I know ADSL uses OFDM, but calls it DMT (Discrete MultiTone). But unlike RF channels which are relatively narrow with characteristics that don’t change much over the channel width, the copper twisted pair channel is wide and varies dramatically with frequency. Due to much higher attenuation at higher frequencies, nulls due to bridge taps, and crosstalk that varies with frequency. So DMT has a dynamic “bit loading table” communicated between the transmitter and receiver, so that each subcarrier is assigned a modulation level based on its SNR. Closest in our world would be 2 or 3 carrier radios with carrier aggregation that might have different QAM levels for let’s say U-NII-3, U-NII-5 and U-NII-7.
From: AF <[email protected]> On Behalf Of Chuck Sent: Thursday, July 10, 2025 6:43 PM To: [email protected] Cc: [email protected] Subject: Re: [AFMUG] RAKE vs OFDM for multipath Did digital TV broadcast use OFDM, i know it was considered for multipath rejection. But i think they went with VSB8 or something like that. Sent from my iPhone On Jul 10, 2025, at 1:32 PM, Bill Prince <[email protected] <mailto:[email protected]> > wrote: Formatted more better. Combining OFDM and RAKE receivers in wireless communication systems Orthogonal Frequency Division Multiplexing (OFDM) and RAKE receivers are two distinct techniques for addressing multipath fading in wireless communication. While they approach the problem differently, they can be combined in certain scenarios to enhance performance. 1. Understanding OFDM and RAKE * OFDM: Divides a high-speed data stream into multiple lower-speed data streams, modulating each onto separate, orthogonal subcarriers. These subcarriers are closely spaced but don't interfere because of their orthogonality. OFDM is primarily used to combat intersymbol interference (ISI) caused by multipath fading and simplify channel equalization by moving it to the frequency domain. * RAKE Receiver: Primarily used in spread spectrum systems (like CDMA), a RAKE receiver uses multiple "fingers" or correlators to identify and combine delayed copies of the transmitted signal that arrive at the receiver through different paths (multipath components). By combining these components with appropriate weights, the RAKE receiver improves the signal-to-noise ratio (SNR) and mitigates multipath fading effects. 2. Complementary strengths and potential synergy OFDM handles multipath fading in the frequency domain, while RAKE receivers operate in the time domain. Combining them can leverage their complementary strengths, potentially resulting in better performance in challenging multipath environments. 3. Potential benefits of combining them * Improved Interference Mitigation: OFDM-RAKE systems can potentially combat interference more effectively by combining the frequency-domain processing of OFDM with the time-domain multipath component combining of RAKE receivers. * Enhanced Performance in Specific Scenarios: Combining beamforming and RAKE reception can be particularly useful in non-line-of-sight (NLOS) underwater acoustic communication to handle multipath interference from long delay waves. * Optimizing Performance in OFDM-CDMA Systems: In OFDM-CDMA systems, which combine OFDM with Code Division Multiple Access, RAKE receiver functions can be integrated to further improve performance against multipath fading. 4. Considerations and potential challenges * OFDM's inherent multipath resistance: OFDM, with its cyclic prefix and frequency-domain equalization, already provides a significant level of resistance to multipath fading. In channels where the delay spread is within the OFDM system's capabilities, adding a RAKE receiver might be redundant and add unnecessary complexity. * Increased Complexity: Combining these techniques will inherently increase the complexity of the receiver architecture, requiring additional processing capabilities and potentially higher power consumption. * Potential Trade-offs: The potential benefits of combining these techniques need to be carefully weighed against the increased complexity, cost, and power consumption. 5. Applications While not a standard combination in all OFDM systems, combining RAKE-like functionalities or considering them for specific aspects like channel estimation and combining can be beneficial in certain scenarios, including: * Wireless Networks: In wireless networks, RAKE receivers could potentially contribute to improving the system's performance, especially in environments with weak signals and challenging multipath conditions. * High-Speed Data Transmission: RAKE receivers can be employed in high-speed data transmission systems to minimize errors caused by transmitting signals over long distances and through complex channels. In summary, while OFDM already offers inherent resistance to multipath fading, combining it with RAKE receiver principles can be advantageous in specific scenarios to further enhance performance, particularly when dealing with severe multipath, long delay spreads, or when optimizing performance in OFDM-CDMA systems. However, careful consideration of the potential benefits, increased complexity, and potential trade-offs is crucial when deciding to combine these techniques. bp <part15sbs{at}gmail{dot}com> On 7/10/2025 11:53 AM, Adam Moffett wrote: RAKE is a new term for me. I don't know anything about it. My experience with OFDM is that how well it works scales with power consumption of the device, which in turn is dependent on its signal processing capability. An 802.11 device does whatever the commodity silicon does. An LTE base station needing hundreds of watts of power can produce data out of almost nothing. WiMax produced middling outcomes, and those base stations needed 30W-100W depending on vendor. This is speculative, obviously, but it's based on what I saw with my own eyeballs. I'd love to see something showing us SNR before and after signal processing. If we did, I think the outcomes would track with power consumption like I'm saying. _____ From: AF <mailto:[email protected]> <[email protected]> on behalf of Ken Hohhof <mailto:[email protected]> <[email protected]> Sent: Thursday, July 10, 2025 2:15 PM To: 'AnimalFarm Microwave Users Group' <mailto:[email protected]> <[email protected]> Subject: [AFMUG] RAKE vs OFDM for multipath Any RF communication nerds here? I have read that both OFDM and RAKE deal with multipath but are not usually used together. How true is this? The company that begins with T implies they implement a RAKE receiver but I haven’t heard them explicitly use that term. Do they actually have multiple receivers for each “finger” of the rake, at both base and remote node? Myself, I don’t totally buy that OFDM deals with multipath in the way claimed. Yes, the cyclic prefix is a trick to make a continuous signal look cyclical so that DFT can do the work of FFT, and yes the cyclic prefix needs to be long enough so that delayed multipath reflections don’t invalidate that. But it doesn’t seem to process each reflection separately and then add them back together. Maybe as an armchair RF engineer I just don’t understand how it works, that’s entirely possible. I also can’t wrap my brain around how a RAKE receiver would separately process multipath versions of the signal if they are identical but exactly 180 degrees out of phase. If they cancel, they cancel, I don’t see how you avoid a fade in that situation. But again, maybe I just don’t understand. But tl;dr does the T company use both OFDM and RAKE at BN and RN, despite conventional wisdom that you don’t use both, and that RAKE is mostly used with DSSS? -- AF mailing list [email protected] <mailto:[email protected]> http://af.afmug.com/mailman/listinfo/af_af.afmug.com
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