Design of Bluetooth Baseband Controller Using FPGA

Transcription

1 Journal of the Korean Physical Society, Vol. 42, No. 2, February 2003, pp Design of Bluetooth Baseband Controller Using FPGA Sunhee Kim and Seungjun Lee CAD and VLSI Lab.,Department of Information Electronics Engineering, Ewha Womans University, Seoul (Received 23 April 2002) This paper presents the design of a Bluetooth baseband controller. Bluetooth, which is intended to replace the cables connecting portable and/or fixed electronic devices, is a universal shortrange radio link via an ad-hoc network and is specifically designed to provide low-cost and robust networking by applying fast frequency hopping and a shaped, binary FM modulation. Our baseband controller is designed in Verilog -hardware description language (HDL) and is implemented into a field programmable gate array (FPGA). The FPGA implementation integrated with Link Manager software was tested with Bluetooth spec version 1.1 and was shown to be fully functional. The controller carried out the baseband protocols and other low-level link routines and supported about a maximal 700 kbps at a 1 Ms/s symbol rate and about a maximal 5 Mbps at a 4 Ms/s symbol rate. PACS numbers: Bh Keywords: Bluetooth baseband I. INTRODUCTION Recently, the increasing demand on wireless personal area networking (WPAN) has resulted in various standards, such as Home RF, IEEE and Bluetooth, as well as systems applied to those. Due to the high demand for 2.4 GHz RF circuits, many CMOS RF solutions with low power consumption are being introduced to the market, and the cost for those devices is getting lower [1,2]. Bluetooth is a short-range radio link intended to replace the cable(s) connecting portable and/or fixed electronic devices [3]. For instance, Bluetooth built into both a cellular telephone and a laptop will replace the cumbersome cable used today to connect the laptop to a cellular telephone. Printers, personal digital assistants (PDAs), desktops, fax machines, keyboards, joysticks, and virtually any other digital device can be part of the Bluetooth system. In addition, Bluetooth provides a universal bridge to existing data networks, a peripheral interface, and a mechanism to form small private ad-hoc groupings of connected devices away from fixed network infrastructures [4]. Bluetooth establishes ad-hoc voice and data connections and operates in the 2.4 GHz unlicensed ISM band. Its specification is open and royalty-free. The symbol rate is 1 Ms/s to exploit a maximum available channel bandwidth of 1 MHz. Fast frequency hopping is applied to combat interference and fading. A shaped, binary FM modulation is applied to minimize transceiver complex- ity [3]. Figure 1 shows the Bluetooth protocol stack. The basic protocols that all Bluetooth systems must have are a radio, a baseband (BB), a link manager (LM), and a logical link controller. The radio takes care of sending and receiving modulated bitstreams. The BB takes care of the timing, and the framing, as well as packets, flow control, error detection, and correction. The LM takes care of managing states and packets and of controlling flow on the link. The logical link controller takes care of multiplexing user protocols, as well as segmentation and reassembly of larger datagrams into packets, and management [3]. This paper presents the hardware design of a Bluetooth baseband controller which supports an optional high data-rate mode of 5 Mbps at a 4 Ms/s symbol rate, as well as a normal data rate of 700 Kbps at 1 Ms/s slee@ewha.ac.kr; Fax: Fig. 1. Bluetooth protocol stack.

2 Design of Bluetooth Baseband Controller Using FPGA Sunhee Kim and Seungjun Lee Fig. 2. Block diagram of the designed baseband controller. Fig. 3. State diagram of the designed baseband controller. symbol rate, as specified in Bluetooth 1.1 specification. II. EVOLUTION OF BLUETOOTH STANDARD Since the Bluetooth specification version 1.1 was announced in February, 2001, the medium rate mode (MED) and the Bluetooth spec 2.0 high rate mode have been discussed as extensions to the current Bluetooth spec. MED aims to meet the demand for a higher data rate for new applications by supporting 2 3 times higher data rate than Bluetooth 1.1 while maintaining most of existing Bluetooth 1.1 functions. Bluetooth 2.0 tries to support up to an 8 Mbps gross air rate by adopting new network topology and a simple modulation scheme. It may also coexist in the Bluetooth 1.1 piconet. In this paper, we present a high data rate baseband controller that can support up to an 8 Mbps gross air rate. A 4-level Gaussian frequency shift keying (GFSK) modulation/demodulation scheme at a 4 Ms/s symbol rates is implemented by modifying only the data path and the modem part of the baseband controller such that Bluetooth 1.1 PHY can be reused without modification. The packet length was changed to accommodate increased user data, as well as the modulation type. However, the access code format remains the same as it is in Bluetooth 1.1. III. HARDWARE DESIGN As shown in Figure 2, the designed baseband controller consists of a register file, a controller, a data path, a modem, a hop selector, a clock generator and interface blocks. The register file is used to exchange information between LM and BB. The controller takes care of timing and state changing, as well as low-level link control. The data path composes the packet to be transmitted and decomposes the received packet. The modem takes care of smoothing the packet in the transmit path and recovering the data and the clock in the receive path. The hop selector selects the hop frequency for packet transmission, and the clock generator supplies clock signals to all the other blocks. 1. Register File The BB controller communicates with the LM processor through the register file. The register file has local device information, remote device information, current status information, transmitted and received packet information, and interrupt flags. Because most of the information requires only a few bits, the data in the register file can be accessed either as a byte, a half-word, or a word. 2. Controller The controller is divided into three parts: a timing controller, a state controller, and a link controller. The channel is divided into time slots that are numbered according to the Bluetooth clock of the piconet master. Each slot, 625 µs in length, corresponds to an RF hop frequency. A time-division duplex scheme is used in which the master and the slave alternately transmit [5]. Therefore, a piconet must be synchronized to the master Bluetooth clock, and each slave must calculate the clock offset when it receives the packets transmitted by the master to estimate the master Bluetooth clock. For that reason, we designed the time controller based on a half-slot time duration, which is the minimum time period needed for the ID packets, the shortest of all packets. The time controller counts from 0 to 2499 during a half slot, determines when both a master and a slave start their transmission, and estimates when the slave receives the packet transmitted by the master. Figure 3 shows the state diagram of our controller. There are seven states: STANDBY, page, page scan,

3 -202- Journal of the Korean Physical Society, Vol. 42, No. 2, February 2003 Fig. 5. Verilog simulation result of the modulator. Fig. 4. Block diagram of the data path. master response, slave response, NR, active and HR active. The NR active state corresponds to the active state specified in Bluetooth spec. 1.1, and the HR active state stands for high data rate active. The HR active state is added to support the higher data rate of a 4 Ms/s symbol rate with 4-level GFSK modulation, which has been implemented as a possible extension of current Bluetooth specification. In order to establish new connections, a master and a slave go through a paging and a page-scan procedure, respectively. Command from LM changes the current state to either a page or a page-scan state. During the paging procedure, the state must be changed upon receiving a packet or at time-out. In those cases, the BB controller analyzes the received packet or checks out the timers for each state. Therefore, the BB controller changes the state to NR active through a page response and carries out routines for each state. The link controller takes care of low-level link control, such as the automatic repeat request (ARQ) scheme and flow control. These are carried out independently of the other units. Thus, there is a separate ARQ-FLOW-Flag register for each AM-ADDR that is used to distinguish among the active members. 3. Data Path Each packet consists of three entities: the access code, the header, and the payload. The packet composer adds the header error check (HEC) to the header information read from the register file and the link controller, and then scrambles the header with a whitening word and encodes it at a rate of 1/3 forward error correction (FEC). The payload information plus the cyclic redundancy check (CRC) is scrambled, encrypted, and coded at a rate of 2/3 FEC. The packet decomposer extracts the header and the payload information from received packet in reverse order from the packet composer. CRC and/or FEC may be included optionally based on the packet type. Figure 4 shows a block diagram of the data path. Fig. 6. Block diagram of the demodulator. A TX buffer is provided separately for each AM- ADDR, but a RX buffer is shared. Each buffer consists of two fifo registers, two switches, and full-flag registers for each register. The switches determine which register can be accessed by the LM processor and which register can be accessed by the BB controller. All the TX and the RX buffer switches are controlled by the BB controller. 4. Modem The modulator consists of a symbol mapper and a gaussian low pass filter (GLPF). Data arrive at the mapper at the input bit rate and is demultiplexed as encoded symbols. In the 2-level GFSK mode, the data are directly passed to the GLPF. For a 4-level GFSK, the symbol rate is one-half of the bit rate, and each symbol is composed of two bits. However, the access code is directly passed to the GLPF because it is used for timing synchronization. Figure 5 shows the Verilog -hardware description language (HDL) simulation results. In the 4-level GFSK mode, the bit data (b) which corresponds to the access code is mapped to the 2-level symbol while the accesscode-valid signal (a) is high, and the bit data are mapped to the 4-level symbol when the access-code-valid signal is low. The signal (d) shows the Gaussian-filtered output. As shown in Figure 6, the demodulator consists of the correlator, the clock recovery, and the symbol demapper. We assume that the sample rate of the analog to digital converter (ADC) is four times the symbol rate, and its output with a resolution of 8 bits is passed to the slicer. The threshold value of the slicer is updated from the default value, 127, to the measured value by the clock recovery after the correlator is triggered. The correlator has the structure of a matched filter. The clock recovery

4 Design of Bluetooth Baseband Controller Using FPGA Sunhee Kim and Seungjun Lee Fig. 7. General block diagram of the hop selector. finds the symbol boundary by using the known, regular pattern of the access code trailer and recovers the symbol clock. 5. Hop Selector The hop selector generates hopping sequences - page hopping sequence, page response sequence, inquiry sequence, inquiry response sequence, and channel hopping sequence - for the 79-hop and the 23-hop systems. As shown in Figure 7, hopping frequency is selected by using control signals - X, Y, A, B, C, D, E, and F- that are derived from the upper address part (UAP)/ lower address part (LAP) of Bluetooth device address (BD- ADDR) and the Bluetooth clock. The Bluetooth clock may appear in two different forms: CLKN, which is a free-running native clock, and CLK, which is a master clock and is derived from CLKN by adding an offset. However, the Bluetooth spec. 1.1 has an obscure description of the page response sequence. X and Y for the slave page response sequence are given by X = [CLKN16(15) 12 + N] mod 32(16) (1) Y = CLKN 1. (2) It is written in the spec. that the value of N is increased by one each time CLKN 1 is set to zero, which corresponds to the start of a master TX slot [3]. However, the master CLKN 1 and the slave CLKN 1 don t correspond because CLKN is never adjusted and is never turned off. Thus, the hopping frequency of a master may be different from that of a slave when the master transmits an frequency hop synchronization (FHS) packet. To resolve this problem, we modified the equation such that the value of N was increased not by CLKN 1 but by CLK 1. When the slave receives an ID packet in the page scan state, it can infer that the master CLKN 1 0 is either 00 or 01, so it sets CLK 1 0 to 01. For a returning Fig. 8. Example comparing the spec and the proposed method of paging hop frequency. response, CLK 1 0 becomes 11, and the value of N is increased at the next half slot. The master transmits an FHS packet a half or one slot after it receives a response packet. Therefore, the slave awaits the arrival of an FHS packet when CLK 1 0 is either 00 and 01. If the slave receives an FHS packet when CLK 1 0 is 01, it resets CLK 1 0 to 00. Figure 8 shows one case - initially the master has CLKN , and the slave has CLKN to compare the spec. and the proposed method. 6. Clock Generator Table 1 shows all the clock signals classified according to their functionality, but the target field programmable gate array (FPGA) only supports four global clock buffers. Therefore, externally supplied clocks, CLK-BUS and CLK-IN, and divided clocks, CLK-MAIN and CLK- BIT, are assigned the clock buffers. The frequency of CLK-BIT is changed according to the operating mode (HR-Active/NR-Active) and the current slot type. CLK- SYMBOL, CLK-SAMPLE and CLK-FILTER are replaced by CLK-IN and enable signals. 7. Interface Block In our test environment the BB controller acts as an advanced high-performance bus (AHB) slave to communicate with the LM processor. Therefore, the Interface block has the AHB decoder to generate bus response signals.

5 -204- Journal of the Korean Physical Society, Vol. 42, No. 2, February 2003 Table 1. Clock signals classified functionally. CLK-BUS AHB bus clock 20 MHz CLK-MAIN Timing controller clock 8 MHz Normal rate High rate CLK-BIT TX bit processing clock 1 MHz 8 MHz RX bit processing clock 1 MHz 8 MHz CLK-SYMBOL TX mapping clock 1 MHz 4 MHz RX demapping clock 1 MHz 4 MHz CLK-SAMPLE RX sampling clock 4 MHz 16 MHz CLK-FILTER Gaussian filter clock 8 MHz 32 MHz Table 2. Implementation results using a Xilinx Virtex XCV2000EFG FPGA. Number of Slices 6,099 Number of Slice Flip Flops 2,996 Total Number of 4 input LUTs 9,825 Number of bonded IOBs 136 Number of Block RAMS 52 Number of GCLKs 4 Total equivalent gate count for design 947,288 a XCV2000E FPGA board for our design, the BB controller. In our test set-up, we connected data and frequency value pins between two boards with cables to isolate the BB from the RF channel noise. The LM commands one to start paging and the other to start page scan, so a connection is established between them under the management of the state controller and the hop selector. To test the link controller, we inserted regularly an error bit to a received packet, we checked the average data rate decrease and the packet error rate difference between the data-medium rate (DM) packet coded with 2/3 FEC and the data-high rate (DH) packet not coded. Also, we intentionally kept RX buffer full in order to check the flow control. As a result, we get about a maximum 700 kbps in the NR-active state and about a maximum 5 Mbps in the HR-active state. VI. CONCLUSIONS Fig. 9. Test environment. IV. IMPLEMENTATION RESULTS The baseband controller was designed in Verilog-HDL and was verified using the Verilog-XL T M simulator. The Verilog description is mapped onto Xilinx Virtex XCV2000. Table 2 shows the implementation results. The total number of 4-input LUTs is 9,825, and the number of bonded IOBs, including test pins, is 136. Fiftytwo block RAMs, each 256-bit, are used for TX and RX buffers. The operating frequency is 32 MHz, and the total equivalent gate count for design is 947,288. V. TEST RESULTS As shown in Figure 9, the test board consists of a ARM7TDMI development board for LM software and In this paper, we present the design of a Bluetooth baseband controller that complies with Bluetooth spec. version 1.1 and supports an extended data rate. It is designed in Verilog-HDL and implemented using a Xilinx Virtex XCV2000. The implementation is tested with LM software executed in an ARM7TDMI. The designed baseband controller was verified to be fully functional to carry out the baseband protocols and other low-level link routines. It supports about a maximal 700 Kbps at a 1 Ms/s symbol rate 2-level GFSK and about a maximal 5 Mbps at a 4 Ms/s symbol rate 4-level GFSK. ACKNOWLEDGMENTS This work has been supported by the Electronics and Telecommunications Research Institute and partly by Brain Korea 21. REFERENCES

6 Design of Bluetooth Baseband Controller Using FPGA Sunhee Kim and Seungjun Lee [1] Gun Sang Lee, Je Kwang Cho, Jae Shin Lee, Suki Kim and Nam Ki Min, J. Korean Phys. Soc. 39, 14 (2001). [2] Ickjin Kwon and Hyungcheol Shin, J. Korean Phys. Soc. 40, 4 (2002). [3] Bluetooth Special Interest Group, The Bluetooth System: Part B: Baseband Specification Draft Ver1.1, (2000). [4] Bluetooth document/overview, com/v2/document/default.asp (1999). [5] Dan Sonnerstam, BLUETOOTH DOC. Document No. 1.C.40/0.9 (1998).