Abstract:
Methods and apparatus for providing channel diversity to wireless terminals (WTs) in a manner that reduces the latency between the time a WT encounters satisfactory channel conditions are described. A plurality of communications channels with different physical characteristics are maintained in a cell by a base station (BS). Each WT monitors multiple channels and maintains multiple channel estimates at the same time so that rapid switching between channels is possible. Channel quality information is conveyed from each WT to the BS. The WT or BS selects a channel based on the measured channel quality. By supporting multiple channels and by introducing periodic variations into the channels in various embodiments, the time before a WT encounters a channel with good or acceptable channel conditions is minimized even if the WT does not change location. Multiple antennas are used at the BS to support numerous channels simultaneously, e.g., by controlling antenna patterns.

Description:
RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 10/763,944, filed Jan. 23, 2004, now allowed, entitled METHODS AND APPARATUS OF PROVIDING TRASMIT DIVERSITY IN A MULTIPLE ACCESS WIRELESS COMMUNICATION SYSTEM, which claims the benefit of U.S. Provisional Patent Application Ser. No. 60/442,008, filed Jan. 23, 2003, entitled “METHODS AND APPARATUS OF PROVIDING TRANSMIT DIVERSITY IN A MULTIPLE ACCESS WIRELESS COMMUNICATION SYSTEM” and U.S. Provisional Patent Application Ser. No. 60/509,741, filed Oct. 8, 2003 entitled “METHODS AND APPARATUS OF PROVIDING TRANSMIT DIVERSITY IN A MULTIPLE ACCESS WIRELESS COMMUNICATION SYSTEM” each of which is hereby expressly incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to communications systems and, more particularly, to methods and apparatus for providing transmit diversity in a multiple access cellular communications network. 
     BACKGROUND 
     In a wireless communication system, a base station, situated at a fixed location, communicates with a plurality of wireless terminals, e.g., mobile nodes that may move throughout its cell. A given base station, with a single fixed antenna may have a fixed antenna pattern. Consider a single base station; its antenna pattern will support variable levels of channel quality between the base station and mobile nodes, depending on the mobile node&#39;s location with respect to the antenna pattern. Now consider that an adjacent base station, with its own antenna pattern, may be creating different levels of interference at different locations. The channel quality between the base station and a mobile node will vary as the mobile node moves to different locations within the cell. The mobile node may experience fading resulting in a degradations or loss of communication. Certain areas within the cell may be considered dead zones where the channel quality is too poor to establish communications. Methods and apparatus are needed that reduce fading and dead zones within cells. 
     In a system, with many mobile nodes, there will typically be a large diversity among the population of users, e.g., for any given antenna pattern there will be some users with good channel condition, some users with poor channel conditions, and other users with varying levels of channel conditions. At any given instant of time each mobile node experiences quasi-static channel conditions. Pilot signals may be broadcast to the mobile nodes; each mobile node&#39;s channel quality may be measured and reported back to the base station. Therefore, a base station could schedule mobile nodes with good channel quality, and hold-off scheduling mobile nodes with poor channel quality. When such a method is used in a strict manner, a mobile node, with poor channel quality, might have to move to a location with acceptable channel quality in order to be scheduled by the base station. 
     In another approach, the base station could periodically readjust its antenna pattern, again send pilot signals, wait for channel quality reports from the mobile node and schedule those mobile nodes with good channel quality. This second approach may lead to a long delay for a mobile node situated in a location of poor channel quality before the base station antenna pattern is adjusted to an acceptable level. In addition, this second approach favors one set of mobile nodes at the expense of another set of mobile nodes. The scheduling delays involved with either of these approaches may be unacceptable for certain types of delay-sensitive traffic such as voice. In some cases, if the traffic of the user has stringent delay constraints, the base station may, be forced to schedule a user even when channel conditions are not favorable resulting in a poor quality of service. Thus, for real time applications such as voice, it is often important to minimize the time period between transmission to a wireless terminal. 
     In cases where a channel&#39;s conditions are varied, practical constraints limit the rate at which the conditions in a particular channel may be varied without negatively impacting communications system performance. From a wireless terminal&#39;s perspective, rapid changes in a communications channel are difficult to track. Furthermore, rapid changes often result in a channel estimate used to decode a received signal being inaccurate since the channel conditions may have changed significantly since the channel measurements upon which the channel estimate is based were made. The use of feedback loops between a base station and a wireless terminal for power control and other purposes limits the rate at which communications channels can be varied since varying channel conditions at a rate faster than the rate at which channel condition information is measured by a wireless terminal and fed back to the base station can lead to the base station having largely inaccurate channel condition information. 
     In view of the above discussion, is should be appreciated that there is a need for improved methods and apparatus for supporting communication to multiple wireless terminals in a cell which may be distributed throughout the cell. Improved methods for providing a mobile with suitable channel conditions for receiving information from a base station are needed. From a scheduling perspective, it would be beneficial if the time interval between periods where a wireless terminal in a cell encounters good channel conditions could be minimized so that the wireless terminal need not have a long delay before encountering suitable transmission conditions. If intentional channel variations are used, it is desirable that the rate at which variations are introduced into a channel be slower than the rate at which channel measurements are made by the wireless terminals and/or the rate at which channel condition information is feed back to the base station. It would be desirable if at least some new methods address the problem of the relative duration of a mobile node&#39;s quasi-static channel condition relative to an acceptable scheduling latency. Methods and apparatus that address ways to mitigate interference effects from adjacent cells would also be beneficial. Methods that exploit the user diversity of the system, rather than be constrained by it, would also be beneficial. Such improved methods could increase user satisfaction, increase quality of service, increase efficiency, and/or increase throughput. 
     SUMMARY 
     The present invention is directed to methods and apparatus for improving reducing scheduling latency in a communication system. In accordance with the present invention, multiple communications channels are maintained by a basestation with different physical characteristics and each of the communications channels occupies a portion of the available communications resource. The physical partition of the available communications resource into multiple parallel communication channels with different physical characteristics can be done in a variety of ways such as in frequency, in time, or in code, or some combination of these. In some embodiment, the communications channels are orthogonal to each other. 
     Each wireless terminal measures the channel conditions on different communications channels. A pilot signal is periodically transmitted in each of the communications channel to facilitate the measurement of the channel conditions. From the measured channel conditions, it is possible to determine which channel has the best channel conditions from the wireless terminal&#39;s perspective at a particular point in time. The wireless terminal provides channel condition information in messages to the base station. This information is used for power and rate control and/or transmission scheduling purposes. In some embodiments, each individual wireless terminal feeds back channel condition information and the base station selects, based on the channel condition information, which channel to use to transmit information to the wireless terminal. The base station will normally select the channel with the best conditions, e.g., highest SNR, from the plurality of channels for which a wireless terminal provides channel condition information. If that best channel is not available, the base station may select the next best channel. To reduce the amount of information required to be transmitted from a wireless terminal to the base station on a recurring basis, in some embodiments the wireless terminals select, based on channel condition measurements of multiple channels, which channel is to be used for transmitting information to the wireless terminal at a particular point in time. The wireless terminal communicates the channel selection as part of the channel feedback information supplied to the base station on a periodic basis. In such embodiments, the feedback information transmitted from a wireless terminal to the base station normally includes a channel identifier and channel quality information, e.g., a signal to noise ratio (SNR) or a signal to interference ratio (SIR). 
     The base station services many wireless terminals and, multiple wireless terminals may select the same channel to be used to transmit information during the same time period. In cases where a communications channel has been selected to be used by multiple wireless terminals, the base station takes into consideration the channel quality reported by the individual wireless terminals and gives a preference to the wireless terminals reporting higher channel quality than those reporting lower channel quality. Other quality of service and/or fairness criterion is also taken into account when the base station makes the scheduling decision in at least some embodiments. Scheduling latency is reduced as compared to systems using a single communications channel as a result of using multiple channels with differing physical characteristics which are reflected in the channel quality reported by the wireless terminals. 
     In various embodiments channels are implemented as a partition of an air link resource where each channel corresponds to a different portion of the air link resource in terms of time and/or frequency. To avoid requiring a wireless terminal to switch between multiple carrier frequencies, in some embodiments the carrier frequency used to transmit signals to a wireless terminal is the same on the plurality of different communications channels. In such an embodiment a wireless terminal can switch between channels without having to change the frequency used to mix a received signal from the passband to the baseband as part of a demodulation process. This has the advantage of allowing for rapid switching between communications channels which allows for switching to occur without interfering with ongoing Internet Protocol sessions even when the channel used to communication the voice or data packets is changed during an ongoing IP communications session. 
     To provide for the ability to switch between channels on a rapid basis, in some embodiments, wireless terminals maintain channel quality estimates and/or channel estimates for a plurality of different communications channels at the same time. In such embodiments at least two channel quality estimates and/or channel estimates are maintained at the same time. The two channel estimates are normally for the two best channels to the wireless terminal, as determined by the wireless terminal&#39;s measurements of the different channels. In some embodiment 3, 4 or more channel estimates are maintained. Each of the channel estimates is usually maintained independent of the other channel estimates so that the individual channel estimate will properly reflect the particular physical characteristics of the channel to which it corresponds. Channel estimates are normally based on multiple channel measurements which occur at different points in time. 
     In some embodiments multiple static communications channels are used. In at least one such embodiment at least 3 different channels are used. However the use of more channels with different physical characteristics, e.g., 4, 8 or even more in a cell is possible. 
     While use of multiple static channels with differing characteristics provides scheduling advantages over embodiments where a single channel is used, even greater benefits can be obtained by introducing variations into one or more of the different communications channels. 
     In some embodiments, beamforming methods of the type described in U.S. patent application Ser. No. 09/691,766 filed Oct. 18, 2000 which is hereby expressly incorporated by reference, are used on individual channels to deliberately induce channel variations. Multiple transmitter antennas are used in such an embodiment to facilitate introducing variations into the communication channel. This method results in channel variations that can be exploited by an opportunistic scheduler such as that used in the base station of the present invention. 
     By combining the opportunistic beamforming method, e.g., the introduction of intentional channel variations, with the use of multiple parallel communications channels, scheduling latency can be reduced beyond the latency reduction benefits that can be achieved using opportunistic beamforming alone. In fact, in some cases latency can be reduced by an amount directly related, if not proportional to, the number of different channels supported in the cell for communication information to the wireless terminals. The reduction in latency can be to a level that would not be possible using a single channel and beamforming since the rate at which beamforming can be used to change a channel in a productive manner is limited by the rate at which a wireless terminal measures the channel and provides channel quality information to a base station. 
     The use of parallel communications channels with multiple opportunistic beams creates an improved version of transmit antenna diversity which may be exploited using channel selection by the wireless terminal and/or base station based on channel quality measurements. Each of the parallel communications channels will normally exhibit a distinct wireless channel quality, thereby allowing the scheduler to take advantage of the diversity with a latency that will be a fraction of that possible when a single channel is used. 
     In accordance with the present invention, in the case where intentional variations are introduced into a communications channel, the rate at which the channel variations occur is usually slower than the rate at which the wireless terminals measure the quality of the particular channel which is being varied. In addition, the rate at which the wireless terminal provides channel feedback information, e.g., on a single channel, is usually faster than the rate at which channels are intentionally varied. In such embodiments the periodicity of the introduced channel variations is usually longer, e.g., in some cases at least twice as long, as the rate at which quality measurements of the particular channel are made and reported back to the base station. In such cases the relatively gradual change in the channel which is intentionally introduced should not have a significant impact on the accuracy of the channel estimate maintained by the wireless terminal or the channel condition information returned by a wireless terminal to a base station. 
     In order to reduce the possibility of repeated periods of interference affecting the same wireless terminal, the rate at which channel variations are introduced into channels of adjoining cells is controlled to be different. Thus, the base stations of adjoining cells, in some embodiments, introduce channel variations at different rates. 
     While the use of multiple transmission elements, e.g., multiple antennas, at a base station is not essential to the present invention, numerous embodiments of the present invention are implemented using multiple antennas. In some of these embodiments, control coefficient sets are maintained and used to control processing of signals transmitted from a base station using different antennas. In such embodiments, different antennas may be used for different communications channels. Alternatively, the same set of antennas can be shared by the different communications channels with signal processing being used to introduce amplitude and/or phase variations into the signals corresponding to the different parallel communications channels. The antenna pattern corresponding to a particular channel is varied in some embodiments to thereby vary the gain of the channel in a particular direction. The gain of multiple channels may be changed in unison to main a uniform difference between the channels to the extent possible. 
     The method and apparatus of the present invention may be used in a wide range of systems including frequency hopping, time division and/or code division based communications systems. 
     Numerous additional features and benefits are described in the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  illustrates an exemplary wireless communication system implemented in accordance with the invention. 
         FIG. 2  illustrates an exemplary cell of the communication system of  FIG. 1 , exemplary communications channels, and exemplary signaling in accordance with the present invention. 
         FIG. 3  illustrates an exemplary base station, suitable for use in the system of  FIG. 1 , implemented in accordance with the present invention. 
         FIG. 4  illustrates an exemplary wireless terminal, suitable for use in the system of  FIG. 1 , implemented in accordance with the present invention. 
         FIG. 5  illustrates the construction of exemplary parallel pipes, using a time partition method, between a base station and wireless terminals, in accordance with the invention. 
         FIG. 6  illustrates the construction of exemplary parallel pipes, using a frequency partition method, between a base station and wireless terminals, in accordance with the invention. 
         FIG. 7  illustrates the construction of exemplary parallel pipes, using a combination of frequency division/time division methods, between a base station and wireless terminals, in accordance with the present invention. 
         FIG. 8  illustrates exemplary parallel pipes using frequency division for exemplary 5 MHz CDMA/OFDM systems, in accordance with the present invention. 
         FIG. 9  illustrates exemplary parallel pipes in a 1.25 MHZ CDMA or OFDM system using time division, in accordance with the present invention. 
         FIG. 10  is a diagram of an exemplary transmitter using parallel pipes and multiple antennas, in accordance with the present invention. 
         FIG. 11  is a graph illustrating opportunistic beamforming for a single beam, in accordance with the present invention. 
         FIG. 12  is a graph illustrating opportunistic beamforming for two exemplary beams in accordance with the present invention. 
         FIG. 13  illustrates the use of two exemplary downlink parallel pipes (constructed by frequency division) and uplink signaling including channel quality reports (including pipe selection by WTs), in accordance with the present invention. 
         FIG. 14  illustrates a portion of an exemplary wireless communications system showing an embodiment of the invention suited for applications where channels are constructed using time division multiplexing. 
         FIG. 15  illustrates a portion of an exemplary wireless communications system showing an embodiment of the invention suited for applications where channels are constructed using frequency division multiplexing. 
         FIG. 16  is a drawing illustrating alternate pipes in alternate time slots, in accordance with the invention. 
         FIG. 17  is a drawing illustrating parallel pipes during the same time slots, in accordance with the invention. 
         FIG. 18  is a drawing illustrating four parallel pipes with different transmission characteristics which are varied over time. 
         FIGS. 19-22  show changes in antenna patterns over time, in accordance with the present invention. 
         FIG. 23 , which comprises the combination of  FIGS. 23A ,  23 B,  23 C, is a flowchart illustrating an exemplary method of operating a wireless communications system in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is an illustration of an exemplary wireless communications system  100 , implemented in accordance with the present invention. Exemplary wireless communications system  100  includes a plurality of base stations (BSs): base station  1   102 , base station M  114 . 
     Cell  1   104  is the wireless coverage area for base station  1   102 . BS  1   102  communicates with a plurality of wireless terminals (WTs): WT( 1 )  106 , WT(N)  108  located within cell  1   104 . WT( 1 )  106 , WT(N)  108  are coupled to BS  1   102  via wireless links  110 ,  112 , respectively. Similarly, Cell M  116  is the wireless coverage area for base station M  114 . BS M  114  communicates with a plurality of wireless terminals (WTs): WT( 1 ′)  118 , WT(N′)  120  located within cell M  116 . WT( 1 ′)  118 , WT(N′)  120  are coupled to BS M  114  via wireless links  122 ,  124 , respectively. WTs ( 106 ,  108 ,  118 ,  120 ) may be mobile and/or stationary wireless communication devices. Mobile WTs, sometimes referred to as mobile nodes (MNs), may move throughout the system  100  and may communicate with the base station corresponding to the cell in which they are located. Region  134  is a boundary region between cell  1   104  and cell M  116 . 
     Network node  126  is coupled to BS  1   102  and BS M  114  via network links  128 ,  130 , respectively. Network node  126  is also coupled to other network nodes/Internet via network link  132 . Network links  128 ,  130 ,  132  may be, e.g., fiber optic links. Network node  126 , e.g., a router node, provides connectivity for WTs, e.g., WT( 1 )  106  to other nodes, e.g., other base stations, AAA server nodes, Home agents nodes, communication peers, e.g., WT(N′),  120 , etc., located outside its currently located cell, e.g., cell  1   104 . 
       FIG. 2  is a drawing  200  of cell  1   104  illustrating exemplary communications channels and exemplary signaling in accordance with the present invention.  FIG. 2  includes communications within cell  1   104  between BS  1   102  and WTs (WT( 1 )  106 , WT(N)  108 ). BS  1   102  includes multiple transmit antennas, e.g., transmitter antenna  1   202 , transmitter antenna N  204 . The base station  502  can transmit by multiple antennas  202 ,  204  to each WT  106 ,  108 . 
     In the illustration of  FIG. 2 , the two solid lines ( 206 ,  208 ), one from each antenna ( 202 ,  204 ) to WT( 1 )  106 , represent a first pipe to WT( 1 )  106 . Similarly, the two dashed lines ( 210 ,  212 ), one from each antenna ( 202 ,  204 ) to WT( 1 )  106 , represent a second pipe to WT( 1 )  106 . Thus, solid lines ( 206 , 208 ) correspond to one set of communications signals which combine in the air to operate as one downlink communications channel to WT( 1 )  106 , while dashed lines ( 210 ,  212 ) represent signals which combine in the air and operate as a second downlink communications channel to WT( 1 )  106 . 
     Similarly, the two solid lines ( 214 ,  216 ), one from each antenna ( 202 ,  204 ) to WT(N)  108 , represent a first pipe to WT(N)  108 ; the two dashed lines ( 218 ,  220 ), one from each antenna ( 202 ,  204 ) to WT(N)  108 , represent a second pipe to WT(N)  108 . Thus, solid lines ( 214 ,  216 ) correspond to one set of communications signals which combine in the air to operate as one downlink communications channel to WT(N)  108 , while dashed lines ( 218 ,  220 ) represent signals which combine in the air and operate as a second downlink communications channel to WT(N)  108 . From the perspective of each WT  106 ,  108  they are coupled to BS  1   102  by two separate pipes from which information may be received at any given time. The wireless terminals ( 106 ,  108 ) provide feedback information to base station  1   102  as represented by arrows ( 222 ,  224 ) proceeding from each WT ( 106 ,  108 ), respectively, to base station  102 . Feedback signals to the base station may include information on each of these pipes. Based on this feedback information, the BS  102  may determine which pipe to use and when to transmit data to the WT( 1 )  106  and/or WT(N)  108 . In some embodiments, each WT ( 106 ,  108 ) sends a signal to the BS  102  indicating which of the pipes should be used at any point in time. 
       FIG. 3  illustrates an exemplary base station  300 , implemented in accordance with the present invention. Exemplary BS  300  may be a more detailed representation of any of the BSs, BS  1   102 , BS M  114  of  FIG. 1 . BS  300  includes a receiver  302 , a transmitter  304 , a processor, e.g., CPU,  306 , an I/O interface  308 , I/O devices  310 , and a memory  312  coupled together via a bus  314  over which the various elements may interchange data and information. In addition, the base station  300  includes a receiver antenna  216  which is coupled to the receiver  302 . The base station  300 , as shown in  FIG. 3 , also includes multiple transmitter antennas, (antenna  1   318 , antenna n  322 ) which are physically spaced apart from each other. Transmitter antennas  318 ,  322  are used for transmitting information from BS  300  to WTs  400  (see  FIG. 4 ) while receiver antenna  216  is used for receiving information, e.g., channel condition feedback information as well as data, from WTs  400 . 
     The memory  312  includes routines  324  and data/information  326 . The processor  306 , executes the routines  324  and uses the data/information  326  stored in memory  312  to control the overall operation of the base station  300  and implement the methods of the present invention. I/O devices  310 , e.g., displays, printers, keyboards, etc., display system information to a base station administrator and receive control and/or management input from the administrator. I/O interface  308  couples the base station  300  to a computer network, other network nodes, other base stations  300 , and/or the Internet. Thus, via I/O interface  308  base stations  300  may exchange customer information and other data as well as synchronize the transmission of signals to WTs  400  if desired. In addition I/O interface  308  provides a high speed connection to the Internet allowing WT  400  users to receive and/or transmit information over the Internet via the base station  300 . Receiver  302  processes signals received via receiver antenna  216  and extracts from the received signals the information content included therein. The extracted information, e.g., data and channel condition feedback information, is communicated to the processor  306  and stored in memory  312  via bus  314 . Transmitter  304  transmits information, e.g., data, and pilot signals to WTs  400  via multiple antennas, e.g., antennas  318 ,  322 . Transmitter  304  includes a plurality of phase/amplitude control modules, phase/amplitude control module  1   316 , phase/amplitude control module n  320 . In the illustrated example of  FIG. 3 , a separate phase/amplitude control module, ( 316 ,  320 ) is associated with each of the transmit antennas ( 318 ,  322 ), respectively. The antennas  318 ,  322  at the BS  300  are spaced far enough apart so that the signals from the antennas  318 ,  322  go through statistically independent paths, and thus the channels the signals go through are independent of each other. The distance between antennas  318 ,  322  is a function of the angle spread of the WTs  400 , the frequency of transmission, scattering environment, etc. In general, half a wavelength separation between antennas, based on the transmission frequency, is usually the sufficient minimum separation distance between antennas, in accordance with the invention. Accordingly, in various embodiments, antennas  318 ,  322  are separated by one half a wavelength or more, where a wavelength is determined by the carrier frequency f k  of the signal being transmitted. 
     The phase and amplitude control modules  316 ,  320  perform signal modulation and control the phase and/or amplitude of the signal to be transmitted under control of the processor  306 . Phase/amplitude control modules  316 ,  320  introduce amplitude and/or phase variations into at least one of a plurality, e.g., two, signals being transmitted to a WT  400  to thereby create a variation, e.g., an amplitude variation over time, in the composite signal received by the WT  400  to which information is transmitted from multiple antennas  318 ,  322 . The control modules  316 ,  320  are also capable of varying the data transmission rate, under control of the processor  306 , as a function of channel conditions in accordance with the present invention. In some embodiments, phase/amplitude control modules  316 ,  320  change phase and/or amplitude by changing coefficients. 
     As mentioned above, the processor  306  controls the operation of the base station  300  under direction of routines  324  stored in memory  312 . Routines  324  include communications routines  328 , and base station control routines  330 . The base station control routines  330  include a transmit scheduler/arbitration module  332  and a receiver scheduler/arbitration module  334 . Data/Information  326  includes transmission data  336  and a plurality of wireless terminal (WT) data/information  338 . WT data/information  338  includes WT  1  information  340  and WT N information  342 . Each WT information set, e.g., WT  1  information  340  includes data  344 , terminal ID information  346 , channel condition information  348 , and stored customer information  350 . Stored customer information  350  includes modulation scheme information  352 , transmission antenna information  354 , and transmission frequency information  356 . Transmission data  336  includes data, e.g., user data, intended to be transmitted to WTs  400 , located within the cell of BS  300 . Data  344  includes user data associated with WT  1 , e.g., data received from WT  1  intended to be forwarded to a communication peer, e.g., WT N, and data receiver from a peer of WT  1 , e.g., WT N, intended to be forwarded to WT  1 . Terminal ID information  346  includes a current base station assigned identity for WT  1 . Channel condition information  348  includes feedback information from WT  1  such as, e.g., downlink channel(s) estimation information and/or a WT  1  selected downlink channel. 
     The transmit scheduler/arbitration module  332  schedules when transmission data  336  will be transmitted, e.g., downloaded, to WTs  400 . As part of the scheduling process module  332  arbitrates between the needs of various WTs  400  to receive data. The receiver scheduler/arbitration module  334  schedules when WTs  400  will be allowed to upload data to the BS  300 . As with the transmit scheduler  332 , the receiver scheduler  334  may arbitrate between several WTs  400  seeking to upload data at the same time. In accordance with the present invention, modules  332 ,  334  perform scheduling operations as a function of received channel condition feedback information, e.g., WT  1  channel condition information  348 . Communications routines  328  determine the frequency and data rate as well as the appropriate encoding or modulation technique to be used for communications with each WT  400 . Communications routine  328  can access the stored channel condition information and customer information, e.g., WT 1  channel condition information  344  and WT  1  stored customer information  350  to obtain relevant information used by the routines  324 . For example, communications routines  328  can access channel condition information  348  obtained from feedback to determine the appropriate data rate to be used in communicating to a WT  400 . In addition, other stored customer information  350  such as modulation scheme information  352 , transmission antenna information  354 , and transmission frequency information  356  can be retrieved and used to determine the appropriate modulation scheme, number of transmission antennas, and transmission frequency to be used when communicating with a particular WT  400  scheduled to receive information. 
     While in some embodiments a single antenna is used to transmit information to a WT  400 , the use of multiple physically separated antennas  318 ,  332  allows the same information to be transmitted from different locations with controlled phase and/or amplitude differences being introduced into at least one of the transmitted signals to produce an artificial signal variance at the receiving WT  400 . 
       FIG. 4  illustrates an exemplary wireless terminal  400 , implemented in accordance with the present invention. Exemplary wireless terminal  400  may be a more detailed representation of any of the WTs  106 ,  108 ,  118 ,  120  of exemplary system wireless communication system  100  of  FIG. 1 . WT  400  includes a receiver  402 , a transmitter  404 , I/O devices  406 , a processor, e.g., a CPU,  408 , and a memory  410  coupled together via bus  412  over which the various elements may interchange data and information. Receiver  402  is coupled to antenna  414 ; transmitter  404  is coupled to antenna  416 . In some embodiments, a single antenna may be used in place of the two individual antennas  414  and  416 . 
     Downlink signals transmitted from BS  300  are received through antenna  414 , and processed by receiver  402 . Transmitter  404  transmits uplink signals through antenna  416  to BS  300 . Uplink signals include downlink feedback channel estimation information and/or information identifying a selected downlink channel over which WT  400  requests that downlink data be transmitted, in accordance with the invention. I/O devices  406  include user interface devices such as, e.g., microphones, speakers, video cameras, video displays, keyboard, printers, data terminal displays, etc. I/O devices  406  may be used to interface with the operator of WT  400 , e.g., to allow the operator to enter user data, voice, and/or video directed to a peer node and allow the operator to view user data, voice, and/or video communicated from a peer node, e.g., another WT  400 . 
     Memory  410  includes routines  418  and data/information  420 . Processor  408  executes the routines  418  and uses the data/information  420  in memory  410  to control the basic operation of the WT  400  and to implement the methods of the present invention. Routines  418  include communications routine  422  and WT control routines  424 . WT control routines  424  include a channel condition measurement module  426  and a channel selection module  428 . 
     Data/Information  420  includes transmission data  430 , stored base station information  432 , and user information  434 . User information  434  includes base station identification information  436 , terminal ID information  438 , assigned downlink channel information  440 , a plurality of channel measurement information (channel  1  measurement information  442 , channel N measurement information  446 ), a plurality of channel estimate information (channel  1  estimate information  444 , channel N estimate information  448 ), and selected channel information  450 . Transmission data  430  includes user data, e.g., data/information to be transmitted to BS  300  intended for a peer node in a communication session with WT  400 , downlink channel feedback information, and/or a selected downlink channel. Stored base station information  432  includes information specific to each base station, e.g., slope values that may be used in hopping sequences, carrier frequencies used by different base stations, modulation methods used by different base stations, beamforming variations that are base station dependent, etc. User information  432  includes information being currently used by WT  400 . Base station ID information  436  includes identification information of the base station in whose cell WT  400  is currently located, e.g., a value of slope used in a hopping sequence. Terminal ID information  438  is a base station assigned ID used for current identification of WT  400  by the BS  300  in whose cell WT is located. Assigned downlink channel information  440  includes a downlink channel assigned by the BS  300  for the WT  400  to expect user data to be transmitted on. Channel  1  measurement information  442  includes measurements of received signals corresponding to channel  1 , e.g., measurements of a pilot signal transmitted on downlink channel  1  such as SNR (Signal to Noise Ration), SIR (Signal Interference Ratio), etc. Channel N measurement information includes measurement of received signals corresponding to channel N, e.g., measurements of a pilot signal transmitted on downlink channel N such as SNR, SIR, etc. Channel  1  estimation information  444  includes downlink channel  1  estimates, e.g., based on channel  1  measurement information  442 . Channel N estimation information  448  includes downlink channel  2  estimates based on channel N measurement information  446 . Selected channel information  450  includes information identifying which channel WT  400  has identified as the more desirable downlink channel, e.g., which of the beam formed downlink channels  1 , N is better suited at the present time for WT  400 . Selected channel information  450  may also include channel measurement information corresponding to the selected channel. 
     The communications routine  422  controls the transmission and reception of data by transmitter  404  and receiver  402 , respectively. Communications routine  422  may vary the data transmission rate, in accordance with the present invention based on channel conditions. In addition, communications routine  422  is responsive to scheduling information, received from BS  300  to insure that transmission data  430  is transmitted by the WT  400  at the times authorized by the BS  300 . Communications routines  422  transmits channel condition information, e.g., channel measurement information  442 ,  446 , selected channel information  450 , and/or amplitude/phase feedback information to the BS  300  via transmitter  404 . Communications routines  422  are also responsible for controlling the display and/or audio presentation of received information to a WT user via I/O devices  406 . 
     Channel condition measurement module  426  measures channel conditions obtaining channel  1  measurement information  442 , channel N measurement information  446 . Channel condition measurement module  426  also processes the channel measurement information  442 ,  446  and obtains channel estimate information  444 ,  448 , respectively. Channel condition measurement module  426  also supplies the amplitude and/or phase feedback information to the communications routine  422 . Channel selection module  428  compares channel measurement information, e.g., channel  1  measurement information  442 , channel N measurement information  446 , selects which channel is better, stores the selection in selected channel information  450 , and supplies the selected channel information  450  to the communications routine  422 . Communications routine  422  then transmits channel measurement information  442 ,  446 , selected channel information  450 , and/or amplitude/phase information to the BS  300  via transmitter  404 . 
       FIG. 5  illustrates an exemplary embodiment of the construction of parallel pipes, e.g., downlink channels between BS  300  and WT  400 . In the time partition method of  FIG. 5 , the time is divided into parallel pipes, each of which can be used simultaneously to transmit signals during a different time slot but using the same bandwidth.  FIG. 5  is a graph  500  of frequency on the vertical axis  502  vs time on the horizontal axis  504 . The air link resource represented by box  506  is partitioned in time into an exemplary four parallel pipes  508 ,  510 ,  512 ,  514 . In the time partition method, each of the parallel pipes  508 ,  510 ,  512 ,  514  occupies the entire bandwidth  516  but within different time slots  518 ,  520 ,  522 ,  524 . 
       FIG. 6  illustrates another exemplary embodiment of the construction of parallel pipes, e.g., downlink channels between BS  300  and WT  400 . In the frequency partition method of  FIG. 6 , the bandwidth is divided into parallel pipes, each of which can be used simultaneously to transmit signals in parallel.  FIG. 6  is a graph  600  of frequency on the vertical axis  602  vs time on the horizontal axis  604 . The air link resource represented by box  606  is partitioned in frequency into an exemplary five parallel pipes  608 ,  610 ,  612 ,  614 ,  616 . In the frequency partition method, each of the parallel pipes  608 ,  610 ,  612 ,  614 ,  616  occupies a different frequency range  618 ,  620 ,  622 ,  624 ,  626  but occupies the entire time slot  628 . 
       FIG. 7  illustrates another embodiment of the construction of parallel pipes, e.g., downlink channels between BS  300  and WT  400 . The  FIG. 7  embodiment combines the above embodiments of frequency division method ( FIG. 6 ) and time division method ( FIG. 5 ) to construct parallel pipes.  FIG. 7  is a graph  700  of frequency on the vertical axis  702  vs time on the horizontal axis  704 . The air link resource represented by box  706  is subdivided into 12 parallel pipes  708 ,  710 ,  712 ,  714 ,  716 ,  718 ,  720 ,  722 ,  724 ,  726 ,  728 ,  730 . 
       FIG. 8  and  FIG. 9  illustrate exemplary embodiments of using parallel pipes in exemplary CDMA and OFDM systems.  FIG. 8  illustrates parallel pipes in exemplary systems using frequency division. In  FIG. 8 , drawing  850  shows frequency on the horizontal axis  802  corresponding to an exemplary CDMA system having a 5 MHz bandwidth  804  in total, which is partitioned into three carriers  806 ,  808 ,  810  each representing a 1.25 MHz pipe  810 ,  812 ,  814 . Thus, there are three parallel pipes, pipe  1   810 , pipe  2   812 , and pipe  3   814  in that 5 MHz CDMA system. Drawing  850  shows frequency on the horizontal axis  852  corresponding to an exemplary OFDM system also having a 5 MHz bandwidth  854  in total, which is divided into N tones  853 . In the figure, those N tones are grouped into four subsets, pipe  1   856 , pipe  2   858 , pipe  3   860 , pipe  4   862 . Thus, there are four parallel pipes  856 ,  858 ,  860 ,  862  in that 5 MHz OFDM system. 
       FIG. 9  is a graph  900  of frequency on the vertical axis  902  vs time on the horizontal axis  904 . The exemplary illustrated CDMA or OFDM system represented by  FIG. 9  has a 1.25 MHz bandwidth  906  in total, which is shared by two parallel pipes  908 ,  910  in a time division manner. In first time slot  912  (t=t 0  to t=t 1 ), pipe  1   908  is used; in second time slot  914  (t=t 1  to t=t 2 ) pipe  2   910  is used; in third time slot  916  (t=t 2  to t=t 3 ) pipe  1   908  is used; in fourth time slot  918  (t=t 3  to t=t 4 ) pipe  2   910  is used. 
     In various embodiments of the present invention, the bandwidth, number of pipes, number of carriers, number of tones, and/or number of subsets may vary. In various embodiments of the present invention, the partition allocation for each pipe may vary. 
     In accordance with the invention WT  400 , under the control of channel condition measurement module  426 , controls receiver  402  to measure received signals in order to obtain the channel quality of each of the parallel pipes. Channel ( 1 ,N) measurement information ( 442 ,  446 ) is obtained from the received signal. Separate channel measurements of multiple parallel pipes allows the WT  400  to perform pipe selection. The channel ( 1 ,N) measurement information ( 442 ,  446 ) may include signal-to-interference ratio (SIR) and fading characteristics. Each parallel pipe may have its own pilot(s) to facilitate the channel quality measurement, and the densities of pilots used may depend on the partitioning of the air link resource. 
     The WT  400  then reports the measurement results back to the transmission source, BS  300 . In some embodiments, the reporting is frequent and/or periodic. In one embodiment, the channel quality report includes a list of the measurements of channel qualities in individual parallel pipes, e.g., channel ( 1 ,N) measurement information ( 442 ,  446 ). In another embodiment, the channel quality report includes the index of one of the parallel pipes that has the best channel quality and the corresponding channel quality measurement, e.g., selected channel information  450 . 
     In accordance with the invention, for a wireless system, e.g., system  100  equipped with multiple transmitter antennas  318 ,  322  at the base station  300 , the antennas  318 ,  322  are used to create different opportunistic beams for different parallel pipes. For the sake of description, consider the case of two antennas. The same principle can be easily extended to the case of many antennas. Let K denote the number of parallel pipes. 
     Denote the signal to be transmitted at time instant t over the K parallel pipes as
 
   S   ( t )={ S   1 ( t ), S   2 ( t ), . . . ,  S   K ( t )}
 
(Note: In some locations vectors are notated by, lines above the symbol, in other locations vectors are denoted by underlining and/or boldface print. These conventions may be used interchangeably throughout this application.)
 
     In an exemplary general description of the invention, two signals are derived from this basic signal and transmitted over the two transmit antennas respectively. The two derived signals may be described as
 
    S   (1)   ( t )={ c   1 ( t ) S   1 ( t ), c   2 ( t ) S   2 ( t ), . . . ,  c   K ( t ) S   K ( t )}
 
    S   (2)   ( t )={ d   1 ( t ) S   1 ( t ),d 2 ( t ) S   2 ( t ), . . . , d K  ( t ) S   K ( t )}
 
where c k  (t) and d k  (t) are, in general, complex time-varying coefficients superposed on the signal on the k-th parallel pipes over the first and second transmit antenna, respectively. In accordance with the invention, coefficients {c 1 (t), c 2  (t), . . . , c K (t)} and {d 1 (t), d 2  (t), . . . , d K  (t)} are independent of the transmitted signal  S (t).
 
       FIG. 10  illustrates a diagram  1000  of an exemplary embodiment of the invention using multiple transmit antennas ( 1002 ,  1004 ) transmitting over parallel pipes.  FIG. 10  shows k parallel pipes and two antennas. Pipe  1  component  1006 , pipe  2  component  1008 , . . . , and pipe k  1010  correspond to antenna  1   1002 . Pipe  1  component  1012 , pipe  2  component  1014 , . . . , and pipe k  1016  correspond to antenna  2   1004 . 
     Input signal S 1 (t)  1018  is multiplied, via multiplier  1020  by complex time-varying coefficient c 1 (t)  1022  generating pipe  1  component  1006 ; pipe  1  component  1006  is input to combining device  1024 . Input signal S 2 (t)  1026  is multiplied, via multiplier  1028  by complex time-varying coefficient c 2 (t)  1030  generating pipe  2  component  1008 ; pipe  2  component  1008  is input to combining device  1024 . Input signal S k (t)  1032  is multiplied, via multiplier  1034  by complex time-varying coefficient c k (t)  1034  generating pipe k component  1010 ; pipe k component  1010  is input to combining device  1024 . Input signal S 1 (t)  1018  is multiplied, via multiplier  1038  by complex time-varying coefficient d 1 (t)  1040  generating pipe  1  component  1012 ; pipe  1  component  1012  is input to combining device  1042 . Input signal S 2 (t)  1026  is multiplied, via multiplier  1044  by complex time-varying coefficient d 2 (t)  1046  generating pipe  2  component  1014 ; pipe  2  component  1014  is input to combining device  1042 . Input signal S k (t)  1032  is multiplied, via multiplier  1048  by complex time-varying coefficient d k (t)  1050  generating pipe k component  1016 ; pipe k component  1016  is input to combining device  1042 . 
     The circuitry illustrated in  FIG. 10  may be, e.g., part of transmitter  304  in base station  300 . In the  FIG. 10  example a combining device ( 1024 ,  1042 ) is used to combine signals from various pipes for transmission using an antenna. Each of the illustrated combing devices takes signals being transmitted over parallel ‘pipes’ and processes them to generate a signal to be transmitted over a single physical antenna. Combining device  1024  takes pipe  1  component  1006 , pipe  2  component  1008 , . . . pipe k component  1010  and combines them into signal S 1 (t)  1052  which is transmitted over antenna  1   1002 . Combining device  1042  takes pipe  1  component  1012 , pipe  2  component  1014 , . . . pipe k component  1016  and combines them into signal S 2 (t)  1054  which is transmitted over antenna  2   1004 . In the event of pipes created in the time domain, the combining devices  1024 ,  1042  may be implemented as multiplexers. For frequency-domain pipes, the combining devices  1024 ,  1042  may be implemented as ‘summers’ since it is combines signals that belong to different frequency bands. 
     The invention results in transmit diversity gains being realized in the receiver  402  of WT  400 . Denote the channel responses from the two antennas to the receiver as h c (t) and h d  (t) respectively. For the sake of description, it is assumed that the channel response from any antenna  318 ,  322  (in BS  300 ) to the receiver  402  (in WT  400 ) is constant across frequency. However, this assumption does not diminish or constrain the invention in any way. Therefore, the signal received by the receiver  402  (in WT  400 ) is given by
 
   R   ( t )={[ c   1 ( t ) h   c ( t )+ d   1 ( t )h d ( t )] S   1 ( t ), . . . , [c K ( t ) h   c ( t )+ d   K ( t ) h   d ( t )] S   K ( t )},
 
where the k-th element in vector  R (t) is the received signal over the k-th parallel pipe. Hence, when the invention is applied to the system with two transmit antennas and multiple parallel pipes, the composite channel response in k-th parallel pipe from the transmitter to the receiver is effectively given by c k (t)h c (t)+d k (t)h d (t). With a suitable choice of the values of the coefficients {c k (t)} and {d k (t)} at the transmitter  304  (in BS  300 ), at least one pipe should have decent composite channel quality with high probability, although the composite channel responses of other pipes may be of bad quality. In any event, the latency experienced by a receiver  402  (in WT  400 ) in waiting for a time instant when it experiences high channel quality is drastically reduced since it can select between opportune scheduling instants on multiple pipes.
 
     The idea of the opportunistic beamforming paradigm is that the transmitter  304  (in BS  300 ) chooses proper values of the coefficients, the receiver  402  (in WT  400 ) independently measures the channel qualities of the parallel pipes. WT  400  reports to the BS  300  (with transmitter  304 ) the measurement results, and the BS  300  controls the transmitter  304  to send traffic to the receiver  402  with those pipes that have good channel quality. To use the invention, the receiver  402  does not need to estimate h c (t) and h d (t) explicitly. 
     In one of the embodiments of this invention, each of the parallel pipes has its own opportunistic beam.  FIG. 11  is a graph  1100  illustrating opportunistic beamforming for a single beam.  FIG. 11  plots received SNR on the vertical axis  1102  vs time in slots on the horizontal axis  1104 ; the characteristic of the single opportunistic beam  1106  corresponding to a single parallel pipe is shown.  FIG. 12 , is a graph  1200  illustrating opportunistic beamforming for two exemplary beams.  FIG. 12  plots received SNR on the vertical axis  1202  vs time in slots on the horizontal axis  1204 ; the characteristic of the opportunistic beam  1   1206  corresponds to a first parallel pipe, while the characteristic of opportunistic beam  2   1208  corresponds to a second parallel pipe. The complex time-varying weights are adjusted so that the beams are effectively offset from one another. The receiver  402  sees the channel quality varying over time on any particular pipe. In general, the receiver  402  perceives high channel quality on one of the pipes (and corresponding beams) when another pipe (and corresponding beam) offer low channel quality, as illustrated in  FIG. 12 . It is easy to see that using two beams effectively reduces the latency at the receiver  402  in waiting for a time instant when the channel quality is high and the receiver  402  can select between the beams depending on their channel qualities. The receiver  402  is in a position to select the strongest among these rotating beams and report the pipe associated with the selected beam (and the corresponding channel quality) to the transmitter  304 ), such that the transmitter  304  can send traffic to the receiver  402  with the pipe of the best channel quality. 
     In the present invention, with multiple rotating beams being transmitted on parallel pipes, the receiver  402  can see diverse channel quality in a short time period and therefore the latency in getting good channel quality is significantly reduced. 
     The choice of the coefficients {c k (t), d k (t)} is quite flexible. In one embodiment, {c k (t)} is set to a constant, {d k (t)} is set to be a constant-amplitude complex number with phase being rotating with time, and the phase components of {d k (t)} are uniformly with time:
 
 c   k ( t )=1
 
 d   k ( t )=exp( j 2 πft+v   k )
 
where the phase offsets {v k } are uniformly distributed in [0,2π]. For example, for K=3,
 
                   υ   1     =   0     ,       υ   2     =       2   ⁢   π     3       ,       υ   3     =       4   ⁢   π     3       ,           ⁢     
     ⁢     and   ⁢           ⁢   for       ⁢                         K   =   4     ,       υ   1     =   0     ,       υ   2     =     π   2       ,       υ   3     =   π     ,       υ   4     =         3   ⁢   π     2     .             
This particular embodiment results in multiple opportunistic beams that each rotates with frequency f.
 
     As a special case of the embodiment, f can be zero, that is, the opportunistic beams do not rotate. In this case, the coefficients can be chosen in either a random manner, or with the phases uniformly distributed, and can be held constant over at least some time period. This special case is especially attractive when a large number of parallel pipes (K&gt;2) are realized. Given the large number of parallel pipes, it is highly likely that at any given time, the receiver  402  can find at least one pipe that is ‘highly beamformed’. 
     As a generalization to the embodiment, the coefficients can use different and time-varying amplitudes:
 
 c   k ( t )=√{square root over (α k ( t ))}
 
 d   k ( t )=√{square root over (1−α k ( t ))}exp( j 2 πft+v   k )
 
where {α k  (t)} are real numbers.
 
     In general, the number of pipes formed need not be the same as the number of opportunistic beams realized using multiple antennas. Multiple beams (up to the number of transmit antennas) can be realized within the same pipe, with the receivers tracking the signal quality on each of these beams on each of these pipes. In fact, different users can then be scheduled on the different beams within a pipe. For example, in the case of two beams within a pipe, one user may have a null on the first beam and be scheduled on the second beam. Another user may be in a complementary situation, having a null on the second beam and will therefore be scheduled on the first beam. 
     When the pipes are formed by splitting the bandwidth and the total system bandwidth is larger than a coherence bandwidth, the method of beam selection described here can exploit the diversity gains from both the transmit antenna diversity and frequency diversity available in the system without requiring any scheduling latency. 
     In a cellular environment, the channel quality is determined not only by the signal component but also by the interference component. To optimize the channel quality, multiple transmit antennas and parallel pipes can be used such that the receiver  402  is highly beamformed in its desired cell, e.g., cell  1   104  (opportunistic beamforming) and at the same time highly nulled in its adjacent cells, e.g., cell M  116  (opportunistic nulling). In one embodiment of the invention, each cell can independently apply the invention illustrated in the above description except that the frequency of rotation of beams f used in adjacent cells may be different. 
       FIG. 13  illustrates the use of two parallel pipes, indexed as  1  and  2 , constructed by frequency division in a frequency division multiplexed system, e.g., an OFDM system. Graph  1300  illustrates downlink frequency on the vertical axis  1302  vs time on the horizontal axis  1304 . The downlink frequency is subdivided into pipe  1   1306  and pipe  2   1308 . Each box  1310  in graph  1300  represents a downlink traffic pipe segment.  FIG. 1350  illustrates uplink signaling, e.g., downlink channel quality reports, from three exemplary WTs  400  (WT A, WT B, WT C) to BS  300 , in accordance with the invention. 
     WTs  400  (A, B, C) including their respective receivers  402  (A, B, C), measure and estimate the channel quality of each of the parallel pipes using the pilots transmitted by BS  300  in downlink signaling in those pipes. The WTs  400  (A, B, C) then report back the best channel quality value and the associated parallel pipe index, in their respective channel quality reports  1352 ,  1354 ,  1356 . In this example, the opportunistic beamforming is such that the channel quality (SIR) measured by receiver A for the two pipes are 0 dB and 10 dB, the SIR measured by receiver B for the two pipes are 5 dB and −3 dB, and the SIR measured by receiver C for the two pipes are 0 dB and −2 dB. Therefore, WT A reports that the pipe of index  2  has the best channel quality and the SIR is 10 dB, WT B reports that the pipe of index  1  has the best channel quality and the SIR is 5 dB, and WT C reports that the pipe of index  1  has the best channel quality and the SIR is 0 dB. Then, the BS  300 , including transmitter  304 , decides to transmit a segment of traffic  1312  to WT A using the pipe  2 , and in parallel, to transmit another segment of traffic  1314  to receiver B using the pipe  1 . The BS  300  further determines the coding/modulation rate and transmission power to be used in those two segments on the basis of the SIR reports from WTs A and B. A short time later, WTs  400  (A, B and C) send their channel quality reports  1358 ,  1360 ,  1362 , respectively, again. This time, WT A reports that the pipe of index  1  has the best channel quality and the SIR is 3 dB, WT B reports that the pipe of index  1  has the best channel quality and the SIR is 10 dB, and WT C reports that the pipe of index  2  has the best channel quality and the SIR is 6 dB. Then, the base station  300  decides to transmit a segment of traffic  1316  to WT B using the pipe  1 , and in parallel, to transmit another segment of traffic  1318  to WT C using the pipe  2 . 
     Pipes discussed in the present invention represent channels which can be used to communicate information. Different pipes, e.g., different channels, will have intentionally induced channel variations. These per channel variations can be measured by a wireless terminal  400 . The induced channel variations will be reflected in channel feedback reports. In various embodiments, the rate at which measurable channel variations are introduced is the same as or slower than the channel report feedback rate. In this manner, the BS  300  should have accurate channel information which may not be the case if the period of channel variations is shorter than the feedback report period. 
     Various features and embodiments of the present invention will now be discussed further.  FIGS. 14 and 15  show exemplary base stations which can be used to implement the methods discussed below.  FIG. 14  shows a portion of an exemplary communications system  1400  including an exemplary base station (BS)  1402  and two exemplary wireless terminals, WT 1   1404  and WT 2   1406 . BS  1402  includes an exemplary input signal S m    1409 , coefficients  1407 , a coefficient control module  1408 , a transmitter module  1412 , an a plurality of antennas (A 1    1416 , A 2    1418 , . . . , A k    1420 ). The coefficient control module  1408  includes coefficient sets  1410  for a plurality of pipes (e.g., for pipes  1  to n). The transmitter module  1412  includes k processing elements ( 1422 ,  1424 , . . . ,  1426 ) corresponding to the k antennas ( 1416 ,  1418 , . . . ,  1420 ), respectively. The coefficient set for exemplary pipe m is shown where  g   m =[g m,1 , g m,2 , . . . g m,k ] T . In base station  1402 , different sets of transmission coefficients  1410  are used to generate different pipes, e.g., at alternating times. (See  FIG. 16 .) For example at the time when it is desired to transmit over pipe  1 , S m =S 1  and  g   m = g   1 =[g 1,1 , g 1,2 , . . . , g 1,k ] T ; at the time when it is desired to transmit over pipe  2 , S m =S 2  and  g   m = g   2 =[g 2,1 ,g 2,2 , . . . , g 2,k ] T . One exemplary pipe  1403  is shown from BS  1402  to WT 1   1404 ; a second exemplary pipe  1405  is shown from BS  1402  to WT 2   1406 . The coefficients control processing elements ( 1422 ,  1424 ,  1426 ), may be, e.g., gain and/or phase adjusting circuits. The  FIG. 14  embodiment is well suited for cases where different channels are constructed using time divisional multiplexing, e.g., CDMA applications. 
       FIG. 15  shows a portion of an exemplary communications system  1500  including an exemplary base station (BS)  1502  and two exemplary wireless terminals, WT 1   1504  and WT 2   1506 . BS  1502  includes an input signal  S   m    1508 , coefficients  1510 , a coefficient control module  1512  a transmitter module  1514  an a plurality of antennas, (e.g., k antennas, A 1    1516 , A 2    1518 , . . . , A k    1520 ). The coefficient control module  1512  includes coefficient sets  1522  for a plurality of pipes (e.g., for pipes  1  to n).  FIG. 15  illustrates an exemplary two pipe embodiment; other numbers of pipes are possible in accordance with the invention. The transmitter module  1514  includes a pipe control module for each pipe, e.g., pipe  1  control module  1524 , pipe  2  control module  1526 . Transmitter module  1514  also includes k summing elements ( 1528 ,  1530 , . . . ,  1532 ) corresponding to the k antennas ( 1516 ,  1518 , . . . ,  1520 ), respectively. Each pipe control module ( 1524 ,  1526 ) includes k processing elements (( 1534 ,  1536 , . . . ,  1538  for pipe  1 ), ( 1534 ′,  1536 ′, . . . ,  1538 ′ for pipe  2 )) corresponding to the k antennas ( 1516 ,  1518 , . . . ,  1520 ), respectively. The coefficient set for pipe  1  is  g   1 =[g 1,1 , g 1,2 , g 1,k ] 2 . The coefficient set for pipe  2  is  g   2 =[g 2,1 , g 2,2 , g 2,k ] T  Input signal  S   m    1508  includes a S 1  component  1540  and an S 2  component  1521 . S 1  input signal component  1540  is the input signal to pipe  1  control module  1524 ; S 2  input signal component  1542  is the input signal to pipe  2  control module  1526 . 
     BS  1502 , as shown in  FIG. 15 , is suitable for transmitting using multiple pipes in parallel where the different pipes may correspond to different sets of tones, e.g., frequencies. The  FIG. 15  example is particularly well suited for the case where the channels are constructed using frequency division multiplexing, e.g., OFDM applications. 
       FIG. 16  is a drawing  1600  illustrating alternate pipes A and B ( 1602 ,  1604 ) generated by using alternating sets of transmission control coefficients, e.g., using the transmitter shown in  FIG. 14  and changes in coefficient sets over time  1606 . The difference between channel characteristics, e.g., gain, normally differs between channels A and B in any two adjacent slots more than the change in gain introduced in a channel between consecutive time slots used by a particular channel. For example, a large difference is maintained between channels A and B at any given time, while the individual channel A varies slowly over time and individual channel B varies slowly over time. 
       FIG. 17  is a drawing  1700  illustrating parallel pipes A and B ( 1702 ,  1704 ) over time  1706 . Parallel pipes A and B ( 1702 ,  1704 ) are generated using first and second sets of coefficients, e.g., using the transmitter shown in  FIG. 15 . Changes in coefficient sets are made over time to induce channel variations. Differences between channel characteristics, e.g., gain, normally differ between channels A and B in any two parallel channels more than the change in gain introduced in a channel between consecutive time slots used by the particular channel. For example, a large difference is maintained between channels A and B at any given time, while individual channel A is varied slowly over time and individual channel B is varied slowly over time. 
       FIG. 18  is a drawing  1800  illustrating four parallel pipes (pipe A  1802 , pipe B  1804 , pipe C  1806 , pipe D  1808 ) with different transmission characteristics which are varied over time, e.g., which are changed by modifying transmission control coefficients at the end of each transmission time period (t i ). Four transmission periods t 1    1812 , t 2    1814 , t 3    1816 , and t 4    1818  and their corresponding end points  1813 ,  1815 ,  1817 , and  1819 , respectively, are shown. 
       FIGS. 19 ,  20 ,  21  and  22  show changes in antenna patterns over time in accordance with the present invention as induced by using different transmission control coefficients over time for the different pipes, e.g., parallel or alternating channels. While shown as a single fixed antenna pattern during each illustrated time period it is to be understood that the pattern could be changed gradually during the time period resulting in the pattern changing from that shown in one figure to that shown in the next figure by the conclusion of the particular time period. 
       FIG. 19  illustrates an exemplary base station  1902  and an exemplary WT  1904 , implemented in accordance with the present invention. In  FIG. 19  a combined antenna pattern is shown including antenna patterns  1906 ,  1908 ,  1910 ,  1912  corresponding to channels A, B, C, D, respectively. Note each lobe  1906 ,  1908 ,  1910 ,  1912  corresponds to the directional pattern of one channel during illustrated time period T 1   1901 . 
       FIG. 20  illustrates the exemplary base station  1902  and the exemplary WT  1904 . In  FIG. 20  a combined antenna pattern is shown including antenna patterns  2006 ,  2008 ,  2010 ,  2012  corresponding to channels A, B, C, D, respectively. Note each lobe  2006 ,  2008 ,  2010 ,  2012  corresponds to the directional pattern of one channel during illustrated time period T 2   2001 . 
       FIG. 21  illustrates the exemplary base station  1902  and the exemplary WT  1904 . In  FIG. 21  a combined antenna pattern is shown including antenna patterns  2106 ,  2108 ,  2110 ,  2112  corresponding to channels A, B, C, D, respectively. Note each lobe  2106 ,  2108 ,  2110 ,  2112  corresponds to the directional pattern of one channel during illustrated time period T 3   2101 . 
       FIG. 22  illustrates the exemplary base station  1902  and the exemplary WT  1904 . In  FIG. 22  a combined antenna pattern is shown including antenna patterns  2206 ,  2208 ,  2210 ,  2212  corresponding to channels A, B, C, D, respectively. Note each lobe  2206 ,  2208 ,  2210 ,  2212  corresponds to the directional pattern of one channel during illustrated time period T 4   2201 . 
     Note that the difference between the patterns is designed to minimize the time before a wireless terminal  1904 , e.g., mobile, located anywhere in the 360 degree transmission field will have to wait before encountering a channel with an optimal or near optimal transmission pattern which, as can be appreciated, will produce good channel transmission characteristics from the wireless terminal&#39;s, e.g., mobile nodes, perspective. As discussed previously, the BS  1902 , in accordance with the invention, includes a transmit scheduler/arbitration module, (See, e.g., module  332  of  FIG. 3 ) and uses channel feedback information to schedule transmissions to individual wireless terminals. 
       FIG. 23 , which comprises the combination of  FIGS. 23A ,  23 B, and  23 C, is a flowchart illustrating an exemplary method  2300  of operating a wireless communications system in accordance with the present invention. The method begins with start node  2302 , and operation proceeds to step  2304 . In step  2304  first and second base stations and wireless terminals, e.g., mobile nodes, are initialized. For the exemplary wireless node, operation proceeds from step  2304  to step  2310 . For the exemplary first base station, operation proceeds from step  2304  via connecting node B  2306  to step  2326 . For the exemplary second base station, operation proceeds from step  2304  via connecting node C  2308  to step  2340 . 
     In step  2310 , the first wireless terminal in a first cell is operated to measure the quality of each of a plurality of different communications channels. Operation proceeds from step  2310  to step  2312 . In step  2312 , the first wireless terminal is operated to periodically report on measured channel quality on one or more of the different communications channels to the first base station. Operation proceeds to step  2314 . In step  2314 , the first wireless terminal is operated to maintain a plurality of channel estimates and/or channel quality estimates in parallel for use in processing information signals received from said first base station. Channel estimates are normally based on multiple measurements of the channel to which the particular estimate corresponds, In step  2316 , the first wireless terminal is operated to select, based on channel quality measurements, the best one of the different communications channels as perceived by the first wireless terminal Operation proceeds from step  2316  to step  2318 . In step  2318 , the first wireless terminal is operated to periodically transmit a feedback signal to the first base station indicating the selected channel to be used to transmit information to the first wireless terminal and information on the quality of the selected channel, e.g., the SNR and/or SIR of the selected channel, the rate of feedback signaling being the same as or faster, e.g.,  2 ×, the rate at which the first base station changes signal transmission characteristics. In step  2320 , the first wireless terminal is operated to receive information on the selected channel after the first base station switches from a first channel to a selected channel when transmitting information to the first wireless terminal in response to the feedback information. Operation proceeds from step  2320  to step  2322 . In step  2322 , the first wireless terminal is operated to switch between a first channel estimate and a channel estimate corresponding to the selected channel in response to receiving information on the selected channel. In step  2324 , the first wireless terminal is operated to demodulate the information received on the selected channel by performing a passband to baseband conversion operation. 
     In step  2326 , the first base station in the first cell is operated to transmit signals on a plurality of different communications channels, each individual one of the plurality of different communications channels each having a physical characteristic which is detectable by the first wireless terminal, a pilot signal being transmitted on a periodic basis on each channel, information to individual wireless terminals, e.g., corresponding to a communications session, being transmitted according to a schedule. Step  2326  includes sub-step  2328 . In sub-step  2328 , the first base station is operated to periodically change at least one signal transmission characteristic of each of said plurality of communications channels by modifying one or more coefficients used to control the signals transmitted using multiple antennas, said changing occurring at a rate equal to or slower than a rate at which channel condition feedback information is received from a wireless terminal Operation proceeds to step  2330 . In step  2330 , the first base station is operated to receive feedback information from a plurality of wireless terminals to which said first base station transmits signals, said feedback information including feedback information from the first wireless terminal, said first wireless terminal feedback information including information indicating the quality at said first wireless terminal of one or more channels and in some embodiments a channel selected by said first wireless terminal for transmission of information to said first wireless terminal; said feedback information further including information from a second wireless terminal, said second wireless terminal feedback information including information indicating the quality at said second wireless terminal of one or more channels, and in some embodiments, a channel selected by said second wireless terminal for transmission of information to said second wireless terminal Operation proceeds from step  2330  to step  2332 . In step  2332 , the first base station is operated to select between the plurality of communications channels to use to transmit information to the first and second wireless terminals, said first base station selecting the channel for purposes of transmitting to the first wireless terminal a channel identified in received feedback information as having been selected by the first wireless terminal or the channel indicated by the feedback information from the first wireless terminal as having the best transmission characteristics, said selecting resulting in a switching between channels if a selected channel differs from a channel which is currently being used to transmit information to a wireless terminal Operation proceeds from step  2332  to step  2334 . In step  2334 , the first base station is operated to schedule information transmissions to individual wireless terminals as a function of the channel selected for transmitting to the individual wireless terminals, said scheduling including giving priority to wireless terminals to use a channel which reported better channel conditions than other wireless terminals selected to use the same channel. Operation proceeds to step  2336 ; in step  2336  the first base station is operated to transmit information to the wireless terminals at the scheduled times using the selected channels. From step  2336  operation proceeds via connecting node D  2338  to step  2330 . 
     In step  2340 , the second base station is operated in a second cell physically adjoining said first cell to transmit signals on a plurality of different communications channels in the second cell each individual one of the plurality of different communications channels in the second cell having a physical characteristic which is detectable by a first wireless terminal in the second cell, a pilot signal being transmitted on a periodic basis on each channel, information to individual wireless terminals, e.g., corresponding to a communications session, being transmitted according to a schedule. Step  2340  includes sub-step  2342 . In sub-step  2342 , the second base station is operated to periodically change at least one signal transmission characteristic of each of said plurality of communications channels in the second cell by modifying one or more coefficients used to control the signals transmitted using multiple antennas, said changing occurring at a rate equal to or slower than a rate at which channel condition feedback information is received from a wireless terminal, said changing occurring at a rate which is different from the rate at which the said first base station periodically changes at least one signal transmission characteristic. Operation proceeds to step  2344 . In step  2344 , the second base station is operated to receive channel condition feedback information from wireless terminals in the second cell, select channels to transmit information to said wireless terminals and to schedule information transmissions. Operation proceeds from step  2344  to step  2346 . In step  2346 , the second base station is operated to transmit information to wireless terminals in the second cell at scheduled times using selected channels. Operation proceeds from step  2346  to step  2344 . 
     A method of the design of beamforming coefficients, in accordance with the invention will now be discussed. A particular design method of time-varying beamforming coefficients,  g   m (t) will be discussed. (Note: underlining is used to connote a vector.) First the design will be considered for a single pipe case, then it will be extended to multiple pipes. 
     Intuitively, the beamforming coefficient vector should, over time, “sweep” over a large range of possible channel gains such that  g (t) will periodically come close to the optimal beamforming configuration for each user. In general, it is advantageous to vary both the phase and magnitude of the coefficients of the K antenna gains thereby producing a multidimensional sweep. 
     One simple way to sweep over this space is to align  g (t) to a representative “phantom” user. Specifically, the base station internally generates a random fictitious channel gain vector  h (t)=[h 1(t) . . . h   k (t)] according to the distribution function of a typical user in the system. For example, this vector can be generated by having K components, h k (t), be independent and identically distributed lowpass Gaussian random processes. The gain  h (t) can be seen as the channel gain of a hypothetical user. The base station then sets the beamforming coefficients  g (t) to be aligned to this user. That is,
 
   g   ( t )=   h   ( t )/∥   h   ( t )∥.
 
As  h (t) varies in time, the beamforming coefficients  g (t) will sweep over the set of possible optimal beamforming coefficients. If the probability distribution of channel gain  h (t) matches the distribution for the users, the beamforming coefficients  g (t) will have correct distribution to optimally visit each of the possible antenna configurations.
 
     Any lowpass Gaussian random process can be used to generate the components of  h (t). The bandwidth of the process determines the rate of variation of  g (t), and thereby provides an adjustable parameter trading off the sweep frequency with the required channel tracking bandwidth at the users. 
     One simple method of extending a sweeping pattern for a single pipe to multiple pipes is to offset the beamforming coefficients by fixed rotations. Specifically, we first determine the sweeping pattern for some pipe, say pipe  1 . Let  g     1   (t) denote the beamforming coefficient for that pipe.  g   1 (t) can be generated using the method discussed above with respect to  g (t). The beamforming coefficients in the remaining pipes can then be set as some fixed rotation from  g   1 (t). That is,
 
   g     m ( t )= U   m     g     1 ( t ), m=1, . . . , M,  (5)
 
where U m &#39;s are a set of M constant unitary K×K matrices, and where m is the pipe index.
 
     The matrices U m &#39;s should be selected so that, at any time t, the set of coefficients  g   m (t)&#39;s are “maximally” separated, insuring that, for any user at any time, the cannel condition of the best pipe is sufficiently good. To define this criteria more precisely, let 
                 G   ⁡     (       U   1     ,   …   ⁢           ,     U   M       )       =       E   _     ⁢       max       m   =   1     ,   …   ,   M       ⁢              h   ′     ⁢     U   m     ⁢     g   1            2           ,         
where the expectation is over h and g 1 , which we assume to be independent K-dimensional complex Gaussian random vectors. Given a channel gain h, the signal-to-noise ratio (SNR) on pipe m, is proportional to |h′g m | 2 =|h′U m g 1 | 2 . Therefore, the quantity G represents the expected SNR of the best pipe among the M pipes. One way to select the U m &#39;s is to maximize this quantity, i.e.,
 
     
       
         
           
             
               U 
               1 
             
             , 
             … 
             ⁢ 
             
                 
             
             , 
             
               
                 U 
                 M 
               
               = 
               
                 
                   argmax 
                   
                     
                       U 
                       1 
                     
                     , 
                     … 
                     , 
                     
                       U 
                       M 
                     
                   
                 
                 ⁢ 
                 
                   
                     G 
                     ( 
                     
                       
                         U 
                         1 
                       
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                       … 
                       ⁢ 
                       
                           
                       
                       , 
                       
                         U 
                         M 
                       
                     
                     ) 
                   
                   . 
                 
               
             
           
         
       
     
     The maximization problem is essentially equivalent to the problem of finding M vectors uniformly on the K-dimensional sphere. When K=2, the optimal matrices are the rotation matrices, 
                 U   m     =     (           cos   ⁢           ⁢     θ   m             sin   ⁢           ⁢     θ   m                   -   sin     ⁢           ⁢     θ   m             cos   ⁢           ⁢     θ   m             )       ,           ⁢       θ   m     =         (     m   -   1     )     ⁢   π     M             
For higher dimensional K, procedures for finding good suboptimal matrices are available.
 
     Various features of the present invention are implemented using modules. Such modules may be implemented using software, hardware or a combination of software and hardware. 
     Many of the above described methods or method steps can be implemented using machine executable instructions, such as software, included in a machine readable medium such as a memory device, e.g., RAM, floppy disk, etc. to control a machine, e.g., general purpose computer with or without additional hardware, to implement all or portions of the above described methods. Accordingly, among other things, the present invention is directed to a machine-readable medium including machine executable instructions for causing a machine, e.g., processor and associated hardware, to perform one or more of the steps of the above-described method(s). 
     Numerous additional variations on the methods and apparatus of the present invention described above will be apparent to those skilled in the art in view of the above description of the invention. Such variations are to be considered within the scope of the invention. The methods and apparatus of the present invention may be used with CDMA, orthogonal frequency division multiplexing (OFDM), or various other types of communications techniques which may be used to provide wireless communications links between access nodes such as base stations and wireless terminals such as mobile nodes. Accordingly, in some embodiments base stations establish communications links with mobile nodes using OFDM or CDMA. In various embodiments the mobile nodes are implemented as notebook computers, personal data assistants (PDAs), or other portable devices including receiver/transmitter circuits and logic and/or routines, for implementing the methods of the present invention.