Friday, March 2, 2012

Evolving Random Access Channel: II Slotted ALOHA Models

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: III Dumb Access Probes and Smart Access Probes

It is well known that the access channel design of 3G cellular mobile network is based on ALOHA models. More specifically it is based on a single-channel single-hub slotted ALOHA plus open-loop power control.  Indeed, there are many possible variations of slotted ALOHA.  Considering access channel configuration, there are the cases with a single shared access channel and the cases with multiple access channels.  Considering network configuration, there are single hub models and multi-hub models. Though multi-channel slotted ALOHA models have been intensively studied in computer engineering domain, a ALOHA model with more than one participating hub and more than one access channel hasn't received enough attention so far. I guess one reason is the original ALOHA models were proposed for satellite communications, where typically it is a single-cell or single-hub service scenario. Another reason I guess is the current computer engineering focuses more on network topology and MAC layer above issues instead of related channel modeling and PHY layer challenges. My feeling is the study of a multi-hub model with PHY-layer macro-diversity usually belongs to the stronghold of communication engineering. Conceptually the access channel of a cellular network is also more closely related to a multi-hub slotted ALOHA model, which has either single acess channel or multiple access channels.

Access Channel Design Challenges

It is well-known that there is a tradeoff between access capacity and throughput in ALOHA models, including both slotted and pure ALOHA.  Basically say, after a certain point, more access load will result in lower access success rate, which usually in turn results in more access delay due to retransmission. This tradeoff can be illustrated in Figure 1.
Figure 1. The Access Channel Access Capacity and Throughput Dilemma
In addition, a traditional CDMA2000 access channel can be modeled as a classic slotted ALOHA, i.e., single-channel single-hub slotted ALOHA. In this design, each base station only detects the access probes with its own access channel mask. Though it is well-known that additional macro-diversity gain is achievable with multiple base stations detect one access probe and the standards also doesn't prevent a base station chipset manufacture from implementing a base station which can detects the access probes to both itself and neighbor base stations, the resulted implementation complexity increase, in both PHY layer, MAC and upper layers, can be very high. One simple illustration of this tradeoff of diversity gain and complexity increase can be shown in Figure 2.
Figure 2. The Access Channel Search Complexity and Achievable Macro-Diversity Dilemma

Then one question is, what is the big deal of macro-diversity for access channel? It helps reduce transmit power, therefore, reduce interference and improve battery life. It helps mitigate network imbalance. It can also help network positioning.

Macro-diversity can help reduce network imbalance. There are two kinds of related network imbalance issues. One is forward link and reverse link imbalance. The other one is the reverse link load imbalance among base stations. Network imbalance is not only because of the non-uniform distribution of terminals but also because it is hard for a terminal to make a good decision on which base station it should point its access probes to without knowing base station side rise over thermal (RoT) condition.  This is important especially when a network user capacity is high, the cell coverage is large and the network is heavily and non-uniformly loaded.  Further more, since the existing access procedure simply asks a mobile to point its access probes to a sector typically with the strongest forward link pilot channel (F-PICH), this approach apparently is suboptimal. Therefore a new network load aware access probing mechanism is necessary in this case.

Access Channel Design Models

Table 1. A Comparison of Different Slotted ALOHA Models for  Mobile Network Access Channel Design

New Access Channel Proposal

Table 2. A Comparison of Different Access Channel Designs for Mobile Network

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