Pulse coding description - biphase

                              By Patrick Mannion
                              EE Times
                              February 5, 2002 (10:59 a.m. EST)
 
 

                              MANHASSET, N.Y. — The fundamental characteristics of ultrawideband (UWB) radio and its principal
                              of operation help explain the excitement and hyperbole the technology has generated of late.

                              UWB begins with the generation of a narrow pulse of electromagnetic radiation, on the order of 0.5
                              nanoseconds. At the important half-power point of -3 dB, the range is a little over +/- 1 GHz,
                              providing 2 GHz of bandwidth, hence ultrawideband.

                              The first UWB systems used a series of constant-phase pulses, with the position of these pulses
                              relative to each other being modulated according to information contained in the baseband,
                              creating a carrier-less train of tightly controlled, coherent pulses.

                              Unmodulated, this train creates a comb of power spectra with regular power peaks that can severely
                              impact narrowband signals.  However, pulse-position modulation
                              (PPM) disperses this power around a given carrier frequency, thus
                              alleviating this interference. This distribution is further enhanced
                              through the use of temporal coding using a pseudo-random number (PN) which disperses the signals
                              even more, pushing the instantaneous power below the noise floor.

                              Reception of the widely spreadsignal requires the use of a time-gated correlator. A correlator
                              multiplies the received RF signal with a stored template waveform,
                              and integrates the result to get a dc output. Because of the PN
                              coding, the correlators must be time-gated according to the PN sequence so it samples the
                              incoming signal at exactly the right time, otherwise the signal would be lost in the noise.

                              An alternate modulation scheme that has found favor recently is called binary phase-shift keying
                              (BPSK), or bi-phase modulation. This uses the baseband information to modulate the phase of
                              the signal (0° or 180°), instead of modulating the position. Pre-processing of the signals
                              eliminates the spectral lines, or comb effect, thereby allowing bi-phase systems to meet
                              regulatory requirements without having to reduce the total transmit power levels. Bi-phase
                              proponents argue that it holds a 3-dB efficiency improvement over PPM, and reduces jitter
                              requirements as well, as signal recovery isn't as position-dependent.

                              Whatever the chosen scheme, UWB has reveled in controversy and the hyperbole of pundits,
                              some plainly misinformed. The propagandist claim that UWB can transmit information across all
                              possible frequencies at all times probably has its basis in the fact that the PN codes can be as
                              large as a designer wants, hence the dithering range is theoretically infinite. Also, with power
                              levels so low (under 75 nW, typical), who cares if the signal crosses into other bands?

                              In fact, the Federal Aviation Administration cares. The military cares. And so does any carrier
                              that's overspent on valuable spectrum. The FAA is worried about flight safety, while the military is
                              concerned about interference in both its communications and GPS bands. Given the current
                              political climate, UWB proponents would be wise not to go against their concerns. Many have
                              already openly sided with the U.S. Department of Defense.

                              As for cellular operators, they've spent billions on spectrum and are unlikely to willingly share it
                              with anyone competing for their customers.

                              The end result is that UWB will be confined to a very narrow band, if the Federal Communications
                              Commission allows it at all. The regulations will also complicate circuit design and add cost.
                              However, the homodyne UWB transceivers still hold the advantage in cost over their venerable
                              heterodyne cousins.

                              UWB also has inherently high security features, thanks to the tight control over power limits, which
                              can be adjusted according to range requirements in real-time, as well as the coherent nature of
                              the pulses. The coherency of the impulse allows the receiver to resolve the signal even in the face
                              of severe multipath, while also allowing for pulse addition at the receiver. This allows for lower
                              transmission power levels, which can slightly impact on data throughput rates.

                              The PN coding gives UWB two of its most exciting, yet overblown capabilities — processing gain
                              and channelization.

                              Processing gain is defined as the ratio of instantaneous bandwidth to information bandwidth. A
                              figure of merit that can indicate a signal's resistance to jamming runs anywhere from 50 dB to 100
                              dB for UWB.

                              But because UWB is unlike direct-sequence, spread-spectrum (DSSS) signals in that it has no
                              compression at the receive end, the processing gain is effectively much lower than 50 dB.

                              Channelization refers to the number of individual communications channels that can operate over
                              a given frequency range, thanks to the PN coding. UWB's proponents have declared this can
                              number in the thousands, due to the PN coding. But studies have shown that in a practical
                              cellular-like application with a distance of 1 mile, the number drops to under 100 channels.

                              Regulatory controls for UWB also affect its operating range. If passed by the FCC, UWB will most
                              likely operate above 4 GHz, where its propagation capabilities are severely limited for a given
                              power level, which will also be controlled. That pretty much does away with long-distance
                              communications, but allows for personal area networks.

                              Many current arguments involving UWB are largely matters of market positioning. UWB
                              proponents claim that GPS is lobbying against UWB solely for fear of competition, due to UWB's
                              position-location capabilities. Yet the NTIA and independent groups have presented ample
                              evidence that shows lower-cost GPS systems are indeed susceptible to interference. Even if UWB
                              were passed, in a region of the spectrum and at power levels that would prevent interference, GPS
                              vendors have nothing to fear, as UWB is being promoted primarily as an indoor, local-positioning
                              technology, which would complement GPS.