The present invention generally relates to two-way radio frequency (RF) communications, and, more particularly, is directed to a method for reducing multipath interference distortion in a time-division multiple access (TDMA) communications system. 
Recently, there has been an increased demand for data communications and digitally-encrypted voice communications over land mobile radio channels.  Since the radio frequency spectrum is inherently limited, one must devise new system concepts and organizational features to accommodate the increased demand for mobile and portable radio communications services. 
Accordingly, there has been a renewed interest in the research and development of a variety of narrowband amplitude modulation (AM) and frequency modulation (FM) communications systems.  A reduction in channel spacing to 12.5 kHz has been achieved through the use of single sideband (SSB) AM communications systems, e.g., Eastmond, et al.,  U.S. Pat.  No.  4,541,118.  Moreover, reduction of occupied bandwidth to less than 6.25 kHz is feasible with linear predictive coding (LPC) voice encoding techniques for very high frequency (VHF) FM land mobile radios, e.g., see Carney and Linder "A Digital Mobile Radio for 5-6 kHz Channels", IEEE International Conference on Communications, Philadelphia, PA, June 13-17, 1982. 
Geographical co-channel reuse techniques and multiple access schemes are also used to improve the efficiency of spectrum utilization.  For example, trunking concepts, which involve the automatic sharing of a block of communications channels among a large number of users have been known and used extensively in the telephone industry and in 800 MHz FM radio systems, e.g., Lynk, Jr. et al.,  U.S. Pat.  No.  4,012,597.  Moreover, cellular radiotelephone systems were developed to reuse radio channels in a given geographic area by dividing the whole coverage area into contiguous smaller coverage areas (cells) using low power transmitters and receivers within the cell, e.g., Cooper, et al.,  U.S. Pat.  No.  3,906,166.  Trunking and cellular systems are two examples of frequency-division multiple access (FDMA) systems. 
Time-division multiple access (TDMA) is an additional method of achieving more efficient spectrum utilization.  TDMA has been used in point-to-point microwave telephone links and in satellite communications systems for some time.  Unlike high-capacity mobile radiotelephone FDMA systems wherein a large number of users are assigned different frequency RF channels, in a TDMA system, each user is assigned a different time slot on the same frequency channel. 
A digital FDMA system offers two to three times better spectrum efficiency than analog FM, and can also provide additional voice security and data communications services which can co-exist with present analog systems.  Nevertheless, FDMA systems exhibit several practical limitations, such as increased equipment costs, tighter specification tolerances, increased occurrences of intermodulation interference, and complicated channel control.  See K. Kinoshita, et al. " Digital Mobile Radio Telephone System Using TD/FDMA Scheme", IEEE Transactions on Vehicular Technology, Vol.  VT-31 pp.  153-7, November 1982. 
On the other hand, TDMA systems enjoy certain advantages over FDMA approaches.  First, variable data rate transmissions are readily accommodated with TDMA systems through the use of multiple, adjacent time slots.  Second, TDMA base station transmitters allow the use of a common power amplifier without increasing the intermodulation distortion present with FDMA systems.  Furthermore, the size and cost of TDMA mobile and portable transceivers can be reduced through the use of lower speed signal processing devices, since receive and transmit bursts appear at different points in time on the same RF channel. 
In order to achieve spectral efficiencies comparable to those obtainable with FDMA systems, TDMA systems must average the required time slot separations over many channels, i.e., at least five.  For reasonable quality speech, i.e., data rates greater than 9.6 kbps (kilobits per second), the raw information data rate of TDMA systems must be at least 50 kbps.  Allowing a data rate expansion factor of 2 to accommodate channel error correction coding methods, a data rate of at least 100 kbps is then required for a single TDMA channel. 
However, multipath interference significantly limits the maximum permissible data rate for a land mobile RF channel.  Multipath interference is usually described in terms of two effects--Rayleigh fading, and intersymbol interference.  Rayleigh fading pertains to the relative RF phases of the signals received over various transmission paths.  Since the total receive signal is a vector sum of the individual multipath signals received, the signal strength will exhibit large variations depending upon the frequency, echo amplitudes, and relative vehicle speeds.  Intersymbol interference is related solely to the time delay differences between the propagation paths as seen at the TDMA receiver.  Intersymbol interference places a limit on the maximum data rate at which digital information can be sent, since the TDMA receiver may simultaneously be supplied with various overlapping informational elements resulting in a smearing of the intelligence. 
For land mobile radio communications, the channel delay spread (i.e., the r.m.s.  value of the time spread in received energy of a transmitted impulse) varies from 200 nanoseconds for suburban locations to 5 microseconds for urban locations.  An approximation of the transmission rate permissible with intersymbol interference is then: EQU maximum transmission rate=0.2/delay spread
See W. C. Y. Lee, Mobile Communications Engineering, New York: McGraw-Hill, 1982, p.45.)  Hence, the maximum permissible data rate for a land mobile radio channel without encountering significant intersymbol interference degradation is in the range of 80 kbps to 1 megabit per second (Mbps).  This result limits the number of 20 kbps time slots to a maximum of only four.  Thus, in a land mobile radio system, the significant advantages of TDMA over FDMA may never be achieved in practice. 
A need, therefore, exists to provide a method and means for allowing transmission of data over land mobile radio channels at transmission rates in excess over those normally allowed by the multipath characteristics of the RF channel.