Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
2002-12-06
2004-08-31
Urban, Edward F. (Department: 2685)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S562100, C455S560000, C455S522000, C455S069000
Reexamination Certificate
active
06785558
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to wireless communication systems. More specifically, it relates to techniques for transporting signals from a base station hotel to remote transmitters using optical fibers.
BACKGROUND OF THE INVENTION
Wireless communication systems, and cellular system in particular, are evolving to better suit the needs of increased capacity and performance demands. Currently cellular infrastructures around the world are upgrading their infrastructure to support the third generation (3G) wireless frequency spectrum. Unfortunately, the tremendous capital resources required to upgrade the entire cellular system infrastructure inhibits the deployment of these 3G systems. It is estimated that up to 3 million 3G cell sites will be needed around the world by 2010.
Traditionally, a cellular communications system includes multiple remote sites, each providing wireless service to a geographic service area, or cell. As shown in
FIG. 1
, a cellular base station (BTS) is normally located in each remote site
100
, together with an antenna tower, antennas, an equipment room, and a number of other relevant components. This traditional approach of deploying all the cell site equipment locally at each remote site has several drawbacks that contribute to the expense of the infrastructure, and upgrades to the infrastructure. At each remote site, a BTS room or cabinet to host the large base station equipment is required, as well as additional electric power supplies for the base station. This increases both the costs of the equipment at each site, as well as the costs of acquiring and renting the physical location for the equipment. The remote cell site equipment must be designed for future coverage and capacity growth, and upgrades to the equipment require physical access to the remote site.
To mitigate these problems, some cellular systems have been designed with a different architecture, as shown in FIG.
2
. The base stations
240
for multiple remote sites
200
are centralized in a base station hotel
210
, while the antenna towers and antennas remain located at various remote sites at a distance from the base station hotel. Separating the base stations
240
from the antennas, however, makes it necessary to transport RF signals between the base station hotel and the various cell sites that it serves, typically using signal converters
250
, network interface equipment
260
, and a broadband communication network
220
. When broadband fiber optic cables are used, RF signals from the base stations are converted to optical format and communicated over the fiber optic cable and then converted back to analog RF signals at the remote sites. After the optical/RF conversion, the signal is sent to one of several sector transmitters
230
and radiated over the air via the antenna to provide cellular coverage. The BTS hotel concept is especially valuable in metropolitan areas where fiber is abundant but equipment space comes at a premium. In these types of areas it is getting increasingly more difficult to deploy new cell sites due to a variety of factors including regulatory and space constraint issues.
Unfortunately, a significant portion of the metropolitan fiber networks already are configured to carry particular types of traffic such as telephony and data. While there is capacity available for additional traffic it must be transmitted in a format that is compatible with the existing traffic. Simply applying the RF signals to the fiber in an analog fashion would require the use of expensive optical components to optically multiplex the analog signals on to the fiber using some type of wavelength division multiplexing. This assumes that the existing network even supports wavelength division multiplexing which is not always the case. In addition, non-standard access equipment would be required to combine the optical signal carrying the RF signals with the optical signals containing the existing digital traffic.
Several techniques have been proposed for the digital transport of cellular signals over existing switched data networks. The typical approach, such as that disclosed in U.S. Pat. No. 5,627,879 to Russell et al., is to digitize a broadband RF signal comprising several dozen RF carriers using a single A/D converter. The digitized broadband signal is then transmitted to the remote sites where a D/A converter is used to recover the broadband analog signal containing the multiple RF carriers. It should be emphasized that the A/D and D/A converters at each end of the communication link convert an entire broadband RF signal containing multiple RF carriers. U.S. Pat. No. 5,852,651 to Fischer et al. describes a similar technique. Broadband RF signals from different sectors may be combined with each other or may remain separated, but in either case A/D and D/A conversion is performed on the entire broadband signal associated with each antenna. It should also be emphasized that the conversion at the remote site always takes place at the remote site's centrally located interface to the switched network, so that the broadband signal is communicated in analog RF form between the central network interface and the various sector antenna transmitters and their associated antennas.
SUMMARY OF THE INVENTION
The present invention introduces an improved technique for transporting wireless communication signals between a set of base stations in a base station hotel and a set of remotely located cell sites. In contrast with prior techniques that digitize the entire broadband RF signal associated with each antenna, the present invention proposes a technique that separately digitizes each RF carrier signal within the broadband RF signal. Separately digitizing each RF carrier has significant advantages, such as easing the dynamic range requirements on both the receiver and A/D converter. The separately digitized carriers are transmitted over a digital network between the base station hotel and the remote sites. In contrast with prior techniques, however, the present invention provides a technique wherein the digitized carrier signals are not converted to analog format when they first arrive at the remote site, but remain in digital format as they are distributed within the remote site to the various antenna units of the remote site. In other words, the remote site A/D and D/A converters are terminally located at the antenna units rather than positioned at an intermediate point in the signal transport path, such as the remote site's interface with the digital network.
Because signals are transported in a purely digital form until the very end of the digital transport (i.e., all the way up to the antenna units), the method of the invention enjoys some key benefits over prior systems that use analog transport at the remote site to distribute the RF signals to separate antenna units. Optical effects that limit analog systems such as attenuation, dispersion and reflection do not directly affect the cellular signal when digital transport is used. As a result, the system can send signals over much longer distances without degradation. Also, dynamic range is unaffected by distance since the digital samples suffer no degradation due to the transport process as long as reliable communications exist. Signal reconstruction techniques can also be used with digital data to ensure data integrity through the entire transport process. For example, error-coding algorithms can be used to detect and correct bit errors. These benefits apply to both downlink and uplink directions.
In one aspect of the invention, a method of downlink wireless communication is implemented by a system comprising a base station hotel, at least one remote site, and a digital data network (e.g., a fiber optic network) connecting the hotel to the remote site. The base station hotel houses a plurality of base stations and a digital hub which connects the base stations to the digital network. The remote site has a set of antenna units, where transmitters and antennas are located, and a network access n
Stratford Scott B.
Uyehara Lance K.
Yeung Simon P. S.
Young Robin Y. K.
LGC Wireless Inc.
Lumen Intellectual Property Services, Inc
Phan Huy
Urban Edward F.
LandOfFree
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