In the desirable radio frequencies occupied by Public Safety and Utilities organizations, there is only so much radio spectrum to share between everyone who wants to use it.
Traditionally, analog radio modulation methods such as AM and FM limited the amount of information conveyed within a given channel, and each channel could only host a single conversation at any one time -a pair of users, one transmitting and one receiving.
Analog modulation (AM) places a very finite limit on the number of users that can occupy the radio spectrum, and does not cater well to today’s expanding needs.
Digital radio, using various modulation schemes, is moving to alleviate this problem by increasing the amount of information that can be conveyed on a channel.
Digital radio has developed ways in which more than one conversation can be accommodated (multiplexed) inside the same physical RF channel. There are three common ways of achieving this:
1. Frequency Division Multiple Access (FDMA)
2. Time Division Multiple Access (TDMA)
3. Code Divisional Multiple Access (CDMA)
1. FREQUENCY DIVISION MULTIPLE ACCESS (FDMA)
The RF (radio frequency) channel is split into several smaller sub-channels. For example, one 12.5kHz wide narrowband FM channel that previously carried only one conversation becomes two 6.25kHz sub-channels, each capable of carrying a separate conversation.
This technique has been around for decades and is used with either analog or digital radios. A ‘telephone style conversation’ can be set up if one sub-channel is used to transmit and one to receive.
The drawback of this is the more sub-channels you try to fit in to the original channel, the more likely that the users will suffer interference on the call. This is because the reduced channel spacing makes it harder to filter only the intended sub-channel and reject all the others at the receiver.
Consequently, dividing a single narrowband 12.5kHz channel frequency into more than two sub-channels becomes impractical.
2. TIME DIVISION MULTIPLE ACCESS (TDMA)
Instead of splitting the original RF channel in to two RF sub-channels, it is instead split into timeslots. The transmitted RF frequency is identical in each slot, but each slot is still capable of carrying a separate conversation.
In the diagram, the channel is split in to four time slots, so in each slot the speech is transmitted in a quarter of the time taken to say it.
This speech compression is possible using digital technology, but eventually practical near/far limits* are encountered that limit how many slots can be accommodated in a single RF channel before neighboring slots begin to overlap at the Base Station.
Again, a ‘telephone-style conversation’ can be set up, if certain slots are used for transmit and others for receive.
3. CODE DIVISIONAL MULTIPLE ACCESS (CDMA)
Instead of splitting the RF channel in to sub-channels or time slots, each slot has a unique code. Unlike FDMA, the transmitted RF frequency is the same in each slot, and unlike TDMA, the slots are transmitted simultaneously. In the diagram, the channel is split in to four code slots.
Each slot is still capable of carrying a separate conversation because the receiver only reconstructs information sent from a transmitter with the same code.
However this does create a problem, as transmissions on the same frequency with different codes are still received and decoded but simply re-appear as noise. This means the greater the number of users, the higher the noise level on the system, which of course can affect coverage.
Near/far issues also require dynamic power control in CDMA systems to make sure all signals arrive at the Base Station at roughly the same level to ensure that signals from distant transmitters are not lost.
* The near-far problem occurs when a transmitter sends a strong signal to a receiver which then makes it difficult for any weaker signals to be received.
To learn more about the basics of channel sharing, including channel spacing and a focus on FDMA vs TDMA, visit Tait Radio Academy for a series of short, free videos. Don’t forget to register for the Academy to stay up-to-date on our latest videos and courses.
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