Abstract:
A smart external antenna module is proposed for wireless systems. The module allows the advantages of smart antennas to be easily added to existing wireless systems without any modification to the transceiver. Specifically, the smart antenna module weights and combines the signals received by multiple antennas to improve the received signal quality, and then when it detects that there is a signal to be transmitted by the transceiver, it turns off the receiver, turns on the transmitter and uses the receive weights to transmit the signal out of the multiple antennas to improve signal reception at the desired user. Such a module can be added to the external antenna port of an existing transceiver with no change to that transceiver.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/526,340 filed Dec. 2, 2003 the entirety of which is hereby incorporated by reference into this application. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention is directed to a system and method for providing a smart antenna system in which an antenna array can attach to an external port of a transceiver without any modification of the transceiver.  
         [0004]     2. Description of the Related Art  
         [0005]     In the past, companies had turned to wireless local area networks (WLANs) only to support low data intensive applications such as inventory management in warehouses and retail price marking in stores. For these applications, even though wireless network interface cards (NIC) are very costly, the improved efficiency derived from going wireless was worth the investment. The high price and low data rates, however, limited the usefulness and penetration of WLANs for general applications.  
         [0006]     With the introduction of inexpensive, high-performance 11M bit/sec products based on the IEEE 802.11b (Wi-Fi) standard, acceptance of WLANs for home, Small Office Home Office (SOHO) and enterprise applications has been increasing significantly. Home-based LANs have accounted for most of the initial market growth. The use of broadband connections and multiple PCs in the home has spurred home users to employ easy to install WLANs to share the broadband connection and network their computers. Wi-Fi adoption is quickly reaching mainstream levels as a communications technology across the enterprise, service providers and home.  
         [0007]     Growth in enterprise WLAN deployments has also begun to accelerate. It has been found that wireless networks contribute to a better working environment and higher productivity for employees while bringing down the costs of networking.  
         [0008]     Recently, WLANs have begun to get support from traditional wireless and wireline service providers. Public telecom carriers view Wi-Fi 802.11 connectivity as a new type of “data subscription” service that eventually will give users access to different types of wireless networks, either local or wide area, depending on what is available in a given location. Companies such Sprint, Verizon Wireless and SBC Communications have unveiled their Wi-Fi rollout plans. SBC also announced it was rolling out Wi-Fi in public hot spots to support its bundled DSL broadband offering. Traditional wireless carriers are adding Wi-Fi to their product offerings.  
         [0009]     WLAN technology is also becoming standard equipment in portable computers. Notebook computer manufacturers such as IBM, Dell, HP/Compaq and Toshiba are building Wi-Fi into their systems, as notebook computers become the systems of choice for corporate users.  
         [0010]     This explosion in growth comes despite the fact that today&#39;s Wi-Fi semiconductor chips have many shortcomings. The key limitation is that end-users do not experience the same quality of service. End-users can be too far away from an access point, behind a wall, in a “dead spot”, or working off a laptop. In addition, WLANs fall short of expected range when actually deployed. Even though a vendor&#39;s specifications may state that the wireless has a range of 300 feet, for example, obstacles such as walls, desks and filing cabinets can significantly decrease the range and coverage uniformity in some directions.  
         [0011]     Smart antennas can significantly improve the performance of wireless systems by adaptively weighting and combining the signals received by multiple antennas. These antennas can increase receive signal gain, mitigate multipath fading, suppress interference, and increase throughput. Using smart antennas on transmission can also improve performance at the desired user as well. Conventional implementation of smart antennas typically required major modification to the RF, baseband, and MAC portions of the transceiver, which is a time consuming and costly process. Many wireless systems have an external antenna port for the transceiver. However, this port is generally used to attach a fixed, higher gain antenna.  
         [0012]     The three physical layer standards for 802.11 are 802.11b, 802.11g, and 802.11a. 802.11b uses a single carrier with data rates of 1, 2, 5.5, and 11 Mbps, and operates in the 2.4 GHz unlicensed band. At the beginning of each data packet, there is a preamble, the first 20 microseconds of which are available for antenna selection purposes (as used by many 802.11 receivers). 802.11g and 802.11a use OFDM modulation, with data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps, operating in the 2.4 and 5.5 GHz unlicensed bands, respectively. At the beginning of each data packet, the preamble only contains about 2 microseconds for antenna selection purposes. All three techniques use time-division-duplexing (TDD), that is, the same frequency is used for transmission as for reception.  
         [0013]     In consideration of the incorporation of smart antennas into 802.11 systems, one method is to completely redesign the 802.11 transceiver with smart antenna processing as an integral part. This method allows for the optimal performance improvement with M antennas, as well as for design optimization. However, it has the following significant disadvantages. First, it requires a substantial redesign effort by every chip manufacturer without the gains being completely verified until the chip is completed. Second, it results in an across the board increase in the cost of the 802.11 chipset for smart antenna gains that may not be appreciated by all users. Given that the current market is very price sensitive, any undervalued increase in cost will definitely result in lost sales by consumers who may not understand or know if they need the gains of smart antennas, and purchase on price alone. One way for chip vendors to avoid this risk is to create two versions of their chips: one with and one without smart antenna functionality. However, if the chip manufacturers make two versions of their chips without at least a doubling of volume, they risk the price advantage that comes through volume. Finally, consumers that purchased WLANs without smart antennas, and then found that they needed them, would have to completely replace their access points, client cards or both.  
         [0014]     It is desirable to provide a smart external antenna module that provides an antenna array that can attach to an external port of a transceiver without any modification of the transceiver to provide improved performance with low manufacturing costs.  
       SUMMARY OF THE INVENTION  
       [0015]     The present invention relates to a method and system of providing a smart antenna using a smart antenna appliqué. The smart antenna appliqué comprises a smart external antenna module (SEAM) which attaches to the external antenna port of a transceiver without any modification of the transceiver. The only input to the smart antenna module is the signal to be transmitted from the transceiver and the only output is the received signal after smart antenna processing, which is sent on the same cable connection as the transmitted signal.  
         [0016]     The SEAM can be attached to an AP (Access point) to enhance both the transmit and receive range and coverage. Depending on the configuration the SEAM does this by a combination of beamforming and power amplification. SEAM uses a smart antenna integrated circuit to perform beamforming. For example, SEAM can employ the JSA100 Smart Antenna Integrated Circuit manufactured by Motia, Inc., Pasadena, Calif.  
         [0017]     In a default operation, SEAM is in a receive mode. While in a receive mode the unit is performing beamforming operations for any signals it receives. When the AP transmits the SEAM senses this and switches to a transmit mode. In the transmit mode the unit can use power amplification over either a single antenna or all antennas driven with fixed (possibly equal) weights, or the unit can use all its antennas to transmit to the desired user with the weights last calculated for reception from that same user. SEAM can be configured to perform transmit beam forming only without doing receive beam forming. In this (transmit beam forming only) configuration, SEAM uses all its antennas to transmit to the desired user with the weights last calculated for reception from that same user.  
         [0018]     In the receive mode, the weights to be used with input signals from a plurality of antennas can be calculated in a blind manner to improve performance and the weighted signals combined to produce an output signal. It has been found that because of the very short acquisition time for 802.11a/g, the latency of digital signal processing for weight calculation is a severe problem which is reduced by the use of analog processing rather than digital processing. In an embodiment of the present invention, the weights are calculated with analog processing, as digital processing adds significantly more latency.  
         [0019]     In an embodiment of the present invention, the weights are calculated using maximal ratio combining, which maximizes the receive output signal-to-noise ratio. Shortly after receive packet detection, the weights can be frozen and held until the end of the packet is detected for better performance. When the transceiver sends a signal to be transmitted, the smart antenna module detects this signal, turns off the receiver and turns on the transmitter, including power amplifiers. The transmitted signal is then split, weighted by the most recent receive signal weights, and then transmitted from the antennas. In one embodiment, the weights are the equal gain combining weights, which are just the phase of the maximal ratio combining weights, to maximize transmit signal power. In this embodiment, the power amplifiers can all operating at their maximum output power. Alternatively, if the desired user is unknown for a given packet to be transmitted (which may be the case at an access point with multiple users), the transmitted signal can use fixed weights that give an approximately omnidirectional pattern.  
         [0020]     The present invention relates to an implementation derived for 802.11, and the performance of this implementation is presented. The present invention permits seamless integration of smart antenna technology with existing WLAN chipset architecture. Since the 802.11 system uses time division duplexing (the same frequency is used for transmit and receive), smart antennas of the present invention can be used for both transmit and receive, providing a gain on both uplink and downlink, using smart antennas on either the client or access point alone.  
         [0021]     Results show a 13 dB gain with a four element smart antenna of the present invention over a single antenna system with the smart antenna on one side only, and an 18 dB gain with the smart antenna on both the client and access point. The present invention provides a “plug-and-play” adaptive array technology which can provide greater range, average data rate increases per user, and better overall coverage.  
         [0022]     The major advantage of this system is improved performance over the standard AP without the need to use processing resources from the AP. This allows for the flexible provisioning of enhanced performance to those users who require it.  
         [0023]     The appliqué of the present invention has the following advantages: fully 802.11 compliant; works with any transceiver chip set; virtually “plug and play;” applicable to access points and client devices; extends range up to about 4 times; increases throughput by 50% and higher; eliminates Dead Spots; reduces Power Drain up to about 90%; and has low manufacturing and operating costs. Utilizing smart antenna appliqués of the present invention to improve 802.11 systems is a cost-effective way to gain the clear benefits to WLAN performance without requiring the complete re-engineering of wireless equipment.  
         [0024]     The invention will be more fully described by reference to the following drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1  is a block diagram of a system for providing a smart antenna.  
         [0026]      FIG. 2  is an embodiment of a smart external antenna module.  
         [0027]      FIG. 3  is an alternate embodiment of a smart external antenna module.  
         [0028]      FIG. 4  is an alternate embodiment of a smart external antenna module.  
         [0029]      FIG. 5  is an alternate embodiment of a smart external antenna module.  
         [0030]      FIG. 6  is an alternate embodiment of a smart external antenna module.  
         [0031]      FIG. 7  is a schematic diagram of a weight freeze circuit.  
         [0032]      FIG. 8  is a schematic diagram of a transmit detection circuit.  
         [0033]      FIG. 9  is a schematic diagram of a de-glitch circuit.  
         [0034]      FIG. 10  is a schematic diagram of a data packet format.  
         [0035]      FIG. 11  is a schematic diagram of an operational timeline. 
     
    
     DETAILED DESCRIPTION  
       [0036]     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.  
         [0037]      FIG. 1  is a block diagram of a system for providing a smart antenna  10  in accordance with the teaching of the present invention. Appliqué  12  communicates with a plurality of antennas  14   a - d  and transceiver  16  using radio frequency (RF) communication techniques. Appliqué  12  combines signals  13   a - 13   d  from respective antennas  14   a - 14   d  and combines them to generate input signal  17  to transceiver  16 . In this embodiment, four antennas are used in system  10  and appliqué  12  combines the four signals from the four antennas to generate the input signal to transceiver  16 . It will be appreciated in alternative embodiments, various numbers of antennas can be used and the signals from the antennas can be weighted and combined to form an input signal to the transceiver.  
         [0038]     In one embodiment, appliqué  12  provides an adaptive array to weight and combine the received signals for forming input signal  17  to transceiver  16 . The use of an adaptive array provides an array gain, such as 6 dB with four antennas, in all environments, along with a diversity gain in multipath environments.  
         [0039]     For the adaptive array to provide the full diversity gain against multipath fading, the fading at each antenna element should be nearly independent. However, the correlation of the fading among the antennas can be as high as 0.7 before the degradation exceeds 1 dB. The correlation can be kept below this level through the use of the following diversity types. The first diversity type is spatial diversity or the spatial separation of the antennas. In a severe multipath environment, such as indoors or around a handset, only about a quarter of a wavelength spacing is needed to obtain low fading correlation. The second diversity type is polarization diversity. The use of dual polarization, horizontal and vertical, allows one physical antenna to be used for two input signals (using different feeds for each polarization). A third diversity type is pattern diversity, the use of antenna elements with different patterns. The combination of the above types of diversity permits the use of a large number of antennas even in a small form factor, such as a PCMCIA card or handset, with near ideal performance.  
         [0040]     There are several combining techniques for the adaptive array. The simplest is selection diversity, whereby the antenna with the highest received signal power is selected for the output signal. This technique does not use all the received signal power and therefore is limited in its dB gain improvement over a single antenna. Another technique for range increase (i.e., in a noise limited environment) is maximal ratio combining (MRC), whereby each signal is weighted and combined to maximize the output signal-to-noise ratio (SNR). The beamforming weights are then the complex conjugate of the channel transfer characteristic, i.e., the received signals are co-phased and their gain adjusted based on their received signal strength. This technique provides a gain of M along with a diversity gain of M in a Rayleigh fading environment. The combiner weights can be generated by correlating the output signal with the received signal on each antenna. Such a method is referred to as a blind technique, as the adaptive array will maximize the SNR of any received signal and demodulation of the received signal is not required.  
         [0041]     A modification of MRC is equal gain combining, whereby the signals are co-phased, but the gain is the same for each antenna element. This technique provides an improvement that is typically one dB lower than that of MRC and does not use gain adjustments on each antenna. In an environment where interference is the greatest concern, an improved technique is minimum mean square error (MMSE) combining, whereby the received signals are weighted and combined to maximize the output signal-to-interference-plus-noise ratio. This technique uses a method to distinguish the desired signal from interference. In general, this requires demodulation of the received signal (although blind techniques, such as the Constant Modulus Algorithm are possible) and thus weight generation is performed by a non-blind technique.  
         [0042]     In a multipath environment, when the difference in the propagation delays of the various paths between the transmitter and receiver (the delay spread) becomes comparable to the symbol period, frequency selective fading results in intersymbol interference which degrades performance. To overcome this impairment, typically temporal equalization at the receiver or OFDM is used. Both of these techniques are conventionally implemented in 802.11. In this case, spatial processing (i.e., the adaptive array described above) followed by temporal equalization or OFDM demodulation is not optimum, and joint spatial-temporal processing is needed for optimal performance, although close to optimum performance with the former technique can often be achieved if the delay spread is small.  
         [0043]     Transceiver  16  can include RF processor  17  and baseband/MAC processor  18 . RF processor  17  typically downconverts received RF signals for baseband processing and upconverts baseband signals for transmission. Baseband/MAC processor  18  performs processing of baseband signals to receive information from a received signal and to connect information in preparation for transmission. Baseband/MAC processor  18  can also provide a host interface.  
         [0044]      FIG. 2  is a schematic diagram of an implementation of appliqué  12  as smart external antenna module (SEAM)  20 . Signals  13   a - 13   d  received by antennas  14   a - 14   d  are passed through filters  21   a - 21   d  and antenna transmit/receive (TR) switches  22   a - 22   d . Signals RFin 1 , RFin 2 , RFin 3 , RFin 4  from transmit/receive (TR) switches  22   a - 22   d  are weighted and combined using the smart antenna integrated circuit  24 . For example, integrated circuit  24  can be a JSA100 circuit manufactured by Motia, Inc., Pasadena, Calif. In one embodiment, integrated circuit  24  performs beamforming using an implementable version of the MRC algorithm for providing weights, which as a blind technique can work for 802.11b, 802.11g, and 802.11a, as long as the weights can be calculated in this algorithm within 2 microseconds after the signal is received as this is the time allotted for 802.11a/g.  
         [0045]     In typical operation SEAM  20  is receiving signals, performing beamforming with integrated circuit  24 , and sending the signals to a desired location, such as an access point. In preferred embodiments, continuous beamforming over each packet can be done in 802.11b, but in 802.11a/g the module would freeze the antenna weights after 2 microseconds for the duration of each packet so that the weight fluctuations do not affect the performance of the 802.11a/g transceiver.  
         [0046]     When access point (AP)  25  transmits, coupler  26  sends a small signal through transmit/receive switch (TR)  27  to SEAM IN pin  28 , integrated circuit  24  detects the signal and sets SEAM OUT pin  29  to high. This switches input TR switch  30 , antenna TR switches  22   a - 22   d , and turns on the power amplifiers (PA)  32   a - 32   d . When access point (AP)  25  stops transmitting the SEAM OUT pin  29  goes to low, and SEAM  20  returns to a receive mode.  
         [0047]     Since there may be no control signal connections between SEAM  20  and access point (AP)  25 , when access point (AP)  25  transmits on one frequency channel, SEAM  20  has to independently determine that channel. SEAM  20  can use a mode referred to as pass-through mode for channel determination. In pass-through mode, all the channel weights are set to be the same and SEAM  20  passes the signals to and from access point (AP)  25  with no beamforming. In the meantime, signal  27  from access point (AP)  25  is routed through the SEAM  20  receiver input. When access point (AP)  25  transmits, SEAM  20  sets LO  33  to a channel, and SEAM  20  checks the received power in that specific channel.  
         [0048]     After a period of time, SEAM  20  has checked all channels. SEAM  20  selects the channel with the highest power. In this way, SEAM  20  guarantees it has not incorrectly selected a channel due to variations in the background levels. After finding the correct frequency channel SEAM  20  continually checks that SEAM  20  is still on the same channel as access point (AP)  25  to determine if a change to the AP frequency channel has occurred.  
         [0049]     SEAM  20  can be implemented as a standalone module and attached to access point (AP)  25  through a single coaxial cable. No other control signal connections are required between SEAM  20  and access point (AP)  25 . No software or hardware configuration changes are required in access point (AP)  25  during installation. Such arrangement allows SEAM  20  to be used as a standalone accessory. An AC adaptor (not shown) can supply DC power to SEAM  20 .  
         [0050]     In an initial operation, SEAM  20  determines which frequency channel access point (AP)  25  is using, i.e., transmitting. SEAM  20  operates in a pass-through mode, where all receive channels are given equal weighting, and receive (RX) and transmit (TX) signals  27  are simply passed along. Microprocessor  34  sets receive LO  33  to channel  1  and waits for access point (AP)  25  to transmit. When access point (AP)  25  transmits, integrated circuit  24  detects the transmission through a power detector associated with SEAM IN pin  28 , and sets a SEAM OUT signal to high. Integrated circuit  24  is switched to a transmit mode. In this instance, the receive mode is also active. Limiter  35  is used to stop high energy from access point (AP)  25  from damaging the RF output of integrated circuit  24  before input TR switch  30  activates.  
         [0051]     The SEAM OUT signal goes to microprocessor  34 . When the SEAM OUT signal goes high, microprocessor  34  sends a high to OR gate  36 , to keep the transmit circuitry active. Microprocessor  34  commands Rx circuitry of integrated circuit  24 . There is enough transmit signal coming through power amplifiers (PA)  32   a - 32   d  and TR switches  22   a - 22   d  such that the receiver can detect a signal.  
         [0052]     A detection process involves monitoring the power of the received signals filtered by receiver channel filters  21   a - 21   d . Microprocessor  34  measures the power level and the power level is recorded in memory. Microprocessor  34  then switches SEAM  20  to a different channel and waits for the next transmit packet. Once microprocessor  34  has information on all the channels it selects the channel with the highest power level, and enables receive beamforming in integrated circuit  24 .  
         [0053]     In a normal operation, SEAM  20  is in the receive mode. When access point (AP)  25  starts to transmit a signal, coupler  26  couples the power of the transmit signal to the SEAM_IN pin  28  of integrated circuit  24 . A power detector of integrated circuit  24  detects the transmit power and automatically switches integrated circuit  24  to the transmit mode. In the meantime, the SEAM_OUT pin  29  switches the antenna TR switches  22   a - 22   d , input TR switch  30 , and power amplifiers  32  to allow transmission.  
         [0054]     As soon as access point (AP)  25  stops transmitting, the power level at SEAM_IN pin  28  drops and integrated circuit  24  switches back to the receive mode.  
         [0055]     This embodiment of SEAM module  20  is implemented with four antennas for both receive and transmit, and no interaction with the AP microprocessor. This embodiment provides high functionality with easy integration into an existing system. It can support Rx beamforming for any received packet, but for Tx beamforming only the last Rx weights are used for the transmit, i.e., the SEAM always transmits back to the last user that a packet was received from. This may be satisfactory for a client that only receives signals from one AP, but would not be effective for an AP with multiple clients. To avoid this problem, alternatively, transmission can be performed using an omnidirectional beampattern, i.e., without transmit beamforming.  
         [0056]      FIG. 3  is a schematic diagram of an alternate embodiment of SEAM  20  without transmit beamforming. In this embodiment, a single power amplifier  32  is used instead of four power amplifiers  32   a - 32   d  shown in the embodiment illustrated in  FIG. 2 . It is expected that the output power of an embodiment using a single power amplifier  32  is 6 dB higher than the output power of an embodiment using four power amplifiers  32   a - 32   d.    
         [0057]     The normal operation of SEAM  20  in this embodiment is the same as the embodiment of  FIG. 2  except that only one antenna is used for transmission.  
         [0058]      FIG. 4  is a schematic diagram of an alternate embodiment of SEAM  20 . This embodiment does not use internal PAs. Accordingly, power used by these circuits can be saved. This embodiment connects power amplifier  40  between antenna TR switch  22  and input TR switch  30 .  
         [0059]      FIG. 5  is a schematic diagram of an alternate embodiment of SEAM  20 . This embodiment does not use antenna TR switch  22 . This embodiment uses transmit antenna  45  and filter  46   c . Power amplifier  40  is connected between input TR switch  30  and transmit antenna  45 .  
         [0060]     This SEAM module provides lowest cost implementation with low manufacturing costs. It uses one PA  34  to drive an additional antenna for transmission, rather than one of Rx antennas  14  as in  FIG. 4 . This embodiment also has no interaction with the AP&#39;s microprocessor. It has receive beamforming, but does not provide transmit beamforming.  
         [0061]      FIG. 6  is a schematic diagram of an alternate embodiment of SEAM  20 . In an initial operation, the channel frequency is passed from access point (AP)  25  to SEAM  20  via a signaling data channel  48 . A signaling data channel provides digital control data from AP to SEAM necessary to set the antenna weight. Since this data signal is at a different frequency as the RF signal, it can be sent through the same coax cable as the transmit and receive RF signal. Microprocessor  34  programs LO  33  to the desired channel. This embodiment does not need to search for the AP frequency channel.  
         [0062]     In a normal operation, integrated circuit  24  is normally in the receive mode. When a packet arrives at access point (AP)  25 , integrated circuit  24  performs a beamforming operation and provides an enhanced signal to access point (AP)  25 . When access point (AP)  25  detects the signal, it passes the packet ID to microprocessor  34 . Microprocessor  34  samples the antenna weights with an on-chip analog digital conversion (ADC) and stores the weights in a table associated with the receive ID (not shown).  
         [0063]     When access point (AP)  25  wants to transmit to a specific destination, access point (AP)  25  passes the destination ID to microprocessor  34  and instructs microprocessor  34  to load the weight associated with the ID into integrated circuit  24 . Access point (AP)  25  then transmits the packet, and integrated circuit  24  detects the transmit power through SEAM_IN pin  28  and switches to the transmit mode. Once access point (AP)  25  terminates transmission, integrated circuit  24  returns to the receive mode.  
         [0064]     This embodiment uses a signaling data link with the AP&#39;s microprocessor to get information about the next transmit signal. SEAM  20  has full receive and transmit beamforming. In an alternate embodiment, variants of this embodiment and the embodiments shown in  FIGS. 3-5  could be used to provide reduced costs.  
         [0065]     SEAM  20  processes a received signal and passes it to access point (AP)  25 . Access point (AP)  25  further processes the signal to extract data. In order not to interfere with operation of access point (AP)  25 , SEAM  20  can implement a weight freeze. In a weight freeze embodiment, SEAM  20  computes the antenna weights on a packet by packet basis in the first two microseconds of the preamble of each packet using an adaptive algorithm. After the first two microseconds, if SEAM  20  were to continue to adapt the antenna weight for the entire duration of the packet, access point (AP)  25  receiver operation might be affected. Accordingly, it is desirable to freeze the antenna weight right after two microseconds into the preamble to avoid interfering with operation of the access point (AP)  25  receiver.  
         [0066]     An implementation for providing the weight freeze is shown in  FIG. 7 . Comparator  51  compares the received signal power to a fixed threshold (TH 1 ). It is triggered at a lower received signal level to create a timing window. The timing window is two microseconds and is determined by the RC time constant at output of the inverter  51 .  
         [0067]     Comparator  52  triggers at a slightly higher power level, determined by the fixed threshold (TH 2 ). TH 2  is slightly higher than TH 1 . As long as the received signal level exceeds TH 2  within two microsecond window, D flip/flop  54  is latched by OR gate  53  and the antenna weight is frozen.  
         [0068]     Feedback resistor  55  creates a hysteresis (changes TH 1  to TH 1 ′) in Comparator  51  to prevent chattering. As soon as the received power level falls below the threshold TH 1 ′, D flip/flop  54  is reset and the antenna weight is unfrozen.  
         [0069]     SEAM  20  needs to accommodate a very high signal dynamic range. SEAM  20  can provide a step attenuator to accommodate the high signal dynamic range. SEAM  20  detects the received power, and if the received power exceeds a threshold, attenuator  35  is set to a high attenuation. It is possible that access point (AP)  25  receiver operation can be adversely affected when SEAM  20  output power suddenly changes to a different level.  
         [0070]     In order to avoid this problem, high-low gain switch  60  can be used as shown in  FIG. 8 . When the received power exceeds a threshold, SEAM  20  switches to a high attenuation mode. Once it switches, SEAM  20  stays in the high attenuation mode until the next packet arrives. For the second packet, SEAM  20  is already in the high attenuation mode, and no switching occurs, so access point (AP)  25  can receive the packet properly. If the received power still exceeds the threshold, it retriggers the high attenuation mode, and then extends the holding period further to the next packet. This operation continues until the received power falls below the threshold.  
         [0071]     It has been found that if a transmit packet is sent in between two received packets, the transmit power level is typically higher than the received level and it triggers the high attenuation mode immediately. To recover back to the state before the transmit packet occurs, the previous state is pushed into a second D-flip/flop  60 . At the end of the transmit packet (determined by the falling edge of SEAM_OUT pin  29 ). The state stored inside the second D flip/flop  60  is pushed back into D flip/flop  54 . SEAM  20  then reverts back to the state before the transmit packet occurred. By employing such circuit, SEAM can ensure that its high-low gain switching will not affect the AP receiver operation. This allows SEAM to interface to any AP from different vendors.  
         [0072]     SEAM  20  can employ a power detector to detect the transmit signal from access point (AP)  25  at SEAM IN pin  28  and set SEAM OUT pin  29  high if the signal level exceeds a predetermined threshold. This switches input TR switch  30  and antenna TR switches  22 , and turns on PAs  32 . When access point (AP)  25  stops transmitting, SEAM OUT pin  29  goes low, and SEAM  20  is back in the receive mode. The signal power level from access point (AP)  25  can fluctuate by a large amount. One example is the 802.11a and 802.11g system employing orthogonal frequency division multiplexed (OFDM) signals which have a large peak to average power ratio. During the transmit duration, the signal power level could fall below the threshold, causing SEAM_OUT pin  29  to go low, and causing the SEAM operation to fail. A deglitch circuit  70  as shown in  FIG. 9  can provide transmit detection. SEAM_OUT pin signal  29  controls the switch. When SEAM_OUT pin  29  is high, switch  71  goes to high position. When SEAM_OUT pin  29  is low, switch  71  goes to the low position. Deglitch circuit  70  removes the short duration glitch during transmit packet caused by large power level fluctuation.  
         [0073]     System  10  as described above for 802.11 implements time division duplexing (TDD), to use the same antennas for transmission as for reception, with the same weights and obtaining the same performance in both directions with appliqué  12  on just one device. The only requirements are that the channel hasn&#39;t changed between reception and transmission, and the relative delays and gains of each of the receiver chains, as well as the transmitter chains, must be similar. Since the transmit power amplifiers have peak power limitations, under this constraint it has been found that improved performance is achieved by using just the phases of the receive weights for transmission (with the gains equal), such as using the equal gain combining weights.  
         [0074]      FIG. 10  illustrates a data packet format which can be used in an operational timeline shown in  FIG. 11 .  
         [0075]     With the four element smart antenna at both the client and the access point, computer simulation results show a gain of 18 dB over a single antenna system. The gain is less than 26 dB (i.e., twice the 13 dB gain on one side) because the diversity gain is less on the receive side since transmit beamforming provides most of diversity gain possible. Also, since the weights at the transmitter and the receiver are calculated independently, the gain is less than if they were calculated jointly. With ideal transmit and receive weights the gain would be 22 dB. Accordingly, the present invention is only 4 dB from ideal, and this calculation also includes implementation loss (i.e., weight estimation error).  
         [0076]     With delay spread, the 13 dB can be reduced somewhat, even with a temporal equalizer in the receiver. Because the equalizers are not specified in the 802.11b standards, the performance degradation of our spatial processing in delay spread of the present invention will vary with different 802.11b transceivers. It has been found that for a textbook tapped delay line equalizer, the degradation was less than 1 dB (i.e., a 12 dB gain) with delay spreads up to 50 ns rms.  
         [0077]     The performance of the smart antenna appliqué for 802.11a/g is shown in Table 1.  
         [0078]     Computer simulation results are shown for the beamforming gain at a 10% packet error rate (PER) for data rates of 6, 24, and 54 Mbps, short and long packets, and independent Rayleigh fading with different delay spreads. The delay spread model used was an exponential decay with 0 (flat) and 50 ns rms delay spread. Typical indoor environments for 802.11 have delay spreads on the order of 30-50 ns rms. With flat fading, the smart antenna has a gain of 11 to 12 dB, while the gain is reduced to 7 to 10 dB with 50 ns rms delay spread. The gain is further reduced by a few dB with higher delay spreads, typical of outdoor environments.  
                                                                                                                                           Beamforming Gain (dB) @ 10% PER                54 Mbps                    6 Mbps   24 Mbps   Short   Long                    Short   Long   Short   Long   Pack-   Pack-   Sum-           Packet   Packet   Packet   Packet   et   et   mary                        Flat   11   11   12   12   12   12   11˜12       Rayleigh       Fading       50 ns Exp   8   10   7   7   8   9    7˜10       Decay       Rayleigh       Fading                  
 
         [0079]     The above cases show the SNR gain with the smart antenna, which can be related to a range increase (e.g., a doubling of range for a 12 dB gain with a fourth law power propagation exponent typical for indoor environments). However, this gain can also translate into a data rate increase in 802.11 systems since higher SNRs allow for higher data rates (up to the peak data rate).  
         [0080]     It is to be understood that the above-described embodiments are illustrative of only a few of the many possible specific embodiments, which can represent applications of the principles of the invention. Numerous and varied other arrangements can be readily devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention. This includes use of the SEAM in other TDD systems, such as 802.16, as well as frequency division duplex systems.