To meet the need for ever-increasing information capacity in wireless communication systems, research efforts have recently turned to the physical layer to increase spectral efficiency. Multiple access systems in general allow many users to share spectrum allocated to the particular communication system on which they communicate, so that any discrete communication flows between select nodes but not necessarily all nodes on the network. One aspect of this research relevant to this invention is in the area of multiuser receivers. These receivers seek to minimize interference between mutual users of a spread spectrum wireless system, and generally include multiuser detectors, linear decorrelators, and linear minimum mean-square-error (MMSE) receivers.
The bulk of multiuser research has been in the context of cellular wireless communications, in which a base station of a cell simultaneously receives signals from multiple users and the individual users in the cell operate as traditional RAKE or otherwise single user receivers. Such cells are geographically based, and their geographic isolation from other geographically based cells enables a certain flexibility and control that improves spectrum efficiency. For example, where the geographic cell uses a spread spectrum system such as CDMA (in any of its standardized forms), power control among the various users is assumed to be within a narrow range, which is enabled by open or closed loop power control as well documented in the art. This is possible because a single base station (or several working in concert as one) receives all communications from mobile users in the cell and coordinates their transmit power levels. The cell-based paradigm also enables individual cells to adapt various parameters in light of traffic conditions, such as using half-rate signaling that may not be appropriate for traffic conditions in neighboring cells. As with power control, this flexibility is possible due to the base station's directive to all other users in the cell.
But while traditional cell-based mobile systems may represent the most ubiquitous and commercially important application of wireless communication systems, there exist other applications and potential applications for wireless systems where the above cell-based pre-conditions do not hold. Specifically, in a mesh topology network such as described below with reference to FIG. 1, any node of the network may serve as a relay point to receive and re-transmit a message from one network node to another, where the two end nodes are beyond a normal direct range of communication with one another. In a traditional cellular system, the extended range is enabled by a base station that interfaces with base stations of other cells. In contradistinction, the relay node in a mesh network is not a designated node serving that purpose, but may be any node within the mesh network that happens to be advantageously positioned. The relay node may change during a series of related bursts between the same end nodes, and multiple relay nodes may serve between the ultimate two end nodes for any individual burst. A typical environment for such a mesh network is one where at least some nodes are highly mobile, such as two aircraft. In such an environment, Doppler effects can become so great that power control for every burst is not practical. Such a system 20 is shown in FIG. 1 and is described particularly below.
FIG. 1 depicts a series of first through seventh nodes, 22 through 34 respectively, communicating with one another over a wireless network, preferably secure. Assume that the first 22 and sixth 32 nodes seek to exchange messages but are out of direct communication range with one another. A burst from the first node 22 may be received and re-transmitted to the sixth node 32 by the second 24 or third 26 nodes. After a short time, the fourth node 28 is better positioned to serve as the relaying node, so follow on bursts between the first 22 and sixth 32 nodes are through it. Later, the first 22 and sixth 32 nodes are in direct range, so the fourth node 28 is no longer needed as a relay point. The range of the network is not tied to a geographic cell as in cellular communications, but varies based on the availability of other network nodes to serve as relays.
Each of the second 24, third 26 and fourth 28 nodes may also be in communication with other nodes simultaneous with relaying messages between the first 22 and sixth 32 nodes, so each preferably operates with a multiuser receiver to avoid delays that would compound in a high traffic environment (such as where each relay node buffers messages to be relayed to await a transmit slot).
The extended range of such a mesh network undermines some assumptions on which prior art communication waveforms are premised. Because the amount of spectrum available for communication is fixed, spectrum is re-used in different geographic areas to increase capacity. For example, frequency division multiple access (FDMA) has not been widely used since the time that mobile communications expanded significantly because FDMA is spectrum inefficient for two reasons: allocated spectrum goes unused for low-volume users, and the geographic separation by which the allocated spectrum may be allocated to another for simultaneous use is relatively large. Time division multiple access (TDMA) suffers from the same problem of unused allocated spectrum. For each, efficient spectrum use requires efficient allocation that relies on a priori knowledge of a users' volume of data to be sent. In the absence of such a-priori knowledge allocation, spectrum goes unused. Code division multiple access (CDMA or spread spectrum) greatly increases efficiency by spreading packets among available slots of time and frequency, and reusing the same time/frequency slot among multiple users (typically LIP to eight) is enabled by the use of different spreading codes.
The mesh network of FIG. 1 undermines certain efficiencies of any of the above systems as compared to a single cell of a base-station/multiple user paradigm. Specifically, a single CDMA packet that in the traditional system occupied one time/frequency slot now occupies more than one, because it is re-transmitted by a relaying node at a different slot. The increased range of the mesh network 20 relies on potentially more nodes sharing the same spectrum than the prior art would generally allow. This increase in range, along with the premise that nodes move relatively fast in relation to one another, makes traditional CDMA power control among all users in the mesh network 20 impractical.
What is needed in the art is a communication waveform that offers frequency and time resource sharing, has a large user capacity, high message throughput, high bandwidth efficiency, and variable message update rates. It should also preferably offer long range, line-of-sight and relayed communications, and enable a highly dynamic network geometry for high user mobility.