Frequency Hopping vs. Direct Sequence

Blue Streak Modem Feature Identification

Blue Streak OEM Box Configuration

Programming the Blue Streak OEM using the Interface Board


Modem Configuration

Fast Setup

Blue Streak Specification

The Blue Streak Spread Spectrum modems put state-of-the-art frequecy hopping technology to work for you. Designed and manufactured by Aerotron-Repco Systems, these compact and powerful RF modems feature:

Frequency Hopping vs. Direct Sequence

Frequency Hopping (FH)

Most manufacturers, using FHSS, will send single or multiple data packets on one of 50 pseudo-random frequencies. Since the radio operates essentially as a narrow-band radio for this "slice of time", it has the desirable properties of excellent rejection of out of band signals. Since the frequency is changing constantly, if there is interference at a given frequency the packet is re-sent on the next frequency hop.

Direct Sequence (DS)

Most manufacturers of direct sequence radios transform each data bit into a sequence of much shorter bits, known as chips. The sequence is unique and different for ones and zeros. By knowing the unique sequence at the receiver, it is possible to re-transform the "chips" into data at the receiver. The process of looking for only the known unique sequence, will also suppress other DS radios with different sequences, and other narrow band interference.

FHSS Advantages

Fundamentally much simpler to implement. Most implementations use a low-cost microprocessor to control the frequency hopping functions, as opposed to direct sequence radios which employ extensive digital circuitry or custom LSI to implement the encoding and decoding algorithms. This usually translates to a lower cost FHSS radio, particularly if the designers have used low cost chip sets available from the cellular communications industry.

Better range, due to lower receiver sensitivity. Competitive FHSS modems operating at 115 kbaud have receive sensitivites around -103 dBm, where competitive DS radios at 115 kbaud are around -92 dBm. This is due to the practical (not theoretical) difficulty in designing sensitive DS receivers. This translates to greatly improved range for FHSS radios or better reliability at the same range. Many vendors of FHSS radios encourage their prospective customers to try a "side by side test" of FHSS vs DS. We have yet to hear of a DS vendor that recommends this comparison.

Good rejection of in-band interference. Because FHSS radios are essentially narrow-band radios, which change frequency, rejection of interfering signals as much as 70 dB higher than the desired signal can be attained. DS radios will reject interfering signals as a function of their "processing gain", which is generally only about 20 to 30 dB.

Good performance in multipath environments. Since each frequency in the hopping table provides a different path length, deep multipath fades over consecutive hops are very unlikely. Generally, only one packet is lost due to multpath fading, which is sent on the next hop. DS radios will only provide immunity to multipath when the distance between the transmitter and receiver is relatively long. This because the transmitter and receiver are on the same frequency, and the "air delay" between them must be equal in time to at least one chip.

No "near/far" problems. Since FHSS radios employ narrow-band filters, it is possible to have the receiver from System A near a transmitter from System B with the transmitter of System A far from the same receiver. DS systems suffer from problems in this type of environment, because they have relatively little immunity from in-band interference.

DS Advantages

Shorter lock times. Since frequency hopping radios must synchronize the transmitter with the receiver, there is a "lockup period" where the receiver is looking for the transmitter frequency. This period is in the vicinity of two seconds for most FHSS radios. DS radios have the ability to lock up much faster, although some DS radios do not.

More efficient use of channel bandwidth. DS radios require no "guard band" associated with the frequency channel. For this reason, it is possible to have a larger number of direct sequence radios operating in the same band at the same time. This property is exploited by CDMA phones.

Potential for higher data rates. The FCC limits the hopping channel bandwidth to 500 KHz at 915 MHz and 1 MHz at 2.4 GHz for FHSS systems. This limits the practical channel data rate to about 200kbps and 400 kbps respectively, without using complex modulation techniques. DS systems require a minimum 10:1 ratio of chip rate to data rate, so at 915 MHz (26 MHz band), the maximum (simple) on air data rate could be 1.3 Mbps, and at 2.4 GHZ it could be 3.25 Mbps.


Blue Streak Modem Feature Identification

Blue Streak OEM Model

The Blue Streak line also includes a TTL level model. This modem is the same as the heavy duty enclosure but is in a compact package for tight applications. This Blue Streak version does not include the power/interface board or the "N" antenna connector. Antenna conection is made using a SMA connector and Data and power utilize a 10-pin connector.

Blue Streak OEM exploded view and phyical dimensions

Programming the Blue Streak OEM Modem using the Interface Board

Position the Blue Streak Modem upside down with the 10-pin connector facing up. Orient the Interface Board with the terminal block side up (fig. 1). The 10 pins of the modem mates with the left most 10 sockets of the 20 pin connector on the bottom of the interface board. The phillips screws in the modem will line up with the holes in the Interface Board. Be careful to use only the left 10 sockets of the 20-pin connector (fig 2). (Note: we recommend using anti-glare glasses when programming the Blue Streak



Modem Configurations

Modem use: Point to point master, point to point slave, point to multipoint master, point to multipoint slave, broadcast group master, broadcast group slave, repeater.

Data direction control: Full duplex or half duplex.

AT commands: Enable or disable (menu control only).

Powerdown mode: Enable or disabled (RX idle periods).

Powerdown sync interval: 1 to 5 seconds (how often transmitted during idle).

System frequency key: 30 keys available.

RF power level: Low, high, or auto.

Maximum data packet size: 20 - 255.

RF receiver sensitivity: Low or high.

DTE communications format: 8 or 9 data bits.

Data packetizing: enabled or disabled.

Flow control method Hardware flow control enabled/disabled.

Error correction: Broadcast packet repetition factor.

Transmit error control: Fixed transmission repetition, resend if corrupt.

Receive error control: Discard bad packets, accept good or last packet, or diagnostic.

Group ID's: Serial numbers of the modems with which this modem will communicate (menu control only).

Fast Setup

Fast setup allows initialization of a set of modems without the use of a terminal or terminal emulator. Modems in Fast Setup mode exchange compatible communication parameters and serial numbers. Fast Setup can be used to configure point-to-point and broadcast group topology. When Fast Setup is complete, the modems can be placed into service. The modem operation parameters can be modified later via Menu Setup, if desired.

Note: Fast Setup cannot be used to configure a system that includes repeaters.

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