1. Field of the Invention
This invention relates to land mobile radio systems and more specifically to minimizing adjacent channel interference for land mobile radio systems.
2. Description of Related Art
Conventional land mobile radio (LMR) channels employ narrow-band frequency division multiplexed (FDMA) systems with different radio units assigned to different frequency bands. These bands are typically 25 KHz wide. There is an immediate need for an increase in capacity of LMR systems in the U.S. for such applications as public safety trunking. The trend is to increase capacity by splitting each existing 25 KHz channel used in LMR systems into two 12.5 KHz channels. However, this causes adjacent channel interference (ACI). ACI is interference introduced at a receiver from a transmitter broadcasting at a frequency corresponding to an adjacent channel and this is sometimes called adjacent channel `splatter`.
In a typical LMR system, communication between mobile units takes place through a base unit (base station). Each base station serves a certain geographic area. Communication between mobile units and base units takes place on a pair of frequencies that are separated, usually widely, to prevent interference. One frequency is needed for base to mobile communication and the other frequency in the pair is used for mobile to base communication. In some situations, mobile units can communicate with each other directly without going through the base unit. This is called "Talk-Around". A pair of frequencies are also used in Talk Around communications, one for each direction.
A problem occurs if two mobile units employ spectrally adjacent frequencies to communicate with their base units. Usually, mobile units within the same area will not be assigned spectrally adjacent frequencies but mobile units in contiguous geographic areas can use spectrally adjacent frequencies. The same situation exists with respect to frequency assignments to base units.
A measure of the acceptable level of ACI in a system is an ACI protection ratio (ACIPR). ACIPR is defined as the amount, in decibels, that the power of the interferer can be increased relative to the power of the desired signal, until a certain performance threshold is reached. For LMR system the bit error rate (BER) is used as an appropriate measure of performance. For analog FM, the ACIPR is specified to be in the range of 65-70 dB. Some digital modulation schemes offer adequate spectral efficiency but lower ACIPR (in the range of 45-50 dB). The ACIPR values may be augmented by several techniques. However, the problem gets increasingly difficult as the need for capacity and higher spectral efficiency arises.
Some of the commonly adopted techniques to improve ACIPR for digital modulation are antenna diversity in which more than one antenna receives a signal and the receiver chooses the signal from the antenna having a better signal strength. Antenna diversity is useful in providing a margin of 3-5 dB in ACIPR. Antenna diversity is further described in Characterizing the Effects of Nonlinear Amplifiers on Linear Modulation for Digital Portable Radio Communications, by S. Ariyavisitakul and T. P. Liu, IEEE Transactions on Vehicular Technology, Vol. 39, No. 4, pp. 383-389, November 1990.
Another technique to improve ACIPR is interference rejection and cancellation where an estimator is employed in estimating what a signal should be, and subtracting the estimated signal from the actual signal to synthesize an interference signal which is then subtracted from the further received signals. A similar technique is interference rejection using filtering described in Rejection Method of Adjacent Channel Interference for Digital Land Mobile Communications, by S. Sampei and M. Yokohama, The Transactions of the IECE of Japan, Vol. E 69, No. 5, pp. 578-580, May 1986. Interference cancellation is described in Method of Rejecting Adjacent Channel Interference Using an Adaptive Equalizer, by N. Kinoshita and S. Sampei, Transactions of IEICE (section B), J71-B, 10, pp. 1119-1126, October 1988. Interference rejection and cancellation involves complex receiver circuitry and is highly dependent upon the channel conditions and interference power. These techniques can provide up to 6-10 dB of gain if properly implemented.
Transmitter power control is described by Y. Nagata and Y. Akaiwa in Analysis for Spectrum Efficiency in Single Cell Trunked and Cellular Mobile Radio, IEEE Transactions on Vehicular Technology, Vol. VT-35, No. 3, pp. 100-113, August 1987. Transmitter power control offers a larger gain (10-15 dB) in ACIPR by controlling the transmit power of mobile stations. In transmitter power control, the mobile units which are closer to the base station transmit at a lower power in order not to "splash" other mobile units. The base station power is not varied. This scheme is complex and the complexity increases with capacity.
Another commonly suggested approach to providing higher spectral efficiency in a LMR channel is to use Trellis Coded Modulation (TCM) as described in Channel Coding with Multilevel/Phase Signals, by G. Ungerboeck, IEEE Transactions on Information Theory, Vol. IT-28, No. 1, pp. 55-67, January 1982. A typical TCM scheme employing Ungerboeck codes is designed to maximize the separation between transmitted signal states, called the Euclidean distance. However, optimization of Euclidean distance need not improve ACIPR.
A technique for designing trellis codes for band-limited channels is described in Bandwidth-Efficient Trellis Coded Modulation Schemes, by S. Ramseier, Proceedings of the International Conference on Communications, pp. 1517-1521, 1990. This technique designs Trellis Codes based on optimizing the normalized in-band power. This optimization is not as important for an LMR system since normalized in-band power does not maximize ACIPR. And further, the optimization as described by Ramseier does not consider the effects of the receive filter characteristics, which has a critical influence on ACIPR.
A related problem is that bandwidth saving is typically achieved at the expense of power and hence range reduction. The range reduction can be quite a problem where the area over which the base unit and mobile unit communicate is very large. An increased transmission area implies increased cost and complexity at the base station. Therefore, there is a need to find a spectrally efficient modulation scheme that has high ACIPR and offers a transmit range comparable to existing analog FM systems.