The use of wireless networks has increased dramatically over the past few years. Wireless local area networks (“WLANs”) are now commonplace in the small office/home office (“SOHO”) environment as well as the commercial building to commercial building environment. A WLAN offers several advantages over regular wired local area networks (“LANs”). For example, users are not confined to specified locations previously wired for network access, wireless work stations are relatively easy to link with an existing LAN without the expense of additional cabling or technical support; and WLANs provide excellent alternatives for mobile or temporary working environments.
In general there are two types of WLANs, independent and infrastructure WLANs. The independent, or peer-to-peer, WLAN is the simplest configuration and connects a set of personal computers with wireless adapters. Any time two or more wireless adapters are within range of each other, they can set up an independent network. In infrastructure WLANs, multiple base stations link the WLAN to the wired network and allow users to efficiently share network resources. The base stations not only provide communication with the wired network, but also mediate wireless network traffic in the immediate neighborhood. Both of these network types are discussed extensively in the IEEE 802.11 standard for WLANs.
In the majority of applications, WLANs are of the infrastructure type. That is, the WLAN typically includes a number of fixed base stations, also known as access points, interconnected by a cable medium to form a hardwired network. The hardwired network is often referred to as a system backbone and may include many distinct types of nodes, such as, host computers, mass storage media, and communications ports. Also included in the typical WLAN are intermediate base stations which are not directly connected to the hardwired network.
These intermediate base stations, often referred to as wireless base stations, increase the area within which base stations connected to the hardwired network can communicate with mobile terminals. Associated with each base station is a geographical cell. A cell is a geographic area in which a base station has sufficient signal strength to transmit data to and receive data from a mobile terminal with an acceptable error rate. Unless otherwise indicated, the term base station will hereinafter refer to both base stations hardwired to the network and wireless base stations. Typically, the base station connects to the wired network from a fixed location using standard Ethernet cable, although in some case the base station may function as a repeater and have no direct link to the cable medium. Minimally, the base station receives, buffers, and transmits data between the WLAN and the wired network infrastructure. A single base station can support a small group of users and can function within a predetermined range.
In general, end users access the WLAN through WLAN adapters, which are commonly implemented as PCMCIA cards in notebook computers, ISA or PCI cards in desktop computers, or fully integrated devices within hand-held computers. WLAN adapters provide an interface between the client network operating system and the airwaves. The nature of the wireless connection is transparent to the network operating system.
In general operation, when a mobile terminal is powered up, it “associates” with a base station through which the mobile terminal can maintain wireless communication with the network. In order to associate, the mobile terminal must be within the cell range of the base station and the base station must likewise be situated within the effective range of the mobile terminal. Upon association, the mobile unit is effectively linked to the entire LAN via the base station. As the location of the mobile terminal changes, the base station with which the mobile terminal was originally associated may fall outside the range of the mobile terminal. Therefore, the mobile terminal may “de-associate” with the base station it was originally associated to and associate with another base station which is within its communication range. Accordingly, WLAN topologies must allow the cells for a given base station to overlap geographically with cells from other base stations to allow seamless transition from one base station to another. One example of this “association” process is described extensively in IEEE 802.11.
The radio component of WLAN adapters receive and transmit data via radio frequency (“RF”) or infrared (“IR”) media. Currently, it is common for manufacturers of WLAN devices to utilize integrated chip sets from third party developers. In the WLAN field, one such manufacturer is Intersil which manufactures and sells the PRISM I® and the PRISM II® chip set. The PRISM I® chip set is a first generation chip set which provided rudimentary functions to the WLAN developer. However, the PRISM I® chip set did not perform as well in a confined, multi-path environment as it performed outdoors in a single path environment. The PRISM II® chip set is a second generation chip set which is highly integrated. The PRISM II® chip set also has more signal processing capabilities for better performance in a multi-path environment. Since the PRISM II® chip set is highly integrated, the developer must abide by design trade-offs made by the manufacturer. One such design trade-off limits the transmit power of the radio and therefore limits the range of communication associated with devices incorporating the chip set.
While it is possible to increase the output power of the integrated chip set various problems arise that will affect the transmitted signal. Normally, this is observed in the transmit spectrum mask. FIG. 1 illustrates the mandated transmit spectrum mask associated with IEEE standard 802.11. As shown, a conforming device must a have transmit spectral mask of less than −30 dB at the channel center frequency ±11–22 MHz and −50 dB for the channel center frequency ±22–33 MHz. Generally, a problem arises when the developer attempts to increase the transmit power for the integrated chip set beyond that set by the manufacturer. Upon such increase in the transmit power, a device previously conforming to the transmit spectral mask no longer meets the requirements and becomes non-conforming, that is the signal does not achieve the mandated −30 dB decrease at ±11–22 MHz from channel center frequency or the −50 dB decrease from the channel center frequency at +22–33 MHz.
Another problem encountered with current wireless LAN chip sets is that they typically have a transmit power amplifier with a relatively low compression point and a fixed current bias setting. This results in limited transmit output power, in the Intersil PRISM II® chip set, the output transmit power is typically around 30 mW. In addition, while lower output power levels such as 15 mW and 1 mW are possible by decreasing the transmit gain of the I/Q modulator chip, the transmit power amplifier's current drain at the 15 mW and 1 mW output levels is the same as the current drain at the 30 mW output level. Thus, battery life may be extended by lowering the transmit current drain when operating at lower power levels.
The present invention addresses these and other problems encountered in the prior art, to provide a system for increased output transmitter power effective over a broad range of temperatures.