Random access channel access and backoff mechanism

Details Random access channel access and backoff mechanism Random access channel access and backoff mechanism

2000-06-14

2004-04-13

Ho, Duc (Department: 2665)

Multiplex communications

Diagnostic testing

Determination of communication parameters

C370S335000, C370S342000, C370S448000, C375S141000

Reexamination Certificate

active

06721281

ABSTRACT:

BACKGROUND
The invention relates generally to resource allocation in a wireless code division multiple access communication system. More specifically, the invention relates to controlling user equipment access attempts for communicating over a random access channel in a wireless code division multiple access communication system.
FIG. 1
depicts a wireless spread spectrum Code Division Multiple Access (CDMA) communication system
18
. A base station
20
communicates with user equipments (UEs)
22
1-22
N
in its operating area. In a spread spectrum CDMA system
18
, data signals are communicated between UEs
22
1
-
22
N
and the base station
20
over the same spread spectrum. Each data signal in the shared spectrum is spread with a unique chip code sequence. Upon reception, using a replica of the chip code sequence, a particular data signal is recovered.
Since signals are distinguished by their chip code sequences (code), separate dedicated communication channels are created using different codes. Signals from the base station
20
to the UEs
22
1
-
22
N
are sent on downlink channels and signals from the UEs
22
1
-
22
N
to the base station
20
are sent on uplink channels.
In many CDMA systems, a random access channel (RACH) is used and is capable of carrying packets of data from multiple UEs
22
1
-
22
N
. Each packet is distinguishable by a combination of time slot and code.
The transmission is time divided into repeating frames having time slots, such as fifteen time slots per frame. When a packet is transmitted over the RACH, it may last for multiple frames.
A typical UE RACH access attempt is as follows. Prior to communicating over the RACH, a UE
22
1
transmits an access signal to the base station
20
to access the RACH. One type of access signal uses a preamble code (preamble). The UE
22
1
repeats the preamble while incrementally increasing transmission power levels. The UE
22
1
repeats transmission of the preamble unit a response from the base station
20
is received or until a maximum number of repetitions is reached.
In response to receiving the preamble, the base station
20
determines whether the UE
22
1
may utilize the RACH. This utilization determination may be based on the availability of the RACH channel, uplink interference levels or RACH loading. If the access attempt is successful, the base station
20
transmits an acknowledgment signal (ACK) to the UE
22
1
. In response to the UE
22
1
receiving the ACK, the UE sends an uplink packet over the RACH. If the RACH is not available, the base station
20
transmits a negative acknowledgment signal (NAK) to the UE
22
1
. Receiving a NAK or reaching the maximum number of repetitions are unsuccessful access attempts requiring the UE
22
1
to reattempt access at a later time.
The period of time between access attempts is critical to a system's performance. If the period between access attempts is too long, the RACH will be underutilized. If the period is too short, many UEs
22
1
-
22
N
may repeatedly request access resulting in service interruptions.
One approach for controlling UE re-access attempts is to use a fixed backoff parameter. The UE
22
1
will reattempt access for a period of time based on the backoff parameter. The backoff parameter represents a deterministic wait period for an access reattempt. A problem with a fixed backoff parameter is that it can not be adjusted in response to the cell loading. Accordingly, during periods of light loading, the RACH may be underutilized and in periods of high loading service interruptions may result.
Another approach is a rule based approach. The UE
22
1
analyzes its prior access attempt statistics. Based on the access statistics, the UE
22
1
, applying predetermined rules, determines a backoff parameter. To illustrate, if the UE
22
1
had many unsuccessful access attempts, the period between accesses is increased. Since the UE's prior access attempts may not represent current conditions, this approach is not optimal.
Another approach is to broadcast a backoff parameter over a broadcast channel (BCH). The backoff parameter is based on the RACH's loading, uplink interference level and other factors. The backoff parameter, Ubbch, is used to derive a backoff wait period at a time n, B(n), such as by equation 1.
B
(
n
)=2
ubbch
  Equation 1
Due to delays in processing and transmitting the backoff parameter, the backoff parameter may not represent current conditions which is not optimal.
Accordingly, it is desirable to have other approaches for controlling UE access reattempts.
SUMMARY
A base station transmits access control signals on a periodic basis having a first transmission rate. The base station transmits access control modification signals at a second transmission rate higher than the first transmission rate. The user equipment transmits an access attempt signal. The access attempt signal requests the base station to permit the user equipment access to the random access channel. The user equipment receives the transmitted access control signals and access modification signals. A wait period is determined based on in part the received access control signals and access control modification signals. In response to an unsuccessful access attempt, the user equipment delays transmitting a subsequent access attempt signal for the wait period.


REFERENCES:
patent: 5752193 (1998-05-01), Scholefield et al.
patent: 6075779 (2000-06-01), Agarwal et al.
patent: 6366779 (2002-04-01), Bender et al.
patent: 6614772 (2003-09-01), Sexton et al.

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