METHOD AND APPARATUS FOR the effective data rate. Prominent among these factors is
PROVIDING ADAPTIVE MODULATION IN A error correction coding, which decreases efficiency by tak
RADIO COMMUNICATION SYSTEM m8 UP valuable bandwidth. Unfortunately, error correction is
desirable when more advanced modulation techniques are
FIELD OF THE INVENTION 5 used to send data at greater rates, since they are inherently
_ ... .. less immune from channel induced errors. As an example, a
The present invention relates generally to radio commu- m lo^ multi.ievei QAM modulation might also
nication systems, and more particularly to such systems that j an ^ correction code hown as ttelJis coding,
provide adaptive modulation schemes when transmitting which .g ... (0 WQrk wjth multilevel symbols Such
error correcting schemes typically increase the amount of
BACKGROUND OF THE INVENTION overhead, thereby reducing the maximum rate at which data
may be sent.
Wireless data communications systems are known to Lastly, because of the substantially different nature of the include transmitting communication units and receiving hardware required t0 moduiate and demodulate signals sent communication units—e.g.. two-way radios. It is further usi differ modulation techniques, a communication known that commuiucation between these two radio units is is tem seMom t more ^ one modulation tech. facilitated through the use of a data transmission medium. ni for data communication. ... the system designer such as radio frequency (RF) channels. The reliability of the , icaU selects ^ si le technique best suited t0 me communication between the two radio units depends on the eral expected channel conditions. If channel conditions channel conditions, or channel quality. That is. the channel then d de fa reason othef methods mat do not condition—and hence, quality of the transmission—can 20 involve me transmission rate or modulation techvary as a result of external interference, attenuation caused nique must be employed
by buildings, or other obstacles, as well as other factors , ...... . * ■ , . ,
including power variations and environmental conditions. Accordingly, mere exists a need for a wireless dato
These channel condition variations can be relatively abrupt, communications system that is not constrained by the limi
particularly in mobile and portable radio applications. „ taUo,n)s f°und/n me jTM1" f * P^c"lar- such a ...
_ , ... . ... . , , 25 would be able to adaptively respond to varying channel
One approach to handle the widely varying channel conditions_both improvements and degradations-during conditions is to use a data transmission rate low enough to a Emission by seiecting an appropriate transmission accommodate the worst expected conditions. However tihis techni Lastly, such a system that could so adapt without approach results in inefficient use of the available the ^ for modulation and demodulation hardbandwidth, as the instantaneous channel conditions are often ware components £ould ^ ^ improvement over the prior better than the worst expected conditions. art
Another approach is to lower the transmission rate according to the quality of the established connection. As an BRIEF DESCRIPTION OF THE DRAWINGS example, wireline modems typically employ such a methodology by transmitting a known training sequence, from FIG. 1 shows a simplified block diagram of a pair of radio which an assessment can be made of the quality of the 35 units that might employ the present invention; connection. After setting the initial transmission rate, the FIG. 2 shows a more detailed block diagram of one the amount of errors detected at the receiving end is conveyed radio units shown in FIG. 1;
back to the transmitting end. If an excessive amount of piG 3 shows a flow diagram depicting the data transmis
errors are detected, communication is suspended, the train- sion process, in accordance with the present invention; and
ing process is repeated, and the transmission rate is reduced 40 ci,„„,„ „ «„„, A; „„ ,K„ „„„•„„.
r., „ y, , i . ■ . . ., ... FIG. 4 shows a now diagram depicting the receiver
until an acceptable performance level is reached. While this ,„ „
, . r .K. , . , .... c ., operation, in accordance with the present invention,
approach is acceptable where channel conditions are fairly r r
stable, it is inappropriate in situations where the channel DETAILED DESCRIPTION OF A PREFERRED
quality can either degrade or improve frequently over the EMBODIMENT course of a particular data session. That is, because the 45
modem approach allows only for a decrease of transmission Generally, the present invention provides a method for rate, the transmission rate cannot be increased to take transmitting data using a selected one of a plurality of data advantage of improved channel conditions. modulation techniques. Selection amongst the plurality of A third approach that does not rely on modifying the data modulation techniques is automatically made during the transmission rate is commonly referred to as an automatic 50 transmission process. The selected data modulation techrepeat-request (ARQ) system. In such a system, the receiv- nique is then used to transmit the data. By transmitting data ing radio unit detects which data frames are received in error in this manner, the present invention allows for enhanced and sends a feedback message to the transmitting end utilization of the transmission medium, even under widely requesting that a particular frame, or group of frames, be varying channel conditions.
re-transmitted. In this manner, the rate at which the data is The present invention can be more fully described with
sent over the channel is effectively increased or decreased, reference to FIGS. 1-4. FIG. 1 illustrates a data communi
in response to the channel conditions. However, such a cation system that includes radio units 100. 102 that com
method requires a significant amount of overhead, both in municate with each other using radio frequency (RF) chan
channel utilization and in transmission delay. In particular, nels 104,106. Such communication is facilitated through the
when channel conditions are poor over an extended period use of a transmit antenna 108 and a receive antenna 110,
of time, the need for re-transmission is high, and the channel 60 which are used to transmit data from a data source 112 to a
utilization is correspondingly low. data destination 114. Of course, the communication direc
Different techniques for modulating data can be employed tion may be reversed so as to facilitate transmission from a
to achieve effective communication over a particular data source in the radio unit 102 to a data destination in the
channel—e.g., N-ary quadrature amplitude modulation radio unit 100.
(N-QAM); where m is an integer value. The transmission 65 The radio units 100, 102 are typically two-way commu
rate is generally a function of the modulation technique used nication units such as a base station, console station, mobile,
and any other factors that may tend to increase or decrease or portable radio, like those manufactured by Motorola. Inc.