Communications systems or networks generally comprise channels for sending and receiving data, audio and/or video information. A number of wired communications systems or networks, such as cable, Ethernet and Gigabit Ethernet systems, generally include stationary channels. A stationary channel is generally one in which the channel response and the noise statistics do not vary significantly or regularly (if at all) over time. Techniques are available for such systems and networks to reduce the adverse effects of random and/or systematic fluctuations in communications parameters. However, such techniques generally do not affect communications parameters that tend to fluctuate over time.
Certain communications systems or networks, such as powerline systems or networks, include cyclostationary channels. A cyclostationary channel is one in which the channel response, the noise statistics and/or channel attenuation vary periodically. Also, powerline channels, which use conventional AC power lines for communications, experience regular fluctuations in noise and signal attenuation. The period of the noise power function in a powerline channel is generally the inverse of twice its frequency. In a typical case, that period is ( 1/120 Hz), or 8.3 msec. Noise in a powerline channel may also be introduced by other sources. During “spikes” in the noise power in a powerline channel, the signal-to-noise ratio (SNR) can be reduced sufficiently to cause errors in the data. Particularly when the signal strength is low, a low SNR can cause significant reliability problems.
Referring now to FIG. 1, the powerline transmits a 60 Hz or 50 Hz sinusoidal power signal 10 supplying AC. The noise 20 in power lines may change within a period TAC or TAC/2, where TAC is the duration of one cycle of the AC power supply and/or voltage, typically 1/60 or 1/50 seconds (see, e.g., “Modeling of Cyclostationary and Frequency Dependent Power-Line Channels for Communications,” by Katayama et al.). As shown in FIG. 1, under common normal operating conditions, powerline channel noise 20 typically comprises a “zero power” component 30 (representative of the noise in the channel at the points where the AC power curve 10 crosses the zero power axis 15) and a burst noise component 40 (representative of the noise in the channel where the AC power curve 10 is at or near its maximum value[s]), although noise in cyclostationary channels may vary, depending upon operating conditions, equipment, power supply variations, etc. Thus, under normal operating conditions, cyclostationary channel noise 20 may be somewhat related to the absolute value of the AC power signal 10.
A typical packet 100 for burst mode transmission is shown in FIG. 2. The packet 100 typically comprises a preamble 110, a synchronization sequence (or “syncmark”) 120, and data 130. The synchronization sequence 120 is used during frame synchronization. Under burst noise, especially cyclostationary burst noise, there is a relatively high probability that bits in the synchronization sequence 120 may be corrupted. For 10 Kb/s DBPSK modulation (0.1 ms/bit), if the cyclostationary noise has a burst length of half of its cycle (i.e., half of TAC/2=4.15 ms for 60 Hz AC power), about 42 bits could be corrupted by cyclostationary burst noise. This implies that the synchronization sequence 120 should be longer, and perhaps significantly longer, than 42 bits.
In systems without burst noise, the preamble 110 is often a 010101 sequence, and it is often used in carrier recovery. Due to the long, cyclostationary burst noise on the powerline, it is possible that the entire preamble 110 can be corrupted by burst noise, which means a very long preamble (e.g., more than 4.2 ms, or 42 bits at a 10 Kb/s rate) is desirable in a 60 Hz powerline channel to improve the success rate in carrier recovery. However, it is naturally desirable to minimize the number of bits dedicated to non-data (e.g., control and/or identification) functions, particularly over a sometimes noisy and/or problematic medium such as power lines. In some applications (e.g., voice transmissions), one might wish to completely avoid transmitting information during periods of high noise, and thus, one might keep the packet length small to fit packets into the “low noise” periods on the power line.
A need therefore exists to improve communications in channels in which time-dependant fluctuations are a potential source of error or corruption, in order to reduce errors, reduce shutdowns, increase the success rate of such communications and/or increase communications uptime in networks that include such channels.