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1 November 2005

Do the hybrid hop

Improve wireless reliability with fast-hopping hybrid spread spectrum.

By Michael R. Moore and Stephen F. Smith, Ph.D.

Existing wireless technology used in industrial applications hinges on either direct-sequence spread spectrum (DSSS) or frequency-hopping spread spectrum (FHSS). While FHSS has been common within the military because it is easier to deploy in a dynamic network, DSSS has been more commonly used for infrastructured systems, such as code division multiple access (CDMA) cell phones because it provides higher spectral efficiency and is much less likely to upset sensitive electronic equipment in plants. By combining the spectral diversity of FHSS, the multipath resistance of DSSS, and other modulation techniques in a hybrid system that hops the carrier on the order of the bit rate, we can make wireless communication links much more reliable.

Three key benefits with this combination:

1. It reduces the effects of short-length multipath, which is often encountered in industrial settings.
2. It improves the ability to combat the near-far problem.
3. It can adapt to a variety of channel problems.

Channel problems include zones with a complex combination of reflective metal surfaces, potential interference signals, and complicated electro-magnetic environments. Improvements in these effects can allow designers to reduce transmitted power, thereby improving battery life or increasing range. As the industrial wireless marketplace matures, it requires technologies addressing industrial needs to reach the market penetration expected and yield the gains in productivity and efficiencies anticipated. This technology can potentially address industrial issues impeding adoption in current applications.

Conventional radio frequency (RF) digital-data transmission schemes are susceptible to errors caused by multipath propagation and other interference conditions. Historically, DSSS techniques have offered immunity against long-path (outdoor) types of reflections, where the time distribution of the individual successive signal reflections arrival is greater than the effective chipping period (the inverse of the spread-spectrum chipping rate) of the transmitted signal. Outdoor environments often exhibit delay-dispersion profiles in the range of 3–100 µs (~ 25 µs RMS) and are amenable to spreading rates of ~1 Mchips/sec, especially when you use them with time-offset modulation schemes, such as offset quadrature phase-shift keying (QPSK). These are in fact the existing parameters of the IS-95 CDMA cellular-telephone system in wide use throughout the U.S. and in other countries.

However, for the indoor propagation case, the dispersion times are much shorter. Typical figures range from 10 to 250 ns, with a median RMS dispersion value of 50 ns. The longer values imply a minimum spreading rate of ~4 Mchips/sec, while the shorter, worst-case values call for spreading rates of about 100 Mchips/sec and thus at least 100 MHz of signal bandwidth.

Obviously, 100 MHz is not feasible for most Federal Communications Commission spectral assignments. One prevalent option is to employ frequency hopping (FH), so as a result of the periodic carrier-frequency changes, the signal will hop to frequencies that do not exhibit multipath nulls (destructive interferences) from the transmitter at the desired receiver locations. In general, the nulls will cancel the total received RF energy of quite a few of these data bursts (hops). Thus, they'll produce bad data packets. But a majority could be of good enough quality to provide effective link operation. However, the link must meet complex (and delay-inducing) interleaving or error-correction coding algorithms (such as Reed-Solomon), or numerous packet retransmissions will need to successfully transport the data payload. In both cases, latency and concurrent link-rate limitations will result.

A more robust scheme (fewer data errors) would function effectively even in severe multipath environments, such as highly RF-reflective areas, yet avoid the introduction of either complex error-correction hardware or latency into the transmission process. There's still no fully developed solution for the issue of link latencies (delays). This is significant in high-speed control applications, where the delays can cause loop-stability problems for the RF-in-the-loop systems. What we need is a solution to address all these requirements.

Frequency hopping vs. direct sequence

There are some trade-offs between FH and DS spread spectrum. The military prefers FH because it isn't susceptible to the near-far problem, making it more robust for dynamic battlefield conditions. Commercial enterprises with CDMA cell phones use DS because it can achieve greater bandwidth efficiency, measured in bits/sec/Hz, and the near-far problem is lessened by the more static infrastructure conditions to help use power control more effectively.

Fast hopping vs. slow

Fast hopping is a protocol in which the carrier hops or shifts more than once per data bit. A data bit might consist of a pseudorandom noise sequence for hybrid spread spectrum. On the other hand, conventional FH or slow hopping employs a carrier that hops at intervals greater than one data bit period. Most conventional systems hop once per message packet, such as Bluetooth.

Hybrid spread spectrum

Hybrid spread spectrum can have superior qualities compared with most present spread-spectrum radio techniques:

• Multipath-rejection capabilities
• Improved data integrity/security
• Better low-probability-of-detection/low-probability-of-interception (LPD/LPI) properties
• Lower link delay (latency) figures
• Superior narrowband/wideband jamming resistance
• Fast synchronization, higher user density
• Less mutual interference among users in a given area or frequency band
• Increased statistical signal diversity
• Near-far reception properties of FH
• Lower overall peak occupied bandwidths

Compared with conventional DSSS or FHSS systems, the hybrid technique offers improved process gain, jamming margin, and multiple-access capabilities. In addition, the hybrid technique offers the advantage of relative freedom from near-far effects of FH, compared with conventional DS. Since in the hybrid system, the DS component can be of lesser bandwidth for comparable overall performance. The front-end and intermediate-frequency (IF) bandwidths of the hybrid receiver can be significantly smaller, and thus possess greater selectivity than in the standard DS implementation. This means they offer greater filter-based rejection of adjacent-channel, out-of-band, and spurious signals. So, the much higher amplitudes of a nearby but off-channel transmitter will not cause blockages of the weaker (more-distant) desired on-channel signal typical of conventional DS systems.

Processing gain

The classic equation for the generalized process gain of a standard spread-spectrum (DS or FH with contiguous, non-overlapping channels) signal is:

G_p = BW_RF ÷ R_info 

Where G_p is the effective processing gain, BW_RF is the total (two-sided) spread-spectrum RF signal bandwidth, and R_info is the modulating (pre-spreading) data rate or information bandwidth.

For standard DS systems, the gain is in general equal to the spreading-code length; in the case of simple FH systems, the long-term average processing gain for contiguous or non-contiguous channel ensembles is equal to the total number of hopping channels. If both DS and FH methods see concurrent use, assuming the DS signal bandwidth is small compared with the width of the total available RF band (so there are a lot of hybrid hopping channels), the overall hybrid-signal process gain is the product of the two individual process gains:

G_p(FH/DS) = G_p(FH) x G_p(DS).

In decibels, the equation becomes

G_{p(FH/DS) dB} = G_{p(FH) dB} + G_p(DS)dB = 10 log (no. of hopping channels) + 10 log (BW_DS/R_info)

Where G_p(FH/DS) is the hybrid spread-spectrum process gain, G_p(FH) is the FH gain, and G_p(DS) is the straight DS gain.

In the theoretical limit, for a fixed available band width (such as 26 MHz for the 902-928 MHz ISM band) and non-overlapping FH channels, the composite process gain for the hybrid DS/FH system cannot exceed the ratio of the total bandwidth to the information rate. For full-band noise, interference, or jamming, hybrid DS/FH techniques using non-overlapping channel sets theoretically would not provide any more process gain than for the single DS signal; but they would in practice still be superior to the full-band DS format in rejecting multipath-induced errors, in resolving near-far interference effects, and in permitting multiple signals to travel simultaneously within the confines of the selected band.

For systems that include tightly controlled transmission frequencies and time slots, the hybrid channels can overlap to improve process gain.

Coherent detection vs. non-coherent

Coherent detection refers to communications receivers that use the absolute phase of the received signals' carrier to recover information. Alternatively, non-coherent receivers do not require absolute alignment of the phases of the incoming signal and the local oscillator.

Although designers can build a hybrid spread spectrum transmitter to output continuous-phase signals (with simultaneously modulated DS and FH components), a coherent hybrid spread spectrum receiver could, in theory, conversely be capable of continuous-phase reception of the waveform. y

Behind the byline

Michael R. Moore and Stephen F. Smith, Ph.D., are research staff in the RF and Microwave Systems group at Oak Ridge National Laboratory in Oak Ridge, Tenn.