WO2009110899A1

WO2009110899A1 - Interference nullifying apparatus with agc and method of weighting and combining signals from antenna elements

Info

Publication number: WO2009110899A1
Application number: PCT/US2008/055951
Authority: WO
Original assignee: Motia, Inc
Priority date: 2008-03-05
Filing date: 2008-03-05
Publication date: 2009-09-11

Abstract

The present invention employs multiple antenna elements and signal combining techniques to achieve both antenna diversity and combining gain while suppressing interference. To obtain the maximum achievable diversity gain, the received signals at the antenna elements are adjusted for independent fading. With small latency, real time packet by packet beamforming is enabled for superior mobility.

Description

Description Interference nullifying apparatus with AGC and method of weighting and combining signals from antenna elements

Technical Field

[1] This invention relates generally to wireless communication systems. More particularly, it relates to a wireless communication system using a plurality of antenna elements with weighting and combining techniques for optimizing antenna diversity and combining gain and for suppressing interference.

Background Art

[2] Recently, the market for wireless communications has enjoyed tremendous growth.

Wireless technology now reaches or is capable of reaching virtually every location on the face of the earth. Hundreds of millions of people exchange information every day using pagers, cellular telephones and other wireless communication products.

[3] With the appearance of inexpensive, high-performance products based on the IEEE

802.1 la/b/g Wireless Fidelity (Wi-Fi) standard, acceptance of wireless local area networks (WLANs) for home, Small Office Home Office (SOHO) and enterprise applications has increased significantly. IEEE 802.1 lb/g is a standard for a wireless, radio-based system. It operates in the unlicensed 2.4 GHz band at speeds up to 1 IM bits/sec for IEEE 802.1 Ib and 54 M bits/sec for IEEE 802.1 Ig. The IEEE 802.1 lb/g specification sets up 11 channels within the 2.4 GHz to 2.4835 GHz frequency band which is the unlicensed band for industrial, scientific and medical (ISM) applications. IEEE 802.1 Ia is another standard for a wireless, radio-based system in the ISM band. It operates in the unlicensed 5-GHz band at speeds up to 54 M bits/sec. As more devices were deployed and limited spectrum available, interference between devices are becoming an important issue.

[4] It has been found that WLANs often fall short of the expected operating range when actually deployed. For example, although a wireless Access Point (AP) is specified by a vendor as having an operating range of 300 feet, the actual operating range can vary widely depending on the operating environment. In particular, WLAN performance can be greatly degraded by direct and multipath radio interference. Multipath occurs in wireless environments because the radio frequency (RF) signal transmitted by the subscriber is reflected from physical objects present in the environment such as buildings. As a result, it undergoes multiple reflections, refractions, diffusions and attenuations. The base station receives a sum of the distorted versions of the signal (collectively called multipath). Similarly, in any indoor wireless system, multipath interference effects occur when the transmitted signal is reflected from objects such as walls, furniture, and other indoor objects. As a result of multipath, the signal can have multiple copies of itself, all of which arrive at the receiver at different moments in time. Thus, from the receiver's point of view, it receives multiple copies of the same signal with many different signal strengths or powers and propagation delays. The resultant combined signal can have significant fluctuation in power. This phenomenon is called fading.

[5] There are additional elements of performance degradation in a network of

802.1 lb/g WLAN access points (APs). Since the 802.1 lb/g channel bandwidth is approximately 16 MHz, only three non-overlapping channels operating in proximity can be accommodated without interfering with one another. The channel re-use factor imposes a severe restriction on implementation of 802.1 lb/g based systems which requires significantly more effort in the network deployment, and increases the chances of interference and packet collision especially within an environment with a dense user cluster, such as in an office building or apartment building. It is not usual that a receiver can obtain signal from more than 10 different APs simultaneously. Multipath interference further complicates the situation because being physically closer to an AP does not mean the signal from the AP is stronger. Signal propagates from a different path from a remote AP can have stronger power. Thus, site survey to determine the signal propagation is often required for a service provider trying to deploy multiple APs within an office or retail complex.

[6] Several approaches for improving the operating performance and range in a fading environment have been suggested. In one conventional approach, selection antenna diversity is used to reduce the effect of multipath fading. Multiple antennas are located in different locations or employ different polarizations. As long as the antennas have adequate separation in space or have a different polarization, the signal arriving at different antennas experiences independent fading. Each antenna can have a dedicated receiver or multiple antennas can share the same receiver. The receiver(s) checks to see which antenna has the best receiving signal quality and uses that antenna for the signal reception. The performance gain thus achieved is called diversity gain. The performance gain increases with the number of diversity antennas. The drawback of the selection diversity approach using a single shared receiver is that fast antenna switching and signal quality comparison is required. Since an 802.11 (a, b, g) signal has a short signal preamble, only two diversity antennas are typically employed. This achieves a diversity gain of approximately 6 dB in a flat Rayleigh fading environment at the required frame error rate. The diversity gain decreases to 3 dB when delay spread is 50 ns and 0 dB when delay spread is 100 ns.

[7] In another conventional approach, signal combining is used to provide improved performance in a fading environment. Signal combining techniques employ multiple spatially separated and/or orthogonally polarized antennas. The received signal is obtained by combining the signals from the multiple antennas. One technique for providing optimal signal quality is known as maximal ratio combining (MRC). To achieve the best signal quality, the received signal from each antenna is phase-shifted such that the resultant signals from all antennas are in phase. In addition, the signal from each antenna is scaled in amplitude based on the square root of its received signal-to-noise ratio.

[8] IEEE 802.11 standards employ CSMA/CA protocol to allow uncoordinated devices to operate at proximity. Wireless 802.11 devices sense the wireless medium using either power detector or its demodulator/signal detector before they transmit packets. If the wireless 802.11 devices detect that channel are being used (busy), it will withhold transmission until channel is clear. Due to the signal propagation blockage and multipath, some devices, unable to detect channel correctly (hidden nodes), can transmit simultaneously, resulting in collisions. Additionally, the use of microwave oven and cordless phone can occupy the channel for a long duration.

[9] The signal bandwidth of the 802.11 device is 16 MHz while channel spacing is 5

MHz. As a result, signals at adjacent channels overlap each other in frequency. Thus, interference can also come from one or more adjacent channels. Disclosure of Invention

Technical Problem

[10] Wireless devices based on other IEEE standards (such as 802.16) or cell phone standards (such as UMTS) using licensed bands operate in a controlled and coordinated fashion in which a number of fixed locations access points (or base stations) are deployed at strategically locations within the service area to provide services to a number of nomadic nodes or mobile terminals. Base stations within a service area ideally use different frequencies for operation to prevent interference with one another. However due to density, rapid adoption, inherent non-uniformity of signal propagation as a result of building/foliage blockage, terrain, or other different signal propagation conditions, mobile terminals or base stations do experience growing interference.

[11] In U.S. patent 7260370, filed Dec. 10, 2003 entitled 'Wireless Communication

System Using a Plurality of Antenna Elements with Adaptive Weighting and Combining Techniques', a closed loop operation system which can simultaneously perform signal combining using MRC and adjacent channel interference suppression are proposed. It has been observed that this approach can be unstable in some cases and unable to achieve fast convergence and integrator overflow. Subsequently, a U.S. Patent Application No. 11/237,439 filed September 28, 2005 entitled Adaptive Beam Forming Receiver Frontend' resolved the integrator overflow issue.

[12] Both US patent 7260370 and U.S. Patent Application No. 11/237,439 apply to systems operating in a low delay spread environment where any channel is flat fading instead of selective frequency fading. However it has been observed that the performance of RF combining can be degraded if the product of the RMS delay spread and the signal bandwidth is greater than one-half (0.5).

[13] Thus it can be appreciated that what is needed is a method or system which enhances conventional communication systems by providing diversity, combining gain or interference suppression techniques which can be self-aligned and converges to the correct parameter values, independent of process, temperature, and component variations.

Technical Solution

[14] In an embodiment of the present invention, interference suppression is performed with an interference nulling adjustment (INA) in combination with beam forming operations performed with maximal ratio combining (MRC).

[15] The interference nulling adjustment (INA) determines an INA error signal for each antenna element, which is proportional to an envelope of the corresponding 'interference signal' and has a phase equal to 180 degree plus the phase difference of the input signal and the SUM channel, defined as a combined signal equal to the sum of weighted signals from all individual antennas. The INA error signal is determined by the method of complex conjugate multiplication of the individual signals and a 'negative' reference SUM channel signal.

Advantageous Effects

[16] The present invention employs multiple antenna elements and signal combining techniques to achieve both antenna diversity and combining gain while suppressing interference. To obtain the maximum achievable diversity gain, the received signals at the antenna elements are adjusted for independent fading. Interference suppression can be achieved if the signal correlation between any pair of antenna elements is less than 0.5 and less than 1 dB loss in performance as compared with uncorrelated fading can be achieved if the signal correlation between any pair of antenna elements is less than 0.7 for diversity gain.

[17] The present invention can apply, but is not restricted to, spatially diversity, polarization diversity, angular diversity, or pattern diversity. The diversity gain that can be achieved increases with the number of antenna elements. The increase in diversity gain is not a linear function of the number of antenna elements. The incremental diversity gain decreases as the number of antenna elements increases. Most of the diversity gain is achieved with first few antenna elements. As far as interference suppression is concerned, the number of interfering signals that can be suppressed simultaneously is equal to the number of antennae minus one. Typically, 15 dB or higher interference suppression can be achieved, depending on accuracy of the implementation.

[18] The present invention provides a method and system for operating a wireless communication system in which received signals from a plurality of antennas are weighted and combined with a beam forming operation to form an output signal. The beam forming operation determines weights adjusted to increase a desired signal power in the output signal while reducing the power in the output signal of interference components.

[19] The invention will be more fully described by reference to the following drawings.

Reference will now be made in greater detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts.

Description of Drawings

[20] Figure 1 is a schematic of a circuit for computing antenna weights

[21] Figure 2 is a schematic of a circuit for normalizing automatic gain control (AGC)

[22] Figure 3 is a schematic of a circuit for computing the sum channel.

[23] Figure 4 is a simplified schematic of a circuit for low delay spread.

[24] Figure 5 is a schematic of a circuit for selecting a signal in spread spectrum

Best Mode

[25] The present invention described below further comprises a method to adapt the process for a high delay spread environment where the product of the RMS delay spread and the signal bandwidth is greater than one-half (0.5). In an embodiment the operation for high delay spread environment is performed at the baseband.

[26] The method further comprises splitting a broadband signal into a plurality of frequency sub-bands wherein the bandwidth of each sub-band has the characteristic that a product of the RMS delay spread and a bandwidth of each sub-band is less than one-half (0.5). One method of splitting the signal into a plurality of sub-bands is applying the process of fast fourier transform (FFT) operation. Another method is passing the signal through a bank of sub-band filters. Separate, but identical beamforming (MRC and INA) operations, are performed at each sub-band. The resultant antenna weights for the sub-bands can be used to combine signal or suppress interference in each sub-band. An inverse FFT can be used to restore the combined sub-band signals to time domain.

Mode for Invention

[27] The present invention comprises the method of determining an interference nulling adjustment (INA) and determining beamforming antenna weights using maximal ratio combining (MRC).

[28] The MRC determines an MRC error signal for each antenna element, which is proportional to an envelope of the corresponding desired input signal and has a phase equal to the phase difference of the input signal and a combined signal equal to the sum of weighted signals from all individual antennas, defined within the present patent application as the SUM channel. The MRC error signal is determined by complex conjugate multiplication of the individual signal for each antenna element and the reference SUM channel signal.

[29] Each error signal is low pass filtered (or integrated) to become the antenna weight for each channel.

[30] The error signal and signal processing for MRC and the error signal and signal processing of the interference suppression adjustment are substantially identical except for the 'phase' of the error signals which differ by 180 degree.

[31] In an embodiment, simultaneous diversity combining gain and interference sup- pression can be achieved by adding the two error signals in the weight generation to generate antenna weights similar to those of minimum mean squared error (MMSE) combining. Note that the error signal is scaled by a factor 'Scale' 144. This allows the system to adjust the amount of interference suppression versus the signal combining.

[32] In an embodiment one low pass filter may be used for both MRC and INA. In another embodiment each MRC and INA processes has its own filter. The interference signal selector for spread spectrum signal uses a different spreading code.

[33] In the prior filings US patent 7260370 and U.S. Patent Application No.

11/237,439, a channel filter is used for selecting the desired signal for combining while a different filter is used for selecting the adjacent channel signal for interference suppression. When the two error signals (from MRC and INA) are combined, simultaneous combining for desired signal and suppression for interference signal can be achieved. Note also that if only the error signal of MRC is used, the resulting operation is for combining only. Conversely, if only the error signal of INA is used, the resultant operation is for suppression of signal only. Depending on the signal selected via the filter, the INA can be used for suppressing co-channel or adjacent channel interference.

[34] Both MRC and INA use closed loop operation. The present invention further comprises a method to stabilize the closed loop operation the maximal ratio combining (MRC) and of interference nulling adjustment (INA).

[35] An embodiment of invention further comprises filtering the signal from each antenna element with a channel filter(s) to select the signal for processing and then amplifies the resultant signal with a variable gain amplifier. The signal power for each antenna element at output of the variable amplifier is then computed and the difference of the summation of signal power from all antenna elements and a reference level is then fed into an integrator and the output of the integrator is used to control the variable gain amplifier to form an automatic gain control (AGC) loop. The AGC operation is intended to keep the resultant signal level at the output of variable gain amplifier within a desired range for MRC and/or INA operation to generate the antenna weights.

[36] Separately, the antenna weight is applied to the signal from each antenna and the resultant signals from all antennas are then summed together to form the SUM channel signal. In an embodiment, the invention passes a SUM channel signal through a channel filter and variable amplifier. The SUM channel power at the output of variable amplifier is computed and the difference between SUM channel power and a reference level is derived and fed into an integrator. The output of the integrator couples to the variable gain amplifier controlling the output of the variable gain amplifier. Thus an AGC operation on SUM channel maintains a constant level at the output of the SUM channel variable amplifier.

[37] Note that the outputs of the SUM channel and individual channel variable amplifier determine the error signals for the MRC and/or INA antenna weight. [38] The AGC operation stabilizes the closed loop operation while the methods of MRC and INA determine the antenna weights for signal combining or interference suppression. The operation discussed so far is applicable to low delay spread environment in which the product of the RMS delay spread and the signal bandwidth is less than .5.

[39] In the prior invention disclosed in US patent 7260370 and U.S. Patent Application

No. 11/237,439, the antenna weights and combining are performed at the RF frequency, RF combining, instead of at the baseband. Accordingly, in an embodiment of the present invention, a beam former is located between the antenna and the receiver/transmitter interface. RF combining simplifies the interface between the beam former and the transmitter/receiver. Typically, this interface is the same for most vendors whereas the baseband interface differs from vendors to vendors. Accordingly, the approach of the present invention enables beam former processing to be compatible with most vendors. However, the performance of the RF combining can be degraded if the product of the RMS delay spread and the signal bandwidth is greater than .5.

[40] In an embodiment a spread spectrum signal may be split into a plurality of sub- bands and each sub-band may be operated on by the MRC and INA prior to being re- combined in a reverse FFT.

[41] The present invention is distinguished from conventional beam forming by employing closed loop blind beamforming. The closed loop operation is continuously active with or without the presence of signal and/or interference. In contrast to an open loop implementation, in which signal detection, acquisition, synchronization are required before the beamforming operation can be performed, closed loop implementation requires minimum amount of preprocessing and control. The present invention operates on signal power and noise characteristics only. Accordingly, no additional signal format information is needed. Thus, the present invention can be easily adapted to different signal format and signal characteristics.

[42] Figure 1 describes the antenna weight computation which comprises the method of determining an interference nulling adjustment (INA) and determining beamforming antenna weight using maximal ratio combining. Signal from each antenna is processed by an MRC 142 and/or INA block 143. The signal to compute the maximal ratio combining antenna weight is first selected by a signal selector 101 which is typically a channel filter used to remove the radiation from adjacent channel. The output of the signal selector 101 is amplified by a variable gain amplifier 102 and then processed by either a FFT or is passed through a band of filters 104. FFT or filter bank 104 is used to split the broadband signal into a number of sub-band signals with narrower bandwidth. The number of sub-bands or the FFT frequency bins can be determined by the relationship that the product of the rms delay spread and the bandwidth of the sub- band or FFT frequency bin is less than .5. For digital processing, the sampling rate should be equal to or greater than signal bandwidth. The minimum number of frequency bins for FFT is determined by the rms delay spread, which also determines the block size of the time domain samples to be processed within one FFT operation. For indoor operation, a 5~10 MHz sub-band is generally sufficient. A small number of sub-bands are required. The number of samples of signal used in each block of FFT operation is small. Thus, the beamforming operation can be performed without much latency. This overcomes the limitations of the traditional baseband beamforming operation which processes a much higher number of samples, requiring a much higher latency in processing. Generally, traditional baseband beamforming is not suitable for packet-by-packet real time beamforming. The present invention uses a much wider sub-band bandwidth to reduce processing latency and allow real time packet-by-packet beamforming. Note that if a finer frequency bins are used for signal detection, the antenna weights obtained for wider sub-band bandwidth can be interpolated in frequency domain to obtain antenna weights for finer frequency bins, which can be used to combine individual antenna signals at finer frequency bins. Once the subband signals are obtained, they are used to compute the correlation through complex multiplication between the conjugate of the subband signals, i.e. Cl-I, and the corresponding sum channel signal, i.e. SUMl-I at correlators 109, 110, 111, 112. At the output of correlators 109, 110, 111, 112, the error signals from the INA 143 are added. Each resultant error signal is low pass filtered (or integrator) 154, 155, 156, 157 to form antenna weight for each channel. Similar processing is performed for interference suppression (INA) 143.

[43] Note that the error signal and signal processing for MRC and the error signal and signal processing of the interference suppression adjustment are substantially identical except for the 'phase' of the error signals which different by 180 degree. The present invention comprises a plurality of circuits 98-99 each coupled to an antenna 1-2 and further coupled to an MRC circuit 142 and a INA circuit 143.

[44] Referring now to Figure 2, the V_Control signal controlling the variable gain amplifiers 118 is coupled to an integrator 175 which is coupled to an operational amplifier 174 receiving a reference level 166 and the output of a summer which receives a plurality of power detector signals 169-170 associated with each antenna 1-2.

[45] Referring now to Figure 3, an embodiment using FFT to create sub-bands, the source of the Sum signals input to the correlator 109-110 are a plurality of sum circuits each receiving a plurality of weighted results of complex conjugate multiplication of a sub-band signal emitted by a FFT or Filter Band 193 and a plurality of weights 113-114; said FFT or Filter Bank 193 coupled to a variable gain amplifier controlled by V_Control and operating on the output of a signal selector. An inverse FFT can be used to restore the combined sub-band signals 133, 134, 135, 136 to time domain. If subband filters are used, the combined signal can be obtained by adding 133, 134, 135, 136.

[46] In a embodiment for a spread spectrum system, the signal selector comprises complex conjugate multiplier 272 coupled to a broad band filter 271 receiving a signal 270 and a PN code generator 273 receiving a code epoch 275 from a PN code acquisition and tracking circuit 274.

[47] Referring now to Figure 6, a circuit for a simpler embodiment of the present invention a plurality of antennas 98-100 each are coupled to a complex conjugate multiplier receiving a weight 113, a summer receives all the products of complex conjugate multiplication and produces a combined signal, a plurality of signal selectors are coupled to the combined signal and are coupled through a variable gain amplifier to produce a sum signal under the control of V_Control S. V_Control S is coupled to an Integrator 296 receiving the output of on operational amplifier 295 comparing a reference level 294 and the output of a power detector S 292 coupled to the SUM signal 133.

Industrial Applicability

[48] The present invention applies to a wide variety of wireless systems such as WiMax, cellular phone system, and satellite radio and/or video broadcast system. Improved mobility results from packet by packet beamforming due to low latency adjustments.

[49] The present invention provides substantial increase in operating range in a multipath-rich environment; an adaptive antenna null formation, which suppresses the interference arriving from directions other than the desired signal; a reduced deployment effort; cost effectiveness; power efficiency; process, temperature, component variation insensitivity; compactness; fast convergence; and compatibility with existing WLAN systems by exploiting the spatial and polarization antenna diversity and optimal signal combining.

Claims

[1] An apparatus for computing antenna weights for interference nulling and beam steering comprising an INA module 143 and a MRC module 142, and further comprising output signals Weight and Input ports Antenna, V_Control 1-2 and V_Control 1-1.

[2] The apparatus of claim 1 further comprising a circuit generating signals

V_Control 1_2 176 and V_Control 1_1 168 wherein each generated signal is the output of an integrator coupled to an operational amplifier receiving a reference level and the summation value of a plurality of power detectors.

[3] The apparatus of claim 2 wherein INA 143 comprises an interference selector

117 coupling the antenna input port with a variable gain amplifier controlled by V_Control 1_2, and a correlator 125 coupling an input SUM signal 133 to a complex conjugate multiplier receiving a scale 144 and emitting an error signal output 129

[4] The apparatus of claim 3 wherein MRC 142 comprises a signal selector 101 coupling the antenna input port with a variable gain amplifier controlled by V_Control 1_1 and a correlator 109 coupling an input SUM signal 133 to a summer receiving error signal 129 from the INA 143 and emitting an output to a filter/integrator which emits a weight 113 wherein a filter/integrator is one selected from the group of equivalents a filter and an integrator.

[5] The apparatus of claim 4 wherein at least one FFT/Filter bank is coupled to a plurality of complex conjugate multipliers each receiving a weight 196-199 and generating a value for a summer which is coupled to the SUM signals 133-136 of Claim 4.

[6] The apparatus of claim 4 wherein a combined signal is coupled to a signal selector coupled to a variable gain amplifier controlled by V_Control S said amplifer producing the SUM signal of claim 4 wherein said combined signal is the summation of a plurality of complex conjugate multiplications of antenna inputs and weights provided by the apparatus of claim 4 and wherein V_Control S is the output of an integrator coupled to the output of an operational amplifier coupled to a reference level and a power detector S reading the SUM signal.

[7] An embodiment for spread spectrum operation wherein the signal selector of

Claim 6 comprises a complex conjugate multiplier 272 coupled to a broadband filter 271 and a PN code generator 273, said PN code generator receiving a code epoch signal 275 from a PN code acquisition and tracking module 274.

[8] An embodiment for spread spectrum operation wherein the apparatus of claim 3 further comprises a power detector 119 coupled to the output of the variable gain amplifier 118, said output further coupled to a plurality of correlators 125-128.

[9] An embodiment for spread spectrum operation wherein the apparatus of claim 4 further comprises a power detector 103 coupled to the output of the variable gain amplifier 102, said output further coupled to a plurality of correlators 109-112.

[10] The apparatus of claim 3 further comprising a power detector 119 and an FFT/

Filter Bank coupled to the output of the variable gain amplifier 118, said FFT/ Filter Bank coupled to a plurality of correlators 125-128.

[11] The apparatus of claim 4 further comprising a power detector 103 and an FFT/

Filter Bank coupled to the output of the variable gain amplifier 102, said FFT/ Filter Bank coupled to a plurality of correlators 109-112.

[12] The apparatus of claim 6 further comprising a variable gain amplifier 190 coupled to the input of the FFT/Filter Bank 193, said variable gain amplifier 190 receiving signals from a signal selector 101 and controlled by an Integrator 211 which is coupled to the output of an operational amplifier 210 which receives a reference level 208 and a summation of power detector outputs coupled to a plurality of SUM values 133-136; said SUM values resulting from a plurality of summers coupled to a plurality of complex conjugate multipliers each operating on a weight from the MRC and a sub-band from the FFT/Filter Bank 193-195, wherein FFT/Filter Bank means at least one selected from the group of functionally equivalent a Fast Fourier Transform and a Filter Bank.

[13] A method of operating an apparatus for weighting and combining signals from a plurality of antenna elements comprising computing packet by packet dynamic beamforming weights whereby interference can be nullified in real time or as a mobile station encounters changing signal strength as it travels.