Abstract:
The present invention relates to an adaptive frequency hopping apparatus in a wireless personal area network (WPAN) system, wherein predetermined packets of data can be correctly transmitted by estimating the channel qualities of operating bands in advance and transmitting the packets through a proper band.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an adaptive frequency hopping apparatus in a wireless personal area network (hereinafter, abbreviated to “WPAN”) system for estimating the channel qualities of operating bands in the WPAN system to transmit packets through an appropriate band. 
   2. Description of the Prior Art 
   A WPAN system operates in the 2.4 GHz frequency band and includes a Bluetooth system, a Wireless Local Area Network (WLAN) and the like which are well known. The Bluetooth system performs the transmission and reception of data by using a frequency hopping scheme, and the WLAN system performs them by employing the frequency hopping scheme and a direct band spread scheme. 
   Further, the Bluetooth system performs the frequency hopping for all the bands by using 79 bands of the RF frequency, and particularly, a frequency hopping system of the Bluetooth system performs independently the frequency hopping with the same probability throughout the entire bands irrespective of the bands used by the other WPAN apparatuses. 
   Therefore, upon transmission and reception of predetermined data, the Bluetooth system produces a frequency interference with the other WPAN apparatuses using the same frequency band. 
   In particular, due to low power transmission of the Bluetooth system, if a WPAN apparatus having high power transmission operates around the Bluetooth system, the Bluetooth system fails to transmit a packet at the same frequency at that time. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an adaptive frequency hopping apparatus in a WPAN system for allowing predetermined packets of data to be correctly transmitted by estimating the channel qualities of operating bands in advance and transmitting the packets through an appropriate band. 
   In an adaptive frequency hopping apparatus according to the present invention, a master unit and active slave units, which form a piconet when they are connected with one another, transmit and receive data in 625 μs time slots assigned in accordance with a Time Division Duplex (TDD) scheme. The respective time slots are numbered in accordance with the Bluetooth clocks of the master unit, and the master unit and slave units basically allow respective receivers to operate in odd-and even-numbered time slots, respectively. 
   The receivers of the master unit and the slave units in the piconet operate N scan  (N scan ≧N h ) times during a predetermined time interval T scan  (T scan ≧1.25 ms×N h , N h =79) for Nh hop frequency bands. Each receiver monitors the channel quality using the output of an access code (AC) correlator forming the receiver and using the result of detection of a received signal strength indication (RSSI) signal, and stores the information on the channel quality for each 1 MHz band which has been accumulated N scan  times, in its own frequency table. 
   An adaptive frequency hopping controller accumulates the information on the channel quality, i.e. a good or bad channel, for each channel generated by a frequency hopping transceiver among N h  RF channels included in the frequency tables every time slots. The master unit collects frequency tables of each slave unit for all N h  RF channels every T scan  interval, and analyzes the information on the qualities for the RF channels in the piconet. 
   In general cases, the frequency tables of each slave unit may have different information on the qualities of the RF channels. Therefore, the master unit performs the determination of the qualities for the N h  RF channels by integrating the frequency tables collected from the K slave units and then divides them into N g  good channels and N b  (N h =N g +N b ) bad channels using the majority logic. The master unit transmits updated frequency tables for the RF channels to the respective slave units using a link manager protocol. These processes are repeated at the T scan  interval. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will become more apparent from reading the following description of the preferred embodiment taken in connection with the accompanying drawings in which: 
       FIG. 1  is a block diagram showing a configuration of the adaptive frequency hopping apparatus according to the present invention; 
       FIG. 2  is a flowchart showing the operation of the channel quality detector shown in  FIG. 1 ; 
       FIG. 3  is a graph illustrating the performance of the adaptive frequency hopping system on the SOC link according to the present invention; and 
       FIG. 4  is a graph illustrating the performance of the adaptive frequency hopping system on the ACL link according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. 
     FIG. 1  is a block diagram showing the configuration of an adaptive frequency hopping apparatus according to the present invention. As shown in  FIG. 1 , the adaptive frequency hopping apparatus comprises a frequency table  10  for monitoring the current qualities of channels for 79 frequency bands, and storing and outputting information on the channel quality for each 1 MHz frequency band accumulated N scan  times, a frequency hopping transceiver  11  for generating and outputting a frequency pattern in accordance with predetermined rules, a link controller  12  for generating an asynchronous connectionless (ACL) link, which is text data, in accordance with output signals of the frequency table  10  and the frequency hopping transceiver  11  and controlling selection of an operating mode between a channel avoidance scheme and a channel selection scheme, a packet handler  13  for generating packet data by integrating a synchronous connection oriented (SCO) link and the ACL link inputted therein, a gaussian frequency shift keying (GFSK) modulator  14  for performing GFSK modulation for signals outputted from the packet handler  13 , a mode selector  15  for selecting the operating mode between the channel avoidance scheme and the channel selection scheme in accordance with the output signals of the frequency hopping transceiver  11  and the link controller  12 , a frequency synthesizer  16  for synthesizing frequencies in accordance with output signals of the mode selector  15 , a first multiplier  17  for mixing signals from outputted from the frequency synthesizer  16  and the GFSK modulator  14 , and for outputting the multiplied signals as transmission signals, a second multiplier  18  for multiplying the output signals of the frequency synthesizer  16  by the receiving signals, an RSSI detector  19  for detecting the RSSI from output signals of the second multiplier  18 , a GFSK demodulator  20  for performing GFSK demodulation for the output signals of the second multiplier  18 , a packet handler  21  for restoring packet-type data from output signals of the GFSK demodulator  20 , and a channel quality detector  22  for estimating the channel quality by using the output signals of the RSSI detector  19  and the packet handler  21  and storing it in the frequency table  10 . 
   In the adaptive frequency hopping apparatus of the present invention having the configuration as described above, upon transmission of the predetermined data, the link controller  12  performs the mode selection for the frequency hopping in accordance with the SCO and ACL links. In addition, the apparatus operates in a frequency-hopping manner corresponding to each link according to the contents stored in the frequency table  10  and the frequency generated at the frequency hopping transceiver  11 , as described above. Further, the apparatus performs transmission of the signals at a hop frequency generated by means of the relevant frequency hopping method from the finally modulated signals. 
   Further, upon reception of the predetermined data, the second multiplier  18  multiplies the received signals by the output signals of the frequency synthesizer  16  to perform the modulation, and then outputs the multiplied signals to both the RSSI detector  19  and the GFSK demodulator  20 . The RSSI detector  19  and the GFSK demodulator  20  perform the RSSI measurement and the GFSK demodulation for the signals outputted from the second multiplier  18 , respectively. 
   The packet handler  21  receives the signals demodulated at the GFSK demodulator  20  and restores the data, which have been transmitted thereto, in accordance with the types of packets. Then, the channel quality detector  22  estimates the channel quality using the restored data and the detected RSSI value. 
   The channel quality detector  22  operates as shown in  FIG. 2 . If the access code correlator is triggered, the channel quality detector  22  checks a header error check (HEC). If there is not the HEC, the channel quality detector  22  estimates the channel as a good channel, and if there is the HEC, the channel quality detector  22  does the channel as a bad channel. 
   Meanwhile, if the access code correlator is not triggered, the channel quality detector  22  compares the RSSI value with a threshold value Th. As the result of the comparison, if the RSSI value is larger than the threshold value Th, the channel quality detector  22  estimates the channel as a bad channel, and if not, the channel quality detector  22  do not operate any longer. 
   Furthermore, even at a receiving end, a relevant frequency hopping method is selected by comparing the contents registered in the frequency table  10  with the frequency generated at the frequency hopping transceiver  11  in accordance with the types of the transmitted packets. The signals are restored at a hop frequency generated by means of the relevant frequency hopping method from the finally received signals. 
   That is, in case of the ACL link, the master unit and the slave units in the piconet adopt the channel selection scheme in which a long packet is assigned to a good channel and a short packet is assigned to a bad channel by using the registered frequency table  10 . Meanwhile, in case of the SCO link, the channel avoidance scheme, in which voice information is transmitted through a good channel by avoiding a bad channel where the interference exists, is adopted. 
   Hereinafter, the channel selection scheme will be described in detail. 
   The channel selection scheme maximizes the data throughput of total users by transmitting data of the users as little as possible using a segment type 1 or 2 packet for RF channels with high packet error probability, and transmitting a segment type 3 or 4 packet for RF channels with good quality. 
   In a process of packetizing the data of the user to be transmitted, the ACL link can generate a proper type of packet by comparing the sequence of the frequency hopping transceiver  11  with the quality of the RF channel stored in the frequency table  10 . That is, in case of the frequency band corresponding to the bad channel, a short packet of 1 time slot, such as DM 1  (DM: Data Medium) of segment type 1, which ⅔ forward error check (FEC) is applied is generated. In case of the good channel, a relatively long packet of 3 or 5 time slots such as DH 3  (DH: Data High), DH 5 , DM 3  and DM 5  of segment type 3 and segment type 4 is assigned thereto. 
   This channel selection scheme of the ACL link is performed using a link manager and a link controller of the Bluetooth unit which controls the generation of the packets. 
   Generally, while a connection is established, the transmitters and the receivers of the master unit and the slave units hop onto new frequencies at every 625 μs. A channel is divided into 625 μs time slots according to the clocks of the master unit, and each time slot is numbered. According to the TDD scheme, the master unit transmits the data in even-numbered time slot and the slave units transmit the data in odd-numbered time slot. The link controller of the master unit and the slave unit obtain the channel quality information from the frequency table for the hop frequency generated at the frequency hopping transceiver at each transmission time slot. The link controller transfers the information on the quality of the RF channel to the link manager. Furthermore, the channel selection scheme may be used in association with a power control scheme of the Bluetooth. A receiving Bluetooth unit can request a counterpart unit to increase or decrease the transmission power if difference between the measured RSSI value and the threshold value is large. This power control message is defined in the link manager protocol in the existing Bluetooth specification. In the adaptive frequency hopping method of the ACL link, if the RF channel quality is bad, the packets are transmitted using the channel selection scheme with the increased transmission power. However, the power controlling method is not applied to all bad channels. The power control scheme is used in association with the channel selection scheme when the interference level of the bad channel stored in the frequency table is lower than the threshold value. 
   Next, the channel avoidance scheme will be described in detail. 
   Establishing the connection of SCO links, the link manager assigns the slots at intervals of T SCO  (T SCO  is a unit time in which the master unit and the slave units can hop onto all frequency bands) based on ACL link. Accordingly, since the type of the packet to be used is predetermined, an RF channel changing scheme is more advantageous than the channel selection scheme which changes the type of the packet according to the channel condition. 
   Upon reception and transmission of the signals, if the RF channel generated at the frequency hopping transceiver  11  is a bad channel stored in the frequency table  10 , the frequency band used upon reception and transmission of the signals is determined by changing the hop frequency into the good channel. In the channel selection scheme of the ACL link, a transmitting Bluetooth unit determines the type of the packet by estimating the channel quality of the hop frequency. However, in the channel avoidance scheme, both the transmitting and receiving unit must estimate the channel quality and hop onto an identical RF channel among the good channels. At this time, a good channel mapper determines which channel among the good channels is to be used. 
   The implementation complexity of the channel avoidance scheme is affected by an implementation method of the good channel mapper. In order to meet the characteristics of the Bluetooth such as simplicity, the good channel mapper is also implemented as a simple architecture which can use the conventional Bluetooth specification. When the hop frequency is a bad channel, the good channel mapper uses a hop frequency that last hopped onto the good channel. Assuming that the frequency band of the interference signals which can interfere with the Bluetooth system is 20–30 MHz, in practice, the probability in which the hop sequence generated at the frequency hopping transceiver  11  will consecutively be assigned to three or more bad channels is low. 
   Therefore, even if the frequency assigned to the bad channel is replaced with a hop frequency last assigned to the good channel, the random property of the hop sequence is rarely affected. 
   Upon implementation thereof, the link controller updates only a register for storing the hop frequency last assigned to the good channel, and if the RF channel generated at the frequency hopping transceiver is a bad channel as the result of the comparison with the frequency table, the link controller simply uses the channel stored in the register. 
   The master unit transmits the DM x  (x=1, 2, 3) packets, and the slave units transmit the DM y  (y=1, 2, 3) packets. The throughput P SCO  of the conventional frequency hopping system and the adaptive frequency hopping system for the SCO link is given in accordance with HV z  (HV: High-quality Voice, z=1, 2, 3) of each SCO packet as follows:
 
 Psco= 1600/ z·P   T   , z= 1, 2, 3  (1)
 
where P T  is a probability of successful transmission of the packet. Each P T  for the conventional frequency hopping system and the adaptive frequency hopping system according to the present invention can be expressed as the following equations 2 and 3, respectively.
 
 P   T   =Ng/Nh   (2)
 
where Nh=Ng+Nb.
 
 P   T =( Nh−Nb−Nba )/( Nh−Nb+Nga−Nba )  (3)
 
where Nba is the number of occurrence of good channel erroneously estimated as bad channels and Nga is the number of occurrence of bad channels erroneously estimated as good channels.
 
   Meanwhile, the throughput P ACL  of the ACL link can be expressed as follows:
 
 P   ACL =(1600 /W )·( P   T   2   +P   T   3 )  (4)
 
   The probability of successful transmission of the packet is P T =Ng/Nh, and W for the conventional frequency hopping system and the adaptive frequency hopping system of the present invention can be expressed as the following equations 5 and 6, respectively.
 
 W=x+y   (5)
 
where x and y=1, 3, 5,
 
 W= 2·( Nb−Nga+Nba )/ Nh+ ( x+y )·( Nh−Nb+Nga−Nba )/ Nh   (6)
 
where x and y=3, 5.
 
   Considering the length of the packet, the data rate can be expressed as follows:
 
 R   ACL =(1600 /W )· P   T ·(1+ P   T )·[( Nba/Nh )· L 1+{( Nh−Nb−Nba )/ Nn}·L   3/5 ]  (7)
 
where L 3/5  means L 3  or L 5 , and Li is data length of DM i  packet (i=1, 3 or 5).
 
     FIGS. 3 and 4  are graphs illustrating the respective performances of the SCO and ACL links. As shown in  FIGS. 3 and 4 , the adaptive frequency hopping system represents the improved data rate for both the SCO and ACL links. Furthermore, the graphs show that as the channel estimation error Pg=Nba/Ng and Pb=Nga/Nb increase, the data rate decreases. 
   The proposed adaptive frequency hopping scheme monitors the frequency channel quality so that the transmission packet can be less affected by an interference component. Therefore, the entire data rate can be improved.