Methods and apparatus for wireless communication using orthogonal frequency division multiplexing

#The present invention relates generally to communication systems, and more particularly to wireless communication systems such as code division multiple access (CDMA) systems for fixed wireless loop (FWL) and other applications.

#FIG. 1 shows a portion of a conventional omni-beam FWL system 10. The portion of system 10 shown includes four hexagonal cells 12-1, 12-2, 12-3 and 12-4, each with a corresponding base station 14-1, 14-2, 14-3 and 14-4, and a subscriber unit 16. The system 10 will generally include numerous additional cells, base stations and subscriber units configured in a similar manner. It is assumed in this system that the base stations are equipped with omni-directional antennas, and that the positions of the subscriber units are fixed. The base station 14-3 of FIG. 1 is in communication with the subscriber unit 16 in cell 12-3, e.g., for providing a communication channel for an on-going voice or data call. The omni-beam FWL system 10 may be configured using a number of different techniques.

#FIG. 2 shows an example of how the omni-beam FWL system 10 may be implemented using a time division multiple access (TDMA) technique such as that used in the Digital European Cordless Telephone (DECT) standard. In accordance with this TDMA technique, different frequencies are used for the different cells, such that among the cells, users are separated in frequency. A suitable frequency reuse pattern, e.g., a seven-cell hexagonal reuse pattern, may also be used in order to limit the number of different frequencies required. Within a given cell, users are separated in time through the use of a sequence of time slots 20, including time slots 22-1, 22-2, . . . 22-N. The system 10 may also be implemented using a code division multiple access (CDMA) technique. In accordance with this technique, the same frequencies but different codes are used for each of the cells, such that the codes are used to separate users in different cells and within a given cell. Some frequency separation may also be used in conjunction with the code separation in order to reduce interference from other cells. Additional details regarding conventional CDMA systems are described in, for example, Andrew J. Viterbi, “CDMA: Principles of Spread Spectrum Communication,” Addison-Wesley, 1995, which is incorporated by reference herein. Other conventional CDMA systems are described in, for example, TIA/EIA/IS-95A, “Mobile Station—Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,” June 1996, and ANSI J-STD-008, “Personal Station—Base Station Compatibility Requirements for 1.8 to 2.0 GHz Code Division Multiple Access (CDMA) Personal Communication Systems,” both of which are incorporated by reference herein.

#FIG. 1 shows a portion of a conventional omni-beam FWL system.

#FIG. 2 illustrates a conventional TDMA technique for use in the FWL system of FIG. 1.

#FIG. 3 shows a portion of a conventional narrow-beam FWL system.

#The present invention will be illustrated below in conjunction with exemplary wireless communication systems and communication techniques. It should be understood, however, that the invention is not limited to use with any particular type of communication system, but is instead more generally applicable to any wireless system in which it is desirable to provide improved performance without unduly increasing system complexity. For example, it will be apparent to those skilled in the art that the techniques are applicable to omni-beam and narrow-beam fixed wireless loop (FWL) systems, CDMA systems, as well as to other types of wideband and narrowband wireless systems. The term “subscriber unit” as used herein is intended to include fixed terminals such as fixed wireless installations, mobile terminals such as cellular telephones and portable computers, as well as other types of system terminals. The term “separating” as applied, e.g., to uplink and downlink or subscriber units in a given cell of a system, refers generally to implementing the system such that interference between, e.g., the uplink and downlink or the subscriber units, is reduced, minimized, or eliminated.

#The invention provides a number of communication techniques for overcoming the above-noted problems of the prior art. The techniques differ in terms of the manner in which uplink and downlink portions of the system are separated, and/or the manner in which users are separated within a given cell. As noted previously, conventional techniques generally separate uplink and downlink portions of the system using frequency, e.g., FDD as shown in FIG. 7, and separate users within a given cell using, e.g., time slots as shown in FIG. 7 or codes. In a first technique in accordance with the invention, referred to herein as code division duplex (CDD) time-slotted CDMA, uplink and downlink portions of the system are separated using codes, while the users are also separated using codes. In a second technique in accordance with the invention, referred to herein as time division duplex (TDD) time-slotted CDMA, uplink and downlink portions of the system are separated using time slots, while the users are separated using codes. In a third technique in accordance with the invention, referred to herein as orthogonal frequency division multiplexing (OFDM),uplink and downlink portions of the system are separated in frequency, while the users are also separated in frequency. Each of these techniques will be described in greater detail below.

#An illustrative embodiment of the CDD time-slotted CDMA technique of the invention will be described with reference to FIGS. 8 and 9. FIG. 8 shows a single cell 80-1 of a wireless system. The cell includes a base station 82-1 and a number of subscriber units 84. As shown, a single antenna beam 86 generated by the base station 82-1 is directed to several subscriber units, i.e., five subscriber units in this example. The beam 86 is approximately 40° wide, such that there will be a total of nine beams generated in each cell. The additional beams are omitted from FIG. 8 for clarity of illustration. It is also assumed that the beams in the cell 80-1 and the other cells of the corresponding system are electronically steerable. The beam 86 in FIG. 8 is purposely made wider than the typical single-user narrow beam in a conventional system such as system 30 of FIG. 3, in order to target more than one subscriber unit. Although the beam 86 is broader than, e.g., the beam 63 or 65 in FIG. 6, it can be configured to span a smaller portion of its sector. Within a given cell, such as cell 80-1, users are separated by codes, i.e., assigned different codes to prevent the users in the beam 86 from interfering with one another. Among adjacent cells, users are also separated by codes. Thus, when beams from adjacent cells collide, the interference will not be catastrophic since the users in adjacent cells are separated by codes. Standard CDMA techniques, such as those described in the above-cited CDMA references, may be used to separate the users within a cell and among adjacent cells. The technique is “time-slotted” in that the beams are steerable, such that different beams can be activated in different time slots, and may also be referred to as “discontinuous-transmission” CDMA.