Abstract:
An improved multi-mode telephone for use in an improved satellite communication method and system. The telephone is selectively capable of receiving signals at different carrier bandwidths, such that a satellite communication system having a non-symmetrical air interface can be easily implemented without requiring rate conversion hardware or software at the satellite for direct mobile-to-mobile communications.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to satellite communication systems. More particularly, the present invention is directed to a satellite communication system capable of operating in a “mobile-to-mobile” mode in which mobile units communicate substantially directly via a satellite. 
     BACKGROUND OF THE INVENTION 
     At the present time, there are many proposals for mobile satellite systems, in which orbiting satellites would support communication (including voice, data, and fax) with dual-mode (cellular/satellite) mobile or handheld phones. FIG. 1 illustrates a block diagram of a satellite communications system. An orbiting satellite  110  is in communication with at least one ground station or outstations called the HUB  100  as well as with a number of portable mobile stations or phones  120 . The phones are each serviced by an appropriate antenna beam from a multiple spot-beam antenna on the satellite providing high gain in the direction of each phone. The HUB communicates with the satellite using, for example, C-band or K-band frequencies, while the satellite communicates with the phones using, for example, L-band or S-band frequencies. Such a satellite system can operate in multiple modes. In a first mode, communication links can be established between communication devices (e.g., standard telephone or mobile telephone) which, as a result of the respective locations of the communication devices, incorporate Public Switched Telephone Network (PSTN) equipment. In a second mode, a so-called “mobile-to-mobile” mode, communication links are established substantially directly between mobile units via the satellite. The “mobile-to-mobile” mode substantially reduces the signal delay by allowing a first mobile to communicate with a second mobile directly via the satellite transponder without requiring the signal of the transmitting mobile to go to the HUB then back up to the satellite, and finally to the receiving mobile. Mobile-to-mobile communication is discussed in detail in the copending, commonly assigned application entitled “Method and Apparatus for Enabling Mobile-to Mobile Calls in a Communication System”, filed on Jul. 30, 1996 and having Ser. No. 08/681,916, the entirety of which is incorporated by reference. 
     A satellite communication system can have a symmetrical or non-symmetrical air interface. In a symmetrical air interface, both the downlink and uplink carrier waveforms occupy the same channel bandwidth. In a non-symmetrical air interface, the downlink carrier waveform occupies a first channel bandwidth (e.g., 200 kHz) and the uplink carrier waveform occupies a second channel bandwidth (e.g., 50 kHz). In a typical non-symmetrical satellite system operating in a mobile-to-mobile mode, a converter located in the satellite or in the ground station converts signals between the first and second carrier bandwidths. Mobile units in such a system transmit communication signals at a first carrier bandwidth (e.g., 50 kHz) and receive communication signals at a second carrier bandwidth (e.g., 200 kHz). A converter adds power consumption, weight, cost and complexity to the satellite. Accordingly, it would be desirable to eliminate the converter, such that mobile units operating in a mobile satellite system would be able to communicate in a “mobile-to-mobile” mode without requiring a carrier bandwidth conversion at the satellite. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the above-described problems, and provides additional advantages, by providing for a method, system and mobile unit which enables a non-symmetrical mobile satellite system to operate in a “mobile-to-mobile” mode without requiring carrier bandwidth conversion at the satellite. More particularly, according to a preferred embodiment of the invention, an additional mode capability is added to the receiver of each mobile unit, such that, when in the mobile-to-mobile mode, the receiver can receive the first carrier bandwidth (50 kHz) without requiring conversion to the normal second carrier bandwidth (200 kHz). The mobile unit is provided with two receiver paths and a switch. When in a first mode (PSTN-to-mobile), the switch is controlled to select the first receiver path so as to receive signals at the normal second carrier bandwidth. When in the second mode (mobile-to-mobile), the switch is controlled to select the second receiver path so that the mobile unit can receive unconverted signals at the first carrier bandwidth. 
     According to the present invention, a non-symmetrical air interface in a satellite communication system can be supported without requiring bandwidth conversion to be performed at the satellite, thus reducing payload hardware and software. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention can be obtained upon reading the following Detailed Description of the Preferred Embodiments, in conjunction with the accompanying drawings, in which like reference indicators are used to designate like elements, and in which: 
     FIG. 1 shows a satellite communication system in which the present invention can be implemented; 
     FIGS. 2A-B are block diagrams of a conventional mobile phone and a mobile phone according to the present invention, respectively; and 
     FIGS. 3A-B are more detailed block diagrams of a conventional mobile satellite system and a mobile satellite system according to the present invention, respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 2A, major components of a conventional mobile phone are shown in block diagram form. The mobile phone  10  has a receiver path which includes a receive antenna  12  for receiving communication signals (e.g., from a satellite), a first receive filter  14 , a low noise amplifier  16 , a second receive filter  18 , a mixer  20 , a third receive filter  22 , an amplifier  24 , a second mixer  26 , and receiver processing circuitry  28 . A synthesizer  30  exchanges signals with the receiver processing circuitry  28  and provides inputs to the mixers  20  and  26 . The third receive filter  22  is selected according to the frequency of the downlink carrier bandwidth (e.g., 200 kHz). The mobile phone  10  further has a transmitter path which includes an I/Q modulator and upconverter  32 , a power amplifier  34 , a transmission filter  36 , and a transmit antenna  38 , connected as shown. The I/Q modulator  32  receives signals from the receiver processing circuitry  28  and synthesizer  30  and modulates and upconverts the signal intended for transmission. 
     Referring now to FIG. 3A, a block diagram of a conventional satellite communications system having a non-symmetrical interface is shown. The system includes a satellite  40  and a plurality of mobile telephones  42   1 ,  42   2 , . . .  42   n . Each mobile telephone  42   1 , . . .  42   n  is of the type shown and described with respect to FIG. 2A above, and thus transmits communication signals to the satellite at a first carrier channel spacing (e.g., 50 kHz), and receives signals from the satellite at a second carrier channel spacing (e.g., 200 kHz). In the communication system of FIG. 3A, mobile telephone  42   1  is operating in a first mode, in which a mobile link is established between the mobile telephone  42   1  and the satellite  40 , and a feeder link is established between the satellite  40  and a ground station  44  associated with a public switched telephone network (PSTN)  46 . The mobile telephone  42   1  transmits signals (e.g., at a 50 kHz carrier bandwidth B 1 ) to the ground station  44  via a channelizer  46  contained in the satellite  40 , and the mobile telephone  42   1  receives signals (e.g., at a 200 kHz carrier bandwidth B 2 ) from the ground station  44  via a channelizer and router  48  contained in the satellite  40 . In the communication system of FIG. 3A, mobile telephones  42   2  and  42   n  are operating in a second mode (mobile-to-mobile). In this mode, channelization and routing are performed by channelizer  52  and router  54 . Signals transmitted between mobile telephone  42   2  and mobile telephone  42   n  are converted from a first carrier bandwidth (50 kHz) to a second carrier bandwidth (200 kHz) by rate converter  50 . Rate conversion is necessary because the mobile telephones  42   1 , . . .  42   n  can receive only signals transmitted at the second carrier bandwidth (e.g., 200 kHz). 
     Referring now to FIG. 2B, a mobile phone architecture according to a preferred embodiment of the present invention is shown. The phone includes the same principal components shown in FIG. 2A, but also includes an additional receive filter  23 . The receive filter  23  is selected in response to a command from the HUB (or ground station  44 ) to initiate communications in the mobile-to-mobile mode. According to this embodiment, the receive filter  22  is a 200 kHz filter, and receive filter  23  is a 50 kHz filter. A switch  25  operates to select a receiver path which includes one of the receive filters  22  or  23 . This arrangement enables the mobile telephone to receive communication signals from the satellite at either a first carrier bandwidth (e.g., 200 kHz) used by the communication system in a first mode (PSTN-to-mobile) or receive signals from the satellite at a second carrier bandwidth (e.g., 50 kHz) used by the communication system in a second mode (mobile-to-mobile). 
     The switch  25  can select the appropriate receiver path, for example, according to call setup information provided to the mobile by ground station  44 . As will be appreciated by those skilled in the art, call setup information provides a mobile telephone with, for example, frequency time slot assignments which are required to establish communication with the mobile telephone. According to this exemplary embodiment, the ground station  44  recognizes when an anticipated communication link between two mobile phones can be established in the mobile-to-mobile mode. When this condition is recognized, ground station  44  includes, in the call setup information provided to the mobile telephones, a command which causes the switch  25  to select the appropriate receiver path. 
     Referring now to FIG. 3B, a satellite communication system incorporating a non-symmetrical air interface according to the present invention is shown. In this embodiment, the mobile telephones  42   1 , . . .  42   n  have architectures similar to the mobile telephone shown and described with respect to FIG.  2 B. Because such mobile telephones are capable of selectively receiving signals transmitted at different carrier bandwidths, no rate conversion is necessary; therefore, the satellite communication system of FIG. 3B does not include a rate converter  50 . 
     While the foregoing description includes numerous details and specificities, it is to be understood that these are for purposes of explanation only. Many modifications will be readily apparent to those of ordinary skill in the art which are clearly within the spirit and scope of the invention, as defined by the following claims and their legal equivalents.