Saturday, March 24, 2012

Evolving Random Access Channel: III Power Control p.k. Access Timing

Access Channel Enhancements for 1x Rel. F
One Eighth Rate Access Probes for Smart Terminals
Evolving Random Access Channel: I Introduction
Evolving Random Access Channel: II Slotted ALOHA Models

During the discussion of evolving access channels for machine-to-machine (M2M) communication liked services, two major requirements are defined. As stated by 3GPP2 system requirement document, one is "[a]n M2M service shall not impact any ... existing applications and services that are currently being supported". The other one is "[t]he scalability of M2M Device deployment shall be supported allow flexible M2M Device and service deployment scenarios for large numbers of M2M Devices."  More details of these two requirements can be found in 3GPP2 Machine-to-Machine Communication System Requirements, S.R0146-0 and 3GPP M2M Service Requirements, ETSI TS 102-689. Due to the nature of M2M traffic pattern and QoS requirement, recent studies find out that uplinks, especially existing access channels, will become the bottleneck when M2M services are widely deployed in the future. Evolving access channel once again becomes a hot topic since it was optimized for short message service (SMS) in 1990s.

Minimize Collision and Interference: Power Control and Access Timing

Of the two requirements, let's start from how to minimize the impact of new on legacy services at first. Doing so is not only required by service providers but also because these two requirements don't always conflict with each other. From random access channel fundamentals, it is not hard to understand two key factors determining the collision and interference between probes, access timing and power control. Access timing information is about when access probes will start using access channel. Access timing is not only the key assumption for many ALOHACSMA MAC protocols, but also important for new access probes to decide the best time using the shared access channel with the minimum overlap to legacy probes. The second factor is perfect power control. This becomes critical is because when access channel is loaded in the future,  it is impossible to completely avoid the overlapping between access probes even though the perfect timing information of the channel might be available. At this time, perfect power control on new access probes becomes necessary in reducing interference to legacy services. Based on these two factors, two optimal access channel designs will be presented and discussed in the following.

Optimal Access Probes: Dumb Probes p.k. Smart Probes.

If the perfect access timing information of legacy access probes is available, one optimal access channel design is to use very short and smart access probes, which know the activities of the other probes and are able to try its best to arrive inside some access gap. In order to have little overlap with other access probes, it is nature for a smart access probe to be short. While it is necessary to maintain a minimum payload during a short period, a smart and short access probe usually demands high transmit power and data rate. How smart probes work with legacy probes can be illustrated in Figure 1.

Figure 1. A traditional Thinking of Improving Access Probe Design
Though smart probes can have minimum impact on legacy probes, it is very hard to be implemented. In reality, it is extremely challenging for an access probes to know the instant access timing information of other probes, even though they may be co-located next to each. In a typical scenario for mobile communication, each mobile has no timing information of any other mobile without the help of base stations.  When the perfect access timing information is unavailable, one optimal access channel scheme is to send very long and dumb access probes instead. Here “dumb” means each of these access probes has no any access timing information of other probes. However, in order to minimize any impact to other probes, including current and future probes, each dumb probe still wants to take advantages of access gaps of access channel. To achieve this, each probe has to be transmitted at a very low data rate and span a very long duration with perfect power control.  A low transmit data rate spanning a long duration usually means a low transmit power. Though a dumb probe does look "slow" and cautious, it also means potentially more diversity opportunities are achievable.
Figure 2. A New Thinking of Improving Access Probe Design
Now as you can see, there are two different starting points for designing optimal access probes for minimizing the impact to legacy services. For smart probes, timing information is the challenge and key.For dumb probes, perfect power control is the key. A comparison of these two optimal designs can be found in the following table.

Table 1. A Comparison of Dumb Probes and Smart Probes

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