. The apparatus of claim 1, further comprising:

a beam weighting circuit (28) for weighting the user-specific signals using user-specific beam weights corresponding to each beam and providing each weighted user-specific signal to a corresponding beam filter.

10. The apparatus of claim 9, wherein said user-specific beam weights are configured such that radiated energy from said antenna elements is directed to a desired mobile user.

11. The apparatus of claim 5, further comprising:

receive circuitry (22) coupled to the beamforming network;

a signal processor (32) coupled to the receive circuitry (22) for processing signals received on the N narrow beams to estimate received signals and for determining a received uplink average signal power for each beam.

12. The apparatus of claim 6, further comprising:

first and second antenna arrays (12) each comprising an odd number N of antenna elements, where N is a positive integer greater than 1, for transmitting a wide beam covering a majority of a sector cell and comprising said common signal and at least one narrow beam covering only a portion of said sector cell and comprising a mobile user-specific signal;

a first transmit circuit (20) coupled to the first antenna array;

second transmit circuitry (20) coupled to the second antenna array;

a first beamforming network (16) coupled between the first antenna array and the first transmit circuit for receiving the user-specific signal and the common signal and generating N narrow beams to be provided to the first antenna array;

a second beamforming network (16) coupled between the second antenna array and the second transmit circuitry for receiving the user-specific signals and the common signals and generating N narrow beams to be provided to the second antenna array;

first circuitry (24, 26, 28, 29) coupled to the first transmit circuitry for ensuring that user-specific signals and the common signal at the first antenna array element are in phase and time-aligned; and

second circuitry (24, 26, 28, 29) coupled to the second transmit circuitry for ensuring that user-specific signals at the second antenna array and the common signal are in phase and time aligned.

13. The apparatus of claim 12, further comprising:

a first receive circuit (22) coupled to the first beamforming network;

a second receive circuit coupled to the second beamforming network;

a signal processor (32) coupled to the first and second receive circuits for processing signals received from the first beamforming network on the N narrow beams and from the second beamforming network on the N narrow beams to estimate received signals.

14. Apparatus comprising an antenna array (12) and transmit circuitry (20) coupled to the antenna array (12), the antenna array comprising a plurality of antenna elements for transmitting a wide beam covering a majority of a sector cell and comprising a common signal and at least one narrow beam covering only a portion of the sector cell and comprising a mobile user-specific signal, the apparatus characterized by:

circuitry (24, 26, 28, 29) coupled to the transmit circuitry for ensuring that the user-specific signal and the common signal are substantially time-aligned and have a controlled phase difference when received by mobile stations in the sector cell.

15. The apparatus of claim 14, characterized in that the circuitry (24, 26, 28, 29) coupled to the transmit circuitry comprises filtering circuitry (24) configured such that the common signal is transmitted only from a central antenna element in the antenna array.

16. The apparatus in claim 14, characterized in that the circuitry (24, 26, 28, 29) coupled to the transmit circuitry is configured such that a wide beam carrying the common signal is generated using a plurality of antenna elements (14) in the antenna array.

17. The apparatus of claim 14, wherein the circuitry (24, 26, 28, 29) coupled to the transmit circuitry includes filtering circuitry (24) configured to compensate for distortion in the common signal and the user-specific signal associated with converting the common signal and the user-specific signal from baseband frequency to radio frequency.

18. The apparatus of claim 14, further comprising:

a first beam weighting circuit (28) for weighting the user-specific signals with user-specific beam weights corresponding to each antenna and providing each weighted user-specific signal to a corresponding antenna transmit filter (24).

19. The apparatus of claim 18, wherein said user-specific beam weights are configured such that radiated energy from said antenna elements is directed to a desired mobile user.

20. The apparatus of claim 18, further comprising:

a second beam weighting circuit (29) for weighting the common signal using a common signal beam weight corresponding to each antenna and providing each weighted common signal to a corresponding antenna transmit filter.

21. The apparatus of claim 20 wherein said common signal beam weights are configured such that radiated energy from said antenna elements is directed in a desired shape in said sectorized cell.

22. The apparatus of claim 20 wherein said user-specific beam weights and common signal beam weights are complex numbers used to phase rotate and amplify said user-specific signals and common signals, respectively.

23. The apparatus of claim 18 wherein said user-specific beam weights are selected to match an average spatial signature that is a complex-valued measure of an average received signal as a function of angle at which said received signal was received.

24. The apparatus of claim 18 wherein said user-specific beam weights are selected to minimize transmit power allocated to mobile users such that a standard deviation of a phase difference between a common signal received by said mobile users and a user-specific signal is less than or equal to a target value that ensures a desired quality of service.

25. The apparatus of claim 14, wherein said antenna array (12) comprises N antenna elements (14), said apparatus further comprising:

a beamforming network (16) coupled to the N antenna elements (14) for generating N receive beams;

receive circuitry (22) coupled to the beamforming network (16);

a signal processor (32) coupled to the receive circuitry (22) for processing signals received on the N receive beams to estimate received signals and for determining statistics of a channel through which the received signals propagate.

26. The apparatus of claim 14, further comprising:

first and second antenna arrays (12), each comprising N antenna elements (14), for transmitting a wide beam covering a large part of a sector cell and comprising a common signal and at least one narrow beam covering only a part of said sector cell and comprising a mobile user-specific signal;

a first transmit circuit (20) coupled to the first antenna array for providing the user-specific signal and the common signal to the first antenna array;

second transmit circuitry (20) coupled to the second antenna array for providing the user-specific signal and the common signal to the second antenna array;

first circuitry (24, 26, 28, 29) coupled to the first transmit circuitry for ensuring that user-specific and common signals from the first antenna element are substantially time-aligned and have a controlled phase difference when received by mobile stations in the sector cell; and

second circuitry (24, 26, 28, 29) coupled to the second transmit circuitry for ensuring that user-specific and common signals from the second antenna elements are substantially time-aligned and have controlled phase differences when received by mobile stations in the sector cell.

27. The apparatus of claim 26, further comprising:

a first beamforming network (16) coupled to the first antenna array (12);

a first receive circuit (22) coupled to the first beamforming network (16);

a second beamforming network (16) coupled to the second antenna array (12);

a second receive circuit (22) coupled to the second beamforming network;

a signal processor (32) coupled to the first and second receive circuits for processing signals received from the first beamforming network on the N narrow beams and from the second beamforming network on the N narrow beams to estimate received signals.

28. A method for use in a radio node comprising an antenna array (12) comprising a plurality of antenna elements (14), characterized by:

filtering the user-specific signal and the common signal to ensure that the user-specific signal and the common signal are substantially in phase and substantially time-aligned at the antenna array (12), an

Simultaneously transmitting from the antenna array (12) a wide beam covering a majority of a sector cell and comprising the common signal and at least one narrow beam covering only a portion of the sector cell and comprising the user-specific signal.

29. The method of claim 28, further comprising:

the common signal is transmitted only from a central antenna element (14) in the antenna array (12).

30. The method of claim 29, wherein the filtering includes compensating for distortions in the common signal and the user-specific signal associated with frequency conversion of the common signal and the user-specific signal from baseband to radio frequency.

31. The method of claim 29, further comprising weighting the user-specific signals to ensure that the user-specific signals are substantially in phase and substantially time aligned with the common signal at a center element (14) of the antenna array (12).

32. The method of claim 29, characterized in that the antenna array (12) comprises an odd number N of antenna elements (14), where N is a positive integer greater than 1, and a beam forming network in the radio node receives the user-specific signal and the common signal and generates N narrow beams to be provided to the antenna array (12).

33. The method of claim 32, further comprising:

simultaneously transmitting the user-specific signals on the N narrow beams at a power determined using N user-specific beam weights, each user-specific beam weight corresponding to one of the N narrow beams, such that a narrower beam than a wide beam carrying the common signal radiates in the direction of the user.

34. The method of claim 33, wherein each user-specific beam weight is proportional to a function of the average signal power received on the corresponding beam.

35. The method of claim 33, further comprising:

the signals received on the N narrow beams are processed to estimate received signals and determine a received uplink average signal power for each beam.

36. The method of claim 33, wherein the method is implemented in two transmit diversity branches.

37. The method of claim 33, wherein the method is implemented in two receive diversity branches, the method further comprising:

processing signals received from the two receive diversity branches on the N narrow beams to estimate received signals.

38. A method for use in a radio node comprising an antenna array (12) comprising a plurality of antenna elements (14), characterized by:

processing the user-specific signal and the common signal to ensure that the user-specific signal and the common signal are substantially time-aligned and have a controlled phase difference when received by the mobile stations in the sector cell, an

Simultaneously transmitting from the antenna array (12) a wide beam covering a majority of a sector cell and comprising the common signal and at least one narrow beam covering only a portion of the sector cell and comprising the user-specific signal.

39. The method of claim 38, wherein the antenna array (12) includes N antenna elements (14), the method further comprising:

-transmitting said common signal from only one of said N antenna elements (14).

40. The method of claim 38, wherein said antenna array (12) includes N antenna elements (14), said user-specific signal being transmitted from said N antenna elements (14) simultaneously.

41. The method of claim 40, wherein said user-specific signal is transmitted using power and phase rotations determined using N user-specific beam weights (W).

42. The method of claim 41, characterized in that the user-specific beam weights are configured such that radiated energy from the antenna elements (14) is directed to a desired mobile user in the sector cell.

43. The method of claim 41, wherein said common signal is transmitted with a power and phase rotation determined using N common signal beam weights (W).

44. The method of claim 43, wherein the common signal beam weights are configured such that radiated energy from the antenna elements is directed in a desired shape in the sectorized cell.

45. The method of claim 43, wherein said user-specific beam weights and common signal beam weights are complex numbers used to phase rotate and amplify said user-specific signals and common signals, respectively.

46. The method of claim 41, further comprising:

the user-specific beam weights are selected to match an average spatial signature, which is a complex-valued measure of the average received signal that varies with the angle at which the received signal was received.

47. The method of claim 41, further comprising:

the user-specific beam weights are selected to minimize transmit power allocated to mobile users such that the standard deviation of the phase difference between the common signal received by the mobile users and the user-specific signal is less than or equal to a target value that ensures a desired quality of service.

48. The method of claim 44, wherein said user-specific signals and common signals are transmitted simultaneously from said N antenna elements (14) using powers determined by N user-specific beam weights and N common signal beam weights, respectively, each user-specific beam weight and each common signal beam weight corresponding to one of said N antenna elements; it is characterized by also comprising:

selecting the user-specific beam weights to direct radiated energy from the antenna array to a desired mobile user, an

The common signal beam weights are selected to direct radiated energy from the antenna array in a desired shape.

49. The method of claim 38, wherein the processing comprises compensating for distortions in the common and user-specific signals associated with translating the common and user-specific signals from baseband frequency to radio frequency.

50. The method of claim 38, wherein the method is implemented in two transmit diversity branches.