Call-setup method in a digital cellular radio communication system

A call-setup method in a digital cellular radio communication system determines a plurality of parameters defining the current state of the system. By evaluating and combining these parameters into a communication resource request, a communication resource that best matches the request may be allocated in order to optimize system resource usage and/or service quality.

TECHNICAL FIELD 
The present invention relates to a call-setup method in a digital cellular 
radio communication system. 
BACKGROUND 
In present digital cellular mobile radio communication systems, such as the 
GSM and D-AMPS systems, a call-setup allocates a fixed gross bit rate 
channel for a subsequent fixed bit rate speech service. This implies that 
the net bit rate conveying the speech information and also the amount of 
added redundant bits which are used for channel error protection are 
fixed. A compromise has to be made between the quality of the speech 
service, the gross bit rate and the degree of channel error protection: 
On one hand a maximum speech quality requires a high net bit rate and a 
high gross bit rate. 
On the other hand the system resources are limited and the system should be 
able to accommodate a very large number of users at any given time. 
Since the total maximum gross bit rate that simultaneously can be 
transmitted by the system is limited, the system capacity is limited to a 
fixed maximum number of users that may simultaneously use the system 
within one cell. In order to accommodate a maximum number of users of the 
speech service at any given time, the net bit rate, the degree of channel 
error protection and thus the channel gross bit rate are set to certain 
minimum values which still guarantee a certain minimum degree of speech 
quality under various radio conditions. 
The fixed setting of gross bit rate, degree of channel error protection and 
net bit rate and service causes the following problems: 
The amount of channel error protection is fixed to such value that a 
certain level of speech quality is maintained in situations of a low C/I 
level, e.g. at cell borders. In some situations, however, a higher degree 
of channel error protection (with the cost of a lower net bit rate) could 
give a higher speech quality (more robust transmission). On the other 
hand, in situations of a high C/I level the degree of channel error 
protection is unnecessarily high, a considerable amount of protection bits 
being wasted. In such a case the speech service quality could be higher at 
a higher net bit rate (more accurate speech encoding) and a lower degree 
of channel error protection. 
The fixed gross bit rate causes a hard limit of possible simultaneous 
users. Thus, in situations of high system load the risk of overload is 
high. An overload may result in failures of connection establishments and 
lost connections. In the opposite situation of low system load there is a 
lot of unused system capacity which is in principle free to be used for 
transmission at a higher gross bit rate and therefore a higher service 
quality. 
The current inflexible use of a fixed speech service at a fixed gross bit 
rate and a fixed degree of channel error protection makes it impossible 
for an operator to offer selected services that could depend on the 
current situation, e.g. network load, time of day and date, location, etc. 
Moreover, it is also impossible for the user to select a more user-suited 
service. 
In the current inflexible system speech decoding of the bit stream received 
from the mobile station is performed at the network side. This ignores the 
kind and the capabilities of the other terminal. Such a system may lead to 
quality degrading due to speech codec tandem configurations, e.g. in the 
case of a mobile station-mobile station connection. 
SUMMARY 
An object of the present invention is to provide a more flexible call-setup 
method in a digital cellular radio communication system. 
This object is solved by the features of claim 1. 
Briefly, a number of parameters defining the present state of the system 
are determined before call-setup and these parameters are used to 
optimally allocate communication resources for the call.

DETAILED DESCRIPTION 
The invention will now be described in detail, mostly with reference to the 
GSM and D-AMPS systems. However, the invention is also applicable to any 
other digital radio communication system (FDMA, TDMA, CDMA), such as PDC 
(Pacific Digital Cellular), IS-95 (CDMA) or even satellite systems. 
The following description will concentrate on three specific examples of 
resource allocation, namely offered degree of channel error protection, 
offered gross bit rate and offered service level. 
A. ADAPTATION OF CHANNEL ERROR PROTECTION 
In order to adapt the degree of offered channel error protection to the 
current situation during call-setup, at least some of the following 
parameters are determined: 
current location of mobile station (MS), e.g. 
indoor/outdoor 
population density of serving cell 
MS distance to cell center (antenna) 
MS global position 
expected mobility of MS 
current channel quality, e.g. 
C/I level 
bit error rate (BER) 
possible battery-saving mode of MS 
These parameters are combined and possibly supplemented by other 
statistical data, and based on this description of the current state of 
the system a suitable degree of channel error protection is allocated. 
This procedure will be described in more detail below. A few examples will 
illustrate how the above mentioned parameters may be used. 
Example A1 
A current location of the mobile station is classified as indoor, and the 
current C/I level measurement indicates good radio channel conditions. 
Thus, a low degree of channel error protection and a high net bit rate 
(accurate speech encoding) is chosen for the subsequent connection. 
Example A2 
The current location of the mobile station is classified as close to the 
serving base station (and distant from the cell border), and the expected 
mobility of the mobile station is low. Thus, good radio channel conditions 
may be expected for the subsequent connection, which leads to selecting a 
low degree of channel error protection and a high net bit rate. 
Example A3 
The current location of the mobile station is classified as being in a cell 
in a highly populated area, and the expected mobility of the mobile 
station is high. This situation indicates a high probability of strongly 
changing radio channel conditions during the subsequent connection. Thus, 
it is wise to select a high degree of channel error protection and a low 
net bit rate. 
Example A4 
The mobile station is in a battery-saving mode. This fact implies that a 
transmission at low channel error protection is requested, since a low 
channel error protection implies less processing of redundant bits and 
therefore saves battery. If the current C/I level indicates acceptable 
radio channel conditions this request is granted and a low channel error 
protection and a correspondingly high net bit rate are selected. 
B. ADAPTATION OF GROSS BIT RATE 
In order to adapt the gross bit rate used for the service to the current 
situation during call-setup at least some of the following parameters may 
be determined: 
current location of MS, e.g. 
indoor/outdoor 
population density of serving cell 
MS distance to cell center (antenna) 
MS global position 
expected mobility of MS 
current channel quality, e.g. 
C/I level 
bit error rate (BER) 
possible battery-saving mode of MS 
system load 
time of day and date 
Note that the first four parameters are the same as in case A above. As in 
case A these parameters are combined and possibly supplemented by some 
other statistical data for obtaining a decision regarding the gross bit 
rate to be allocated for the requested service. For example, in a TDMA 
based system, e.g. GSM, D-AMPS, the gross bit rate may be adjusted by 
allocation of an appropriate number of TDMA time slots. A few examples are 
given below. 
Example B1 
The current system load is high and the current C/I level indicates an 
acceptable radio channel quality. A low gross bit rate is allocated for 
transmission at a relatively low service quality. 
Example B2 
The system load is relatively low and the time of day or date indicates 
that it is safe to assume that the system load will remain low for the 
duration of a call. In this situation a high gross bit rate may be 
allocated for the service. The high gross bit rate may be used to 
compensate for a poor radio channel by offering a high degree of channel 
error protection (robust encoding) and keeping the net bit rate at a low 
level. On the other hand, if the radio channel is good, a high quality 
service at a high net bit rate may be supported. 
Example B3 
The mobile station is in a battery-saving mode and therefore requests a low 
rate service with transmission at low channel error protection. If the 
current C/I level indicates acceptable radio channel conditions, the 
request is granted and a low gross bit rate is chosen for transmission. 
C. ADAPTATION OF SERVICE LEVEL 
It is desirable to give the operator the freedom to offer a varying range 
of services, for example: 
a low gross bit rate/low speech quality service 
a robust speech service that allocates as much gross bit rate as required 
to meet a certain quality level under the current radio channel conditions 
a high quality speech service 
combined speech and data services 
The following parameters are relevant in deciding what service to use: 
current location of MS, e.g. 
indoor/outdoor 
population density of serving cell 
MS distance to cell center (antenna) 
MS global position 
expected mobility of MS 
current channel quality, e.g. 
C/I level 
bit error rate (BER) 
possible battery-saving mode of MS 
system load 
time of day and date 
user request, e.g. 
a default service 
a specific service according to individual user request 
a specific service according to individual user profile 
a specific service according to an automatic request of the MS, e.g. a 
battery-saving mode 
Note that case C differs from case B only in the last parameter (user 
request). The following examples illustrate this case further. 
Example C1 
The current system load is high and the current C/I level indicates an 
acceptable radio channel quality. A low gross bit rate/low quality 
service, e.g. a half rate channel is chosen. 
Example C2 
The system load is relatively low and the current mobility of the mobile 
station indicates that strongly varying radio channel conditions are to be 
expected during the connection. Thus, a robust speech service is chosen 
with a high gross bit rate and a high degree of channel error protection. 
Example C3 
The subscriber has an important call to make (for example a business call) 
and requests highest possible quality regardless of costs. A high gross 
bit rate with high quality speech encoding and high channel protection is 
chosen. Later he wants to make a private call and requests the least 
expensive service. A low gross bit rate with low quality speech encoding 
and low channel protection is chosen. 
DETERMINING CALL-SETUP AMETERS 
The above mentioned parameters may, for example, be determined in the 
following ways: 
Current Location of MS 
In a well-planned network the indoor/outdoor location of a mobile station 
is known with the knowledge of the current cell. For example, in a GSM 
system this information could be requested from the Mobility Management. 
Furthermore, in a well-planned network knowledge of the current cell also 
specifies whether the cell is in a highly populated area or not. For 
example, in a GSM system this information could be requested from the 
Mobility Management. 
The current systems determine a Timing Advance parameter on the network 
side This parameter may be used to estimate the distance between base 
station and mobile station. With knowledge of the cell topology it is 
possible to deduce an estimate of the distance to the cell center or cell 
border. 
The mobile station may figure out its current global position by means of 
GPS (Global Positioning System) or triangulation (involving the three 
closest base stations). 
Expected Mobility of MS 
The expected mobility of the mobile station may be deduced from the 
knowledge of the "indoor/outdoor" situation. In a detected indoor 
situation the mobility parameter could be set to a low value. In the 
opposite case a detected outdoor situation would set the mobility 
parameter to a higher value. 
Another indicator of the mobility could be the current speed of the mobile 
station. This speed could be determined by monitoring a possible variation 
of the Timing Advance parameter, a possibly detected Doppler radio 
frequency shift, or by applying a differential GPS measurement in the 
mobile station. 
Still another way to estimate the mobility parameter is to count the number 
of cell changes during the connection and to apply a statistical estimate 
of this parameter. 
The last two methods require some type of preliminary connection to perform 
mobility measurements. This is possible to do before the call is finally 
setup. It can also be done regularly on the control channel. 
Current Channel Quality 
In the current systems there are available measurements of the signal 
strength of the received radio signal, e.g. RXLEV in GSM. This measurement 
could be used as indicator of the current C/I level. 
In the current systems there are available measurements of the bit error 
rate, e.g. RXQUAL in GSM. This measurement is an indicator of the current 
bit error rate. 
Possible Battery-saving Mode of MS 
This parameter may become active, for example on a detected bad battery 
status of the mobile station or a user selected long-life mode of the 
mobile station. 
System Load 
Information about the current system load may, for example, in a GSM system 
be obtained from the Radio Resource Management. A suitable value for this 
parameter could be the ratio between the number of currently allocated 
gross bit channels and the number of existing gross bit channels within 
the current cell. Similarly, the load in neighboring cells, calculated in 
the same way, could also be taken into consideration. 
Time of Day and Date 
Provided by system clock. 
User Request 
Type of call, requested quality level, VIP call, requested cost level. 
Statistical parameters may be formed by collecting parameters and forming 
long or short term averages, e.g. running averages. It is also possible to 
include other knowledge into the generation of the statistical parameters, 
e.g. public holidays, dates of fee changes, studies about subscriber 
behavior, etc. 
DECISION ALGORITHM 
The decision about the degree of channel error protection, gross channel 
allocation and the actual service selection may be based on an index, 
which is calculated in accordance to the following algorithm (also refer 
to drawing): 
1. Select (step 12) a set of at least two parameters and possibly a set of 
statistical parameters to base the decision on. 
2. Determine (step 14) a numerical value representing the current status of 
each respective parameter. 
3. Look up (step 16) the values of the statistical parameters to be used. 
4. Weight (step 18) each of the parameter values to be taken into account 
by a suitable weighting factor, which stands for the importance of the 
respective parameter. 
5. Sum (step 20) up all weighted parameter values. 
6. Compare the weighted sum to predefined values in the look-up table and 
find (step 22) that table element which is closest to the weighted sum. 
The index of this table element is the required index. This index will 
define (step 24) the service to be allocated. 
Note that the selection of parameters and statistical parameters to base 
the decision on, the weighting factors as well as the look up table may be 
fixed or adaptive. 
It is also possible to generalize the above method by examining all the 
system components which are involved in the subsequent connection. With 
knowledge of capability of the various components, the connection may be 
established in a mode which offers a maximum quality of service at a 
minimum of required system resources. 
One such example is an MS-MS connection. During call-setup the speech 
coding capabilities of both terminals are determined by means of a 
suitable protocol. If the terminals share some common speech coding 
method, it is possible to avoid the decoding and re-encoding in the 
transcoder unit on the network side. Instead the terminals could agree, by 
a suitable protocol, on the speech coding method to be used. During the 
connection the network transfers the bit stream in a transparent mode from 
one terminal to the other. Transparent means that no source decoding and 
re-encoding is performed on the network side. The gain is that a possible 
quality degraduation due to tandem configurations may be avoided. 
The described method may also be generalized by noting that the flexibility 
obtained at call-setup may also be obtained during a call by repeating the 
method and dynamically reallocating resources during said call. This 
allows a reaction on parameter changes, which would help to maintain a 
high service quality during the entire connection time and to achieve a 
more efficient resource usage. For example, if radio conditions get worse 
during a high cost (VIP, "Gold Card") call a higher gross bit rate may be 
reallocated to enable stronger channel protection. This may even be done 
by "stealing" gross bit rate from a low cost call. 
It is possible to use either different or the same speech encoding, channel 
protection, gross bit rate, etc. in uplink and downlink directions. 
A further service that is possible due to the service flexibility offered 
by the present invention is GSM/DECT or D-AMPS/DECT phones. On incoming 
calls the call-setup procedure determines if DECT access is possible and 
desirable instead of a GSM or a D-AMPS connection. In such a case the call 
is forwarded to the DECT system. 
The benefit of the described flexibility of channel error protection, gross 
channel allocation and service selection at call-setup is a better usage 
of system resources. This helps to provide: 
A maximum of available service quality at any given time and any given 
location. 
A more uniform service quality over the network. 
A higher system capacity in terms of simultaneously accomodated users. 
Moreover, flexibility at call-setup may give: 
The operator the possibility to offer selected ranges of services depending 
on the current situation. 
The user a range of selectable services. 
It will be understood by those skilled in the art that various 
modifications and changes may be made to the present invention without 
departure from the spirit and scope thereof, which is defined by the 
appended claims.