Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-06-12
2002-04-30
Chin, Wellington (Department: 2664)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
Reexamination Certificate
active
06381229
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates in general to the mobile telecommunications field and, in particular, to a method and system for processing multiple random access calls in a Code Division Multiple Access (CDMA) or Wideband CDMA (WCDMA) system.
2. Description of Related Art
For the next generation mobile communication systems, such as the IMT-2000 and Universal Mobile Telecommunications System (UMTS), Direct Sequence-CDMA (DS-CDMA) approaches have been proposed for use in the United States, Europe and Japan. In this regard, similar DS-CDMA approaches are being considered for use in both Europe and Japan, but a somewhat different DS-CDMA concept is being considered for use in the United States. As such, the DS-CDMA concept accepted by the European Telecommunications Standards Institute (ETSI) and ARIB in Japan is often referred to as WCDMA.
These next generation systems will be required to provide a broad selection of telecommunications services including digital voice, video and data in packet and channel circuit-switched modes. As a result, the number of calls being made is expected to increase significantly, which will result in much higher traffic density on random access channels (RACHs). Unfortunately, this higher traffic density will also result in increased collisions and access failures. Consequently, the new generation of mobile communication systems will have to use much faster and flexible random access procedures, in order to increase their access success rates and reduce their access request processing times. In other words, there will be a high demand for much faster and more efficient access in those systems due to the expected substantial increase in packet-switched traffic.
The proposed WCDMA approach includes two different ways to transmit packets, on a common channel and a dedicated channel. However, there will be a high demand for faster and more efficient random access using either transmission scheme. For example, commonly-assigned U.S. patent applications Ser. Nos. 08/733,501 and 08/847,655, and U.S. Provisional Application Serial No. 60/063,024 describe such a random access approach, which can be used for a packet-based service where a mobile station (MS) can transmit packets on a common channel and a dedicated channel. For the common channel case, the packets are included in the random access requests being transmitted. For the dedicated channel case, the random access requests being transmitted include requests for a dedicated channel on which to transmit the associated packets.
The patent applications mentioned directly above disclose a Slotted-ALOHA (S-ALOHA) random access approach. Using this approach, a common transmission medium can be shared by a plurality of users. Essentially, the transmission medium is divided into a plurality of access slots, which are characterized by a time offset relative to the received frame boundary. Each user (MS) randomly selects an access slot and then transmits its message information in that access slot. However, a shortcoming of this approach is that the access slots are not allocated to the users, which can result in collisions between the different users transmissions.
For example, using the S-ALOHA random access approach in the above-described patent applications, a MS generates and transmits a random access request. A diagram that illustrates a frame structure for such a random access request is shown in FIG.
1
. The frame structure shown is used in the first two of the above-described patent applications. As shown, the random access request comprises a preamble and a data field portion. The preamble part is used primarily as a ringing function. The data portion includes the request and/or the data packet. In order to reduce the risk of collisions for requests from different MSs that choose the same access slot, the preamble for each MS's request contains a unique signature (bit) pattern. The MSs randomly select the signature patterns used (preferably from a limited set of signature patterns), which further reduces the risk of collisions.
The following procedure is typically used in an S-ALOHA random access system. First, an MS is synchronized to a base station. The MS “listens” to a broadcast channel over which, for example, the network broadcasts random access codes, broadcast channel transmit power level, and the interference signal level measured at that base station. Next, the MS estimates the downlink path loss, and together with the knowledge of the base station interference signal level and the transmit power level, estimates a transmit power level to use. The MS then selects an access slot and signature pattern, and transmits its random access request on the selected access slot and with the selected signature pattern. The MS awaits an acknowledgment to the access request from the base station. If the MS does not receive an acknowledgment from the base station within a predetermined (time-out) period, the MS selects a new access slot and signature pattern, and transmits a new random access request.
Referring to
FIG. 1
, the preamble portion is modulated with different signature patterns, and spread with a base station-unique spreading code. The signature patterns are used for separating different simultaneous random access requests, and also to determine which spreading/scrambling code to use on the data portion of the requests. Consequently, as mentioned earlier, the requests from two different MSs that use the same access slot but with different signature patterns can be separated. Also, pilot symbols can be inserted into the data portion of the request, in order to facilitate coherent detection. The preamble portion of the request can also be used for coherent detection, but if the data portion is relatively long, the channel estimate has to be updated accordingly.
FIG. 2
illustrates the frame structure of the random access request described in the third of the above-described patent applications. Using the frame structure shown, the data portion is transmitted on the I branch of the channel, and the preamble/pilot is transmitted on the Q branch. This frame structure is used in order to make the random access channel compatible with the other dedicated uplink channels used, which for the WCDMA approach is I/Q multiplexed. In any event, it does not matter whether the data and pilot symbols are time-multiplexed, I/Q multiplexed, or code-multiplexed (which can be performed among other methods by complex scrambling an I/Q multiplexed signal).
A frame is divided into a number of time slots on the dedicated data channel according to the power control command rate. These slots are denoted frame slots. In the proposed WCDMA systems, there are 16 of these frame slots per frame. In a random access scheme, a frame is also sub-divided into a number of access slots, but the purpose is to increase the throughput efficiency of the random access process. An access slot defines a period in which an MS can start its transmission of a random access request. Using the random access approach in the first two of the above-described patent applications, the random access requests can, for example, be transmitted in consecutive access slots as shown in FIG.
3
.
The data portion of the random access requests shown in
FIG. 3
is scrambled by a long code (same length as the data portion). Consequently, an access slot plus a guard time can be equal to N frame slots. Preferably, the preambles from different access slots should not overlap, because there would be too many preamble detectors required in the base station, and the interference (due to the same spreading codes being used) would be increased for the random access detection process. However, for the frame structure used in the third of the above-described patent applications, the throughput efficiency of the random access channel may be reduced, because longer preambles are being used and the preambles of different access requests in different access slots should not overlap.
The random access r
Gustafsson Maria
Narvinger Per
Chin Wellington
Jenkins & Gilchrist,P.C.
Jones Prenell
Telefonaktielbolaget L M Ericsson (publ)
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