Design of a Transceiver for 3G DECT Physical Layer - Rohit Budhiraja
The Big Picture 2G DECT Binary GFSK 1.152Mbps 3G DECT M-ary DPSK 3.456 Mbps DECT - Digital Enhanced Cordless Telecommunications
Overview 2G DECT specifications and 2G transceiver 3G DECT specifications Issues in receiver design Digital FM demodulator Coherent detector for DPSK symbols Results and Conclusion
2G DECT Specifications Multi-Carrier TDMA TDD system RF carriers separated by 1.728 MHz (=B) each in 1880 MHz to 1938 MHz band B = 1.728 MHz F c -3B F c -2B F c -B F c F c +B F c +2B F c +3B f (MHz) Bit rate, R b = 1/T b = 1.152 Mbps GFSK modulation with BT b =0.5
2G DECT Specifications (contd.) t/t b Time domain waveform f (MHz) Magnitude spectrum Nominal frequency deviation of ±288 khz Allowed deviation limits: 70% to 140% of nominal
GFSK Transceiver Bit Stream @1.152 Mbps PAM A T b +A -A Gaussian LPF BT b =0.5 VCO @IF BPF PA (non-linear) F c -IF Acquisition/ Synchronisation/ Data Detection In Software SAW ADC PLL BPF LNA filter FM demod Hard limiter IF 1 -IF 2 F c -IF 1 Samples @ 2.304 MHz
TDMA Frame Structure in DECT 0 ms 5 ms 10 ms downlink uplink 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 Frame S-field A-field B-field Slot Guard Band S-Field: Synchronisation Field A-Field: Control Information B-Field: Data Packet Guard Band
3G Physical Layer Specifications Modulation Schemes : π/2 DBPSK, π/4 DQPSK, π/8 D8PSK π/2 DBPSK π/4 DQPSK π/8 D8PSK Constellation for differential PSK modulation
3G Physical Layer Specifications (contd.) Root-Raised Cosine with 50% excess bandwidth Symbol rate is 1.152 Msps Zero ISI at the output of the matched filter in the receiver Normalized timedomain waveform Power spectrum (frequency in MHz)
3G Physical Layer Specifications (contd.) Allowed combination of modulation schemes Configuration S-field A-field B-field 1a GFSK GFSK GFSK 1b π/2-db π/2-db π/2-db 2 π/2-db π/2-db π/4-dq 3 π/2-db π/2-db π/8-d8 S and A fields always employ π/2-dbpsk can be detected in a non-coherent GFSK receiver
DPSK Transceiver Root-raised cosine filter α=0.5 1010 Bits to symbol mapping Root-raised cosine filter α=0.5 Cos(2πf IF t) Sin(2πf IF t) + - F c -f IF BPF PA (linear) Bit Stream Signal processing in digital domain ADC BPF 2 SAW LNA In Software IF 1 +IF 2 F c -IF 1 Samples @ 3.456 MHz IF 2 = 9.504 MHz
Bandpass Sampling - choice of IF2 IF2 = 9.504 MHz = (5+0.5)*B; B = 1.728 MHz minimum sampling rate, F s = 2B = 3.456 MHz B=1.728 MHz -IF2-5B -4B -3B -2B -B B 2B 3B 4B 5B IF2 f (MHz) -5 π -4 π -3 π -2 π - π π/2 π 2 π 3 π 4 π 5 π w w c
I-Q Demodulation g k (n) 3 r(n) @ 3.456 Msps cos ( nπ/ 2) ( nπ ) sin / 2 y(n) @1.152 Msps g k (n) 3 j Carrier Frequency and Carrier Phase synchronization Clock Frequency and Clock Phase synchronization
Data Detection in the receiver a) non-coherent differential b) coherent differential Q Q θ I ϕ I θ > 0 => bit 1 θ < 0 => bit 0 ϕ = +90 degrees => bit 1 ϕ = -90 degrees => bit 0 - Transmitted constellation points - Received constellation points (in noise), y(n)
Performance of Different Demodulation Schemes
Tasks in the receiver Slot boundary acquisition on power-up/sync loss Clock recovery in every slot Frequency and phase offset estimation Data detection with adaptive carrier phase tracking
Signal Processing in Digital Domain @ 2.304 MHz S & A fields B field (if GFSK) Digital FM demodulator Acq & Sync/ data (for GFSK) Demodulated GFSK data Start of S-field Symbol clock phase (kt b /12, k=0,1 11) Bandpass samples @ 3.456 MHz S-field only Carrier frequency and phase offset estimator B-field Coherent data detector with adaptive phase correction Demodulated DPSK data
FM Demodulation An FM signal r(t) = A cos(2π f c t + φ(t)) ; φ(t) = 2 π k f m(τ)dτ+ 2π f t = A cos (φ(t)) cos(2π f c t) A sin (φ(t)) sin(2π f c t) x c (t) Instantaneous phase φ(t) = tan -1 (x s (t)/x c (t)) x s (t) Instantaneous frequency dφ( t) dt = 2πk m( t) + 2π f f
Digital FM Demodulator r(t) sampled @ 3.456MHz r(n) = x c (n)cos(nπ/2) - x s (n)sin(nπ/2) x c (n) = r(2n+1)(-1) n x s (n) = r(2n)(-1) n x c (n) and x s (n) are not samples at same time instant. Implementations constraints Output of Demodulator should be @ 2.304MHz. Interpolate x c (t) and x s (t) by 4 and then decimate by 3 ALSO For tan -1 ( ) samples of x c (t) and x s (t) should be at same time instant Decimate with different phases
Digital FM Demodulator (contd.) Tan -1 ( ) calculation Calculation of φ(n) = tan -1 (x s (n)/x c (n)) is computation intensive Table Look-up method Trade-off between computational complexity and memory requirement
Soft FM demodulator block diagram r( 2n) x c (n) x ci (n) x cd (n) 4 3 r(n) Demux n ( 1) n ( 1) tan 1 x x (n) sd cd ( n) ( n) φ g(n) Digital Differentiator r( 2n + 1) 2 4 z 3 x sd (n) x s (n) x si (n)
Signal Processing in Digital Domain @ 2.304 MHz S & A fields B field (if GFSK) Digital FM demodulator Acq & Sync/ data (for GFSK) Demodulated GFSK data Bandpass samples @ 3.456 MHz S-field only Carrier frequency and phase offset estimator Start of preamble Symbol clock phase (kt b /12, k=0,1 11) A and B fields Coherent detector with adaptive phase correction Demodulated DPSK data
Soft I/Q Demodulator g k (n) 3 r(n) @ 3.456 Msps cos ( nπ/ 2) ( nπ ) sin / 2 y(n) @1.152 Msps g k (n) 3 j g k (n) - root-raised cosine matched filter g ( ) ( ) k n = g nt kt b / 12, k = 0,1,2... 11, from clock recovery
Symbols in S-field y(n) = (I n +jq n ) e j(nα +θ), where α = 2π.δf.T s S-field (1-0 pattern) always DBPSK y(n)=[i o +jq o ][e jθ, e j(π/2+θ), e jθ), ], δf=0, θ 0 Q θ I
Estimation of δf y(n) = A.[I o +jq o ].[e jθ, e j(π/2+α+θ), e j(2α +θ), ] y 1 (n) = y(2n) = A.(I o +jq o ).e j(2nα+θ) y 2 (n) = y(2n+1) = A.(I o +jq o ).ejπ/2.e j((2n+1)α+θ)) For i=1,2 y i (n).y i* (n-1) = A 2 e j2α = A 2 [cos(2α)+jsin(2α)] Average y i (n).y i* (n-1) over αˆ the preamble to get an estimate of α, denoted by
Compensate for δf Estimation of θ y d ( n) = y( n) e jnαˆ = [ I n + jq n ] e j( α αˆ ) n+ θ [ I n + jq n ] e jθ Form two sequences z e (n), z o (n) z e (n) z z e e (2n) = (2n y d + 1) = (2n) e y d (2n jπ / 2 + 1) z o (n) z z o o (2n) = (2n y d + 1) = (2n) y d (2n + 1) e jπ / 2
Estimation of θ (contd.) z e (n) will be points from one of the following, z o (n) will be from the other Q Q π/2+θ I θ I The average of z e (n) or z o (n) will be small; the other sequence is used to estimate, θˆ
y ( n) Data detection with Phase Tracking u ( n ) v ( n) ˆd ( n) (. ˆ ˆ) e j n α+ θ * w n ( ) _ + v n * ( ) = w ( n). u( n) e( n) = dˆ ( n) v( n) w n w n u n e n * ( ) = ( 1) + µ. ( ). ( ) e (n )
Performance of the LMS Algorithm
Performance of the LMS Algorithm (contd.)
Performance of the Receiver Algorithm
Performance of the Receiver Algorithm
Conclusions Transceiver hardware design for 3G DECT physical layer was presented Issues involved in the receiver design were discussed Carrier synchronization algorithms were discussed Performance results of the receiver were presented