promotion image of download ymail app
Promoted

DQPSK是什麼樣的modulation?! DSSS還是FHSS?!

DQPSK是什麼樣的modulation?! DSSS還是FHSS?!

Bluetooth又是那一個modulation type呢?!

1 個解答

評分
  • Quincy
    Lv 5
    1 十年前
    最愛解答

    PSK (Phase Shift Keying) and SS (Spread spectrum) are very different things. PSK determines how the digital signals (10010111010...) are converted to a radio wave, SS determines how frequency is used.

    First, I'll talk about SS:

    SS specifies how frequencies are used.

    If I can use any radio frequencies frequencies between 900MHz and 950MHz (A total of 50MHz), there are different ways that I can use the frequency:

    Send all messages in one big 50MHz channel, ordivide the 50MHz frequencies into smaller channel (eg. CH1 from 900-910MHz, Ch2 from 910-920MHz, Ch3 from 920-930MHz, Ch3 from 930-940MHz and Ch5 from 940-950MHz.Direct Sequence Spread Spectrum (DSSS) uses method 1. It sends all messages onto one big 50MHz channel. Because it has a high bandwidth, each bit of the message can be repeated many times. Therefore, receiver errors would likely to affect part of the bit, which can be corrected by the other part of that bit.

    Frequency Hopping Spread Spectrum (FHSS) uses method 2. Each user sends its message in an allocated channel. However, their allocation changes rapidly, therefore the users sends part of each bit on different channels. Because each channel does not have as much bandwidth as DSSS, it cannot send as many copies of the bit. However, if there is some interference on one channel, part of the bit can be still received in another channel.

    ---------------------------------

    Modulation:

    I'll just talk about PSK.

    Phase Shift Keying encodes bits into the phase of a sine wave. You can think the phase of a sine wave as the "direction" of the wave (eg. from 0 to 1 to 0 to -1 to 0 is one phase, 0 to -1 to 0 to 1 to 0 is another phase, etc). The simplest PSK is Binary PSK (BPSK), it works similar to this:

    (remember we are talking about sine waves)

    We encode the bit 1 as the phase (0, +1, 0, -1, 0) and bit 0 as the phase (0, -1, 0, +1, 0) - the complete opposite of bit 1.

    To encode 1011, the wave would be like:

    (0, +1, 0, -1, 0, -1, 0, +1, 0, +1, 0. -1, 0, +1, 0, -1, 0)

    1|0|1|1 ^|^|^|^ /\|/\|/\|/\ ------+------+------+------ \/|\/|\/|\/ v|v|v|v

    Quaternary PSK (QPSK) divides the sine waves into 4 different symbols. By convention, they don't start at phase 0 degree like BPSK, but uses +/- 45 degrees and +/- 135 degrees.

    I won't go into too much details, except to say that 1011 might look like:

    10|11 ^|^ /\|/\ ------+------ /\|/\ v|v

    Notice that QPSK is 1/2 the length of BPSK.

    Now for DQPSK: The "D" stands for "Differential" - it changes phase if there is a change in the bits:

    1011 will be encoded as 1110 (1st bit is always a 1, if the next bit is different to the current bit, it is a 1. If no change, it is a 0).

    After the differential encoding, the normal QPSK is applied.

    ----------------------------------------

    Blue tooth uses FHSS. version 1 uses Gaussian Minimum Shift keying, version 2 uses DQPSK

    More information:

    Wikipedia - Phase Shift Keying Wikipedia - Spread Spectrum Rappaport T. S., Wireless Communications: Principles and Practice, Second Edition, Prentice Hall, 2002, Chapters 6.8.2, 6.8.3, 6.11.2, 6.11.3, 6.12.3

    • Commenter avatar登入以回覆解答
還有問題嗎?立即提問即可得到解答。