Radio frequency (RF) spectrum is a valuable commodity in today's world. There are more people desiring to use the RF spectrum than there is spectrum to go around, so use of the spectrum must be regulated. In many countries, the RF spectrum is regulated by governmental bodies. The Federal Communications Commission (FCC) regulates the RF spectrum in the United States.
The RF spectrum is regulated usually in one of two ways. A first way that governmental bodies regulates the RF spectrum is to sell portions of it to the highest bidder. The winning bidder, then has exclusive use of the particular portion of the RF spectrum that he has just purchased. This is way that RF spectrum for cellular telephones, television and radio channels are allocated. Single user allocations are the preferred method for applications where interference from other sources cannot be tolerated.
A second way that the government regulates RF spectrum usage is to create certain bands where anyone can use the RF spectrum as long as they comply with specified spectrum usage rules. For example, in the United States, the FCC has created three such bands. These bands are called the industrial, scientific, and medical (ISM) and the unified national information infrastructure (UNII) bands and are in the 900 MHz, 2.4 GHz, and 5.7 GHz portions of the RF spectrum. Anyone may use the spectrum in these bands as long as they are able to accept interference from other users and do not cause undue interference to other users.
The ISM and UNII bands have created a huge market for wireless consumer electronics products, such as cordless telephones, wireless computer products, and wireless computer networks. However, the popularity of the bands has resulted in a problem that many product developers did not anticipate, namely, performance degradation due to interproduct interference.
In wireless computer networks, the performance degradation is seen mainly in the network's data transfer rates. A wireless network today is capable of delivering a data transfer rate of 11 Mbps or more in an interference free environment, but if interference is introduced, the data transfer rate may drop to only a small fraction of the maximum.
Interference to a wireless computer network may come from many different forms. Sources of interference may include large appliances in the environment, other electronic devices such as pagers, cordless telephones, and microwave ovens, and other wireless computer networks. The relatively simple sources of interference such as appliances and pagers and telephones are relatively easy to deal with because their interference is typically periodic and is usually predictable. Because the interference is predictable, it is easy to avoid.
Interference from other wireless networks are more difficult to deal with due to the bursty nature of computer data traffic and the fact that certain types of networks follow a random transmission pattern, making it extremely difficult to predict where and when they will transmit next.
One solution, often used for single tone (also known as single carrier) wireless networks, is to interleave the transmitted data stream. It is common to encode the original data stream into a stream of encoded bits prior to transmission. Interleaving of the stream of encoded bits involves the dispersal of encoded bits that were adjacent to one another. The purpose of interleaving is to reduce the probability that an error or interferer can damage transmitted bits that are adjacent to one another in the encoded bit stream. Single bit errors can be detected and corrected with low complexity error correcting codes, while error correcting codes that are able to correct multiple adjacent bit errors are much more complex with a complexity that increases dramatically with increasing number of bits.
However, interleaving has several disadvantages that make it inappropriate in some situations. In the case of wireless local area networks, the channel impairments are often dominated by multipath interference, in which the transmitted signal is reflected off bodies lying in between transmitter and receiver. The multiple reflections arrive at the receiver and present multiple delayed copies of the transmitted signal. This results in multiple copies of the transmitted signal to be added together at slightly different delays, causing self-interference. After processing at the receiver, this effect manifests itself as transmitted bits having an “echo” that spills into subsequent bits. To deal with this type of interference, one strategy that is possible is to compensate for and subtract out the after-echo effect of the previous bits. But for this to be effective, bits must be processed at the receiver in the order they were transmitted over the channel. Interleaving is explicitly designed to destroy this ordering and thus makes the “compensation-and-subtraction” strategy unworkable. A main motivation for the present invention is to find a way of dealing with narrowband interference that still allows the use of the “compensation-and-subtraction” strategy against the also-present multipath interference. That is, it is desirable to find a way of combating narrowband interference that does not involve interleaving or any other technique that involves the destruction of the ordering of the transmitted bits.
A need has therefore arisen for a technique that permits dynamic and adaptive solutions to the problem of narrowband interference to single tone wireless networks.