Acoustic Communications

Woods Hole Oceanographic Institution

Micro-Modem Overview

The WHOI Micro-Modem is a small-footprint, low-power acoustic modem based on the Texas Instruments TMS320C5416 DSP.

Use the links to the left to find hardware and software documentation, firmware updates, operating scenarios, ordering information and installation tips .

 

Micro-Modem and Power Amplifier

 

Equipment Resources

 

The Micro-Modem has been in development for several years, now. As a result, many new equipment designs have been created: transducers, shallow water enclosures, deep water enclosures, power amplifiers, matched to most commercially available transducers, and accessory electronics.

This equipment is documented, and much of that documentation is presented here. If you cannot find the information you need using the links to the left, please contact us, and we will be happy to assist you.

 

Specifications

 

The table below shows typical specifications for a Micro-Modem system.

Micromodem Specifications
DSP
Texas Instruments TMS320C5416
100MHz low-power fixed point processor
Transmit Power

< 50 Watts for LLF and RFF Power Amplifier based systems

< 100 Watts for MF Power Amplifier based systems


Both power numbers above depend on the transducer that is used for the installation.

Idle Power

~158 milliWatts @ 12V for single channel systems (i.e. Modem + Power Amplifier)

~211 milliWatts @ 24V for single channel systems (i.e. Modem + Power Amplifier)

~230 milliWatts @ 36V for single channel systems (i.e. Modem + Power Amplifier)

Assumes that external power is supplied to the Micromodem Power Amplifier which then provides power to the Micromodem DSP.

Multi-Channel PSK systems that utilize an acoustic receive array and multi-channel analog interface will draw more power at idle. However, there are means to turn off the array to reduce the idle power consumption. Power numbers can be provide upon request.

Receive Power

~158 milliWatts @ 12V for single channel systems (i.e. Modem + Power Amplifier)

~211 milliWatts @ 24V for single channel systems (i.e. Modem + Power Amplifier)

~230 milliWatts @ 36V for single channel systems (i.e. Modem + Power Amplifier)

While detecting or decoding an FSK packet.

~2 Watts

While detecting and decoding a PSK packet (requires additional hardware)

Sleep Power

~5.8 milliWatts @ 12V for single channel systems (i.e. Modem + Power Amplifier)

~19.3 milliWatts @ 24V for single channel systems (i.e. Modem + Power Amplifier)

~39.6 milliWatts @ 36V for single channel systems (i.e. Modem + Power Amplifier)

Assumes that external power is supplied to the Micromodem Power Amplifier which then provides power to the Micromodem DSP.

Data Rate

80-5400 bps

The Micromodem supports several FSK and PSK packet types.

Data rates higher than 80bps FSK require an additional co-processor card for data to be decoded. Additional harware is required for multi-channel PSK data reception. Refer to our Installation and Testing page for more information.

International Customers: Export restrictions allow us to only provide PSK hardware domestically at this time.

Packet Burst Rate vs System Throughput:

It is important to realize that there is a difference between packet burst rate and throughput, which is a function of many things, including serial traffic in and out of the modem and the time for the modem to decode the PSK data. These details are described below.

When referring to the FSK or PSK data rate we are talking about the burst rate. For example: A Rate 5 PSK packet has a Burst Rate of ~5000bps as that is the user content in bytes divided by the total time for the packet.

Packet Burst Rate (PSK BW = 5kHz, FSK BW = 4kHz):

 

Burst Rate (bps)

Frames / Packet

Frame Size (bytes) Packet Length (s)

Signal Length (s)

(Probe + Null + Packet)

PSK Rate 6

490 6 32 2.95 3.25

PSK Rate 5

 

5388 8 256

3.04

3.34

PSK Rate 4

 

1301 2 256 3.15 3.45

PSK Rate 3

 

1223 2 256 3.35 3.65

PSK Rate 2

 

520 3 64 2.95 3.25

PSK Rate 1

 

498 3 64

3.08

3.38

FSK Rate 0

 

80 1 32 3.90 5 (with CI)

 

Communication System Throughput:

Total throughput depends on many other factors such as serial port speed (on each side of the communication system), travel time in the water, polling time, and processing time on the coprocessor (which varies between single channel and multi-channel operation) etc. We can provide throughput numbers for specific situations (x channels, x baud rate).

Some example aggregate receiver throughput numbers:

Assumptions:
Downlink with no ACK.
Serial Port Baud Rate: 115k on the modems and host on both sides.
Times: Times include serial input to the modem (data request interaction) on the TX modem and the serial port character output from the RX modem.
Rate 5 PSK with 5400 bps packet burst rate

For a single channel receiver at rate 5, such as what would be used for a deep vehicle talking to the surface, the overall timing and rate are actually limited by the transmitter. The speed of the host computer sending data to the modem is what controls the overall throughput. The best-case throughput (with a host computer that can respond immediately to each data request), the time to transmit a packet is 4.9 seconds (and the receiver can process it in 4.6 seconds), so the data rate is 3343 bps.

TX Side Limited PSK Rate 5 Timing (1) Channel: 4.9s
PSK Rate 5 Throughput (1) Channel: 3343 bps

For the multi-channel case at rate 5, such as what would be used in shallow water, the modem is no longer running in real-time because of the additional processing required to use the 4 channels. The time per packet and resulting continuous throughput is:

RX Side Limited PSK Rate 5 Timing (4) Channel: 10.1s
PSK Rate 5 Throughput (4) Channel: 1622 bps

If the time for an ACK is added, then, depending upon the distance, the total cycle time grows to about 15.5 sec (for 1500 m range), and if the system is polled, then an additional cycle-init packet plus travel time is necessary, increasing this by another 2 seconds, for 17.5 minimum.