Frequency Diversity Options in OFDMA

1. Frequency Diversity Options in OFDMA Date: 2015-05-04 Authors: Reza Hedayat, Newracom

2. Summary • OFDMA offers frequency-selectivity gain, but that gain is obtainable if some a priori knowledge of sub-bands is available at the transmitter side • In practical scenarios, an OFDMA-capable AP might end up sending OFDM PPDUs even to OFDMA-capable STAs due to lack of the STA sub-bands’ status • In order to maintain OFDMA gain in practical scenarios, this contribution suggests to allow some frequency diversity technique in DL OFDMA PPDUs so that OFDMA transmission is used more often Reza Hedayat, Newracom

3. Frequency-Selectivity in OFDMA • OFDMA offers frequency-selectivity gain, which depends on the multi-path propagation environment, transmission and sub-band bandwidth • Below simulation shows the potential gain in one example scenario • Simulation assumption • ITU UMi channel model • PPDU bandwidth=40MHz • Baseline is OFDM PPDU with 40MHz bandwidth • Proportionally fair (PF) scheduler for 2.5MHz sub-band offers about 22% median throughput increase vs 40MHz OFDM baseline • With no sub-bands’ status available, a random sub-band assignment has 17% less throughput compared to 40MHz OFDM baseline Reza Hedayat, Newracom

4. Frequency-Selectivity in OFDMA • While there is OFDMA frequency-selectivity gain to harvest, the gain can be achieved if knowledge of the channel has been obtained in advance • If AP does has no or outdated sub-bands’ status of a STA, the AP might be better off to send full-band OFDM frame to the STA vs sub-band OFDMA, otherwise the AP has to pick a larger link margin compared to a full-band OFDM transmission • Simulations show that the additional margin for a 2.5MHz sub-band with respect to full-band 40MHz transmission could be on average 4-7 dB (see appendix). For 80MHz and 160MHz full-band transmission, the additional margin that is required is even larger. Reza Hedayat, Newracom

5. OFDM vs OFDMA • In order to harvest OFDMA frequency-selectivity gain, and at the same time to avoid deep fades, an AP need to obtain the channel status of each STA that is going to send OFDMA PPDU to • To obtain the channel status, an AP needs to gather sub-bands’ status of each STA preferably across the full bandwidth (of the BSS) and gathering STAs’ sub-bands’ status takes time and/or additional frame exchanges • The main gain that OFDMA offers is breaking down the overhead across several frames • Overhead = backoff + IFS + legacy and HE PLCP • In an OFDMA PPDU the overhead is shared among several STAs • In an OFDM PPDU the overhead is spent for one STA Reza Hedayat, Newracom

6. OFDM vs OFDMA The following are some scenarios that an OFDMA-capable AP might end up sending OFDM PPDUs to STAs (not considering legacy STAs): • When STAs are out of sleep/doze mode, they first check Beacon and may send PS-Poll frame and wait for DL. Depending on how long the STA has been inactive, the AP might have outdated STA’s channel status and likely it’s better off to send the DL frame in OFDM PPDU format • If DL OFDMA happens to become an optional feature in 11ax or related certification programs, an AP has to send unicast frames to an incapable STA in OFDM format • For multicast and broadcast frames, AP likely uses OFDM rather than OFDMA since frequency-selectivity patterns varies across STAs Reza Hedayat, Newracom

7. Frequency Diversity Techniques in OFDMA • To enable OFDMA-capable APs to send as many OFDMA frames, there is a need to have a diversity technique within OFDMA frames • Other OFDMA-based technologies, such as LTE, have also adopted diversity techniques within OFDMA frame • 802.11ax has OFDMA and OFDM frame structures, hence in general OFDM frame can be used to offer frequency diversity. But … • In dense environments, frequent use of OFDM would reduce the efficiency of BSS vs if OFDMA were to be used always • Each time a payload, that could have been assigned in an OFDMA frame, is transmitted via OFDM frame it’d cause additional overhead such as backoff, IFS, legacy PLCP, and HE PLCP that is spent on the OFDM frame Reza Hedayat, Newracom

8. Frequency Diversity Options in OFDMA • One diversity technique is to allow non-contiguous sub-band allocation in OFDMA • It is known that spreading a payload across the full bandwidth offers frequency diversity • By assigning non-contiguous sub-bands in a DL OFDMA PPDU, frequency diversity can be enhanced for the payload that is sent to a STA whose sub-band status is unknown at AP • Pros and cons: • It is possible to allow multiple non-contiguous assignments within the same PPDU • Non-contiguous sub-band assignment could make HE SIG lengthier • Depending on duration of the OFDMA PPDU, Non-contiguous sub-band assignment only helps larger payloads Reza Hedayat, Newracom

9. Frequency Diversity Options in OFDMA • Another diversity technique is full-band transmission in DL OFDMA PPDUs • A full-band PSDU before the sub-band transmission region allows APs to send a payload to a STA whose sub-band status is unknown. • Below shows an example of the optional full-band PSDU in 20MHz PPDU • There need to be an indication in HE SIG on whether the full-band PSDU is present or not. If present, the details of the full-band PSDU appears in HE SIG-B • Pros and cons: • Only one full-band PSDU assignment per DL OFDMA PPDU is possible • HE STAs that are not OFDMA-capable can receive a full-band PSDU in OFDMA PPDU • Most useful for DL OFDMA. There would other issues if used for UL OFDMA Reza Hedayat, Newracom

10. Conclusion • Due to various practical reasons an OFDMA-capable AP might end up sending OFDM frames to 11ax devices, due to lack of STAs’ sub-bands status at the AP. This reduces the overall efficiency that OFDMA offers • This contribution highlights the need for a frequency diversity mode in OFDMA. A diversity mode would allow an AP to have assignments even for those STAs whose sub-band status is not known at the AP Reza Hedayat, Newracom

11. Straw-Poll • Do you agree to add the following to 11ax SFD: “4.y.z The specification shall define a frequency diversity mode for DL OFDMA that is optionally present in a DL OFDMA PPDU. Note: Frequency diversity mode is a mode that allows to exploit the frequency diversity across the transmission bandwidth of a PPDU.” • Y/N/A: Reza Hedayat, Newracom

12. Appendix: Simulation Results on Effective SINR difference Reza Hedayat, Newracom