Method for Repeaters to Adaptively Digitally Code Received Analog Information

This invention relates generally to digital coding of analog information and, in particular, to adaptive digital coding of analog information.

Communication systems are known to comprise a plurality of communication units, repeaters, and dispatcher stations. The repeaters and dispatcher stations are physically connected together via a data link, such as a local area network (LAN). The communication units may either be digital communication units (transceive digital information) or analog communication units (transceive analog information). The digital communication units, because they transmit digital information, are able to communicate on a digital infrastructure directly, while analog communication units, because they transmit analog information, are incompatible with a digital infrastructure. For this reason, an analog communication unit cannot function in a digital infrastructure unless the received analog information is digitized before placement on the LAN and then decoded back to an analog signal before transmission to a receiving analog communication unit.

One implementation, which permits analog communication units to operate on a digital infrastructure, converts analog signals into a selected digital coding format. The selected digital coding format is typically based on the bandwidth of the digital infrastructure. For example, the selected digital coding format may be 4.8 kbps vector sum excitation linear predictive (VSELP), a Motorola acronym, or 64 kbps pulse coded modulation, (PCM). In general, the more bits used in the digital coding process, the more accurately the analog information can be recaptured. Conversely, the less bits used to digitally code the analog information, the less accurately the analog information can be recaptured. Thus, the 4.8 kbps VSELP digital coding may be fine for strong analog signals (high signal strength or low signal to noise), but may produce distorted representations of weak analog signals (low signal strength or high signal to noise). Consequently, the quality of the digitally coded analog signal may be unacceptable. In contrast, the 64 kbps PCM digital coding provides acceptable digital coding of weak analog signals and strong analog signals but requires substantial infrastructure bandwidth.

While the above works well when the signal quality remains relatively constant, in practice the signal quality tends to vary.

Therefore a need exists for a method of handling analog communication units within a digital infrastructure that adaptively digitally codes received analog signals based on signal strength and minimizing infrastructure bandwidth requirements.

FIG. 1 illustrates a digital communication system in accordance with the present invention.

FIGS. 2A and 2B illustrates a logic diagram that may be used to implement the present invention.

FIG. 3 illustrates a logic diagram that a repeater and a dispatcher station may use to implement the present invention.

Generally, the present invention provides a method and apparatus that allows repeaters in a digital communication system to efficiently support analog communication units. This is achieved by taking a received analog signal and digitally coding it to produce a digital signal. The digital coding may be pulse coded modulation (PCM), adaptive differential pulse coded modulation (ADPCM), or vector sum excitation linear predictive (VSELP), a Motorola acronym, which is selected based on the quality of the received analog signal.

The present invention can be more fully described with reference to FIGS. 1-3. FIG. 1 illustrates a digital communication system that includes repeaters 101-105, a digital bus 106, a dispatcher station 107, and mobile communication units 115-116. Each repeater 101-105 includes an antenna 108, a receiver 109, an ABACUS, a Motorola acronym, 110, a digital signal processor (DSP) 111, a receive signal strength indication (RSSI) 112, a microprocessor 113, and memory 114. As is known, the digital bus 106 is used to send and receive digital information between repeaters and the dispatcher station in a communication system and may extend over a large geographical area. The ABACUS circuit 110 provides the initial high bit rate analog to digital conversion of the intermediate frequency (IF) signal of the receiver, the DSP is used to demodulate the received signal and further digitally encode/decode the demodulated analog signals. The RSSI function indicates received signal quality of the received analog signal.

FIGS. 2A and 2B illustrates a logic diagram that a repeater may incorporate to implement the present invention. The process begins when a signal is received from a communication unit 200. A determination is made as to whether the received signal is analog or digital 201. The received signal is digital information, if it is a digitally readable signal. Otherwise it is an analog signal. If the received signal is digital the signal is transmitted onto the digital network in a known manner. The received digital signal may or may not be compressed for purposes of reducing bandwidth before being transmitted onto the digital network 202. In either case, the process is known, thus no further discussion will be presented except to facilitate the understanding of the present invention.

If the received signal is analog, the quality of the received analog information is determined 203. The quality is determined by the RSSI which measures the absolute signal strength of the radio frequency (RF) signal and/or signal to noise power ratio of the same. Next the quality of received analog information is compared with two signal quality threshold values 204. For purposes of this discussion, the two signal quality thresholds will be RF signal to noise ratios of 6 dB and 12 dB plus or minus some hysteresis. If the quality of the received analog information is above the first signal quality value, or 6 dB, 205, and below the second signal quality value, or 12 dB, 208 the received analog information is digitally coded based on a first digital coding format 211, such as 32 kbps ADPCM, and then marked as being digitally coded by the first digital coding format (32 kbps ADPCM) 212. If the quality of the received analog information is below the first signal quality value, or 6 dB, 205 the received analog information is digitally coded based on a second digital coding format 206, such as 64 kbps PCM which requires more bandwidth than 32 kbps ADPCM, and then marked as being digitally coded by the second digital coding format (64 kbps PCM) 207. If the quality of the received analog information is above the first signal quality value, or 6 dB, 205 and above the second signal quality value, or 12 dB, 208 the received analog information is digitally coded based on a third digital coding format 209, such as 4.8 kbps VSELP which requires less bandwidth than 64 kbps PCM and 32 kbps ADPCM, and then marked as being coded by the third digital coding format (4.8 kbps VSELP) 210. After the received analog information has been digitally coded and marked as being digitally coded by a particular digital coding format, the digitally coded signal is transmitted onto the digital bus 213.

Next, a check is done to determine whether a predetermined amount of time has expired since the analog signal has been received 214. The predetermined amount of time, or time interval, is based on the mobility of the communication units. For example, if the communication units have high mobility, the time interval may be one second. On the other hand, if the communication units have low mobility, the time interval may be one minute. Basically, the predetermined time interval may be any time value that is appropriate for the communication system. If the predetermined amount of time has expired, the signal quality of the received analog information is determined again 203. If the predetermined period of time has not expired, the repeater determines whether the analog information is still being received 215. If the analog information is no longer being received, the process ends. If the analog information is still being received, the repeater determines whether the predetermined period of time has expired since receiving the analog information 214 and continues as mentioned above until the analog information is no longer being received. As long as the analog information is being received, the quality of the information will be checked at a predetermined time interval. This is done to allow a different digital coding method to be chosen to digitally code the received analog information if the quality of the information changes during the duration of its transmission.

FIG. 3 illustrates a logic diagram that a repeater and/or a dispatcher station may use to implement the present invention. The process begins when a digital packet is received from the digital bus 300. Once the digital packet is received, it is checked for marking 301. The marking indicates whether the digital packet was received analog information that has been digitally coded. The marking also indicates the type of digital coding format used to digitally code the analog information. If the received digital packet is not marked, it is transmitted to the communication unit or to the dispatcher station as digital information 302. If the received digital packet is marked, the digital coding format is identified based on the marking 303. Next the received digital packet is decoded based on the digital coding format indicated by the marking to reproduce the analog information 304. Finally, the reproduced analog information is transmitted to other analog communication units 305 to end the process.

The above discussion described two threshold levels, however, to one skilled in the art, it is easily recognizable that multiple threshold levels could be used. For example, a different threshold value could be used to indicate when to digitally code the received analog information using 64 kbps PCM, 32 kbps ADPCM, 16 kbps ADPCM, 8 kbps VSELP, 4.8 kbps VSELP, etc.

By allowing analog information to be digitized and adaptively compressed, the audio quality of the received analog information is maintained throughout a transmission in which the RF signal quality varies. Since most signals are received with an average RF signal quality that permits a high degree of voice coding compression, the present invention minimizes the average bandwidth requirements on the digital bus, while still preserving the audio quality. In this manner, the digital link can accommodate more calls, in general, than when using only one digital coding format such as 64 kbps PCM for all received analog information.