Getting Started with OFDMA
By Matthew Olfert
Over the past year, upstream peak traffic growth has added pressure to access networks all over the world. Due to this behavior change, operators accelerated their plans to add upstream capacity. Many cable operators have been working on deploying orthogonal frequency division multiple access (OFDMA) to increase upstream spectral efficiency (bits per second per hertz, or bps/Hz). In many cases operators are working on cable plant upgrades to allow for additional upstream spectrum as well. This combination brings forward significant upstream capacity gains to counter the upstream traffic growth and bring forward higher upstream speed offerings.
Spectrum location
Channel spectrum location is the key first choice for any OFDMA deployment plan. The following considerations should be made:
- Are there plant leakage or ingress concerns from over-the-air signals like FM radio or NAV/COM?
- Is there any higher noise profile spectrum within the OFDMA channel spectrum?
- Has the OFDMA overlapped possible diplex filter band changes?
The OFDMA advantage lies in what can be achieved in terms of bps/Hz over single-carrier quadrature amplitude modulation (SC-QAM). With a mid-split capable cable plant, placing the OFDMA channel in the 42 MHz to 85 MHz range is ideal. In a high-split configuration, multiple OFDMA channels are possible. With two OFDMA channels 85 MHz capable modems will become a large concern if you want the lower OFDMA channel going above 85 MHz to maximize the whole high-split spectrum.
Guard band
Since the planned OFDMA is not the only channel in your network, the guard band between your OFDMA channel and SC-QAM channels becomes important. If the neighboring SC-QAM channel is running at 6.4 MHz wide no guard band is required (OFDMA with a cyclic prefix @ 192 and roll-off @ 96). Upon testing, a good receive modulation error ratio (RxMER) was achieved, but we did see a lot of correctable forward error correction (FEC) codewords. By adding a guard band of 400 kHz, this was reduced to 5%. Increasing the guard band to 600 kHz reduced it even further to 0.1%. Based on these learnings, a guard band of at least 500 kHz is recommended.
Channel parameters
When looking at subcarrier spacing, OFDMA has two choices: 25 kHz (4k-FFT) or 50 kHz (2k-FFT) spacing. Not all CMTS vendors currently support 4k-FFT. Therefore, 2k-FFT should provide better resiliency over 4k-FFT at the cost of overhead. Once you have this selected FFT size, cyclic prefix and roll-off period values are required. We have many options, but the general rule is, the lower the number, the less overhead, the higher the number, the higher the resiliency. For cyclic prefix, the options are 96, 128, 160, 192, 224, 256, 288, 320, 384, 512, and 640. For roll-off period the options are 0, 32, 64, 96, 128, 160, 192, and 224. A recommended starting place is a cyclic prefix of 192 samples and roll-off period of 96 samples until you understand your cable plant performance with OFDMA.
Interval usage codes
The last major piece of the OFDMA puzzle is the interval usage codes (IUC). These are very similar to OFDM’s downstream profiles. Each IUC has a modulation and a pilot pattern and supports variable bit-loading. Pilot patterns settings are based on the FFT that has been selected. The lower number reduces the pilots, thereby reducing the overhead. Higher pilot patterns can improve the equalizer performance and overall robustness. The 2k-FFT pilot pattern range is 1-7. The 4k-FFT pilot pattern range is 8-14.
Configuring multiple IUCs allows cable modems to fallback to lower modulation IUCs. This enables individual modems to adjust to changing cable plant conditions. This will also ensure cable modems are running the highest supported modulation for their individual radio frequency (RF) conditions. The majority of DOCSIS 3.1 cable modems on the cable plant tend to cluster around the same IUC.
Operational behavior
Cable modem noise mitigation and channel impairment operation will be different with OFDMA channels then traditional SC-QAM channels. OFDMA operational behavior will be different for each cable modem termination system (CMTS) vendor, but the general principles will be the same. As a cable modem comes online it will be assigned IUC 13, typically this will be a lower modulation order (QPSK, 16-QAM). Next, the CMTS will determine the average RxMER of the OFDMA channel for each cable modem registering. This RxMER value determines which IUCs each cable modem can likely support without error. Next, the cable modem will start utilizing the highest capacity IUC that is supported by the cable modem’s RxMER value. Now that the cable modem is operational on the OFDMA channel with two IUCs (typically 13 and another one at a higher capacity), codewords should start to increment on the higher capacity IUC. This process provides a baseline for the cable modem to start on OFDMA. A second process based on FEC codeword errors during transmission will change the active IUC for the cable modem. If unreliable FEC codewords occur, the CMTS will change from the highest capacity IUC to the next highest IUC. This fallback behavior quickly moves a cable modem to a more reliable IUC. Cable modems will continue this fallback process until stability has been achieved. It may even impair the OFDMA channel and move to partial service, if needed. Once the fallback process has stabilized, a recovery process will begin. Typically, the CMTS will have a hold-off timer which waits for a time before any recovery of the channel or higher capacity IUC. This timer will slow possible flapping between two different IUCs if the cable modem is sitting on the edge between the two IUCs. A hold-off timer of 15 minutes provides a good balance between capacity recovery and IUC stability.
Additional Considerations
Once cable modems start utilizing OFDMA, new issues will emerge. Since OFDMA requires the latest cable modems, this new technology creates a divide between OFDMA-enabled cable modems and older cable modems. Understanding the behavior of the CMTS’s scheduler is important to ensure older cable modems still have access to enough upstream capacity to provide good service. It is also very important to check each cable modem model against current active firmware. This can be the biggest challenge for initial OFDMA deployments since cable modem firmware may not have been qualified for OFDMA yet. We have seen modems abandon SC-QAM channels after OFDMA is enabled. We have also seen patterned packet loss while the cable modem is utilizing the OFDMA channel due to a cable modem firmware problem. OFDMA may also require a different type of monitoring. FEC codeword errors are less meaningful, since the fallback and recovery processes are very effective. Monitoring RxMER or lower capacity IUC usage are likely better indicators of OFDMA performance on a given cable plant.
Matthew Olfert,
Shaw Communications
Matthew Olfert is a Senior Network Architect at Shaw Communications for the Next Generation Access Network Team. His primary areas of focus include access network platforms to bring the latest broadband Internet, Wi-Fi, video, and digital phone services to customers in Canada.
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