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. 

Thursday, February 2, 2012

Evolved Macro-Diversity: CDMA2000 and UMTS

Macro-diversity typically means a special communication mode between a single mobile station and multiple base stations in a cellular network. It has been in CDMA standards as soft handoff since the beginning. The people in the industry usually think "soft handoff" and "macro-diversity" are interchangeable in most scenarios. Recently it has been employed for CDMA2000 BCMCS and UMTS MBS too. The basic idea is to coordinate multiple base stations to deliver the same data stream to a mobile receiver in the down links and receive the signals from a mobile station from multiple base stations. Macro-diversity is possible for CDMA soft handoff because there is no hybrid automatic repeat request (HARQ) for voice data and no fast retransmission is necessary due to the strict delay requirement of voice service. The benefits of doing soft handoff on voice service include reduced transmission power and seamless mobility. On the other hand, it also challenges the mobile station's capability to handle additional multipaths.

In the scenario of high-rate data delivery, HARQ is necessary for taking advantage of inaccurate channel estimation as well as channel fluctuation. Since the channels between the mobile and each base station in its active list are generally different and there is no fast link among involved base stations, therefore it is inherently difficult to do soft handoff for high-rate data delivery service.  In addition, the amount of fading resulted from soft-combining multiple channel may diminish and this may result less achievable time diversity gain. Therefore, macro-diversity is only applicable for the data delivery services, where there is no HARQ, for example, BCMCS and UMTS MBS. For the case of the macro-diversity of soft over the air combining, the achievable ergodic capacity is

C1 = B * E{ log2[ 1 + ( S1 + S2) / N ] }

However, things changed a little bit more recently. the simultaneous communication between multiple base stations and a single mobile is proposed for EV-DO Rev. C in a new term, single-carrier multi-link. For LTE-Advanced (LTE Release 10/11) , it is called Coordinated Multi-Point transmission and reception (CoMP).  When two data streams from two base stations are independent from each, it essentially is a way of spatial multiplexing, in which interference cancellation is one of the key receiver element for the mobile station to achievable maximum throughput. However, considering the independent fast multiuser scheduling and HARQ are used by each access network or eNodeB, it is very challenging for mobiles to do successive interference cancellation. For the case of the spatial multiplexing without interference cancellation, the achievable ergodic capacity is

C2 = B * E{ log2[ 1 + S1 / ( N + S2 ) ] } + B * E{ log2[ 1 + S2 / ( N + S1 ) ] } ≤ C1