TELECOM Digest OnLine - Sorted: Re: Walkie Talkie

Re: Walkie Talkie

Steve Watt (steve@Watt.COM)
Tue, 22 Mar 2005 00:28:58 UTC

In article <telecom24.103.8@telecom-digest.org>,
jason <cheanglong@gmail.com> wrote:

> Hello all,

> I need to know more about walkie talkie and how the frequency range
> work. Let'say a walkie talkie with frequency range work from 2400 to
> 2500 Mhz, while the IF is 5 Mhz.

> So how will the channels be allocated for transmitting and receiving
> if it is a single duplex type?

The IF doesn't have anything to do with the channel spacing at all;
many older handheld transceivers have IFs of 455KHz, and that has no
impact on the channel spacing.

In simplified terms, minimum channel spacing is determined by the
modulation bandwidth. If you've got FDMA (i.e. normal AM, SSB or FM
modulation) then the channels are slightly more than the modulation
bandwidth apart. So if you have a 3KHz modulation bandwidth, the
channel center frequencies would be 5KHz apart.

If your modulation scheme is based on spread-spectrum techniques, then
basically everything changes, and everyone shares the same frequency
band with separation of signals provided by receive correlation.

Now, to your other question of how the channels will be allocated?
That's only barely a technical issue. The regulators in the
particular country you intend to operate that transmitter in have
regulations and band plans that say what can be transmitted where.
They usually specify bandwidths and center frequencies for FDMA, add
slot timing to TDMA, and specific spreading codes or classes of
spreading codes for CDMA.

So, in the US, the FCC controls channel spacing for duplexing, and I
don't remember (and don't have at hand) the spacing in that frequency
band in the US. -- Steve Watt KD6GGD PP-ASEL-IA ICBM: 121W 56' 57.8"
/ 37N 20' 14.9" Internet: steve @ Watt.COM Whois: SW32 Free time?
There's no such thing. It just comes in varying prices...

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