Noise codes are generated of a type termed "code mates" having correlation unctions which, upon detection in a matched filter, provide an impulse autocorrelation function. More particularly, code mate pairs of at least two code bits each are generated and shown, for purposes of illustration, utilized in a communications system and wherein one code mate is comprised of two signal bits of first and second or opposite polarity, one bit of which has a larger amplitude than the other bit and wherein the other code mate is comprised of two signal bits of said second polarity, one bit of which has a like larger amplitude than the other bit.

Base radio stations are spatially arranged in a radio transmission system in accordance with a cellular system, separating the message transmission channels from adjacent base stations being effected either by using the frequency-division multiplex method or by using the code-division multiplex method or by using a combination of these multiplex methods. In appropriately large spatial distances the same set of channels can be repeated in a further radio cell. If a mobile radio station moves during the conversation from one radio cell into another, then it is necessary to hand-over the then existing radio connection. To avoid the necessity of using additional receivers in each base station and to prevent a repeated hand-over in the event of high co-channel interferences, the measurements used for hand-over decision are effected in the mobile radio station. As part of the method the reception quality of the co-channel message transmission channels of remote base stations are additionally measured in the mobile station during the existence of a radio connection to a near base station.

A cellular digital radio transmission system which increases the number of available communication channels between base stations and mobile stations in such system. Message transmission from a base station to its assigned mobile stations is effected by a combination of time-division, code-division and frequency-division multiplexing. Message transmission from the mobile stations assigned to a particular base station is effected by at least one of such multiplexing modes.

A digital (de)modulation system uses a larger code set of M codes for N length codes, where M>N, to provide an increased data rate while maintaining the coding gain. For example, the system can use 16 different codes each having a length of 11 chips in a code set while the conventional M-ary keying systems use a code set size of 8 for 11-chip codes or 8-chip codes. By extending the code set size, the system increases the data rate of the system. With 16 codes and the ability to change the sign of the code to be transmitted, the system can encode 5 data bits on both I and Q, so a total of 10 data bits can be encoded per code symbol. In this embodiment, a code symbol contains an 11 chip code on a I modulation branch and an 11 chip code on a Q modulation branch. As such, using 11 chip codes and a chip rate of 11 Mhz, the system provides a data rate of 10 Mbps while conventional M-ary keying systems can only achieve 8 Mbps using the same code length and chip rate. By extending the code length, the processing gain is increased. The extended code set is not orthogonal, so a non-zero cross-correlation value results between the different codes of the code set. However, the resulting noise and multipath performance degradation can be kept small by choosing code sets with small cross-correlation values (nearly orthogonal). The magnitudes of both cross-correlation values and auto-correlation sidelobes should preferably be below half a code length. In some embodiments, the code set is derived from orthogonal codes which are modified to reduce the autocorrelation sidelobes associated with the orthogonal codes. In other embodiments, the code set is derived using a complementary code which provides low autocorrelation sidelobes and is modified to reduce the cross-correlation values between the codes.

A digital modulation system provides enhanced multipath performance by using modified orthogonal codes with reduced autocorrelation sidelobes while maintaining the cross-correlation properties of the modified codes. For example, the modified orthogonal codes can reduce the autocorrelation level so as not to exceed one-half the length of the modified orthogonal code. In certain embodiments, an M-ary orthogonal keying (MOK) system is used which modifies orthogonal Walsh codes using a complementary code to improve the auto-correlation properties of the Walsh codes, thereby enhancing the multipath performance of the MOK system while maintaining the orthogonality and low cross-correlation characteristics of the Walsh codes.