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| communication:basics [2008/04/04 08:08] – external edit 127.0.0.1 | communication:basics [2008/07/19 23:33] (current) – external edit 127.0.0.1 |
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| * Lower frequency | * Lower frequency |
| * Narrower bandwidth (but lower data rates result). | * Narrower bandwidth (but lower data rates result). |
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| {{communication:pathloss_b.xls|G8KHW Path Loss Calculator}} | {{communication:pathloss_b.xls|G8KHW Path Loss Calculator}} |
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| | [[communication:examples|Practical Examples]] |
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| ===== Packet Radio Testing ===== | ===== Packet Radio Testing ===== |
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| the tests suggest that the limit of line of sight communication is of the order of 40Km with such a set-up - certainly greater than 20Km and definitely a lot less than 100Km. This indicates that communication at distances in excess of 100Km are possible with 300 baud 200Hz FSK equipment (HF packet Radio) - in fact the MiHAB 2 radio system achieved the equivalent of over 280Km with such a system. | the tests suggest that the limit of line of sight communication is of the order of 40Km with such a set-up - certainly greater than 20Km and definitely a lot less than 100Km. This indicates that communication at distances in excess of 100Km are possible with 300 baud 200Hz FSK equipment (HF packet Radio) - in fact the MiHAB 2 radio system achieved the equivalent of over 280Km with such a system. |
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| ===== Error Correcting Codes ===== | ===== Error Correcting Codes ===== |
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| Another factor that can increase communication distance (for a given power) is the use of error correcting codes. These codes add additional data to the transmission which are used to correct any errors introduced by "noise" on the communication channel. These codes can be used to reduce the amount of transmit power needed to achieve reliable communication. One of the most famous examples of these codes was the use of the Reed Solomon codes by the voyager spacecraft to get pictures of neptune back at over 57,000,000Km using just 20 watts of transmit power. Reed-Solomon (RS) codes can typically reduce the amount of power needed to achieve reliable communication by a factor of 4x (6dB) or so (depending on error rate needed and size of the transmitted data block) - so the use of a RS codes can effectively double the communication range without increasing power requirements. | Another factor that can increase communication distance (for a given power) is the use of error correcting codes. These codes add additional data to the transmission which are used to correct any errors introduced by "noise" on the communication channel. The codes can be used to reduce the amount of transmit power needed to achieve reliable communication. One of the most famous examples was the use of the Reed Solomon codes by the voyager spacecraft to get pictures of neptune back at over 57,000,000Km using just 20 watts of transmit power. Reed-Solomon (RS) codes can typically reduce the amount of power needed to achieve reliable communication by a factor of 4x (6dB) or so (depending on error rate needed and size of the transmitted data block) - so the use of a RS codes can effectively double the communication range without increasing power requirements. |
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| A more recent coding scheme are "Turbo Codes" (used in 3G mobile phone systems) - these have a better performance that RS codes - improving on them by a factor of about 1.5x (2db) in terms of power - equivelent to about 20% further in distance. Turbo Codes can come close to the theoretical maximum for radio system perfomance. | A more recent coding scheme is "Turbo Codes" (used in 3G mobile phone systems) - these have a better performance that RS codes - improving on them by a factor of about 1.5x (2db) in terms of power - equivelent to about 20% further in distance. Turbo Codes can come close to the theoretical maximum for radio system perfomance. |
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| The experince gained on the MiHab2 balloon radio link (which was un-corrected) would suggest that a range of 1000Km is not infeasable using a 10mW 434MHz licence exempt transmitter (and modest recieve system) if Turbo Codes were used on the transmission link. | The experince gained on un-corrected balloon radio links would suggest that a range of 1000Km is not infeasable using a 10mW 434MHz licence exempt transmitter (and modest recieve system) if Turbo Codes were used on the transmission link. |
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| **Note:** The above makes some fairly gross asumptions about underlying modulation schemes and other factors. | **Note:** The above makes some fairly gross asumptions about underlying modulation schemes and other factors. |
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