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Make the most of the unlicensed ISM band

Posted: 16 Sep 2005 ?? ?Print Version ?Bookmark and Share

Keywords:ism? 915mhz? fcc? spectrum? dsss?

Due to its unlicensed nature, the 915MHz U.S. Industrial, Scientific and Medical (ISM) band is popular for establishing wireless data links with short-range wireless devices. As its popularity grows, the number of devices operating in this band increases. This situation can create congestion in the frequency spectrum, which manifests itself in interference from other devices operating in the band, leading to a degradation of performance of the intended link.

The ISM band is an unlicensed band intended for low-data-rate communications and extends from 902MHz to 928MHz. By definition, any transceiver operating in an unlicensed band does not require that the end-user obtain a government permit to use the device. However, the device itself must be certified by the governing authority in the operating country.

Spread-spectrum systems are known for their interference rejection and anti-jamming capability in multiple access systems. Spread-spectrum signals used to transmit digital information are distinguished by the characteristic that their bandwidth, W, is much greater than the information rate, R, in bits/s. The bandwidth expansion factor, Be = W/R for a spread-spectrum signal, is much greater than unity. The large redundancy inherent in spread-spectrum signals is needed to overcome the potentially severe interference levels that are encountered when transmitting digital information over radio channels. Radios operating in the 915MHz ISM band are subjected to interference from other unlicensed devices in the same band that could significantly degrade the operating radio's sensitivity.

Another important element used in the design of spread-spectrum systems is pseudo-randomness, which makes the transmitted signals appear similar to random noise and difficult to demodulate by the receivers, other than the ones intended to receive the information. The benefits of spread-spectrum systems are:

? Combating or suppressing the interference's detrimental effects due to jamming, interference arising from other users of the same channel and self-interference due to multipath propagation;

? Achieving message privacy in the presence of other listeners;

? Hiding a signal by transmitting it at low power and making it difficult for an unintended listener to detect in the presence of background noise. Thus, these signals have a low probability of intercept.

Two types of spread-spectrum systems exist: direct-sequence (DSSS) and frequency-hopping (FHSS). In DSSS, a pseudo-noise (PN) generator generates PN codes at the chip rate, which is much faster than the data rate. The data output at the data rate and the PN generator output at the chip rate are Modulo-2 added and fed to the phase-shift-keying (PSK) modulator. At the receiver (Rx), the PN codes' complex correlation properties decode the message sequence. DSSS is an expensive solution and more complex to implement due to the stringent synchronization requirements. DSSS also needs a coherent modulation technique such as binary PSK. These factors exclude DSSS as a suitable choice for simple, low-cost, low-data rate ISM band transceivers. The alternative, FHSS, is better suited for such applications.

FHSS systems

In an FHSS system, the available channel bandwidth is subdivided into a large number of contiguous frequency slots. The U.S. ISM band lends itself to low-data-rate FHSS systems. In any signaling interval, the transmitted signal occupies one of the available frequency slots. Frequency slots are selected pseudo-randomly during each signaling interval. User-defined protocols can determine the hopping sequence.

The transceiver transmits or receives during dwell time. The time it configures its registers to transmit or receive at another frequency is called blank time. What occurs during dwell time is the Tx (or Rx) preamble, start bit, data sequence and post-amble at a particular frequency in the hopping sequence. And the actions during blank time are pseudo-random frequency generation, configuring the transceiver registers to operate at the randomly generated frequency, and waiting for the PLL to lock. To conserve battery power, the blank time should be as short as possible.

There's a good reason to use frequency hopping for transceivers operating in the ISM band. FCC regulates the operation of unlicensed devices in this band. Under part 15.247 of FCC regulations, frequency-hopping systems are permitted to transmit at powers of up to 30dBm EIRP. This higher power operation (i.e. higher-link budget or, ultimately, range) combined with the benefits of spread-spectrum systems makes frequency hopping an attractive option for unlicensed radio devices in the ISM band.

Single-channel general-purpose data devices share the same 902MHz to 928MHz frequency band as hoppers, but operate at reduced power levels. There's no hopping requirement for single-channel devices. The maximum output power is about -1.25dBm, and the maximum harmonic power is about -41.25dBm.

Frequency-hopping wireless UART chipset takes advantage of interference-rejection features and the higher transmit power option offered by frequency hopping systems. The chipset that operates in the 902-928MHz ISM band is a true wireless UART solution (data-in RF-out, RF-in data-out) where all the hardware and software aspects of a wireless link have already been implemented to yield a ready-made solution. The chipset can implement a wireless link that end-applications can interface to as a peripheral, shielding the application from the frequency-hopping system's complex implementation details.

The chipset's transceiver is identified by a 16bit unique transceiver ID, a 16bit network ID and a 16bit system ID, forming a 48bit address. Each transceiver is also configured to use one of 16 distinct frequency-hop sets, which consists of 50 frequency channels that are pseudo-randomly arranged. Two transceivers with different hop-set configurations will not communicate.

The data packet consists of a header, data payload and trailer with the checksum. The data communication protocol supports acknowledgments and retries (up to 20) for ultrareliable data transfer. The chipset is configured to support point-to-point and point-to-multipoint (broadcast) network topologies.

Frequency-hopping protocol

Each transceiver is designed to hop pseudo-randomly across 50 channels in the 902-928MHz band as configured by the hop-set ID. Because the Tx carrier frequency hops pseudo-randomly, the Rx needs to generate frequencies in the same pseudo-random order to ensure proper frequency lock, demodulation and detection of the signal. Thus, there has to be time synchronization between the Tx and Rx. This synchronization happens in two phases. Acquisition is the initial phase where the receiver recognizes the transmitter. The tracking phase happens upon successful acquisition. While tracking, the Tx and Rx need to be in continuous synchronization until data transmission and/or reception is complete. Both phases can be implemented in firmware.

Tx and Rx devices must be set to identical hop tables. The originating device, once activated transmits data on a random channel determined by the configured hop set. The Rx device scans each channel looking for the Tx preamble consisting of a 0101 sequence. Once the Rx device determines a valid preamble, it remains on the valid channel. When the originating device transmits the 70ms preamble, it sends the sync pattern (with the sequence of 00110011), and the receive device syncs up with the originating device and prepares to receive valid data. Upon receiving valid data, the Rx hops to the next channel predetermined by firmware to transmit an acknowledgement to the originating device. The device then goes into receive mode after transmission and listens for the acknowledgement on the next channel determined by firmware. Upon successful communication, the originating device passes a successful transmission acknowledgement from the intended Rx to the host.

- ShreHarsha Rao

RF Systems and Applications Engineer

Texas Instruments Inc.

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