The present invention relates to the field of wireless communications systems, and in particular, to a method and system for separating and/or distinguishing between signals associated with multiple remote user terminals occupying the same channel.
One advance in increasing the capacity of communication systems has been in the area of resource sharing or multiple access. Examples of multiple access techniques include code division multiple access (CDMA), frequency division multiple access (FDMA), and time division multiple access (TDMA). For example, in a TDMA system, each remote user terminals communicates with a hub communication device (e.g., a base station) in a frequency channel shared with other remote user terminals, but in its own (i.e., non-overlapping) time slot. As such, in a TDMA system, multiple remote user terminals may communicate with the hub communication device within the same frequency channel, but within non-overlapping time slots. (The term xe2x80x9cchannelxe2x80x9d as used herein refers to any one or a combination of conventional communication channels, such as frequency, time, code channels).
Typically, a wireless communication network will include a number of hub communication devices distributed over a geographic region to service several remote user terminals, as well as to allow the same channels to be reused. For example, in voice and/or data cellular communication networks, the same channel may be used by more than one hub communication device if the possibility of interference is limited, for example, due to geographic separation of hubs occupying the same channel(s) or geographic obstructions. Generally, there is a trade-off between channel reuse and capacity of remote user terminals that can be accommodated in a wireless communication network.
Although antennas have sometimes been one of the neglected components of wireless systems, relatively recent development in the field of (adaptive) antenna arrays and xe2x80x9csmart antennasxe2x80x9d have not only realized significant improvements in geographic capacity, but also in signal-to-noise ratio (SNR), interference reduction, directionality, spatial diversity, power efficiency, and security. Accordingly, employment of antenna arrays has been proposed in a number of different wireless applications, including, but not limited to, radio communication systems, cellular systems, television broadcasting, paging systems, medical applications, etc.
Antenna arrays typically include a number of antennas that are spatially separated and coupled to one or more processors. Adaptive antenna arrays, or simply, adaptive arrays, periodically analyze the signals received from each of the antennas in an array to distinguish between one or more desired signals (e.g., from a desired source, such as cellular telephone or other communication device) and one or more undesired signals (e.g., interference from users of other hubs sharing the same frequency, interference from other radio frequency (RF) emissions overlapping in channel, Johnson noise, multipath, or other interference sources, etc. Because adaptive array systems may sometimes be able to distinguish between spatially distinct sources (e.g., two cellular user units at different locations), such systems are sometimes referred to as xe2x80x9cspatial processingxe2x80x9d or xe2x80x9cspatial division multiple access (SDMA)xe2x80x9d systems. In general, adaptive array systems provide relatively significant improvement in performance relative to single antenna element systems.
FIG. 1A is a diagram depicting a simplified radiation pattern of an antenna array system, according to the prior art. In the system shown in FIG. 1A, an antenna array 10 transmits (downlink) signals to and/or receives (uplink) signals from a desired source 12, such as a mobile or stationary remote user terminal (e.g., a cellular voice and/or data communication device, a PDA having wireless capability, etc.). As shown, a beamforming pattern 8, which represents the transmission and/or reception directional gain pattern (depicted for only two dimensions) for the antenna array 10, includes a region of enhanced gain 6, as well as a region of relatively minimized gain or xe2x80x9cnullxe2x80x9d region 2 and another region of relatively minimized gain or null 4.
The null regions 2 and 4 represent one of the advantages of adaptive arrays and xe2x80x9csmart antennaxe2x80x9d processing. In particular, each of the nulls 2 and 4 represent a represent a region or direction of relatively minimized gain with respect to the beamforming pattern of the antenna array 10. As such, the antenna array 10 typically directs a null in the direction of an interfering source. To this end, the null 2 is directed toward an interfering source 14, while the null 4 is directed to the interfering source 16. The interfering sources 14 and 16 each may represent a moving car, another mobile or stationary remote user terminal in communication with the antenna array 10 or another communication device (e.g., a base station that may or may not include an antenna array), etc., which may cause interference. As such, null generation may be viewed as interference mitigation, and each xe2x80x9cnull regionxe2x80x9d may be referred to as an interference mitigated region.
By enhancing the gain in the direction of desired source, while diminishing (and ideally reducing to zero) the gain in the direction of one or more interfering sources, the antenna array 10 may xe2x80x9cdirectionallyxe2x80x9d receive and transmit signals, and as such, increase system capacity, decrease interference to the desired source(s), etc.
FIG. 1B is a graphical representation of a beamforming pattern for the antenna array 10 shown in FIG. 1A, according to the prior art. In FIG. 1B, the level of the transmission (downlink) and reception (uplink) gain of the antenna array 10 is depicted on the vertical axis and (spatial) direction is shown on the horizontal axis. As shown, there is relatively greater gain in the direction of the desired source 12, which corresponds to the enhanced gain region 6, than there is toward the interfering source 14, which corresponds to the null region 2, or the interfering source 16, which corresponds to the null region 4.
It should be appreciated that the term xe2x80x9cnullxe2x80x9d as used in the context of adaptive array systems does not necessarily mean, and often does not mean, a region of zero electromagnetic energy, since nulls may often include some level of gain, though typically less than the enhanced region 6. Ideally, a communication device employing an adaptive array would direct a null having zero gain toward an interfering source. Furthermore, the closer the gain value of a null is to zero, the more intense or xe2x80x9cdeepxe2x80x9d the null is. Thus, the xe2x80x9camount of nullingxe2x80x9d that an antenna array or adaptive array system may generate may be defined one or both the number of nulls and the intensity of such nulls, such that the greater the amount of nulling, the greater the number of nulls generated and/or the more intense/deep one or more nulls are. As shown in FIG. 1B, for example, the null 2 is less intense than the null 4, since the latter represents less gain (and greater nulling intensity).
Regardless of the type of communication system or multiple access technology, a xe2x80x9chubxe2x80x9d communications device, such as a base station or portion thereof, generally should be able to: (1) sufficiently recover the desired signal from all the signals received on a given channel; and (2) determine the identity of the source of each desired signal. For example, in a cellular communication system that includes a number of base stations each acting as a hub, when two or more remote user terminals are communicating with their corresponding hubs on the same reused channels, each cellular base station should be able to perform steps (1) and (2) for each signal or set of signals associated with a particular xe2x80x9cdesiredxe2x80x9d remote user terminal, while discarding the signal(s) associated with interfering sources.
In a spatial processing system, wherein the hub communication device may be part of a base station that accommodates xe2x80x9cspatial channels,xe2x80x9d two or more remote users may often share the same frequency and time slot for transmission and/or reception with the hub device. As such, two remote user terminals may simultaneously be xe2x80x9cdesiredxe2x80x9d sources, yet be relatively indistinguishable by a hub communication device because they share the same channel. Although a spatial processing-capable hub communication device may be able to distinguish between the transmissions of the remote user terminals in some instances, in other instances (e.g., if the two sources have similar spatial or spatio-temporal characteristics), the transmissions from the two or more remote user terminals may be inadequately indistinguishable.
Thus, what is needed is a method and apparatus to separate and/or distinguish between signals from two or more wireless communication devices (e.g., remote user terminals) simultaneously occupying the same channel.
A method and apparatus is provided for distinguishing between two or more signals, each associated with a particular remote user terminal, in a shared-channel wireless communication system. A communication device, for example, a base station, causes an offset (e.g., a time and/or a frequency offset) between the transmission of first and second uplink signals""that simultaneously occupy the same channel and that are transmitted by first and second remote user terminals, respectively. According to one aspect of the invention, the offset is relatively small enough such that the first and second uplink signals remain within the same channel, but relatively large enough such that based on the offset, the communication device may identify that the first uplink signal is associated with the first remote user terminal.