Patent Document

CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit, under 35 U.S.C. §119(e)(1), of U.S. Provisional Application No. 60/651,451, filed Feb. 8, 2005, and Provisional Application No. 60/746,458, filed May 4, 2006, and incorporated herein by this reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present embodiments relate to wireless communications systems and, more particularly, to improved handover for Digital Video Broadcast-Handheld (DVB-H) for a wireless communication system. 
     Wireless communications are prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. One such advancement includes the use of spread spectrum communications, including that of code division multiple access (CDMA) which includes wideband code division multiple access (WCDMA) cellular communications. In CDMA communications, user equipment (UE) (e.g., a hand held cellular phone, personal digital assistant, or other) communicates with a base station, where typically the base station corresponds to a “cell.” CDMA communications are by way of transmitting symbols from a transmitter to a receiver, and the symbols are modulated using a spreading code which consists of a series of binary pulses. The code runs at a higher rate than the symbol rate and determines the actual transmission bandwidth. In the current industry, each piece of CDMA signal transmitted according to this code is said to be a “chip,” where each chip corresponds to an element in the CDMA code. Thus, the chip frequency defines the rate of the CDMA code. WCDMA includes alternative methods of data transfer, one being frequency division duplex (FDD) and another being time division duplex (TDD, where the uplink and downlink channels are asymmetric for FDD and symmetric for TDD. Another wireless standard involves time division multiple access (TDMA) apparatus, which also communicate symbols and are used by way of example in cellular systems. TDMA communications are transmitted as a group of packets in a time period, where the time period is divided into time slots so that multiple receivers may each access meaningful information during a different part of that time period. In other words, in a group of TDMA receivers, each receiver is designated a time slot in the time period, and that time slot repeats for each group of successive packets transmitted to the receiver. Accordingly, each receiver is able to identify the information intended for it by synchronizing to the group of packets and then deciphering the time slot corresponding to the given receiver. Given the preceding, CDMA transmissions are receiver-distinguished in response to codes, while TDMA transmissions are receiver-distinguished in response to time slots. 
     New standards for Digital Video Broadcast (DVB) standards are currently being developed to permit streaming video reception by portable user equipment. DVB packets or data streams are transmitted by Orthogonal Frequency Division Multiplex (OFDM) transmission with time slicing. With OFDM, multiple symbols are transmitted on multiple carriers that are spaced apart to provide orthogonality. An OFDM modulator typically takes data symbols into a serial-to-parallel converter, and the output of the serial-to-parallel converter is considered as frequency domain data symbols. The frequency domain tones at either edge of the band may be set to zero and are called guard tones. These guard tones allow the OFDM signal to fit into an appropriate spectral mask. Some of the frequency domain tones are set to values which will be known at the receiver, and these tones are termed pilot tones or symbols. These pilot symbols can be useful for channel estimation at the receiver. An inverse fast Fourier transform (IFFT) converts the frequency domain data symbols into a time domain waveform. The IFFT structure allows the frequency tones to be orthogonal. A cyclic prefix is formed by copying the tail samples from the time domain waveform and appending them to the front of the waveform. The time domain waveform with cyclic prefix is termed an OFDM symbol, and this OFDM symbol may be upconverted to an RF frequency and transmitted. An OFDM receiver may recover the timing and carrier frequency and then process the received samples through a fast Fourier transform (FFT). The cyclic prefix may be discarded and after the FFT, frequency domain information is recovered. The pilot symbols may be recovered to aid in channel estimation so that the data sent on the frequency tones can be recovered. A parallel-to-serial converter is applied, and the data is sent to the channel decoder. 
     Referring to  FIG. 1 , rectangles  100  and  102  represent DVB packets of a current data stream  104 . The time between the start of DVB packets  100  and  102  is the delta-t time. Time between the DVB packets  100  and  102  is off time. The delta-t time is transmitted with other header information in each DVB packet to inform the DVB-H receiver when the next packet will arrive. The delta-t time is relative rather than absolute, so the DVB-H clock only needs to accurately measure the time from one packet to the next packet. Moreover, if a packet is lost, the DVB-H receiver may continue to monitor the carrier frequency  104  until the next packet arrives. This form of time slicing advantageously permits the DVB-H receiver to enter a low power mode or sleep mode after packet  100  is received. The DVB-H receiver subsequently wakes up in response to a timed interrupt to receive the next data packet  102 . This method of operation greatly reduces power consumption by the DVB-H receiver and prolongs battery life. Alternatively, the DVB-H receiver may use this time between packets to monitor alternative carrier frequencies of nearby cells. These alternative carrier frequencies are provided in a Network Information Table (NIT) for each network. The NIT includes an NIT-actual, having a list of frequencies for the current network, and several NIT-other lists, each having a list of frequencies for an adjacent network. 
     Referring now to  FIG. 2 , there is an exemplary DVB multi-frequency network (MFN). The MFN includes three cells  200 ,  202 , and  204  operating at frequencies f 1 , f 2 , and f 3 , respectively. Cell  200  has a maximum radius d=3.2 km, representing approximately 0 dB gain for 16 QAM at ⅔ code rate. Radii d/2 and 2d represent 10 dB and −10 dB gain, respectively. For transmitter power of 5 kW, a digital video broadcast handheld (DVB-H) receiver  210  moving at 120 km/h would receive seamless quality of service (QoS) if a handover is completed in 48 seconds or less. This represents d/2 or 1.6 km. There are several problems, however, that are somewhat unique to DVB handovers. 
       FIG. 3A  illustrates two data streams from neighboring cells. The upper data stream includes a series of OFDM packets N through N+5. The lower data stream includes a series of OFDM packets N+3 through N+7. The upper data stream is transmitted from a current DVB transmitter  200  operating at frequency f 1  which is not in synchronization with a neighboring DVB transmitter in another cell  204  operating at frequency f 3 . A first packet N on frequency f 1  is followed in time by packet N+3 on frequency f 3 . A second packet N+1 on frequency f 1  is followed in time by packet N+4 on frequency f 3 . This problem typically occurs due to differing internet backbone delays and precludes a simple handover from cell  200  to cell  204 . 
       FIG. 3B  illustrates another problem when multiple data streams are transmitted with significant time shifts. Here, packets N, N+1, and N+2 of a first data stream and packets M, M+1, and M+2 of a second data stream are transmitted on frequency f 1  from DVB transmitter  200 . The same data streams are transmitted on frequency f 3  from DVB transmitter  204 , but packets M+1, M+2, and M+3 are shifted to the left in time.  FIG. 3C  illustrates yet another problem of time shifting with multiple data streams. Here, packets N, N+1, and N+2 of a first data stream and packets M, M+1, and M+2 of a second data stream are transmitted on frequency f 1  from DVB transmitter  200  as in  FIG. 3B . However, packets N+4, N+5, and N+6 of the first data stream and packets M+4, M+5, and M+6 of the second data stream are transmitted on frequency f 3  from DVB transmitter  204  with a significant time shift. These problems significantly complicate handovers as the DVB-H moves from cell to cell in the MFN. 
     BRIEF SUMMARY OF THE INVENTION 
     A wireless receiver of the present invention provides seamless handovers in a digital video broadcast environment. The wireless receiver receives a first signal from a first transmitter and measures the signal strength. The signal strength is compared to a first threshold to determine if a handover is necessary. The wireless receiver receives a second signal from a second transmitter in response to the comparison. Both the first and second signals are sent to an application processor. The application processor determines when to stop receiving the first signal in the handover. 
     Other devices, systems, and methods are also disclosed and claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a block diagram of a OFDM data packets showing off time and delta-t time; 
         FIG. 2  is a diagram of a multiple frequency network; 
         FIG. 3A  is a diagram showing an elementary data stream on each of frequencies f 1  and f 3 ; 
         FIGS. 3B and 3C  are diagrams showing multiple elementary data streams on each of frequencies f 1  and f 3  and having significant packet offsets; 
         FIG. 4  is a block diagram of a DVB-H receiver of the present invention; 
         FIG. 5  is a state diagram showing candidate frequency list development and maintenance according to the present invention; 
         FIG. 6  is a state diagram showing operation of the DVB-H during handover according to the present invention; 
         FIG. 7  illustrates handover for an elementary data stream on each of frequencies f 1  and f 3 ; and 
         FIG. 8  illustrates handover for multiple elementary data streams on each of frequencies f 1  and f 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A DVB-H user initially selects a service for reception in much the same way as one might select a channel on television. It is an object of the present invention to maintain seamless or error free reception of that service until it is complete or until the user selects a different service. Error free service, therefore, requires the DVB-H receiver to maintain uninterrupted service as the user moves from cell to cell within a single MFN or from a current MFN to a different MFN. The maintenance of service requires handovers the DVB-H from one transmitter to another as the user moves from cell to cell. In general, the DVB-H handover requires a candidate list of frequencies that may replace the current frequency. This may be achieved by use of a frequency list descriptor or by a cell frequency link descriptor. The Network Information Table (NIT) provides the frequency list descriptor for each network. The NIT includes an NIT-actual, having a list of frequencies for the current network, and several NIT-other lists, each having a list of frequencies for an adjacent network. The cell frequency link descriptor is similar to the frequency list descriptor, but it also identifies the cells for which the frequencies are valid. This information is transmitted to all network users in the Transmission Parameter Signaling (TPS) bits in the data packet header. If the cell frequency link descriptor is temporarily unavailable, the DVB-H may use service identification information to complete the handover. This information is transmitted over the network every 100 ms in the Program Association Table (PAT) which is part of the Program Specific Information (PSI). 
     The preferred embodiments of the present invention provide seamless handovers for a handheld digital video broadcast receiver (DVB-H) in a wireless communication system. A wireless receiver of the present invention that performs the handover is shown at  FIG. 4 . The DVB-H preferably includes a wireless receiver circuit  100  and an application processor circuit  120 . The wireless receiver circuit  100  includes a radio frequency (RF) front end  104  coupled to antenna  102 . The RF front end relays the received data signals to analog-to-digital (ADC) converter circuit  106  to produce digital data signals. These digital data signals are applied to orthogonal frequency division multiplex (OFDM) demodulator circuit  108 . Control processor  110  relays the OFDM signals to the application processor circuit  120 . The application processor circuit  120  includes digital signal processor circuit  122  to decompress and decode the signals and application processor  124  to assemble the data signals. Each burst of the digital data signals includes a header with burst information including a packet number. Processor  124  uses this information to combine corresponding data burst packets into a contiguous data stream. The contiguous data stream is applied to LCD controller  126 , so that it may be viewed on the DVB-H. The application processor circuit  120  includes other controllers  128  which may operate a digital camera, GPS system, heart rate monitor, or other suitable application. The application processor  120  is also referred to as a media processor and by other similar names. The DVB-H also includes a power management circuit  112  which controls sleep and wake up modes of the wireless receiver to conserve power as will be explained in detail. 
     Referring now to  FIG. 5 , there is a state diagram showing development and maintenance of a candidate frequency list. The candidate frequency list is generated at power up and updated when a new program is selected or an old program is being removed. The candidate frequency list is preferably maintained during off time between data packet reception of the selected service. The list is preferably short and may only have 3 to 6 candidate frequencies. A list of other frequencies in a MFN is given in the NIT for the current network and in NIT-other for other networks. A short candidate frequency list can be selected based on geographical location of neighboring cell and a data stream that carries the same service of interest. This is based on the internet protocol (IP) platform identification and service identification. List creation begins at state  500 . If the candidate list is not empty and the currently received signal power is degraded below a predetermined threshold, the wireless receiver  100  measures the signal quality indicator (SQI) of each frequency over several intervals. This SQI can be the averaged receive signal strength indicator (RSSI) or other quality indicator. If the SQI of the current cell (source) is low enough and the candidate SQI is high enough, the cell identification is verified at state  504 . If the cell identification fails, the candidate will be moved to the end of the candidate list for lower priority monitoring, and the verification will be continued on the next best candidate until verification is succeeded or the candidate set is empty. 
     Turning now to  FIG. 6 , there is a state diagram showing operation of the DVB-H during a handover after the candidate set of frequencies is completed. A normal operating state  600  represents a DVB-H wakeup, receiving a data packet from the current selected service, and updating the SQI. If the SQI is sufficient, the DVB-H returns to sleep mode until it receives another wake up. The DVB-H remains in this state as long as the SQI remains above a threshold value T 0 . When the SQI falls below threshold value T 0 , the DVB-H transitions to monitor state  602  to prepare for a possible handover. A best handover candidate is selected from the candidate set of frequencies. If the SQI of the current frequency improves to a value greater than T 2 , the DVB-H returns to normal operation and no other action is necessary. Alternatively, if the SQI continues to deteriorate to a value less than T 1  and the best candidate&#39;s SQI is higher than the current SQI by a hysteresis margin H, the DVB-H moves to handover state  604 . Here, data from the best handover candidate is processed together with the current data. Both streams are provided to application processor circuit  120 . Application processor circuit  120  will buffer the data from both streams until data packets from the current stream duplicate data packets from the best handoff candidate. When a seamless replacement is completed, the handover is successful and the DVB-H returns to normal operation state  600 . If the handover fails, however, the DVB-H moves to state  606  and the best candidate is rejected. The DVB-H then moves to normal operation state  600 . If the current SQI is still inadequate, the process is repeated with another best candidate until the handover is successful. Note that the candidate set is created and maintained as previously described with regard to  FIG. 5 . 
     Operation of the DVB-H will now be explained in detail with reference to  FIG. 7 . The diagram of  FIG. 7  illustrates reception of two elementary data streams from neighboring transmitters of a multiple frequency network prior to a handover. The upper data stream, including packets N through N+4, is currently being received by the DVB-H on frequency f 1 . When the wireless receiver  100  determines a handover is necessary, it selects frequency f 3  from the candidate frequency set as a possible handover candidate. This corresponds to the monitor state  602  ( FIG. 6 ). If the RSSI of frequency f 1  subsequently falls below threshold T 1 , wireless receiver  100  initiates the handover in state  604 . In this state, wireless receiver  100  processes packet N on frequency f 1 , then packet N+3 on frequency f 3 , then packet N+1 on frequency f 1 , then packet N+4 on frequency f 3 , and so on. All packets on both frequencies are sent to application processor circuit  120  ( FIG. 4 ). Application processor circuit  120  captures the data packets and concatenates them to form a contiguous data stream for LCD controller  126  to display, for example. When the in sequence concatenated packet delivery is successful, application processor circuit  120  directs processor  110  to stop receiving data on frequency f 1  via control bus  116 . Frequency f 3  then becomes the current frequency, and the DVB-H returns to normal operation state  600 . 
     Referring now to  FIG. 8 , there is a diagram showing a handover for multiple elementary data streams. The diagram of  FIG. 8  illustrates reception of elementary data streams N and M from neighboring transmitters of a multiple frequency network prior to a handover. The upper data stream, including packets N through N+2 and M through M+2, is currently being received by the DVB-H on frequency f 1 . The lower data stream, including packets N through N+3 and M through M+3, is currently being received by the DVB-H on frequency f 3 . The shaded area between the packets shows the time available for the DVB-H to monitor and update the RSSI of other candidate frequencies. As previously discussed, when the wireless receiver  100  determines a handover is necessary, it selects frequency f 3  from the candidate frequency set as a possible handover candidate. This corresponds to the monitor state  602  ( FIG. 6 ). If the RSSI of frequency f 1  subsequently falls below threshold T 1 , wireless receiver  100  initiates the handover in state  604 . In this state, wireless receiver  100  processes packet N on frequency f 1 , then packet N on frequency f 3 , then packet M on frequency f 1 , then packet M+1 on frequency f 3 , and so on. All packets on both frequencies are sent to application processor circuit  120  ( FIG. 4 ). Application processor circuit  120  receives the data packets and puts them in sequence to form two contiguous data streams for LCD controller  126  to display. When the above process detects a certain number of successfully received duplicate packets from both frequencies, application processor circuit  120  directs processor  110  to stop receiving data on frequency f 1  via control bus  116 . Frequency f 3  then becomes the current frequency, and the DVB-H returns to normal operation state  600 . 
     Still further, while numerous examples have thus been provided, one skilled in the art should recognize that various modifications, substitutions, or alterations may be made to the described embodiments while still falling with the inventive scope as defined by the following claims.

Technology Category: h