Abstract:
A transmitter comprises a channel encoder to encode data bits; a modulator to modulate the encoded data bits, wherein the modulator selects one of a first modulation scheme and a second modulation scheme for each of a plurality of time slots; wherein, for each time slot, the channel encoder: includes a prefix and a suffix at the beginning and end, respectively, of the encoded data bits corresponding to the respective time slot, the prefix and the suffix being selected based on the selected modulation scheme for the respective time slot; and passes the prefix, the encoded data bits, and the suffix to the modulator; wherein, for each time slot, the modulator: modulates the prefix, the encoded data bits, and the suffix received from the channel encoder according to the selected modulation scheme for the respective time slot; and applies a window function to the prefix and the suffix.

Description:
BACKGROUND 
     In some communication systems, inter-symbol interference may occur when switching between modulation schemes. For example, in a Global System for Mobile communication (GSM) network implementing Enhanced Data rate for GSM Evolution (EDGE), two modulation schemes are defined and supported in the base stations and wireless devices. The transmitters in the base stations and the wireless devices need to switch from one modulation scheme to the other whenever there is a need to do so. For example, a first time slot may use Gaussian Minimum Shift Keying (GMSK) modulation and a second time slot may use GMSK or 8 phase shift keying (PSK) modulation and vice versa. The transition from GMSK to 8PSK and vice versa should be smooth to reduce the creation of frequency spikes due to switching. In other words, phase discontinuity when the modulation switching occurs should be minimized to reduce the creation of high frequency interference. 
     SUMMARY 
     In one embodiment, a transmitter that transmits modulated signals over a communication link is provided. The transmitter comprises a channel encoder to encode data bits; a modulator to modulate the encoded data bits, wherein the modulator selects one of a first modulation scheme and a second modulation scheme for each of a plurality of time slots; wherein, for each time slot, the channel encoder: includes a prefix and a suffix at the beginning and end, respectively, of the encoded data bits corresponding to the respective time slot, the prefix and the suffix being selected based on the selected modulation scheme for the respective time slot; and passes the prefix, the encoded data bits, and the suffix to the modulator; wherein, for each time slot, the modulator: modulates the prefix, the encoded data bits, and the suffix received from the channel encoder according to the selected modulation scheme for the respective time slot; and applies a window function to the prefix and the suffix. 
    
    
     
       DRAWINGS 
       Understanding that the drawings depict only exemplary embodiments and are not therefore to be considered limiting in scope, the exemplary embodiments will be described with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIG. 1  is a high level block diagram of one embodiment of a wireless communication system. 
         FIG. 2  is a block diagram of one embodiment of communication devices. 
         FIGS. 3A and 3B  are diagrams depicting one embodiment of exemplary time slots. 
         FIG. 4  is a data flow diagram depicting one embodiment of data flow in a modulator. 
         FIG. 5  is a flow chart depicting one embodiment of a method of communicating data. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the exemplary embodiments. Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific illustrative embodiments. However, it is to be understood that other embodiments may be utilized and that logical, mechanical, and electrical changes may be made. Furthermore, the method presented in the drawing figures or the specification is not to be construed as limiting the order in which the individual steps may be performed. The following detailed description is, therefore, not to be taken in a limiting sense. 
       FIG. 1  is a high level block diagram of a wireless communication network  100 . In particular, the exemplary implementation shown in  FIG. 1  depicts a Global System for Mobile communications (GSM) network. GSM network  100  is used for cellular wireless networks and is typically used with licensed radio frequency spectrum. However, it is to be understood that other communication network standards can be used in other implementations of network  100 . Network  100  comprises a plurality of base station transceivers (BTS)  102 - 1  . . .  102 -M, which are controlled by a Base Station Controller (BSC)  104 . BTS  102 - 1  . . .  102 -M each comprise a transmitter  110  and a receiver  112  to transmit and receive wireless signals from one or more wireless devices  106 - 1  . . .  106 -N. Exemplary wireless devices include mobile phones, personal digital assistants (PDA), laptops, and any other device configured to connect to network  100 . Each of wireless devices  106 - 1  . . .  106 -N also includes a transmitter and receiver, not shown, to transmit and receive wireless signals. Although the transmitter  110  and receiver  112  are shown as separate devices in  FIG. 1 , in some implementations, the receiver  112  and transmitter  110  are integrated into a single device (sometimes referred to as a “transceiver”). An exemplary transmitter  110  and an exemplary receiver  112  are shown and described in more detail in  FIG. 2 . 
     Wireless devices  106 - 1  . . .  106 -N can make a voice call to telephone  116  via Mobile Switching Center (MSC)  108  which provides circuit-switching to a public switched telephone network (PSTN)  114 . In addition, wireless devices  106 - 1  . . .  106 -N can access non-real-time data, such as web pages, email, etc., stored on content servers  124 , over GSM network  100 . In particular, BSC  104  includes a packet control unit (PCU) function  105  that processes packet data received from wireless devices  106 - 1  . . .  106 -N. Although PCU  105  is implemented as part of BSC  104  in this embodiment, it is to be understood that in other embodiments PCU  105  can be implemented as a separate device. PCU  105  passes the packets to a serving general packet radio services (GPRS) support node (SGSN)  118 . SGSN  118  uses stored location information to route data packets to and from wireless devices  106 - 1  . . .  106 -N. Gateway GPRS support node (GGSN)  120  provides an interface to network  122 . Network  122  can be the internet, a local area network (LAN), or a wide area network (WAN), etc. Data from content servers  124  is sent over network  122  and provided to wireless devices  106 - 1  . . .  106 -N via GGSN  120 , SGSN  118 , and BSC  104 . 
       FIG. 2  is a block diagram of an exemplary transmitter  210  and receiver  212 . The transmitter  210  comprises a channel encoder  226  and a modulator  228 . The channel encoder  226  encodes data bits received from a data source  201  using techniques known to one of skill in the art. The data source  201  includes higher layer functionality that provides data for transmission such as, but not limited to, email, multimedia capture (image, video, sound), and Voice over Internet Protocol (VOIP). In addition, after modulating the encoded data bits, the transmitter  210  transmits the encoded data bits in time slots. For example, in this exemplary implementation of a GSM system, transmitter  210  transmits 148 bits in each time slot and 8 time slots per time division multiple access (TDMA) frame. The channel encoder  226  pre-pends a prefix at the beginning of data corresponding to a time slot and appends a suffix at the end of the data corresponding to the time slot. The prefix and suffix are chosen based on the modulation scheme selected for that corresponding time slot. 
     In particular, the encoded data bits for each time slot are modulated according to one of a plurality of modulation schemes. In this exemplary embodiment, two modulation schemes are used. For example, the modulation schemes can include, but are not limited to, a phase-shift keying (PSK) modulation scheme, such as a Global System for Mobile communications (GSM) 8-PSK modulation scheme or a Gaussian Minimum Shift Keying (GMSK) modulation scheme. Although specific exemplary modulation schemes are mentioned herein, it is to be understood that other modulation schemes can be used which map a binary sequence of bits to a plurality of symbols which each represent a plurality of bits. In this exemplary embodiment, the prefix and suffix corresponding to an 8-PSK modulation scheme each comprise a symbol repeated a predefined number of times. For example, in this implementation, the symbol is repeated 4 times and chosen from the 8 possible symbols. The prefix and suffix corresponding to a GMSK modulation scheme in this implementation, comprise a bit value repeated a predefined number of times. In particular, in this implementation, the bit value is logic value 1 and repeated 4 times. 
     The modulator  228  modulates the encoded data bits received from the channel encoder  226 . In particular, as discussed above, the encoded data bits for each time slot comprise a prefix, data from data source  201 , and a suffix. The processing functionality  230  of modulator  228  modulates the encoded data bits for each time slot according to the modulation technique selected for that time slot. The modulation technique used changes based, for example, on the type of data being transmitted, the required bandwidth for the data, and the conditions of the communication link  236 . 
     After modulating the encoded data bits, the processing functionality  230  applies a window function to the prefix and suffix of each time slot. The window function is selected such that discontinuities between time slots of different modulation techniques are minimized. For example, due to the inherent 3π/8 rotation involved in the 8-PSK modulation used in this embodiment, modulation of the prefix creates a sine wave of 2708333/5.333˜51 kHz at the beginning of the time slot. A similar sine wave is created by modulating the suffix at the end of the time slot. Application of the window function to the sine wave creates a smooth ramp up and ramp down signal as shown in  FIG. 3  and described in more detail below. In this implementation a Hanning window function is applied to the constant prefix and suffix of each time slot. However, it is to be understood that other window functions can be used in other embodiments. In addition, in some implementations, the communication link includes a control channel. In some such implementations, the modulator  228  only applies the Hanning window to the prefix and the suffix of consecutive time slots on the control channel when the selected modulation scheme corresponding to the consecutive time slots is different from one time slot to another. 
     The modulated encoded data bits for each time slot (also referred to hereinafter simply as “time slots” for purposes of explanation) are transmitted to receiver  212  via communication link  236 . In particular, the modulated time slots are transmitted such that at least a portion of the prefix and suffix are transmitted during a period when power levels on the communication link  236  are at a minimum level. For example, in this exemplary embodiment, the modulated time slots are transmitted such that at least a portion of the prefix is transmitted during a guard period or band prior to the scheduled start of the time slot. Link  236  comprises any wired or wireless medium suitable for communication signals, such as, but not limited to, fiber optic cable, coaxial cable, twisted pair cable, and wireless radio frequency (RF) communication signals. Hence, modulator  228  is operable to modulate the time slots for transmission over link  236 . For example, in this exemplary embodiment, the transmitter  210  further comprises an RF module  234 . The RF module  234  receives the modulated time slots produced by the modulator  210  and produces an RF signal suitable for transmission on the link  236 . For example, in one implementation where the modulator  210  outputs a digital baseband modulated signal having in-phase (I) and quadrature (Q) components, the RF module  236  performs an up-conversion operation to up-convert the baseband signal to an appropriate RF frequency and performs a digital-to-analog (D/A) operation to produce an analog signal suitable for transmission. 
     The receiver  212  comprises an RF module  238 , a demodulator  240  and a channel decoder  242 . The RF module  238  receives the RF signal transmitted on the link  236  and produces a digital baseband signal, comprising the encoded data bits, suitable for use by the demodulator  240 . For example, in one implementation where the demodulator  240  is configured to received a digital baseband modulated signal having I and Q components, the RF module  238  performs a down-conversion operation to down-convert the RF signal to baseband and to separate out the I and Q components. The RF module  238 , in such an implementation, also performs an analog-to-digital (A/D) operation to produce digitized versions of the baseband signals suitable for use by the demodulator  240 . 
     Demodulator  240  demodulates the modulated data bits and provides the encoded data bits to the channel decoder  242 . Channel decoder  242  decodes the demodulated data bits and provides the decoded data bits to a data sink  246 , such as, but not limited to, a mobile phone, television system, etc. 
     Processing functionality  230  in modulator  228  can be implemented using software, firmware, hardware, or any appropriate combination thereof, as known to one of skill in the art. For example, processing functionality  230  can include or interface with hardware components and circuitry that support the demodulation of received symbol sequence y. By way of example and not by way of limitation, these hardware components can include one or more microprocessors, memory elements, digital signal processing (DSP) elements, interface cards, and other standard components known in the art. Any of the foregoing may be supplemented by, or incorporated in, specially-designed application-specific integrated circuits (ASIC) and field programmable gate arrays (FPGA). 
     In one implementation of the embodiment shown in  FIG. 2 , at least a portion of the RF module  234 , the modulator  228 , and/or the channel encoder  226  in receiver  210  are implemented in software that executes on a suitable programmable processor. For example, such a programmable processor can be implemented using a digital signal processor (DSP) that executes software that implements at least a portion of the functionality described herein as being performed by the RF module  234 , the modulator  228 , and/or the channel encoder  226 . Such software comprises a plurality of program instructions tangibly embodied on a processor-readable medium such as modulation and window function instructions  250  stored on memory  232 . In other examples, the programmable processor is a part of another type of programmable device such as an ASIC or FPGA. Similarly, in one implementation of the receiver  212  shown in  FIG. 2 , at least a portion of the RF module  238 , the demodulator  240 , and/or the channel decoder  242  are implemented in software that executes on a suitable programmable processor. 
     The memory  232  can be implemented as any available media that can be accessed by a general purpose or special purpose computer or processor, or any programmable logic device. Suitable processor-readable media may include storage or memory media such as magnetic or optical media. For example, storage or memory media may include conventional hard disks, Compact Disk-Read Only Memory (CD-ROM), volatile or non-volatile media such as Random Access Memory (RAM) (including, but not limited to, Synchronous Dynamic Random Access Memory (SDRAM), Double Data Rate (DDR) RAM, RAMBUS Dynamic RAM (RDRAM), Static RAM (SRAM), etc.), Read Only Memory (ROM), Electrically Erasable Programmable ROM (EEPROM), and flash memory, etc. Suitable processor-readable media may also include transmission media such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as a network and/or a wireless link. 
       FIGS. 3A and 3B  are diagrams depicting one embodiment of exemplary time slots transmitted over link  236  above. As described above, a time slot includes a prefix, a set number of encoded data bits (148 bits in the exemplary implementation described above) and a suffix. As shown in  FIG. 3A , when transmitted, time slots  303 - 1  . . .  303 -N are offset from the scheduled periods  1  . . . N corresponding to the time slots  303 - 1  . . .  303 -N. That is, transmission of a given time slot  303  begins prior to the period scheduled for the corresponding time slot  303 . In particular, at least a portion of the prefix is transmitted prior to the start of the corresponding period. For example, as shown in  FIG. 3A , the constant bit prefix “1111” corresponding to GMSK time slots and the constant symbol prefix “SSSS” corresponding to 8-PSK time slots are transmitted during a guard band prior to the corresponding period. As shown in  FIG. 3B , after applying a Hanning window function to the prefix and suffix, a smooth ramp up and ramp down of each time slot minimizes discontinuities between time slots of different modulation techniques. In deed, in some embodiments, the Hanning window is only applied when the modulation technique of a given time slot is different from the modulation technique of the immediately previous time slot. 
       FIG. 4  is a data flow diagram depicting one embodiment of data flow in modulator  228 . time slots  303 - 1  and  303 -N, which correspond to GMSK modulation, enter the modulator  228  at block  402 . Time slots  303 - 1  and  303 -N are modulated according to the GMSK modulation scheme at block  404 . Similarly, time slot  303 - 2 , which corresponds to 8-PSK modulation, enters the modulator  228  at block  406 . At block  408 , the data bits in time slot  303 - 2  are mapped to symbols. At block  410 , the symbols in time slot  303 - 2  are rotated by 3π/8. Then at block  412 , a Gaussian filter is applied to the time slot  303 - 2 . At block  414 , the real component of the time slots  303 - 1  and  303 -N are multiplexed with the real component of the time slot  303 - 2 . Similarly, at block  416 , the imaginary component of time slots  303 - 1  and  303 -N are multiplexed with the imaginary component of the time slot  303 - 2 . In particular, the real and imaginary components are multiplexed such that at least a portion of the prefix of each time slot is occurs prior to the scheduled transmission period of the corresponding time slot. At block  418 , a Hanning window is applied to the real components and at block  410  a Hanning window is applied to the imaginary components. The real and imaginary components are then output to the RF module for transmission on link  236 . 
       FIG. 5  is a flow chart depicting a method  500  of communicating data. At block  502 , the data to be communicated is received from a data source. For example, voice data from a microphone, video data from a camera, etc. is received by a transmitter in a device such as one of wireless devices  106 . At block  504 , a modulation scheme is selected for each time slot in which the received data is to be transmitted. For example, the modulation scheme may be selected based on various factors, such as the conditions of the communication link over which the data is to be transmitted, etc., as described above. 
     At block  506 , the received data is encoded. Encoding the data includes pre-pending the data with a prefix and appending the data with a suffix. As described above, the prefix and suffix for a given time slot are chosen based on the selected modulation scheme for the respective time slot. At block  508 , for each time slot, the encoded data with the prefix and suffix is modulated according to the selected modulation technique. For example, the modulation techniques to be selected from in this example include, but are not limited to, 8-PSK and GMSK. At block  510 , a window function, such as a Hanning window is applied to the prefix and suffix. At block  512 , each time slot is transmitted such that at least a portion of the prefix is transmitted when power levels on the communication link are at a minimum level. For example, in one embodiment, at least a portion of the prefix is transmitted during a guard band immediately prior to the scheduled transmission time of the respective time slot. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.