Wednesday, February 29, 2012

Evolving Random Access Channel: I Introduction

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


Random access channel (RACH) is an contention-based uplink common channel resource, which is shared by multiple access terminals. The typical application scenarios, which trigger random access procedure, include
  • initialize access from power up or radio link failure,
  • initialize access from idle state,
  • initialize access to the handover target,
  • initialize access when uplink synchronization is unavailable.
In addition, it can also be used by a terminal for registration update, location update and to send a small amount of data on the uplink. A key feature of random access channel is that messages are not scheduled and there is no certainty of collisions. This is different to a dedicated channel, which is exclusively assigned to one user at each time.

In 3GPP, it is defined as a common transport channel in the uplink, which is one-to-one mapped onto physical random access channels (PRACHs).  In current UMTS standard, RACH can also be used to send small amount of data on the uplink, which is quite different from the approach taken by 3GPP. In 3GPP2, there are reverse link access channel (R-ACH) defined in IS-95 and reverse link enhanced access channel (R-EACH) in IS-2000. 

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