HK1078693A - Simple smart-antenna method and apparatus for mud-enabled cellular networks - Google Patents

Description

Method and apparatus for a simple antenna for MUD-enabled cellular networks

Technical Field

The present invention relates to a smart antenna, and more particularly, to a smart antenna for use in a cellular network in which wireless network transmit/receive units and/or base stations of the network use multi-user detection (MUD).

Background

Thereafter, a wireless transmit/receive unit (WTRU) includes, but is not limited to, a user equipment, mobile base station fixed or mobile subscriber unit, pager, or any other type of device capable of operating in a wireless network environment. A base station as referred to hereafter includes, but is not limited to, a base station, node B, site controller, access point or other interfacing device in a wireless network.

A smart antenna system generally refers to a combination of a plurality of transmit and/or receive antenna elements that together use a signal processing protocol using the antennas to improve the quality of signal reception and the capacity of the cellular network.

Smart antennas have been extensively studied and have shown a high potential in terms of performance enhancement. However, one major drawback of most current smart antenna technologies is the significant complexity required to achieve more than a modest increase in performance. This problem is exacerbated in systems that also use multi-user detection (MUD) techniques, and current smart antenna techniques can only maximize the received power at each WTRU, but cannot cope with interference to or from other users.

Disclosure of Invention

A method and apparatus for a smart antenna for use in a cellular network is disclosed, wherein the wireless network transmit/receive units and/or base stations of the network use multi-user detection (MUD). The present invention takes into account interference to or from other users to maximize the desired signal while eliminating or reducing the interference signal, and thus the overall system capacity will be increased.

Drawings

Figure 1 illustrates a cellular region having isolated areas in the coverage area associated with a single WTRU in connection with the present invention.

Fig. 2 is a logical block diagram of a simple smart antenna process for a transmitter in accordance with the present invention.

Fig. 3 is a physical block diagram of a simple smart antenna process for a transmitter in accordance with the present invention.

Fig. 4 is a logic block diagram of a simple smart antenna process for a receiver in accordance with the present invention.

Fig. 5 is a physical block diagram of a simple smart antenna process for a receiver in accordance with the present invention.

Detailed Description

The present invention will now be described with reference to the drawings, wherein corresponding reference numerals indicate corresponding parts, and wherein the general principles herein are applicable to transmitter and receiver processes of some variations. In both embodiments, a beam pattern is established for each user so that the signal power received from or transmitted to the direction of the target user is stronger than the signal power received from or transmitted to the direction of the other users.

Figure 1 shows a beam pattern 10 for use by a single Wireless Transmit Receive Unit (WTRU) WTRU-112. Signals transmitted to or received from the WTRU-112 may be enhanced in its area and reduced in strength in other areas. Thus, the overall signal-to-noise ratio from or to the WTRU-112 will increase. It is also apparent from figure 1 that the coverage area may also be quite complex, and in the case of many WTRUs, it is unlikely that the signals of each WTRU will be completely separated, however, even partial separation provides a significant improvement in signal-to-noise ratio.

The technique shown in fig. 1 is referred to as "beamforming" when applied to transmit signals, and "smart antenna reception" when applied to receive signals. The benefits of the present technique include 1) requiring only a lower number of antennas, e.g., only three (3) for an omni-directional coverage area, or less than a coverage area that has been oriented as a cell sector; 2) low complexity processing; and 3) processing with MUD-compatible technology at the receiving end.

FIG. 2 is a logical block diagram of a system for processing a transmitted signal at a transmitter. Although the block diagram focuses on processing signals for a single WTRU, such as WTRU-112, similar structures may be used for each WTRU. As shown in fig. 2 and 3, the WTRU-1 signal is linearly processed in terms of reducing the degree to which the signal contributes in the direction of other WTRUs or groups of other WTRUs by the introduction of one or more nulls. For example, with WTRU-1, the WTRU-1 data 20 is processed in the linear processor 24 in an inactive manner for WTRU-2 through WTRU-5, and once processed, the WTRU-1 signal is added to similarly processed signals in the adder 26 that includes other WTRU signals, which are then transmitted via the antenna array 30.

FIG. 3 is a physical block diagram for implementing the architecture of FIG. 2. Only three WTRU's are considered for simplicity. Antenna complex weighting { w_i.jIs calculated to produce an invalidation, where i represents the designed user and j represents the antenna, which removes or reduces interference and maximizes the composite channel power for the desired user.

The goal is to maximize the received power of the desired user WTRU-112 and minimize the interference power to other users WTRU-214 and WTRU-316. Mathematically, assuming that the desired user is denoted as 1, the antenna weights can be calculated as follows:

limit | w_i|＝1，i＝1，2，3；

Equation 1

WhereinAnd H_i＝[h_1ih_2ih_3i]And h is_ijIs the channel impulse response from antenna i to user j, andequation 1 can be effectively expressed as equation 2:

equation 2

Wherein λ_imaxIs a matrix pair (R)_i， ) The generalized eigenvalues of (1). It is noted that the method is very different from the TXAA (transmit adaptation array) used in the current 3GPP standard, and the system only maximizes the WTRU receive power and does not deal with interference to other users. The capacity of the overall system may also be increased because the invention proceeds to take into account interference to other users.

The processing at the receiving end also performs a similar structure as the logical and physical block diagrams shown in fig. 4 and 5. The main differences between the sending end and the receiving end are: 1) the antenna weights are selected to maximize the desired signal power and minimize interference to other users; 2) the processing of the smart antenna is continued after a multi-user detection (MUD) processor, which internally combines the inputs from different antennas for each user.

However, this assumes that the channel impulse response is the same in both the uplink and downlink directions, so that complex antenna weights in either the uplink or downlink direction can also be applied to the other uplink or downlink direction.

The present invention is preferably implemented in both base station transmitters and base station receivers when any smart antenna application is used in a cellular environment, thereby providing a performance advantage. As described with reference to equations 1 and 2, the method of the present invention requires knowledge of the channel impulse response for each antenna. When the channel impulse response is sufficiently known, an accurate response estimate for the channel impulse response of each WTRU in each antenna will typically be obtained during normal receiver processing, and different standard non-smart antenna conventional approaches can be used to obtain such estimates.

Obtaining channel impulse responses (i.e., channel impulse responses from base station transmit antennas to WTRUs) for a transmitter in a Frequency Division Duplex (FDD) system can be a problem because different frequency bands are used for downlink and uplink transmissions, and thus implementation of the present invention in the transmitter portion of the system can be somewhat difficult.

However, in Time Division Duplex (TDD) systems, the same frequency band is time shared between the uplink and downlink, which allows the downlink to be evaluated using the channel impulse response resulting from transmissions received from the uplink. After obtaining a complete channel impulse response estimate in a TDD system, the difficulty arises for some number of time slots, however, in many TDD systems such as the proposed TDD mode of UMTS W-CDMA, the time slot duration is short enough that the channel estimate is also valid for one or more time slots for channels with slow fading, such as those used for gated or general applications, which are typically found in microcellular or picocellular environments. The present invention is therefore particularly suited to these environments.

A more efficient application is to use a MUD type receiver in a cellular network with a multi-cell environment. In such networks, receiver performance is typically limited primarily by inter-cell interference between base stations and between WTRUs located in neighboring cells. For example, where linear MUDs are used, the MUD using a decorrelated/forced-zero type receiver has an effective signal-to-noise ratio (SIR) at its output as given in equation 3 below:

equation 3

Wherein H_KK ⁺Is [ k, k ]]H of the assembly^-1(ii) a H is a function of, for example, user signature sequence, data rate and channel stateA matrix determined by the inter-cell configuration and environment of the cell; and σ²The total power due to thermal noise and inter-cell interference, except in very large cells, the inter-cell noise is sufficient to represent all σ 2 values; the MUD using a Minimum Mean Square Error (MMSE) type receiver has an effective signal-to-noise ratio (SIR) at its output as shown in equation 4 below:

equation 4

Where I is the same matrix co-dimensional with H.

From equations 3 and 4, when σ²Approaching zero, the SIR approaches infinity. Therefore, reducing intercell interference is the most important thing for MUD-enabled networks. Cross interference within a cell is at σ²The approximation infinity can be fully corrected by the MUD, unlike network systems with RAKE and matched filtering base receivers, where intra-cell interference is also important. Sigma²Values include inter-cell interference, hot channel noise, and noise introduced by receiver processing, among which inter-cell interference is typically the most dominant factor, and thus, reducing inter-cell interference results in significant reductions, e.g., σ²The measured total interference power has the largest impact.

Intercell interference is particularly large in TDD systems where node Bs significantly interfere with WTRUs receivers located at the cell edge, similar interference is common in microcell and picocell systems where the cell size is small. The present invention is ideally suited for use in the reduction of inter-cell interference in the environment, by selectively targeting the transmitters of WTRUs or groups of WTRUs in those cells, the base station significantly reduces the total amount of energy spread in any particular direction, thus reducing the overall inter-cell interference. By limiting the angular range over which energy is collected from the receiver, the base station can limit the effect of interference from other cells on any single WTRU receiver input signal, and the resulting improvement in performance is significant. For example, reducing the inter-cell interference into the decorrelating receiver by half can increase the 3 db performance of the MMSE receiver.

It should be appreciated that with the prior art, the present invention is ideally suited for reducing inter-cell interference, particularly where the base station significantly interferes with WTRUs receivers located at or near the edge of a cell via transmitters that are selectively targeted to interfere with each WTRU. The number of antenna weights, the number of resulting complex weights, and the number of transmitter and receiver antennas may be implemented as desired without departing from the spirit and scope of the present invention.

Although specific processing functions have been described above and performed by specific components, it should be understood that the performance of processing functions may also be distributed as desired by network components.

Although the present invention has been described in detail, it must be understood that the invention is not limited thereto and various changes may be made without departing from the spirit and scope of the invention, which is defined in the claims.

Claims (12)

1. A base station for communicating using multi-user detection, comprising:

a plurality of j antennas;

a transmitter, comprising:

a plurality of i parallel data signal inputs for transmission;

a plurality of weight number generators for generating a composite weight index according to the number of antennas j and each data signal i;

at least one linear processor for processing the complex weights by reducing the magnitude of the signal contribution to unintended points while maximizing the power gain to desired points; and

a vector adder for adding the output of the linear processor to form channel impulse response data for transmission by the plurality of j antennas.

2. The base station of claim 1 wherein the j antennas form a smart antenna that generates beam forming in response to the channel impulse response data.

3. The base station of claim 1 wherein j-3.

4. A base station using multi-user detection, comprising:

a plurality of j antennas for receiving input of i channel impulse response data;

a receiver, comprising:

a plurality of weight number generators for generating a composite weight index according to the number of antennas j and each data signal i;

at least one linear processor for processing the composite weight values, which reduces the strength of the signal contribution to unintended points when maximizing the power gain to a desired point; and

a multi-user detection processor for combining the weighted signals from the j antennas to reconstruct the i data signal.

5. The base station of claim 4 wherein the j antenna forms a smart antenna that generates the smart antenna that receives the channel impulse response data.

6. The base station of claim 4 wherein j-3.

7. A method for reducing inter-cell interference between base stations, comprising:

storing a set of antenna impulse response data for each of a plurality of base stations;

selecting at least two plane points within the cell as interference locations for each of the plurality of sources;

determining a set of antenna weights from the set of antenna impulse response data and the plane point for each source that maximizes a signal at each of the plurality of sources when the signals at the other sources are reduced to zero; and

the antenna weights are applied to produce the cell division in which the strength of each source signal in its region is enhanced and the strength in other regions is reduced.

8. The method of claim 7 wherein the set of antenna impulse response data is determined by a base station.

9. The method of claim 7 wherein a downlink includes the set of antenna weights selected to maximize the desired signal power and minimize interference to other users and cells.

10. The method of claim 7 wherein an uplink includes the set of antenna weights selected to maximize the desired signal power and minimize interference to other users and cells.

11. The method of claim 7 wherein a downlink channel and an uplink channel are the same, and the set of antenna weights is then selected from the set of antenna impulse responses for the downlink or uplink channel.

12. The method of claim 7 wherein the smart antenna has at least three antenna elements for transmitting and receiving.