Abstract:
Multi-pattern transmission of frames. The method of operations comprises transmitting a first portion of a frame using a first radiation pattern. The frame comprises one or more preambles and a single data portion associated with the one or more preambles. Thereafter, an operation is conducted to switch the radiation pattern from the first radiation pattern, used to produce the first portion of the frame, to a second radiation pattern. A second portion of the same frame is produced using the second radiation pattern.

Description:
CROSS-REFERENCE OF RELATED APPLICATIONS 
     This is a continuation application of U.S. patent application Ser. No. 12/432,614, now U.S. Pat. No. 8,223,072, the entire contents of which are incorporated by reference. The applicant(s) hereby rescind any disclaimer of claim scope in the parent application(s) or the prosecution history thereof and advice the USPTO that the claims in this application may be broader than any claim in the parent application(s). 
    
    
     FIELD 
     The present invention relates to wireless digital networks, and in particular, to the problem of transmitting information in dense or crowded RF environments and improving the reliability of transmissions in any environment. 
     BACKGROUND OF THE INVENTION 
     Wireless digital networks, such as those operating to IEEE802.11 Ethernet standards, use wireless access nodes connected to controllers and provide a wide range of services to wireless clients, such as access to infrastructure devices and services such as printers and file servers, as well as to the greater Internet. In RF-dense environments like those found in corporate offices, it is common to have many devices, wireless access nodes and wireless clients both, operating in close proximity. 
     When devices operate in close proximity, both in terms of physical location and radio frequencies, opportunities for interference arise. Such interference can occur not only when devices operate on the same channel, but when devices operate on partially overlapping or adjacent channels. As an example, in the U.S. 2.4 GHz ISM band, the three non-overlapping channels are 1, 6, and 11. Channels 1 and 3, for example, partially overlap. Channels 1 and 6 are adjacent, but they can still interfere with each other depending on the proximity of the devices and strength of transmissions. This 2.4 GHz band is also shared with services such as Bluetooth, wireless telephones, microwave ovens, and other devices which intentionally or unintentionally radiate RF energy. 
     The design of IEEE802.11 protocols alleviates these problems to a certain degree by implementing carrier sense and collision avoidance; before a device transmits on a channel, it first listens for activity. If it detects activity on the channel, it backs off for a minimum predetermined time or a randomly chosen time within a predetermined range, and checks again. In this “carrier sense” approach, the device senses for energy and carrier at the transmitter and defers the transmission if energy or carrier is detected, and it does not have the necessary intelligence to determine if the detected energy or carrier would actually interfere with its own transmission and vice versa. 
     It is known to the art that the “interference range” of a device is commonly greater than the “communications range,” that is, a device is capable of causing interference to other devices at greater distances than it is capable of establishing communications. 
     What is needed is a better method of operating in RF environments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention in which: 
         FIG. 1  shows antenna patterns, 
         FIG. 2  shows a frame format, 
         FIG. 3  shows a block diagram of a transmitter and antenna system, and 
         FIG. 4  shows another block diagram of a transmitter and antenna system. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention relate to methods of transmitting wireless data frames. According to the present invention, a digital device contains a transmitter feeding an electronically steerable antenna system where the radiation pattern produced by the antenna system may be electronically selected. A first portion of a wireless data frame such as a data frame according to one or more IEEE 802.11 standards is transmitted using an a first radiation pattern, and a second portion of the same frame is transmitted using a second radiation pattern. The first and second portions may be transmitted using an electronically steerable antenna system that supports the ability to switch or electronically alter radiation patterns. The antenna system may use one antenna with switchable elements, or may use different antennas with different radiation patterns. When multiple antennas are used, the first antenna may be an omnidirectional antenna, and the second antenna may be one of a group of antennas providing beamforming or sectorized coverage. 
       FIG. 1  shows a representation of antenna radiation patterns. Omnidirectional antenna  100  when used to transmit information produces an omnidirectional radiation pattern, represented by circles  110  and  120 . In wireless digital systems such as those operating under the various IEEE 802.11 standards, information is transmitted (and received) on channels in the 2.4 GHz and 5 GHz bands. 
     As is understood in the RF arts, interference can limit the effective range of communications. A transmitter and antenna such as  100  in  FIG. 1  has an effective communications range shown by circle  120 . Particularly in the 802.11 environment where the range of frequencies used by RF channels overlap and collision avoidance techniques are used, transmissions on the same or adjacent channels can cause interference to other devices, and can cause interference outside the ranges at which solid data transfers are possible. This interference range is shown as circle  110  of  FIG. 1 , covering more area than the communications range shown by circle  120 . 
     An approach to improving communications is to use directional antennas to direct more RF energy to the desired receiver, such as by using antennas with directional patterns rather than omnidirectional patterns. Also shown in  FIG. 1  are idealized radiation patterns for 120-degree sectorized antennas  130   a ,  130   b , and  130   c . Antenna  130   a  has a radiation pattern filling sector  140   a , antenna  130   b  has a radiation pattern filling sector  140   b , and antenna  130   c  has a radiation pattern filling sector  140   c . Other approaches in addition to such sectorized, adaptive beam-forming, or smart antennas are often described as electronically steerable and rely on switching antennas and/or antenna elements, or altering the phasing between elements to increase antenna gain to try and mitigate interference and improve communications. Such electronically steered antennas may be used for transmission, reception, or both. 
     As is understood by the art, IEEE 802.11 wireless systems practice collision avoidance; prior to transmitting on a channel, a device monitors that channel for a predetermined period of time. If the device senses energy or carrier present on the channel, it backs off for a predetermined period of time, thus avoiding collisions which would occur if the device had started transmitting. It is understood that devices that are not within the directional pattern of another device, but are within the communication range, may not be able to hear the directional transmissions and hence may attempt to transmit causing further delays, collisions or interference. A well understood solution is to transmit a separate frame such as short CTS (Clear-to-Send) or equivalent frames in omnidirectional or another pattern prior to the directional transmission to prevent the devices outside of the directional pattern from attempting to transmit. This additional transmission of a separate frame prior to the beam-forming pattern incurs additional overhead thereby limiting the overall capacity. 
     According to an aspect of the invention, a device transmitting a digital frame of information transmits a first portion of the frame using a first antenna radiation pattern, switching to a second antenna radiation pattern at a predetermined point in the frame and transmitting the second portion of the frame using the second antenna radiation pattern. Transmit power delivered to the antenna system may also be varied between the first and second portions of the frame. The point at which the switching from one pattern to another occurs may vary per-frame depending on the nature and mode of transmissions. In one embodiment of frame-steering, the device transmits the first portion of the frame using a wide radiation pattern, such as the omnidirectional pattern  120  of  FIG. 1 , and the second portion of the frame using a narrower pattern, such as that shown using a sectorized antenna such as  130   a  producing pattern  140   a  of  FIG. 1 . The selection of the pattern used for the second portion of the frame depends on the location of the receiver. In an alternate embodiment, the first antenna radiation pattern may combine a wide radiation pattern with a narrower pattern, with only the narrower pattern being used for the second portion. 
       FIG. 2 . shows a sample frame  200  according to 802.11 standards, in this case a high-throughput (HT) frame typical of IEEE 802.11n communications. Such a frame consists of a broadcast or legacy portion  215  which has a non-HT preamble  210 , legacy protection and PLOP data (L-SIG)  220 , followed by the HT portion  235  which includes HT-sig  230 , HT-training  240 , and HT-data  250 . One of the purposes of the L-SIG header is to allow HT 802.11n frames to be identified by older legacy 802.11a/b/g devices which cannot decode 802.11n. The HT portion  235  of the frame includes identification  230  and training  240  fields as well as the data  250  field. For some frames, such as HT frames, different modulation methods may be used for the broadcast or legacy portion  215  of the frame and the data or HT portion  235  of the frame. As an example, for 802.11n, the broadcast or legacy portion  215  of frame  200  consists of legacy long and short training sequences and robust BPSK-OFDM modulation signal field, while the HT portion  235  of frame  200  consists of HT long and short training sequences, HT-SIG with BPSK-OFDM and HT-Data with one of the less robust and more efficient OFDM modulations, BPSK, QPSK 16-QAM or 64-QAM. 
     According to an aspect of the invention, for multi-pattern frames, the broadcast portion  215  of frame  200  is transmitted using the first antenna radiation pattern, and the second portion  235  of frame  200  is transmitted using the second narrower antenna radiation pattern. By transmitting broadcast portion  215  of frame  200  over a wider area, nearby devices will sense this portion of the frame and back off, while switching to a narrower antenna pattern for second portion  235  of frame  200  which includes data  250 , and allows more RF energy to be delivered to the target device. 
       FIG. 3  shows an embodiment of the invention. While the embodiment shows sectorized or switched antennas, it is equally applicable to other electronically steerable antenna systems. Antenna  100  is a broad-pattern antenna such as an omnidirectional antenna. Antennas  130   a ,  130   b , and  130   c  are higher gain directional antennas such as sectorized antennas. As an example, three 120-degree sector antennas may be used, or four 90-degree sector antennas may be used. 
     These antennas  100 ,  130   a ,  130   b ,  130   c  are connected respectively to radio frequency (RF) switches  310 ,  312 ,  314 , and  316 . While these switches are preferably PIN diode switches, other technologies may also be used, provided they have the required switching speeds and isolation. As an example gated power amplifiers may be used. PIN diode switches for RF are known to the art, and are described for example in  The PIN Diode Circuit Designers&#39; Handbook  published in 1998 by Microsemi Corporation, incorporated herein by reference. PIN diodes are available from numerous sources including Microsemi, Infineon, Vishay, and Avago Technologies. Switches are selected using control lines  318 . 
     Switches  310 ,  312 ,  314 ,  316  are fed by RF distribution network  300 , which may be a separate RF splitter such as those available from Mini-Circuits Corporation, or this functionality may be incorporated along with switches  310 ,  312 ,  314 ,  316 . 
     The overall radiation pattern of an antenna may also be altered or steered by selecting elements of the antenna to feed, or by altering phasing among elements of an antenna or antenna array. Such an embodiment would have a block diagram similar to that of  FIG. 3 , where multiple elements  130   a ,  130   b ,  130   c  may be selected at any time, and/or the phasing of elements is varied. Switching antenna elements and/or altering phasing of elements may also be accomplished using PIN diode switches. 
     Transmitter  350  shown in block diagram form includes power amplifier  360  producing RF output  365  which feeds RF distribution network  300  and the switches and antennas. As shown, transmitter  350  includes antenna sequencer  370  which drives control lines  318  and controlling switches  310 ,  312 ,  314 ,  316 . Transmitter  350  receives a transmit datastream  380  and antenna selection data  390  from controller  500 . Portions of the transmitter such as local oscillators, mixers, I/Q modulators and the like not necessary to understand the invention are not shown. 
     According to the invention, when transmitting a multi-pattern frame, controller  500  provides transmitter  350  and its antenna sequencer  370  with information on which antenna pattern to use in the default configuration, such as for receiving, which antenna pattern to use for the first portion of the frame, and which antenna pattern to use for the second portion of the frame. Switching between the first and second patterns is initiated by transmitter  350  and its antenna sequencer  370 . 
     As an example, transmitter  350  when transmitting a multi-pattern frame  200 , first selects switch  310  and antenna  100  for the first portion of the frame, for example, broadcast portion  215  of  FIG. 2 . During transmission of the second portion  235  of frame  200 , transmitter  350  switches off switch  310  and antenna  100  and switches on one of switches  312 ,  314 ,  316  and accompanying antenna  130   a ,  130   b ,  130   c . Thus the first portion  215  of frame  200  is transmitted using a wide pattern, and the second portion  235  of frame  200  is transmitted using a narrow pattern. 
     In another example, transmitter  350  when transmitting a multi-pattern frame  200  selects switch  310  with antenna  100 , and one of switches  312 ,  314 ,  316  and accompanying antenna  130   a ,  130   b ,  130   c . The first portion  215  of frame  200  is thus transmitted using wide-coverage antenna  100  and one of the sectorized antennas. For the second portion  235  of frame  200 , switch  310  and thus antenna  100  are disabled, so only the enabled sectorized antenna  130   a ,  130   b , or  130   c  is used for transmitting. This results in the first portion  215  of frame  200  being transmitted using a combined wide and narrow pattern, with the second portion  235  only being transmitted using the narrow pattern. 
     According to the invention, a second embodiment is shown in  FIG. 4 . Where transmitter  350  of  FIG. 3  generates antenna switching signals  318  directly, and thus must be designed and implemented in accordance with the invention, the embodiment of  FIG. 4  uses an unmodified transmitter  350  and implements antenna switching along side the transmitter. This embodiment may be more applicable for use with standard designs and/or prebuilt transmitter and transmitter/receiver assemblies. Antenna selection  318  is provided by antenna controller  400  which receives antenna data  410  from controller  500 . In such an embodiment, antenna pattern switching is timing based. When a multi-pattern frame is to be transmitted, controller  500  sends to antenna controller  400  information on which antenna pattern is to be enabled for the first period, the time of the first period, and information on which antenna pattern is to be used for the second period. 
     As an example, using a frame  200  such as shown in  FIG. 2 , the time required to transmit the first portion  215  of frame  200  is predetermined by controller  500 . This time may vary from frame to frame, and over frame types. Controller  500  sends  410  antenna selection information and timing information to antenna controller  400 . When controller  500  begins sending transmit data  380  to transmitter  350 , controller  500  also signals  410  for antenna controller  400  to start its timing cycle. Assume antenna controller  400  has a counter chain which has been loaded with the time required to transmit the first portion  215  of frame  200 . Antenna controller  400  begins counting when transmission begins, as signaled by controller  500 . When the count completes, antenna controller  400  switches antennas as selected. Microsecond resolution is adequate for such a counter. This may be implemented, for example, using programmable logic such as a CPLD or FPGA for antenna controller  400 , a counter-driven state machine, or even using a high-speed counter chain in a dedicated microprocessor. Suitable CPLDs and FPGAs are available from manufacturers such as Lattice Semiconductor, Altera, Xilinx, Atmel, and Cypress. 
     As with the embodiments of  FIG. 3 , in a first embodiment of  FIG. 4  when transmitting a multi-pattern frame  200 , controller  500  commands antenna controller  400  to select switch  310  and antenna  100  for the first portion of the frame, for example, broadcast portion  215  of  FIG. 2 . Controller  500  also sends antenna controller  400  the duration of the first portion of the frame, and the antenna to select when this portion is complete. Controller  500  then signals the start of the transmission, sending data  380  to transmitter  350 . When the counter in antenna controller  400  expires, it switches off switch  310  and antenna  100  and switches on one of switches  312 ,  314 ,  316  and accompanying antenna  130   a ,  130   b ,  130   c.    
     In an alternate embodiment, when transmitting a multi-pattern frame  200 , controller  500  commands antenna controller  400  to select switch  310  with antenna  100 , and one of switches  312 ,  314 ,  316  and accompanying antenna  130   a ,  130   b ,  130   c . Controller  500  also sends antenna controller  400  the duration of the first portion of the frame. Controller  500  then signals the start of the transmission, sending data  380  to transmitter  350 . The first portion  215  of frame  200  is thus transmitted using wide-coverage antenna  100  and one of the sectorized antennas. When the counter in antenna controller  400  expires, it switches off switch  310  and antenna  100 , so only the enabled sectorized antenna  130   a ,  130   b , or  130   c  is used for transmitting. 
     According to another embodiment of the invention in  FIG. 3  or  4 , controller  500  also sends the duration of the second portion of the frame, the antenna pattern to be used for receiving subsequent response frames, and duration for staying in the same pattern for receive mode, in addition to the first portion&#39;s duration and antenna patterns. The antenna controller switches to receive mode after the duration of the second portion is completed and/or transmit-to-receive transition is detected by other means so as to receive a response frame such as 802.11 Acknowledgement or 802.11 Block Acknowledgment using the same pattern used for the second portion of the frame, or a different pattern. 
     In all embodiments, the antenna controller is instructed to use a specific or default pattern for reception. As an example, in 802.11 the radio is always in receive mode unless the radio is transmitting a frame or sequence of frames. 
     According to an aspect of the invention, for multi-pattern frames, the broadcast portion  215  of the frame  200  is transmitted using the first antenna radiation pattern and a first transmit power level, and the second portion  235  of the frame  200  is transmitted using the second narrower antenna radiation pattern and a second power level. By transmitting broadcast portion  215  of frame  200  using a different transmit power, the interference range is controlled effectively, while switching to a narrower antenna pattern and a different transmit power for second portion  235  of frame  200  which includes data  250 , and allows more RF energy to be delivered to the target device increasing the reliability of the communication. The differences in power levels may be implemented using a combination of power-amplifier control, controlling the drive level to the power amplifier, and/or switching attenuators between the transmitter and the antenna. 
     It should be noted that not all frames need be transmitted using the multi-pattern capability; selection of frames for multi-pattern transmission, as well as transmit power levels and the antenna patterns to be used is made by controller  500 , or by other control elements of the larger system in which the transmitter is embedded. 
     While the invention has been described in terms of various embodiments, the invention should not be limited to only those embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is this to be regarded as illustrative rather than limiting. The applicant(s) hereby rescind any disclaimer of claim scope in the parent application or the prosecution history thereof and advice the USPTO that the claims in this application may be broader than any claim in the parent application.