A communications network is any system or mechanism that provides for the exchange of information or data between participants. As used herein, the term “participant” refers to a device or mechanism that communicates with other devices or mechanisms. In some communications network arrangements, one of the participants is designated as a master participant, or simply as the “master.” The master generally initiates and controls communications with the other participants, which are conventionally referred to as slave participants, or simply as “slaves.” Masters may also be assigned other functions to perform. Although a master participant typically performs additional functions than the other participants, any participant is capable of performing these functions if elected as the master participant.
A frequency hopping (FH) protocol is an approach for wireless communications in a communications network that uses a frequency hopping signal transmission technique in which information or data is transmitted over a set of frequencies in a communications frequency band. A frequency hopping communications system is a system that uses a FH protocol. The order in which the communications network hops among the set of frequencies is known as the hopping sequence.
In contrast to FH systems, a non-frequency hopping (NFH) system is simply a communications system whose carrier does not hop over a set of frequencies. A typical NFH system may occupy a portion of the communications frequency band corresponding to several frequencies used by an FH system.
With the FH approach, the frequency band is broken up into separate frequencies, often referred to as “channels.” The FH system transmits data on one channel, hops to the next channel in the hopping sequence to transmit more data, and continues by transmitting data on subsequent channels in the hopping sequence. The switching of frequencies may occur many times each second. The use of an FH protocol helps to reduce problems with interference from other communications systems and other interference sources. Frequency hopping also helps with fading of transmissions and power consumption and also provides security for the transmission so that others may not intercept the data being transmitted because others do not know the hopping sequence. An example of a frequency hopping protocol is the Institute of Electrical and Electronics Engineers (IEEE) 802.15.1 Wireless Personal Area Network Standard, which is based on the Bluetooth™ wireless personal area network (WPAN) technology from the Bluetooth Special Interest Group. The BLUETOOTH trademarks are owned by Bluetooth SIG, Inc., U.S.A. The Bluetooth protocol uses 79 individual randomly chosen frequency channels numbered from 0 to 78 and changes the frequencies 1600 times per second. Examples of NFH systems include the IEEE 802.11 b Wireless Local Area Network (WLAN) and the IEEE 802.15.3 next-generation WPAN, both of which operate in the 2.4 GHz Industrial, Scientific, Medical (ISM) band, which is an unlicensed portion of the radio spectrum that may be used in most countries by anyone without a license.
An FH communications system may be either point-to-point, meaning that communications paths are provided from one participant to another participant, or point-to-multipoint, meaning that communications paths are provided from one participant to multiple participants.
Typically one device of the network, such as the master, determines the hopping sequence that will be used by all participants of the network. The other participants of the network, such as slaves, are time synchronized with the master to maintain communications with the master. For example, the Bluetooth and IEEE 802.15.1 frequency hopping protocols are point-to-multipoint systems in which the frequency hopping sequence is determined by the physical address of the master and the phase of hopping is determined by the master's clock. The “physical address” of the master is a unique identifier, such as a media access control (MAC) address, that identifies the device anywhere in the world on a network.
Some FH communications system are configured such that certain devices transmit at specified times or timeslots. For example, slaves may respond to the master at assigned timeslots. As another example, in a Bluetooth FH communications system, the master may transmit at even-numbered timeslots on the hopping sequence and the slaves listen at those regular intervals. The master will address one slave (or all slaves in a “broadcast” mode), and the addressed slave returns back to the master at the next odd-numbered timeslot.
A preamble, which is known to all the participants of the FH network, is used to identify the network and for the slaves to synchronize with the master. For example, in Bluetooth and IEEE 802.15.1, the known preamble is called the “channel access code.”
One problem with frequency hopping communications systems is that coexistence problems arise between the FH communications system and NFH communications systems that operate in the same frequency band. While the FH communications system hops over the entire frequency band, the NFH communications systems occupy separate parts of the frequency band. When the FH communications system hops over part of the frequency band occupied by an NFH communications system, there may be interference between the systems. Although the use of a FH protocol helps to lessen the interference problem because not all of the FH channels will interfere with other communications systems, there nevertheless remains interference on those channels that coincide with the NFH communications systems. An example of the interference situation is the coexistence problem between the frequency hopping IEEE 802.15.1 WPAN and the non-frequency hopping IEEE 802.11b Wireless Local Area Network (WLAN) because both share the 2.4 GHz ISM band.
Further, because of the dynamic nature of interference due to the use of devices at different times and locations, eventually all FH channels will experience some degree of interference at some time. Interference may change depending on when the communications systems use the band and the relative locations of the participants of each system to participants of another system. Because the participants may be mobile, interference may vary depending on the movements of the participants of one system relative to the locations of participants of other systems. In addition, interference may arise from other sources, such as some consumer appliances like microwave ovens, resulting in a degradation of performance of the FH communications system.
Interference results in data transmission errors, such as an increase in the bit error rate (BER) or the loss of data packets, resulting in reduced transmission quality and performance and the need to retransmit the data.
One approach for managing the coexistence problem is to increase the power used in the transmissions so that the other interfering system have less of an impact on the system transmitting at the increased power. However, this increased power approach drains batteries used by the participants, and thus the required power increase may be impractical. Also, the increased power approach only benefits the system using the increased power and results in a bigger interference impact on other systems.
Another approach for managing the coexistence problem is to skip a “bad” channel that suffers from interference, such as by moving onto the next channel in the sequence or by jumping to another randomly selected channel. However, this skipping approach does not necessarily avoid other bad channels because the next channel used may also have an interference problem. Also, known “bad” and “good” channels may change over time due to the transient nature of some types of interference.
Based on the need for wireless communications and the limitations of conventional approaches, an approach for managing interference in communications system, such as the coexistence problem between frequency hopping communications systems and non-frequency hopping communications systems, that does not suffer from the limitations of the prior approaches is highly desirable.