digitalmodulationbasics(編輯修改稿)
2024-10-07 14:54 本頁(yè)面
【文章內(nèi)容簡(jiǎn)介】 oving the carrier signal to obtain the original signal waveform ? Detection – extracts the symbols from the waveform – Coherent detection – Noncoherent detection 29/01/2020 Property of R. Struzak 19 Coherent Detection ? An estimate of the channel phase and attenuation is recovered. It is then possible to reproduce the transmitted signal and demodulate. ? Requires a replica carrier wave of the same frequency and phase at the receiver. ? The received signal and replica carrier are crosscorrelated using information contained in their amplitudes and phases. ? Also known as synchronous detection 29/01/2020 Property of R. Struzak 20 Coherent Detection 2 ? Carrier recovery methods include – Pilot Tone (such as Transparent Tone in Band) ? Less power in the information bearing signal, High peaktomean power ratio – Carrier recovery from the information signal ? . Costas loop ? Applicable to – Phase Shift Keying (PSK) – Frequency Shift Keying (FSK) – Amplitude Shift Keying (ASK) 29/01/2020 Property of R. Struzak 21 NonCoherent Detection ? Requires no reference wave。 does not exploit phase reference information (envelope detection) – Differential Phase Shift Keying (DPSK) – Frequency Shift Keying (FSK) – Amplitude Shift Keying (ASK) – Non coherent detection is less plex than coherent detection (easier to implement), but has worse performance. 29/01/2020 Property of R. Struzak 22 Geometric Representation ? Digital modulation involves choosing a particular signal si(t) form a finite set S of possible signals. ? For binary modulation schemes a binary information bit is mapped directly to a signal and S contains only 2 signals, representing 0 and 1. ? For Mary keying S contains more than 2 signals and each represents more than a single bit of information. With a signal set of size M, it is possible to transmit up to log2M bits per signal. 29/01/2020 Property of R. Struzak 23 Geometric Representation 2 ? Any element of set S can be represented as a point in a vector space whose coordinates are basis signals ?j(t) such that ? ? ? ?? ?? ? ? ?210 , 。 ( = a r e or t hog ona l )1 。 ( nor m a l i z a t i on )T he n ijiNi ij jjt t dt i jE t dts t s t???????????????? ? ???????29/01/2020 Property of R. Struzak 24 Example: BPSK Constellation Diagram ? ? ? ? ? ? ? ?? ? ? ?12122c os 2 , c os 2 。 。 0 e ne r gy pe r bi t 。 bi t pe r i od F or t hi s si gna l se t , th e r e i s a si ngl e b a si c si gna l2 c os 2 。 0 bbB P SK c c bbbbbcbbB P SKEES s t f t s t f t t TTTETt f t t TTSE??????? ? ? ???? ? ? ? ? ?? ? ? ???? ? ? ?? ? ? ?????? ? ?? ? ? ? ?? ?11,bbt E t??? ? ? ??? ? ? ??Eb ?Eb Q I Constellation diagram 29/01/2020 Proper
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