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Description  |
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BACKGROUND
The present invention relates generally to radiocommunication systems and,
more particularly, to techniques and structures for reducing the delay
priority of comfort noise transmissions.
The cellular industry has made phenomenal strides in commercial operations
both in the United States and the rest of the world. Growth in major
metropolitan areas has far exceeded expectations and is rapidly
outstripping system capacity. If this trend continues, the effects of this
industry's growth will soon reach even the smallest markets. Innovative
solutions are required to meet these increasing capacity needs as well as
to maintain high quality service and avoid rising prices.
In addition to the challenges posed by a need for greater system capacity,
the designers of the remote units used in radio communication systems,
e.g., mobile phones, have their own unique set of challenges. For example,
a well known challenge in the industry is to extend the time between
recharging batteries used to power these remote units, while at the same
time continuing to provide additional functionality to satisfy subscriber
demand. These competing desires have lead remote unit designers to search
for creative ways in which to further reduce energy consumption in remote
units.
As the radiocommunication industry matures, various subscriber usage
patterns have been recognized. For example, it has been found that during
a typical voice connection between two subscribers, the actual voice
activity transmitted over the air interface accounts for less than 50% of
the total connection time. Therefore, in an attempt to conserve power,
remote units have been designed to actuate the transmission circuitry only
during the voice-active portion of a call and render the transmission
circuitry inoperative during periods of silence. This has been
implemented, for example, using a detector for detecting voice activity
and a discontinuous transmitter (DTX) that becomes inoperative when the
voice activity detector (VAD) detects a pause in the user's speech. As one
skilled in the art will appreciate, this technique has been shown to
reduce interference thereby leading to higher system capacity. The DTX
technique also reduces power consumption of the remote units by turning
off the transmitting circuitry for extended periods of time. Additional
information regarding the use of a VAD in a discontinuous transmission
system is described in commonly assigned, U.S. Pat. No. 5,881,373 to R.
Elofsson et al., entitled "Muting the Microphone in Radiocommunication
Systems," the entirety of which is incorporated by reference herein.
While this technique has been shown to reduce interference and the power
consumption of remote units, it poses a problem for reproduction of the
voice signal at the receiving side. Specifically, since the transmitter is
turned off during periods of silence, the background noise that would
otherwise be transmitted over the air interface is not received by the
other subscriber's equipment. This, in turn, results in a reproduced voice
that is discontinuous and includes audible artifacts, e.g., popping
sounds.
One way to overcome this difficulty is to generate artificial background
noise for reproduction at the receiving side when no voice signal is
transmitted. This artificial background noise is commonly referred to as
"comfort noise". Comfort noise can be generated by adaptive functions that
monitor the background noise picked up by the microphone of a remote unit.
When a pause in speech is detected, the comfort noise functions generate
comfort noise information that is transmitted over the air interface
instead of speech information. This information takes relatively little
time to transmit, thereby allowing the transmitter to be turned off during
most of each period of silence. At the receiving end, the comfort noise
information is used to generate background noise so that the listener is
not troubled by the discontinuity in transmission.
Such a comfort noise generation technique is currently available in GSM.
Therein, a comfort noise evaluation algorithm is used in a remote unit's
speech encoder to create parameters that include information on the level
and spectrum of the background noise. The evaluated comfort noise
parameters are then encoded into a Silence Descriptor (SID) frame for
transmission to the receiver. The SID frame is transmitted at the end of a
speech burst, i.e., before the transmitter is switched off. As such, the
SID frame also serves to initiate the comfort noise generation on the
receiver side. If, after transmission of the first SID frame, the period
of silence continues, SID update frames are transmitted by the remote
unit. A SID update frame performs several functions. It indicates not only
that the period of speech inactivity continues, but also that the cellular
connection is still present. Moreover, the SID update frame serves to
update the background noise detected at the remote unit.
The interval at which these SID update frames are transmitted depends on
the type of speech coder employed. For example, for Full Rate (FR) and
Enhanced Full Rate (EFR) speech coders in GSM, the rate at which SID
frames are transmitted is FN MOD 104=52, where FN is the Frame Number.
This corresponds to SID frames being transmitted approximately every 480
ms. For a Half Rate speech coder, the rate at which SID frames are
transmitted is doubled, i.e. every 240 ms. Moreover, for the newly
developed Adaptive Multi-Rate (AMR) speech coder, the SID transmit rate is
predicted to be up to four times higher than for the FR or EFR coders,
i.e. every 120 ms.
While speech has been and will continue to be an important part of mobile
communications, usage of mobile communication equipment for transmission
of data rather than speech has become increasingly popular by consumers
over the past decade. The possibility to send and receive electronic mail
and to use a web browser to obtain world-wide-web access is frequently
discussed as services that will be more and more used in wireless
communication systems.
There are fundamental differences between requirements for data
communication and e.g., speech communication. For example, delay
requirements are higher for speech, which is a real time service, and the
error requirements are higher for data communication, while the delay
constraints are lower. The use of packet data protocols, which are more
suitable for transmission of data than circuit-switched protocols, starts
to find its way into cellular communication systems. Packet service
integration in both GSM cellular systems as well as DAMPS cellular systems
is presently being standardized. As a result, cellular components (e.g.,
base transceiver stations, base station controllers, etc.) are being
adapted to handle packet data services.
Therefore, when introducing packet data services into, for example, a GSM
cellular system, designers need to consider the delay requirements of the
information transmitted. For example, it is well established that speech
is the most delay sensitive traffic and should be put into the highest
priority class. Other services, which are more delay tolerant, such as
packet data transmissions, can be put into a lower priority class.
However, the conventional DTX technique within GSM is not conducive to
such prioritization. For example, as set forth above, the conventional DTX
technique in GSM specifies specific intervals at which SID update frames
are transmitted. Under the conventional DTX scheme, these SID update
frames, which are not as delay-sensitive as actual speech frames, are
processed by the base transceiver station and base station controller at
the same level of priority as speech frames. This is evident from the fact
that the conventional DTX scheme correlates speech calls using the same FN
counter irrespective of the speech activity. As such, the gain of
statistical multiplexing (i.e., the ability to integrate calls based on
the statistics of the speech source), which is common to the DTX scheme,
is reduced thereby leading to higher bandwidth demands or longer speech
delays.
There exists a need for a system and method that prioritizes information
transmitted in a radiocommunication system so as to allow for better
utilization of transmission resources.
SUMMARY
The present invention seeks overcome the above deficiencies in the art by
providing a system and method for reducing the delay of speech processing
in a discontinuous transmission system. According to exemplary embodiments
of the present invention, a base transceiver station assigns priority
indicators to information received therein. When the information is speech
or a first SID frame, a high priority indicator is assigned. When the
information is a SID update frame, a low priority indicator is assigned.
The information is transferred from the base transceiver station to a base
station controller in an order determined by the priority indicators. As
such, the delay in transferring and processing speech information is
reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
The above objects and features of the present invention will be more
apparent from the following description of the preferred embodiments with
reference to the accompanying drawings, wherein:
FIG. 1 illustrates an exemplary system into which the prioritization
technique of the present invention may be employed;
FIG. 2 illustrates a base transceiver station and base station controller
for processing mobile station transmissions according to the conventional
DTX scheme;
FIG. 3 illustrates an exemplary base transceiver station and base station
controller into which the prioritization technique of the present
invention may be employed; and
FIGS. 4A and 4B illustrate exemplary time charts that depict the queuing
and the Abis interface transmission delay associated with packets
transferred from the router to the base station controller when speech
frames are received over each of a plurality of time slots and when SID
update frames are received over one of a plurality of time slots,
respectively.
DETAILED DESCRIPTION
In the following description, for purposes of explanation and not
limitation, specific details are set forth, such as particular circuits,
circuit components, techniques, etc. in order to provide a thorough
understanding of the present invention. However, it will be apparent to
one skilled in the art that the present invention may be practiced in
other embodiments that depart from these specific details. In other
instances, detailed descriptions of well-known methods, devices, and
circuits are omitted so as not to obscure the description of the present
invention.
FIG. 1 illustrates an exemplary system into which the prioritization
technique of the present invention may be employed. In FIG. 1, the
exemplary system includes a plurality of mobile stations (MS) 100.sub.1
-100.sub.N, a plurality of base transceiver stations (BTS) 110.sub.1
-110.sub.N, two base station controllers (BSC) 115 and 116, a mobile
services switching center (MSC) 120, a packet mobile services switching
center (PMSC) 130 that is connected to a packet network 140, such as the
Internet, and a public switched telephone network (PSTN) 150. In
operation, the plurality of mobile stations 100.sub.1 -100.sub.N
communicates with a respective base transceiver station in a well known
manner. According to the present invention, each mobile station comprises
a discontinuous transmitter and voice activity detector (not shown) that
operate in the manner described above. As such, each mobile station
transmits speech frames, as well as SID frames. Each mobile station is
also capable of transmitting packet data and fax/modem information.
The base transceiver stations 110.sub.1 -110.sub.N, in conjunction with the
base station controllers 115 and 116, receive information from the mobile
stations, process the information and forward the processed information to
the MSC 120. One skilled in the art will appreciate that a BTS and BSC
pair also receives information from MSC 120 and transfers such information
to one or more mobile stations.
At the MSC 120, information from the base station controller is routed to
its destination (e.g., the PSTN 150 when the information is speech or
fax/modem data and through the PMSC 130 to its destination on the packet
data network 140 when the information is packet data). One skilled in the
art will appreciate that the network illustrated in FIG. 1 would typically
include other components, such as routers (not shown), that aid in the
reception, processing and transmission of information in the network.
Moreover, the number of each of the components depicted in FIG. 1 is
reduced for the sake of simplicity. Radio communication systems generally
include multiple base station controllers, MSCs, PMSCs, etc.
FIG. 2 illustrates a base transceiver station and base station controller
for processing mobile station transmissions according to the conventional
DTX scheme. As illustrated, the base transceiver station 210 receives
information from, and transmits information to, a group of mobile stations
using a plurality of transceivers TRX1-TRXx. Each transceiver is capable
of handling speech communications. Base transceiver station 210 also
comprises a router 212 which controls the routing of information over the
interface 230 (known in GSM as the "Abis" interface) that connects the
base transceiver station 210 and the base station controller 215.
The base station controller 215 comprises a transcoder (TRA) 220 that
processes information received in the base station controller 215. One
skilled in the art will appreciate that a typical base transceiver station
and base station controller would generally include more components than
those illustrated in FIG. 2 that aid in receiving, transmitting, and
processing of information.
In order to illustrate the shortcomings of the conventional DTX scheme,
consider the following example. Assume that each of the plurality of
transceivers TRX1-TRXx receives either speech frames or SID frames during
a first time slot. The received frames are decoded by a respective TRX,
placed into packets and transferred to the router 212. If the base
transceiver station's internal bandwidth (i.e., the bandwidth of the bus
connecting a respective TRX to the router) is high enough to allow for a
TRX's entire packet to be transferred, then internal queuing within the
TRX can be avoided. If the bandwidth is not high enough, then each TRX
will contain an internal queue (not shown) that allows for temporary
storage of the packet. A typical bandwidth of a base transceiver station's
internal bus is generally greater than 2 Mbps.
In the router 212, the packets formed by the TRXs containing information
received during the first time slot are then transferred to the base
station controller. Depending on the bandwidth of the Abis interface, some
packets may have to be queued in the router prior to transfer (i.e, if the
Abis interface bandwidth is less than x*bandwidth of the internal bus,
packets will be queued prior to transfer). A typical bandwidth on the Abis
interface is 2 Mbps. Regardless of packet queuing, all packets received
during the first time slot are sent to the base station controller prior
to any packets from the second time slot being forwarded. The packets from
the second time slot are generally ready one burst time (.about.0.6 ms in
this exemplary GSM embodiment) after those of the first time slot.
In the base station controller 215, the transcoder 220 processes the
packets received during the first time slot irrespective of the packet's
contents. That is, the transcoder 220 processes packets that contain only
SID update information with the same priority as it processes packets
containing speech information. As such, processing of speech information,
which is the most delay-sensitive traffic, is delayed in those instances
where packets containing SID update information have been received prior
to the speech information.
The present invention seeks to overcome the above-identified deficiencies
by providing a system and method that prioritizes the information received
at a transceiver of a base transceiver station. For example, the present
invention recognizes that SID update frames are not as delay-sensitive as
speech information. In fact, in those instances where SID update frames
are not received, a slow muting procedure is generally initiated to
provide acceptable background sound. As such, delays in processing SID
update frames are not the most significant factor in providing acceptable
received signal quality to cellular users.
According to exemplary embodiments of the present invention, the
conventional DTX scheme is split into at least two different processes. A
first process handles high priority transfers (i.e., those packets that
contain either speech information or the first SID frame) while a second
process handles lower priority transfers (i.e., those packets that contain
lower priority information, such as SID update frames, packet data, etc.).
FIG. 3 illustrates an exemplary base transceiver station and base station
controller in which the prioritization technique of the present invention
may be employed. In FIG. 3, the base transceiver station 310 comprises a
plurality of transceivers 312.sub.1 -312.sub.x which are connected to a
router 314 via separate internal buses. The router 314 comprises a
priority detection unit 316 and a queue 318. Similar to the conventional
system described above with respect to FIG. 2, the router 314 is connected
to the base station controller via the Abis interface 330. The base
station controller 315 comprises a transcoder 320 which includes two
speech decoder processes: a high priority process for handling speech
decoding and comfort noise generation for the first SID frame and a low
priority process for handling lower priority information, such as comfort
noise updates (i.e., SID update frames). One skilled in the art will
appreciate that, as an alternative, transcoder 320 could actually be two
separate transcoders, one for handling high priority packets and one for
handling low priority packets. Moreover, while the router in FIG. 3 is
depicted as having a single queue, it will be appreciated that the router
could, alternatively, comprise multiple queues for storing together
packets having the same priority. In such an event, the base transceiver
station may also include a scheduler for scheduling the routing of the
packets stored in the multiple queues over the Abis interface.
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