Method of registering/-reassigning a call in a dual mode communication network

Registration operates by sensing (106) the availability of a first communication system. A quality factor of a channel of the system is measured (111). A channel of the first system is selected if the quality factor of the channel exceeds a threshold (113). If selected, the subscriber attempts to register the call (114). If the registration fails, the availability of a second communication system is sensed (107). A quality factor of a channel of the second system is measured (111) and selected (113), if the quality factor exceeds a threshold. The subscriber will then attempt to register (114) with the second system. Reassignment operates by determining if the first system has reached a capacity limit (126). If it has, it determines if a channel on the second system is available (128). If available, a transfer of the call from the first to the second system will be attempted (132).

RELATED INVENTIONS 
The present invention is related to the following invention which is 
assigned to the assignee of the present invention: 
Dual Mode Communication Network, invented by Morton Stern et al., having 
U.S. Ser. No. 906,785, and filed on Jun. 30, 1992; and 
Method and Apparatus for Frequency Hopping a Signalling Channel in a 
Communications System, invented by Borth et al., having U.S. Ser. No. 
07/955,793 and filed on Oct. 2, 1992, now U.S. Pat. No. 5,381,433. 
FIELD OF THE INVENTION 
The present invention relates, in general, to communication systems and, 
more particularly, to a method of registering/reassigning a call in a dual 
mode communication network. 
BACKGROUND OF THE INVENTION 
In current cellular communication systems, pedestrian users will access the 
mobile cellular network. This mobile cellular network provides continual 
overhead measurements used by the system to maintain channel quality or 
perform hand-off functions. Since these measurements require the same 
amount of processing whether the user is mobile or not, a pedestrian user 
is charged the same fee for using their phone as the user who is mobile. 
Therefore, there exists a need in the industry for a personal communication 
system (PCS) which would provide a low tier system for pedestrian users at 
a reduced cost. The low tier system would provide access via radio 
frequency (RF) link to a basic cellular network which may or may not 
provide hand-off capability. For purposes of this discussion, a pedestrian 
user is one who roams slowly (10 kph, kilometers per hour, or less) as 
opposed to a mobile (up to 100 kph or more) user. 
In order to avoid the necessity of having two separate subscriber units 
(handsets), it is desirable to provide a dual mode network in which only 
one subscriber unit is required, such as the network described in the DUAL 
MODE COMMUNICATION NETWORK patent application, incorporated herein by 
reference, described above. 
Once such a network is established, there exists a need to provide a method 
of selecting which system in the dual network will be used to originate a 
call. Along the same lines, it will also be desirable to transfer an 
existing call between the two systems in the dual mode network. Such as, 
for example, in situations where a user is currently conducting a call 
while driving in a car. This call would normally be connected in the high 
tier, more expensive system. When the user stops the car, it would be 
desirable to have the call transferred to the low tier, more economical 
system. 
SUMMARY OF THE INVENTION 
In one embodiment of the present invention, a method of registering a call 
in a dual mode communication network having first and second communication 
systems is provided. The method operates by first sensing the availability 
of the first communication system at power-up of a subscriber unit. A 
quality factor of a channel of the first communication system is then 
measured. The first communication system channel is then selected if the 
quality factor of that channel exceeds a threshold of the first 
communication system. If selected, the subscriber attempts to register the 
call with the first communication system on that channel. If the attempted 
registration of the call on the first communication system fails, the 
availability of the second communication system is sensed. If the second 
communication system is available, a quality factor of a channel of the 
second communication system is measured. If the the quality factor of that 
channel exceeds a threshold of the second communication system then the 
subscriber will attempt to register the call with the second system on 
that channel. 
In another embodiment of the present invention, a method of reassigning a 
call, within a coverage area, from a first mode of a first communication 
system to a second mode of a second communication system in a dual mode 
communication network is provided. It is first determined if the first 
communication system has reached a capacity limit within the coverage 
area. If it has, it is determined if a channel on the second communication 
system is available. If the channel on the second communication system is 
determined to be available, a transfer of the call from the first 
communication system to the second communication system will be attempted.

DETAILED DESCRIPTION OF THE DRAWINGS 
Referring initially to FIG. 1, a block diagram of a cell structure, 
generally designated 10, of a communication system embodying the present 
invention is illustrated. Cell structure 10 consists of a plurality of low 
tier cells 12, each having a base site 13, grouped in some form of traffic 
channel reuse pattern (21 cell, 7 cell, etc.). For purposes of this 
description, the term low tier denotes a communication system which, in 
exchange for reduced operating cost, provides a low delay performance, 
shorter range, and lower speed hand-off as compared to present day 
cellular systems. In addition to the low tier pedestrian portion of the 
system, there is a need to have the mobile cellular system function as a 
high tier portion of the PCS. For purposes of this description, the term 
high tier denotes a communication system which provides at least the same 
type of performance, range and hand-off capability as present day cellular 
systems. This high tier system is represented by cells 11, having a base 
site 15, which, in this preferred embodiment, are in a single cell reuse 
pattern. The high and low tier systems function together to provide a 
transparent single service to the user. The low tier system being 
controlled by a low tier controller 14 and the high tier by controller 16. 
Optionally, an overall network controller 17 is provided. Controller 17 
may be comprised of portions of controllers 14 and 16. 
An example of the operation of this type of network is a pedestrian user 
who is walking down a street using an RF telephone in the low tier system. 
The user then, during a call, enters a vehicle and drives off. The system 
must be able to determine that a change has occurred and transfer the call 
from the low tier system to the high tier system in a fashion transparent 
to the user. 
As an alternative, the user may desire to control the mode of the 
subscriber unit. To accomplish this, a manual switch, or soft key, is 
provided on the subscriber unit for the user to change between high and 
low tier. In the scenario provided above, when the pedestrian enters the 
vehicle and drives off, the call would be discontinued by the low tier 
system once the user reached a speed beyond the capability of the low tier 
system. 
In a further alternative, a reduced price subscriber unit can be provided 
which only has low tier capability. This type of unit would be used in 
pedestrian situations (e.g. home, work, shopping, etc.); but would not 
function with the high tier system. A cost saving would result from the 
ability to eliminate various components from the subscriber unit (such as 
forward error correction and interleaving). 
However, in order to eliminate the need to carry multiple phones, or dual 
type phones, it is desirable to provide a dual mode system in which the 
high tier and low tier systems are compatible with each other such that a 
single transceiver (subscriber unit) can be utilized. Therefore, the 
present invention provides a dual mode system wherein the traffic channel 
protocols for each system operate on the same frame structure so that a 
single subscriber can be provided to operate at either mode. 
In Table 1 below, the specifications for the traffic channels for the low 
tier (pedestrian) and high tier (mobile) systems are provided. 
TABLE 1 
__________________________________________________________________________ 
DUAL MODE TRAFFIC CHANNEL PROTOCOLS 
SPECIFICATION LOW TIER HIGH TIER 
__________________________________________________________________________ 
SPEECH CODER 32 kbps ADPCM 16 kbps LD-CELP 
Forward Error Cor. 
NONE RATE 1/2 
BIT RATE 500 kbps 500 kbps 
CHANNEL SING 
400 KHz 400 KHz 
ACCESS METHOD TDM/TDMA 10 SLOTS 
SFH-CDMA 10 SLOTS 
FRAME DURATION 
2 ms 2 ms 
TRAFFIC CHANNELS 
750 750 
MODULATION QPSK QPSK 
CONTROL CHANNEL 
YES, DEDICATED SLOT 
YES, DEDICATED SLOT 
DUPLEX METHOD FREQUENCY DIVISION 
FREQUENCY DIVISION 
HAND-OFF CAPABILITY 
YES YES 
DIVERSITY SWITCHED ANTENNA 
MAX RATIO COMBINING 
FREQUENCY HOPPING 
NO YES 
TX POWER (AVG) 
10 mW 100 mW 
__________________________________________________________________________ 
In the low tier (pedestrian) communication system, a traffic channel 
protocol using a 32 kbps (kilobits per second) ADPCM (Adaptive Delta Pulse 
Code Modulated) speech coder is utilized to provide toll quality calls. No 
error correction or equalization is required in the low tier system. In 
the high tier system, a 16 bit LD-CELP (Low Delay - Code Excited Linear 
Predictive) speech coder is used with a rate 1/2 forward error correction 
(FEC). However, a 32 kbps ADPCM using two slots per frame or an 8 kbps 
coder using one slot in every other frame would also provide acceptable 
high tier coding alternatives. If a 32 kbps ADPCM coder is used for both 
tiers, then two slots in the high tier would be required to transmit the 
32 kb (kilobits) when the 1/2 rate FEC is used. This effectively reduces 
the number of channels from ten to five, but reduces the price and 
complexity of the unit by only requiring one type of coder. 
As can be seen from TABLE 1, and as illustrated in FIG. 2, the frame, 
generally designated 15, for the high tier system is a 20 hop interleaver 
frame 16. Each hop consists of a 10 slot TDMA (Time Division Multiple 
Access) frame 17. Each TDMA slot consists of 100 bits which consists of 6 
ramp up bits, 20 pilot bits, 68 coded data bits (speech bits), and 6 ramp 
down bits. The 68 speech bits consist of interleaved speech, FED, and 
signalling bits. Each slot is 200 .mu.sec (microseconds) long. This 
results in a TDMA frame being 2 msec (milliseconds) and the interleaver 
frame being 40 msec. Since this protocol utilizes both slow frequency 
hopping code division multiple access (CDMA) (i.e. the hopping sequence) 
combined with a time division multiple access method (TDMA) (multiple slot 
configuration) this protocol could best be characterized as a combination 
CDMA/TDMA method. 
A block diagram of the operation of a high tier modem, generally designated 
30, is illustrated in FIG. 3. A speech/information signal is received at 
one input of a framing device 31 and a signalling signal is received at a 
second input. In the preferred embodiment the speech is received at 16 
kbps and the signalling at 0.5 kbps. The output from framing device 31 is 
a 16.5 kbps signal. This frame is input to a forward error detection (FED) 
device 32 which adds an additional 0.5 kbps signal onto the 16.5 kbps 
signal from framer 31. The output from FED 32 is input to a forward error 
correction (FEC) device 33. This takes the 17 kbps input and codes it to 
provide a 34 kbps output signal. The 34 kbps signal is then interleaved in 
interleaver 34. The ramp up, pilot, and ramp down bits (16 kbps), block 
36, are then added, in framer block 35, to the signal frame which provides 
the 50 kbps traffic channel output. This compares with the 100 bit slots 
provided in FIG. 2 since the frames in FIG. 2 are 2 ms each or 500 frames 
per second. With each frame being 100 bits, the rate calculates out to the 
same 50 kbps figure. Likewise, the 32 bits per frame provided for ramping 
and pilot bits would be 16 kbps for 500 frames per second. 
Referring now to FIG. 4, a low tier frame, generally designated 25, is 
illustrated. Since the low tier system is not hopped, there is no 
interleaving frame set. Therefore, the highest order frame in the low tier 
system is TDMA frame 17, having 10 slots. As in the high tier system, each 
slot contains 100 bits which consists of the 6 ramp up bits, 2 
differential bits, 9 signalling bits, 64 speech bits, 13 FED bits, and 6 
ramp down bits. Also, as with the high tier system, each slot has a 
duration of 200 .mu.sec, making each TDMA frame 2 msec. While the 
transmission from the subscriber is a TDMA protocol, the transmissions 
from the base site may be either TDMA, where only the needed slots are 
used, or time division multiplexed (TDM) where all of the slots are filled 
whether being actively used or not. Therefore, the low tier system could 
be characterized as having either a TDMA or a TDM/TDMA protocol. 
In FIG. 5, a block diagram of the operation of a low tier modem, generally 
designated 50, is illustrated. The low tier modem uses many of the same 
functions as the high tier modem, which may or may not operate in the same 
fashion. In FIG. 5, framing device 31 receives the speech signal at 32 
kbps and the signalling information at 4.5 kbps. These are combined in 
framer 31 to form a 36.5 kbps signal. The 36.5 kbps signal is provided to 
FED 32 which adds 6.5 kbps for error detection. The resulting 43 kbps is 
added, in framer block 35, to a 7 kbps signal consisting of ramp up, 
differential, and ramp down bits, block 51. This results in a 50 kbps 
traffic signal. 
As can be seen in a comparison of FIGS. 2 and 4, the low tier TDMA frame 
set matches the TDMA portion of the CDMA/TDMA frame set utilized by the 
high tier. By utilizing the same frame sets in both the high and low tier 
systems, a single transceiver can be designed to operate in both tiers 
which utilizes many of the same components, making a smaller, less 
expensive communication unit possible. 
In FIG. 6, a general block diagram of a modem, generally designated 90, 
which will operate in either high tier or low tier is illustrated. Modem 
90 consists of a high tier portion 93, a low tier portion 94, and a common 
portion 85 of components which are used in both the high and low tiers. 
The operation of modem 90 is controlled by a control device 91. 
Control device 91 may operate based upon one or more parameters to select 
whether modem 90 operates in high or low tier. In one example, control 
device 91 may be a simple manual switch which the user controls to set 
modem 90 into either high tier or low tier operation. Alternatively, 
control device 91 may base the selection on availability of the low tier. 
For example, if the user is not within an area having low tier coverage 
(e.g. a sparsely populated area), control 91 would have to select the high 
tier to obtain service. 
Another control parameter would be the bit error rate (BER) or word error 
rate (WER). If the BER or WER were excessive, control 91 would select the 
high tier. In another example, the user may start in the low tier mode and 
be transitioned, or handed-off, to the high tier mode when the users speed 
increased to a level where the BER or WER was unacceptable. Measuring the 
carrier-to-interference (C/I) ratio would have the same effect. 
Referring now to FIGS. 7 and 8, a flow diagram of a process, generally 
designated 100, embodying the present invention is illustrated. Process 
100 illustrates the method used to access a network. Process 100 begins at 
power-up of the subscriber, step 101. The subscriber may be provided with 
a manual override switch to place the unit in a mode where either the low 
tier or high tier system is specified. If this mode is active, decision 
step 102, the process 100 senses whether the selected tier is available, 
decisions step 103. If the selected system is available, registration is 
attempted using registration subroutine 104 which is described in more 
detail in FIG. 8. If the selected system is not available or if the 
registration in subroutine 104 fails, process 100 ends, step 105. 
If the mode select switch is not active, decision step 102, then the 
subscriber unit will first attempt to connect with the low tier system. To 
accomplish this, it is first determined if the low tier system is 
available, decision step 106. If the low tier is available, registration 
is attempted using subroutine 104. If the low tier system is not available 
or if for some reason registration on the low tier system failed, process 
100 then senses the availability of the high tier system, decision step 
107. If the high tier system is available, registration is attempted using 
subroutine 104. If the high tier system is not available or if 
registration failed, then the call is not completed and process 100 ends, 
step 105. 
Subprocess 104, illustrated in the flow chart of FIG. 8, is entered from 
one of the various avenues illustrated in FIG. 7 at step 102. Subprocess 
104 then measures a quality factor of each channel available for the 
identified system, step 111. The quality factor may be any of a Received 
Signal Strength Indicator (RSSI), a Bit Error Rate (BER), a Word Error 
Rate (WER) , a carrier-to-interference (C/I) factor, or an InterSymbol 
Interference (ISI) factor. 
Once the quality measurements are made, the channels are ranked in order of 
quality by system, step 112. The subscriber then selects the best, untried 
channel of the system, step 113, and attempts to register the call on that 
channel, step 114. If the registration is successful, decision step 115, 
subprocess 104 ends, step 116. 
However, while the signal transmitted from the base to the subscriber may 
appear to be a good quality signal to the subscriber, this does not mean 
that the signal from the subscriber to the base has the same quality. 
Therefore, the base may deny registration to the subscriber on that 
particular channel. If this occurs, process 100 continues to decision step 
117 where it is determined if there are any more untried, qualified 
channels available in the system. If there are, subprocess 104 returns to 
step 113 and repeats from there. 
If the registration fails, decision step 115, and there are no more 
qualified channels in the low tier, decision step 117, then subprocess 104 
exits and returns to process 100, step 118. 
As an option to process 100, a method of reducing the quality threshold 
level(s) can be implemented and the process repeated in an attempt to 
complete the call at a somewhat reduced quality. This is illustrated in 
FIG. 7 by the dashed blocks. If the registration attempt at the current 
threshold level fails, process 100 can determine if the quality threshold 
level is set to a minimum allowable, decision step 108. If it is at a 
minimum, process 100 will end, step 105, as before. If the threshold is 
not at a minimum, the quality threshold is reduced, step 109, and process 
100 loops back to either decision step 103 or decision step 106. Process 
100 will then attempt to complete the call at the reduced quality level. 
The above method provides for call connection, or registration. Another 
aspect of the present invention is to provide for the transfer of a call 
between tiers during a call. For example, if the user is in the low tier 
and starts moving too fast, the call can be transferred to the high tier. 
Conversely, if the user is in the high tier and slows down, then the call 
could be transferred to the low tier. Another example is where a user is 
in the low tier, but, because of overload, the quality of the signal has 
degraded below some acceptable level. The network, or subscriber on its 
own initiative, could transfer the call to the high tier. 
An example of a process, generally designated 125, of transferring between 
tiers in a dual mode system is provided in FIG. 9. Process 125 begins at 
step 126 when the first system, the low tier system in this example, 
reaches capacity. The decision that capacity has been reached would 
typically be made by a system controller and would be based on one or more 
capacity, quality, or mobility factors. The capacity factors include items 
such as the amount of time the system has been at full capacity (e.g. all 
channels being used) and the blocked call rate. The quality factors 
include RSSI, BER, WER, C/I, and ISI, as described above in conjunction 
with process 100. The mobility factors would be such things as having the 
same channel available over an extended period (indicating non-movement of 
the subscriber) and various Doppler effects (e.g. frequency offset). 
Once the first system determines it has reached capacity, it will look for 
subscribers to transfer to the other tier. The system will notify a 
subscriber unit, step 127, that the system is at capacity and a hand-off 
to the other tier is to be attempted. The subscriber unit will then sense 
the availability of the other tier, step 128, and, if present, make 
quality measurements of the available channels, step 129. 
The available channels are then ranked according to their quality factors, 
step 130, and the best, untried channel is selected from the list, step 
131. Step 131 may be conducted by either the subscriber or the base site 
(after having the measurements transmitted thereto by the subscriber). The 
subscriber will then attempt to register with the high tier system, step 
132. If registration is successful, decision step 133, the call is 
transferred and process 125 ends, step 134. 
If the attempted registration fails, decision step 133, then process 125 
determines, in decision step 135, if there are more channels available. If 
more channels are available, process 125 loops back to step 131 and 
repeats If there are no more channels available, then the transfer fails, 
step 136. Note that since the transfer has failed, the call remains on the 
low tier system. The call is not otherwise effected. 
Thus, it will be apparent to one skilled in the art that there has been 
provided in accordance with the invention, a method of 
registering/reassigning a call in a dual mode communication system that 
fully satisfies the objects, aims, and advantages set forth above. 
While the invention has been described in conjunction with specific 
embodiments thereof, it is evident that many alterations, modifications, 
and variations will be apparent to those skilled in the art in light of 
the foregoing description. Accordingly, it is intended to embrace all such 
alterations, modifications, and variations in the appended claims.