1. Technical Field
The present invention relates generally to radio frequency (RF) signal circuitry, and more particularly, to multi-channel/frequency front-end integrated circuits for time domain duplex communications.
2. Related Art
Due to population growth and increased mobilization, modern metropolitan areas suffer from substantial congestion that result in decreased productivity, wear to transportation infrastructure, increased fuel consumption, increased risk of bodily harm by accidents, and so forth. A number of systems currently in development under the umbrella term of intelligent transportation systems, or ITS, contemplate the application of information technology to solve such transportation-related problems. These applications include onboard navigation systems that have real-time traffic update capabilities and map update capabilities, as well as signal control systems that request data from passing vehicles to determine and regulate traffic flow. Additionally, some applications contemplate one vehicle being able to communicate with another for collision avoidance and the like.
A variety of networking standards for intelligent transportation systems have been proposed that consider the specific needs and environmental limitations. One readily available networking modality is the cellular telephone network such as Global System for Mobile Communications (GSM), Wideband Code-Division Multiple Access (W-CDMA), and the like. Another networking modality current in development is Wireless Access in Vehicular Environments (WAVE), which is based off the Institute of Electrical and Electronics Engineers (IEEE) 802.11 wireless local area network (WLAN) standards. Specifically, the WAVE standard is referred to as IEEE 802.11p, and a number of car-to-car and car-to-infrastructure applications are being developed with the standard. As WAVE is an 802.11 variant, transceivers therefor are functionally compatible with standard WLAN systems. However, whereas 802.11 b/g WLAN utilizes the 2.4-2.5 GHz Industrial-Scientific-Medical (ISM) band and 802.11 a WLAN utilizes the 4.9-5.85 GHz band, WAVE utilizes the 5.9 GHz band.
Generally, WLAN systems transmit and receive signals on a single channel of frequency. In order to share the single channel, the transmit and the receive signals are time-domain duplexed. That is, for a predetermined period of time, the transmitter generates a burst signal, and for another predetermined period of time, the other transmitter generates another burst signal to be detected by the receiver. It is understood that the transmit signals and the receive signals do not overlap in the time domain. Where the receiver detects errors in the burst signal via checksums and other well-known techniques, the other transmitter may be directed to retry. Errors may be caused in part by increased noise from the surrounding environment, obstacles, and so forth. If there are a substantial number of retry attempts, data throughput is decreased.
One pertinent feature of WAVE is the use of at least two channels at different operating frequencies, one of which is designated a control channel and another that is designated a public safety channel. The control channel is understood to operate at a fixed frequency, and is dedicated for vehicle control data. The public safety channel(s) are understood to be dedicated for safety data to indicate events such as emergency brake activation, left and right turn signal activation, lane changes, etc. Other channels may be utilized for map downloads, traffic updates, Internet connections, and other general-purpose data transfers. WAVE systems encompass vehicle-mounted transceivers, also referred to as on-board units (OBUs), as well as stationary transceivers placed alongside the road, also referred to as road-side units (RSUs).
For the public safety and control channels, it is desirable to have highly sensitive signal detection. Even minor transmission delays that result from the above-mentioned retry attempts may have dire consequences for the lives of vehicle occupants due to the speed at which they are travelling and the criticality of the transmitted information.
Signal reception problems in WAVE systems are commonly attributable to multi-path propagation phenomena, where a single signal reaches the antenna via two or more different paths. At the RF signal level, destructive interference and phase shifts may occur. Multipath propagation may be caused by reflection from mountains, tall buildings, and other such structures. For example, when a car is moving with high speed in crowded environments, the signal between the on-board unit and the road side unit may reflect off nearby buildings, bridges, and other cars. The substantially weakened signal may not be recoverable by the respective receives.
One approach to solve this problem is understood to employ two antennas that are physically separated from each other. The probability that the RF signals reaching both of the antennas with different phases is known to be miniscule, so the dual-antenna configuration is understood to exploit this low probability. In further detail, this approach involves two receive chains, each connected to a separate antenna. This configuration is known as receive antenna diversity. The receiver may select the signal of higher power as the proper signal to be decoded and further processed. Other, more sophisticated techniques such as power combining, maximum likelihood, and so forth for signal extraction are also known in the art. Receive and transmit antenna diversity is particularly useful for deployment on on-board units, as some road side units may be screened by opposite surfaces of the vehicle.
As indicated above, WAVE implementations operate on two distinct channels. Thus, in order to additionally implement antenna diversity, connections to four separate antennas may be necessary. This implementation, however, is expensive and the cables to the antennas may be difficult to manage, in addition to being undesirable as having to mount so many antennas to the external surface of the vehicle.
Accordingly, there is a need in the art for multi-channel or frequency front-end integrated circuits for time domain duplex communications systems such as WAVE.