Multiplex communications – Communication over free space – Combining or distributing information via code word channels...
Reexamination Certificate
1999-07-13
2002-12-10
Maung, Nay (Department: 2681)
Multiplex communications
Communication over free space
Combining or distributing information via code word channels...
C370S324000, C370S335000, C370S350000, C370S442000
Reexamination Certificate
active
06493334
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to packet transmission in multiple access communication systems.
BACKGROUND OF THE INVENTION
In this patent document, the references referred to in square brackets, eg [1], are listed at the end of the disclosure.
The 3G CDMA system is supposed to support simultaneous voice/packet data/circuit data operations [1] with different QoS requirements. Since the traffic characteristics and the quality of service requirements for packet data services are quite different from those of voice traffic, system re-design in certain essential aspects is necessary for an integrated voice-data system without impacting voice quality or sacrificing data performance.
Packet-type services are designed for on-off sources, which generate no information during often prolonged, but unpredictable, off intervals. For many data applications the intermittent nature of the traffic is orders of magnitude greater than that of voice signals as shown in FIG.
1
. The system capacity can be greatly increased if no signal is transmitted during the off period, since the CDMA system is interference limited.
The efficient provision of packet-type services is particularly difficult on reverse links (from mobile to base station) of CDMA systems, because of the need to rapidly regain synchronization of spreading sequences when the source resumes transmission after a period of silence. One possible solution to the link maintenance problem when bursty transmission is required on reverse links of CDMA systems is to assign a separate low bit rate physical control channel (created using code division multiplex) to each portable in a given cell. This approach, proposed by some investigators [2], can be called a continuous transmission medium access control scheme (CTX-MAC), and it unfortunately causes increased multi-user interference introduced by continuously transmitted maintenance signals. Increased interference naturally leads to reduced traffic capacity. Also its implementation is characterized by significantly increased complexity of base station receivers, where each physical control channel (serving one portable) would require a separate despreading correlator. If the duty cycle (ratio of the on-period to off-period of the mobile's transmitter) as shown in
FIG. 1
is small, then considerable savings in both base station hardware and system capacity may be achieved if transmission is discontinued during the off-periods and the hardware is shared among different users. This is the discontinuous transmission medium access control (DTX-MAC) scheme. With the DTX-MAC scheme, when a particular user has a data packet to send, the base station needs to be notified by the mobile of its intention to transmit, and synchronization needs to be quickly resumed. To achieve that an access request message (ARM) is transmitted from the mobile to base station to acquire synchronism, and to inform the base station of the mobile's identity. An access reservation channel (ARC) is allocated for such a purpose. All mobiles in a cell (or sector) use the same PN code to send their ARMs on the ARC; this avoids the need to have a separate receiver for each mobile, even in the off mode. If the number of sources is large, then more than one ARC may be used.
A MAC protocol is required on the ARC for the mobiles to access the base station with individual ARMs. One possible approach is to allow the mobile units to send ARMs in an asynchronous fashion—ADTX-MAC protocol. Another is a synchronous approach—SDTX-MAC protocol, in which a slotted frame structure is used for the access control of ARMs from all users.
Since the user sending a transmission request has already been registered and has acquired synchronization before, the timing ambiguity is mainly caused by the propagation delay uncertainty due to the movement of the mobile in a cell during the silent period. Hence, it is relatively easier to synchronize again than to acquire initial synchronization during the registration phase involving the access channel. On the other hand, the synchronization needs to be regained very quickly so that the arriving data burst can be transmitted without undue delay. Due to these reasons, the specific structure of the IS-95 access channel [3] which is used by the mobile station to initiate communication with the base station and to respond to paging channel messages, is no longer suitable, since its access probe occupies several 20 ms frames in time. Instead, much shorter access request messages are used within the access reservation channel structure significantly different from the IS-95 access channel. Such an approach dramatically reduces access delay and increases throughput.
SDTX-MAC Protocol
In the SDTX-MAC system, a time-slotted frame structure is used to accommodate access requests from different users on the access reservation channel (ARC). Specific time slots in the frame equal in length to the duration of the access request message (if necessary, a small guard time may be added) are assigned by the base station to mobile users after registration (one slot per user). Collision is thus avoided and there is no need of inserting an identifier appendix after the synchronizing part of the ARM. A method of step-increase of power is used for ARMs. After each ARM, the mobile monitors the paging channel for an acknowledgment (ACK) from the base station. If the elapsed time before the ACK is received is longer than the prescribed maximum, the corresponding ARM is regarded as a failure, and next one is sent at a power level increased by a fixed step. For each user, a fixed number of ARMs of increasing powers form an ARM sequence. If the synchronization is still not acquired after the sequence of ARMs is sent, a new sequence is started, with the transmitting power starting from the lowest level. Identical ARM sequences are repeated up to a maximum allowed number of repetitions, which is assumed large enough to ensure acquisition. The synchronism acquired through the ARC may be coarse, but it is sufficiently accurate to receive data packets from the user in question on a dedicated data channel. Each data packet sent on that channel is preceded by a short synchronizing preamble to refine synchronism.
The number of slots in one slotted frame of the access reservation channel (ARC) is designed to accommodate the expected number of registered users in a cell or sector. Once a user is registered, a slot is assigned to him on the ARC, and the base station subsequently uses that assignment to identify a user sending an access request. An example is shown in
FIG. 2
wherein there are four slots within one frame, T
p
, T
F
are the slot and frame lengths, respectively, and there are three ARMs within one access message sequence.
ADTX-MAC Protocol
The asynchronous discontinuous transmission medium access control (ADTX-MAC) protocol is in general terms similar to that used in the IS-95 access channel. It is a spread slotted ALOHA [4]-[6] with p-persistence [7] (plus sequence back-off) between ARM sequences. As shown in
FIG. 3
a fixed number of ARMs forms an ARM sequence (three ARMs within an ARM sequence in this figure), with the transmission power increasing consecutively by a fixed increment. There is a random back-off T
R
between ARMs in the sequence, if the ACK is not received before time-out, T
A
. If the second ARM sequence is required, there is a random back-off T
S
before a p-persistence test, for the next ARM sequence to start. To reduce the probability of collision, the exact transmission time of each user is pseudo-randomly delayed from the slot start time, the delay being known to the base station.
Since all the users use the same PN code to spread their ARMs, each ARM consists of one synchronization preamble and an appendix for user identification. This appendix is designed to have the same length as the preamble, and is block-coded to provide the necessary error protection. The ARM for ADTX-MAC is of twice the length of an ARM
Krzymien Witold A.
Sun Songsong
Maung Nay
Stallman & Pollock LLP
Telecommunications Research Laboratories
Vuong Quochien B.
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