Collision avoidance

Details Collision avoidance Collision avoidance

2001-02-08

2003-01-14

Bocure, Tesfaldet (Department: 2631)

Pulse or digital communications

Spread spectrum

Direct sequence

C370S441000

Reexamination Certificate

active

06507601

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to spread-spectrum communications, and more particularly to code-division-multiple-access (CDMA) cellular, packet-switched systems. The inventive concepts involve use of a start of message indicator as a collision avoidance mechanism, to reduce the impact of remote terminal mistakes in perceived successful channel access attempts.
BACKGROUND
Recent developments in wireless communications technologies have allowed expansion of service offerings from the original voice telephone service model to include a number of services supporting packet data communications. As customers become increasingly familiar with data services offered through landline networks, they are increasingly demanding comparable data communications in the wireless domain, for example to maintain service while mobile subscribers roam freely or to provide remote service in locations where wireless loops are preferable to landline subscriber loops. A number of technologies that support packet data communications in the wireless domain utilize code division multiple access (CDMA), which involves direct sequence spread-spectrum encoding. In such systems, different physical channels are defined by use of different channelization and/or spreading codes, as part of the direct sequence modulation process.
A CDMA-based random access channel (RACH) provides uplink packet transport from a mobile station (MS) to a base station (BS), with a random slotted-ALOHA type procedure to access the channel resources. U.S. Pat. No. 6,169,759 to Kanterakis et al. discloses a common-packet channel (CPCH), which provides a similar uplink transport for transmitting variable size packets from a mobile station (MS) to a base station (BS). The disclosure of U.S. Pat. No. 6,169,759 to Kanterakis et al. is entirely incorporated herein by reference.
The RACH and CPCH channels do not need direct resource allocation. The channel resource allocation of these channels is contention based. The mobile station transmits an access preamble corresponding to a channel that the mobile station desires to use. The base station responds with a matching preamble that signals successful access to the selected channel resource.
FIG. 1
is a simplified example of the signals exchanged between a mobile station and a base station for a CPCH service. The mobile station selects one of the available uplink channels through a base station. The access phase involves the MS-spread-spectrum transmitter sending one or more access preambles (AP) over an uplink physical channel, in access slots defined in relation to a frame-timing signal derived from receipt of the common synchronization channel (not shown). The access preamble (AP) contains a signature corresponding to the selected uplink channel, that is to say the one channel that the mobile station is attempting to access from among those available by the base station. When the base station receives an access preamble (AP) at an adequately detectable power level, it transmits back an acknowledgement (ACK), containing a signature that corresponds to the access preamble signature, over the indicator channel.
The mobile station ceases transmission of the access preamble (AP) when it receives the corresponding ACK signal from the base station. If the mobile station successfully receives the acknowledgement corresponding to the access preamble that the station transmitted, the mobile station proceeds to the next phase in the transmission process, shown generally as transmission of data and control information over the uplink channel in FIG.
1
. Alternatively, the mobile station will cease its transmission of access preambles if the mobile station has transmitted the maximum allowed number of access preambles. In this later situation, the mobile station assumes that its access attempt has failed, so the station backs off and waits for some period of time before initiating another access attempt.
As shown in
FIG. 2
, the message part carried over the downlink physical control channel (DPCCH) is divided into 10 msec frames. Each 10 msec frame is split into 15 slots (
0
,
1
,
2
, . . .
14
), each of length T
slot
=2,560 chips. As shown, each slot carries 10 bits of information. Each 10-bit slot of the downlink control channel contains fields comprising Pilot, CCC, TFCI and TPC. The TPC field carries transmission power control (TPC) bits. The TFCI field carries the transport format combination indicator, and the CCC filed carries four data bits for CPCH control command information.
Although these CDMA-based communication technologies offer enhanced packet data communications, problems still arise that cause collisions. It has been found that there is roughly a 30% chance that two or more preambles from mobile stations will arrive at the base station in any given 50 ms time-window. One mobile station may then mistake a subsequent acknowledgement signal intended for another mobile station to be one intended for itself. The mistaken mobile station will then transmit its packet on a channel intended for use by another mobile station. This will lead to excessive interference. Moreover, if the mistaken mobile station sends its packet over an already busy channel, there is a great chance of a cell-shut-down. This is extremely undesirable in cellular telephony as users in the neighboring cells will also be adversely affected.
Various methods of collision detection, collision resolution and channel assignment were developed to reduce the occurrences of these collision. A CPCH system, such as that disclosed by Kanterakis et al., utilizes a collision detection phase at the start of the data and control communications over the uplink transportation channel and the downlink control channel, to allocate the uplink channel to a mobile station that successfully avoids collision. If two or more mobile stations are still attempting access to the same channel at the time of the collision detection phase, the base station may respond with at most one matching collision detection preamble, effectively allocating the channel to one mobile station. In some cases, the base station will not be able to resolve the collision detection and will not send back any collision detection preamble. A mobile station that fails to receive its matching collision detection preamble from the base station aborts its access attempt.
The collision detection approach does reduce collision problems. However, none of these methods can totally eliminate the chance of collision due to unintended errors. For example, a mobile station may misinterpret its channel assignment and proceed to transmit in a channel other than the one it was assigned, even though the other channel may be assigned to and in use by a different mobile station. There are several situations that may lead to such a mistake. For example, in CPCH, two mobile stations, MS
A
and MS
B
, have sent in AP
1
and AP
2
, respectively. The base station has only responded with an AP ACK
1
, which corresponds to AP
1
. MS
A
has correctly identified the ACK, however, MS
B
has mistaken the ACK to correspond to AP2. The mistaken mobile station may then proceed with uplink transmission over the channel corresponding to AP
2
, which may already be in use by another station.
Although the above-described collision detection procedure reduces the probability of such collisions between stations attempting access on the same channel at about the same time, mistakes regarding attempted accesses to different channels still can occur as a result of substantially concurrent collision detection phases for different channels. In this situation, assume that mobile station MS
B
is waiting for a collision detection acknowledgement. At about this time, the base station sends a CD acknowledgement for the mobile station MS
A
that was attempting to access another channel. The one mobile station MS
B
, however, mistakes the CD acknowledgement as an acknowledgement of its own CD preamble and begins transmitting over the channel that it is attempting to access, even though that c

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