Receiver sensitivity- Packet Detection v/s Packet capture

Discussion in 'Wireless Internet' started by Vinay, Oct 13, 2005.

  1. Vinay

    Vinay Guest

    Hello All,
    There is a notion of "detecting the signal" and "capturing the
    signal" based on the received signal strength. The packet reception
    success depends upon the noise on the channel too (SNR). For simplicity
    of this discussion, let us ignore noise for the time being.

    According to the best of my knowledge, the only physical entitiy
    present(which impacts the discussion) in the data sheet of a wireless
    receiver is the "receiver sensitivity". If the received signal level is
    higher than this sensitivity (noise ~= 0), then the receiver is able to
    successfully receive the packet.

    However, in certain research work and simulators, "capture threshold"
    and "reception threshold" variables are also introduced. Here is the
    main point of this topic. Are these abstract entities? In a simple
    scenario where a receiver is placed at some distance from transmitter.
    Is there a possibility of sensing the packet but not being able to
    decode it (assume noise ~= 0 again) ? If not, then the whole idea of
    capture threshold is meaningless. If yes, then why doesnt the wireless
    cards have it in their specs?

    My thinking is that a signal can be captured properly if the
    demodulation phase succeeds. There is always something on the channel.
    If the demodulator can figure out the envelope wave (depending on the
    modulation type), it should be able to demodulate the wave and capture
    the digital content from the analog signal. Is there a possibility that
    we detect the envelope wave but still not able to demodulate? (Again
    assuming noise ~= 0). If this is so, then there can be carrier sensing
    but incorrect reception.
    Since the whole envelope wave is sent with the same power, is there any
    reason of not decoding it after detecting it.

    Wlog, I guess we can assume that this effect can be stated even when
    any noise present on ths channel. There is still capture/detect effect,
    but now we have to consider SNR ratio instead of just signal strength.

    Any pointers to this topic is greatly appreciated. Sorry if the
    question is very trivial to DSP/communication people! :)

    Thanks and Regards,
    Vinay, Oct 13, 2005
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  2. Not really. In Wi-Fi, receiver sensitivity is measured at a specific
    BER (bit error rate) which implies a specific signal to noise ratio
    ratio Eb/No.
    Generally, the reference BER is either 10^5 or 10^6.

    Signal "capture" is an FM effect that decrees that FM signals over a
    specific margin (usually 1-2dB) will completely "capture" or cause the
    lower level signal to disappear. This works with 802.11 1-2 mbits/sec
    which is pure FM, but not with higher speeds, which include an AM
    Wrong. The SNR must be above a specific margin or the demodulator
    will belch garbage (errors). The measurement reference SNR is NOT a
    good place to operate. At 1 error every 10^5 bits with 512 byte
    average packets, that's one error every 250 packets, which sucks. With
    a 10^5 BER, it even worse with one error every 25 packets. + That's
    what "fade margin" is all about. A minimum fade margin of 20dB is
    typical, which yields a much better BER for adequate communications.
    Neither term has any real meaning to me. I'll guess that reception
    threshold is acutally minimum detectable signal and capture threshold
    deals only with FM modulated signals. Neither of these have any EIA
    or ISO specs for their measurment.
    You were right the first time. Receiver threshold is meaningless as
    it really a measure of how much ambient noise is in the local
    environment and how much signal it takes to overcome this noise. Not
    a very useful or reproduceable measurement. Instead of 0dB S/N (which
    is where the signal equals the noise) a BER reference makes more sense
    and is reproduceable.
    Captured properly? That implies an interfering signal.
    What does that mean? Are you making measurements with an antenna
    connected? If so, you won't get consistent results. The receiver
    sensitivity should be measured in a pollution free environment with a
    reproduceable test setup. Ambient RF pollution is not reproduceable.
    Yeah, that's what demodulators do. With direct sequence spread
    spectrum, it's a bit different as there is no analog signal. Most of
    the common chipsets used today are all digital and use a DSP to
    provide the necessary digital demodulation without first converting to
    Sorry. You lost me.
    Again, nobody trys to demodulate a high speed digital signal with 0dB
    SNR. The higher data rates and more exotic modulation schemes require
    a much higher SNR. For example, at 11Mbits/sec with CCK modulation,
    the minimum SNR is about 11dB for a 10^6 SNR. Look at the curve at:
    There are modulation schemes that will work with 0dB SNR, but the
    thruput isn't anything near as fast as Wi-Fi.
    You lost me again.
    You most certainly should consider SNR. If insufficient, you get
    noise on your demodulation output instead of data.
    You're probably asking in the wrong newsgroups. I can't tell what
    you're working on or what you're trying to accomplish to advise as to
    the proper newsgroup.

    Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
    831.336.2558 voice AE6KS Skype: JeffLiebermann
    Jeff Liebermann, Oct 14, 2005
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