TECHNICAL FIELD OF THE INVENTION The present invention relates in general to synchronizing cipher encoding between two transceivers and in particular to encoding and decoding synchronization signals using two unique words.

BACKGROUND OF THE INVENTION Wireless communication systems transfer information, such as voice and data, between two or more transceivers by modulating and broadcasting a carrier frequency. When the information transfer is intended to go from one transmitter to multiple receivers, the broadcast property of wireless communications is advantageous. However, when the information transfer is intended to go from one transmitter to a limited number of authorized receivers, the broadcast property of wireless communications can be a disadvantage. Accordingly, wireless communications are often enciphered to restrict unauthorized access.

Typically, the radio link between the two transceivers must be established in the clear (i. e., not enciphered), because the transceivers have not yet agreed upon a cipher key. Once a cipher key is transferred, the cipher process can begin. However, the start of ciphering of transmitted information at the first transceiver must be synchronized with the start of deciphering of received information at the second transceiver. Similarly, the start of ciphering at the second transceiver must be synchronized with the start of deciphering at the first transceiver. Further, the reverse must occur if a transition back to clear mode is required.

The communicating transceivers typically agree to transition from clear mode to ciphered mode based on the exchange of network layer messages such as"start cipher command","cipher started response"and"cipher mode complete". These messages can be lost due to the bursty nature of the wireless link. Multipath fading can also cause messages to be received in error and therefore lost. Protocol fault timers must be relied upon to recover from errors and exception conditions.

A typical cellular communications system employs network layer messages and fault timers. When the link is of high quality and interference free, network layer messages work fine. However, when there are errors or exceptions, the protocol timers expire causing transceivers to revert to the original state (e. g., clear mode) and start the cipher synchronization process all over again. This causes call setup

delay in such systems. This delay is tolerable in cellular systems as compared with satellite systems because the propagation delay from the mobile terminal to the base station is very short, on the order of microseconds. The round trip propagation delay in a satellite system is more than 500 milliseconds.

Some personal communications systems (PCS) use additional bits in each burst to indicate the cipher status of that particular burst, i. e., whether it is ciphered or not. This has two problems. First, the cipher status bits increase the overhead bandwidth and reduce the power and bandwidth efficiency of the system. Second, if these bits are not interleaved over several bursts, they are unprotected against fading and other link impairments.

SUMMARY OF THE INVENTION Accordingly, the present invention provides an improved method and apparatus for encoding information to facilitate synchronizing cipher encoding between two transceivers. The inventive method reduces the likelihood of losing information during a transition from a clear mode to a cipher mode and/or from a cipher mode to a clear mode.

In accordance with a first aspect of the invention, a method for controlling a transceiver is disclosed. The method begins by operating in a first mode and receiving a first unique word.

Subsequently, the transceiver receives a second unique word. The transceiver then transitions to a second mode of operation in response to the reception of the second unique word.

In some preferred embodiments, the first mode comprises a clear mode and the second mode comprises an enciphered mode. In other embodiments, the first mode comprises an enciphered mode and the second mode comprises a clear mode.

In accordance with another aspect of the invention, a method is disclosed for synchronizing cipher encoding at a transceiver.

The method begins by receiving clear information coupled to a first unique word. Subsequently, the transceiver receives clear information coupled to a second unique word. In response, the transceiver transmits enciphered information coupled to the second unique word. Assuming zoo errors occurred, the transceiver then begins receiving enciphered information coupled to the first unique word. In response, the transceiver begins transmitting enciphered information coupled to the first unique word. If and when, the transceiver eventually receives enciphered information coupled to the second unique word, it transmits clear information coupled to the second unique word.

In some preferred embodiments, the transceiver comprises a mobile terminal. In other preferred embodiments, the transceiver comprises a portable telephone. In some embodiments, the step of receiving clear information coupled to a second unique word further

comprises receiving a cipher start message. In certain embodiments, the step of transmitting enciphered information coupled to the second unique word further comprises transmitting a cipher response message. In any of the foregoing embodiments, the step of receiving enciphered information coupled to the second unique word further comprises receiving a cipher stop message. In some preferred embodiments, the step of transmitting clear information coupled to the second unique word further comprises transmitting a cipher response message.

In accordance with a further aspect of the present invention, a method for synchronizing cipher encoding at a transceiver is provided. The method comprises the step of receiving clear information coupled to a first unique word. The method further comprises the step of transmitting clear information coupled to a second unique word. Still further, the method comprises the step of receiving enciphered information coupled to the second unique word. Additionally, the method comprises the step of transmitting enciphered information coupled to the first unique word in response to receiving enciphered information coupled to the second unique word. The method further comprises the step of receiving enciphered information coupled to the first unique word. Still further, the method comprises the step of transmitting enciphered information coupled to the second unique word.

Additionally, the method comprises the step of receiving clear information coupled to the-second unique word. The method further

comprises the step of transmitting clear information coupled to the first unique word in response to receiving clear information coupled to the second unique word.

In some preferred embodiments, the transceiver comprises a network device. In other preferred embodiments, the step of transmitting clear information coupled to a second unique word further comprises transmitting a cipher start message. In some embodiments, the step of receiving enciphered information coupled to the second unique word further comprises receiving a cipher response message. In certain embodiments, the step of transmitting enciphered information coupled to the second unique word further comprises transmitting a cipher stop message. In any of the foregoing embodiments, the step of receiving clear information coupled to the second unique word further comprises receiving a cipher response message.

In accordance with a yet another aspect of the present invention, an apparatus for synchronizing cipher encoding is provided.

The system comprises a detector for determining a unique word associated with a received message. The system further comprises a cipher synchronizer for determining a transmission cipher state, a transmission encoding state, and a reception cipher state, based on the unique word and a previous reception cipher state. Still further, the system comprises a delay unit for storing a reception cipher state determined by the cipher synchronizer.

In some preferred embodiments, the apparatus further comprises an encipher unit for enciphering a message to be transmitted based on the transmission cipher state. In other preferred embodiments, the apparatus further comprises a multiplexer for encoding a message to be transmitted with a unique word based on the transmission encoding state. In some embodiments, the apparatus further comprises a decipher unit for deciphering the received message based on the reception cipher state.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features and advantages of the present invention will become more apparent from a detailed consideration of the following detailed description of certain preferred embodiments when taken in conjunction with the drawings in which: FIG. 1 is a block diagram of a satellite system that implements the method and apparatus of the present invention; FIG. 2 is a block diagram of a transceiver, suitable for use as one of the transceivers depicted in FIG 1; FIG. 3 is a state transition diagram illustrating the functionality of a cipher synchronizer, such as the cipher synchronizer depicted in FIG. 2; and, FIG. 4 is a flow diagram illustrating a method of cipher synchronization which may be employed by a pair of transceivers such as the transceivers depicted in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Although the following description focuses on a mobile terminal transceiver and a network transceiver in a satellite communication system, encoding/decoding cipher and clear modes of communication, persons of ordinary skill in the art will readily appreciate that the techniques of the present invention are in no way limited to mobile terminais, network terminals, satellite communication systems, or cipher/clear modes of communication. On the contrary, any communication system which might benefit from synchronizing two transceivers on one of two or more states may employ the techniques shown herein. Such systems might include other wireless or wired links such as a cellular communication system or a computer communication system. Further, persons skilled in the art will readily appreciate that other elements could be employed in the spirit of the present invention.

A satellite system capable of implementing the present invention is illustrated in FIG. 1. A first transceiver (10a, 10b, or 10c) transmits a signal (e. g., a radio signal) to second transceiver 12, which in turn relays the signal to a third transceiver 14. In a preferred embodiment, the first transceiver may be a mobile terminal 1 Oa, such as

a portable telephone or portable computer. Alternatively, the first transceiver may be a fixed ground based system 10b or a space based system 10c. Preferably, the second transceiver 12 is a satellite, and the third transceiver 14 is a gateway to a network. However, any of the foregoing devices (10a, 10b, 10c, 12,14) may be replace with any device capable of transmitting and receiving signals. For example, a computer configured to transmit and receive signals over a network could employ the teachings of the present invention.

FIG. 2 is a block diagram of a transceiver 10 suitable for use as one of the transceivers (10a, 10b, 10c) depicted in FIG 1. The radio signal is captured by a receiver 20 and recovered by a demodulator/synchronization detector 22 in a known manner. The radio signal contains information (in either clear or enciphered mode) coupled (e. g., adjacent in time) to a unique word (e. g., UWA or UWB). The UW consists of a predetermined pattern chosen because of its excellent cross correlation properties. The demodulator/synchronization detector 22 estimates the signal's phase by performing minimum mean squared error techniques on the signal's UW portion.

However, in the method and apparatus of the present invention, the state of the UW is also used as an input to a cipher synchronizer 25 to determine the correct mode of operation. By using the state of the UW, no overload bits are added to the messages.

Further, the UW is less susceptible to loss from bursts and multipath

fading due to its cross correlation properties. Accordingly, the demodulated signal is processed by the demodulator/synchronization detector 22 to determine the current state of the received UW. The detector 22 determines which of two or more unique words (e. g., UWA or UWB) is coupled to the information in the signal by comparing the received UW with predetermined patterns UWA and UWB. One of the detector outputs, S1 (i. e., the current UW state, UWA or UWB), is then sent to the cipher synchronizer 25.

The cipher synchronizer 25 uses the current UW state S1 and the current deciphering state S2 (i. e., currently receiving clear or enciphered information as discussed in more detail below) to determine three outputs (S3, S4, and S5) used in cipher synchronization (see FIGS.

2 and 3). One of the synchronizer 25 outputs is the next deciphering state S3. The next deciphering state S3 determines if the next signal received should be deciphered or not. In other words, it indicates if the next packet of data received is to be clear (not enciphered) or enciphered. Accordingly, before the output S3 is used to enable a decipher unit 30, the output S3 is delayed by a delay unit 28. The delay unit 28 may be implemented in a variety of know manners. For example, in a preferred embodiment, the delay unit 28 could be implemented using a semiconductor gate. Alternatively, the delay unit 28 could be implemented using software instructions operating in a

microprocessor. After the delay is performed, the next deciphering state S3 becomes the current deciphering state S2.

The current deciphering state S2 is used by the decipher unit 30 as an enable/disable signal. If the current deciphering state S2 is"clear", the decipher unit 30 does nothing except pass demodulated data from the demodulator/synchronization detector 22 on to a decoder 24 and to other systems requiring received data 32. On the other hand, if the current deciphering state S2 is"decipher", the decipher/decoder unit 30 deciphers the demodulated data in a known manner before passing it on to the decoder 24 and other systems requiring received data 32. The current deciphering state S2 is also used as an input by the cipher synchronizer 25 as mentioned above and discussed below.

Another one of the synchronizer 25 outputs is the next enciphering state S4. The next enciphering state S4 determines if the next data signal transmitted 34 should be enciphered or not. The enciphering state S4 is used by an enciphering unit 36 as an enable/disable signal. If the enciphering state S4 is"clear", the enciphering unit 36 does nothing except pass transmit data 34 encoded by an encoder 38 on to the modulator/synchronization word multiplexer 40. On the other hand, if the current enciphering state S4 is"encipher", the enciphering unit 36 enciphers the encoded transmit data in a known manner before passing it on to the modulator/synchronization word multiplexer 40.

Yet another of the synchronizer 25 outputs is the next encoding state S5. The next encoding state S5 determines if the next signal transmitted should be coupled with UWA or UWB. Accordingly, transmit data 34 is optionally enciphered and then coupled with a UW by the modulator/synchronization word multiplexer 40 in a known manner. Finally, encoded and optionally enciphered data is modulated and transmitted in a known manner by the modulator/synchronization word multiplexer 40 and a transmitter 42.

A state transition diagram describing the functionality of the cipher synchronizer 25 is illustrated in FIG. 3. Two inputs (S1 and S2) are used to determine three outputs (S3, S4, and S5). The cipher synchronizer 25 may implement the functionality of the state transition table in a variety of known ways in order to determine the correct mode tof operation. For example, a logic circuit such as a multiplexer or a microprocessor executing software instructions could implement the functionality of the state transition table.

The first rule in the state transition diagram (FIG. 3) declares that if the current decode state S1 is UWA (i. e., the signal just received uses UWA) and the current decipher state S2 is"clear" (i. e., the signal just received did not require deciphering), then the next decipher state S3 will be"clear"also (i. e., no change to the decipher state), the next encipher state S4 will be"clear" (i. e., do not encipher the next transmission), and the next encode state S5 will be UWA (i. e., couple

the next information packet to be transmitted with UWA). This rule is invoked over and over under normal clear mode transmission and reception.

The second rule in the state transition diagram (FIG. 3) declares that if the current decode state S1 is UWB and the current decipher state S2 is"clear", then the next decipher state S3 should change to"decipher", the next encipher state S4 should change to "encipher", and the next encode state S5 should be UWB. This rule is invoked once (assuming no errors occur) upon a transition from clear mode to cipher mode. The reception of UWB signals the transition from clear mode to cipher mode, and the transmission of UWB acknowledges the transition from clear mode to cipher mode.

The third rule in the state transition diagram (FIG. 3) declares that if the current decode state S1 is UWA and the current decipher state S2 is"decipher", then the next decipher state S3 should remain"decipher", the next encipher state S4 should remain"encipher", and the next encode state S5 should be UWA. This rule is invoked over and over under normal cipher mode transmission and reception.

The fourth rule in the state transition diagram (FIG. 3) declares that if the current decode state S1 is UWB and the current decipher state S2 is"decipher", then the next decipher state S3 should change to"clear", the next encipher state S4 should change to"clear", and the next encode state S5 should be UWB. This rule is invoked once

(assuming no errors occur) upon a transition from cipher mode to clear mode. The reception of UWB signals the transition from cipher mode to clear mode, and the transmission of UWB acknowledges the transition from cipher mode to clear mode.

A flowchart of a program that can be implemented by a pair of transceivers to synchronize cipher/clear mode in accordance with the teachings of the present invention is illustrated in FIG. 4. The programmed steps are typically performed by a control circuit such as a microprocessor or application specific integrated circuit as is conventional. In this example, one of the transceivers is a mobile terminal 10a and the other is gateway to a network 14. Further, in this example the mobile terminal 10a initiates the transaction (block 50), but optionally the network gateway 14 could initiate the transaction (block 54 or block 56). Still further, this example assumes no errors in transmission.

The mobile terminal 10a initiates the transaction by transmitting information in the clear using UWA (block 50).

Subsequently, the network 14 receives the information (block 52).

Before the network 14 responds to the reception, it determines if it should enter cipher mode (block 54). For example, a predetermined algorithm or user command may indicate that the system should attempt to communicate in cipher mode. If it is not yet time to enter cipher mode, the network 14 also transmits information in the clear using UWA

(block 56). Subsequently, the mobile terminal 10a receives the information (block 58) and the network 14 waits for a clear/UWA response (block 52).

This clear mode transmission and reception sequence iterates until the network 14 determines that the system should enter the cipher mode (block 54). At that time, the network 14 transmits one more message (for this sequence) in the clear, but uses UWB to signal a transition to cipher mode (block 60). Of course, the determination to enter cipher mode need not ever occur. In such an instance the system continues to operate in clear mode.

Once the mobile terminal 10a receives the clear message using UWB (block 62), it transmits its next message enciphered using UWB (block 64). The reception of UWB signals the transition from clear 'mode to cipher mode, and the transmission of UWB acknowledges the transition from clear mode to cipher mode. Optionally, the transmission/reception of the clear message using UWB (block 60 to block 62) inclues a"CipherStart"message to signal the transition from clear mode to cipher mode. Similarly, the transmission/reception of the enciphered message using UWB (block 64 to block 66) includes a "CipherResponse"message to acknowledge the transition from clear mode to cipher mode.

In response to receiving the enciphered message using UWB (block 66), the network 14 transmits its next message enciphered using

UWA (block 68). By using UWA, the network 14 is indicating to the mobile terminal 10a that the transition from clear mode to cipher mode was successful. Subsequently, the mobile terminal 10a receives (block 70) and transmits (block 72) in the enciphered mode using UWA.

Accordingly, the network 14 receives the information in the enciphered mode using UWA (block 74). Before the network 14 responds to the reception, it determines if it should exit cipher mode (block 76). For example, a predetermined algorithm or user command may indicate that the system should attempt to communicate in clear mode. If it is not yet time to enter clear mode, the network 14 also transmits enciphered information using UWA (block 68). Subsequently, the mobile terminal 10a receives the information (block 70) and the network 14 waits for an enciphered/UWA response (block 74).

This enciphered mode transmission and reception sequence iterates until the network 14 determines that the system should exit the cipher mode (block 76). At that time, the network 14 transmits one more enciphered message (for this sequence), but uses UWB to signal a transition to clear mode (block 78). Of course, the determination to enter clear mode need not ever occur. In such an instance the system continues to operate in cipher mode.

Once the mobile terminal 10a receives the enciphered message using UWB (block 80), it transmits its next message in the clear using UWB (block 82). The reception of UWB signals the transition

from cipher mode to clear mode, and the transmission of UWB acknowledges the transition from cipher mode to clear mode.

Optionally, the transmission/reception of the enciphered message using UWB (block 78 to block 80) inclues a"CipherStop"message to signal the transition from cipher mode to clear mode. Similarly, the transmission/reception of the clear message using UWB (block 82 to block 84) includes a"Cipher_Response message to acknowledge the transition from cipher mode to clear mode.

In response to receiving the clear message using UWB (block 84), the network 14 transmits its next message in the clear using UWA (block 56). By using UWA, the network 14 is indicating to the mobile terminal 10a that the transition from cipher mode to clear mode was successful. Subsequently, the mobile terminal 10a transmits (block 50) and receives (block 58) in the clear mode using UWA.

In summary, persons of ordinary skill in the art will readily appreciate that a method and apparatus for synchronizing cipher encoding has been provided. Systems implementing the teachings of the invention can enjoy synchronization of cipher encoding between two transceivers without the overhead associated with additional bits or network layer messages.

The foregoing description has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teachings.

It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.