Part Six: Downlink Capacity
Even though LPWA tends to consist of uplink centric data models, downlink capacity looms as a potential bottleneck for the following reasons:
- As we saw in the Part 5, there is downlink control required for scheduling uplink transmissions and acknowledging uplink transmissions (especially for frequent small packet communication).
- Based on the enhanced link budgets of LPWA approaches, the supported downlink rates of the channel is far lower which exacerbates the downlink bottleneck.
- The amount of downlink power available for cellular LPWA is severely limited. Less power means less capacity. The LPWA cellular is either adjacent to voice/data channels or is contemplated to be in the stop-band. In either case, transmit power must be minimized to the point of not interfering with voice/data communication – the cash-cow of the industry. This is yet another example of all technical decisions being driven (understandably) by voice/data needs at the expense of the machines.
…limiting the ability to acknowledge transmissions would be very disappointing to the applications that plan on using the cellular LPWA network. Reliable delivery is a crucial requirement for LPWA devices.
Fundamental Cellular Issue
An Overview of 3GPP Enhancements on Machine to Machine Communications published in IEEE Communications Magazine gives a sense of the problem: “HARQ retransmissions are directly triggered by receiving a new assignment over the MPDCCH.” Having to send downlink full overhead messages on an already overstressed link just to signal retransmissions cuts deeply into downlink capacity. As a result, the downlink is bursting at the seams. For this reason, in TR45.820 : Cellular system support for ultra-low complexity and low throughput Internet of Things (CIOT), all simulations claim only a 50% acknowledgment rate. That is because there is likely no overhead left over for downlink data.
In our experience, limiting the ability to acknowledge transmissions would be very disappointing to the applications that plan on using the cellular LPWA network. Reliable delivery is a crucial requirement for LPWA devices.
How RPMA Solves the Issues
Our first generation RPMA system had similar issues before we optimized for our second generation. This is one of the benefits of 8 years of focusing on customers’ needs. We solved this by using an outer loop Reed-Solomon erasure code coupled with a spectrally efficient “stop indication” which reduced downlink signaling for uplink acknowledgment by a factor of 20. The TDD spectrum of RPMA (as opposed to the FDD approach of Cellular LPWA) helps here as well to accurately optimize downlink data rate.
This post is a part of the series Is the cellular standard roadmap (3GPP/GSMA) the answer to Low Power Wide Area (LPWA) Connectivity? Click a link below to learn more, or download our free eBook, How RPMA Works: The Making of RPMA.
- Part 1: Introduction
- Part 2: Cellular LPWA Availability
- Part 3: 3GPP/GSMA is NOT Providing a Graceful Evolution Path for Machines
- Part 4:. Cellular LPWA Complexity
- Part 5: Cellular LPWA Performance Issue 1: Uplink Capacity
- Part 6: Cellular LPWA Performance Issue 2: Downlink Capacity
- Part 7: Cellular LPWA Performance Issue 3: Firmware Download
- Part 8: Cellular LPWA Performance Issue 4: Robustness
- Part 9: Cellular LPWA Performance Issue 5: Power Consumption