Multimode PLL for Adaptive Modulation Scheme in Satellite Communication † Kenta Umebayashi, ‡ Robert H. Morelos-Zaragoza and † Ryuji Kohno, † Yokohama National University ‡Department of Electrical Engineering, Graduate School of Engineering San Jose State University
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Abstract This paper describe the application of the multimode PLL which can recover the carrier and identify the modulation schemes whitout any supplementary information. To design the multimode PLL for Adaptive Modulation Scheme (AMS), one of the problems is modulation identification error by noise, initial phase offset and the positions of the transmitted signals bias in I-Q plane. This paper proposed two methods which are Sub-Phase Lock Detector (S-PLD) and Status Transition Model (STM) to solve the previous problem. The results show the proposed methods can decrease the modulation identification error effectively. In addition, this work shows blind approach can apply to SDR.
Keywords Software Defined Radio, Carrier Recovery, Modulation Identification, Adaptive Modulation
1. Introduction Recently, a multimode transceiver, known as Software Defined Radio (SDR) is receiving much attention in the wireless communication field [1]. Such multiple functions in the physical layer of a SDR can be configured such that modulation scheme, data rate, coding scheme and so on. Specifically, multiple modulation schemes can be considered for adaptive modulation or hierarchical modulation. Thus such a multimode transceiver, a function to identify a change operation is necessary. For this problem, there are two main approaches, which can be classified as the non-blind and the blind techniques. The non-blind approach employs supplementary information to identify the mode change. On the other hand, in the blind approach, there are some identification algorithms instead of using such a supplementary information. In fact, this approach can be considered as a concept in wireless communication, since it presents the possibility of realizing a universal transceiver, which does not require supplementary information, and estimates the environment to reconfigure to the optimum configuration still on research. For the multimode transceivers, a ”Multimode PLL” which can achieve not only carrier recovery but also blind modulation identification has been proposed in [1][3], here, basic performances which are the carrier lock probability and the acquisition time are discussed. However, the studies of the applications for the multimode PLL are not articulate are incommensurate. Therefore, this paper focuses on Adaptive Modulation Scheme (AMS) as the application of multi-mode PLL. Considering AMS, the multimode PLL has some problems which causes modulation identification errors. This paper clearly shows the causes which are the initial phase off-
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set and the positions of the transmitted signals bias, in addition, categorizing the error models to make easy to take measures to meet the problems. By the aid of the error models, this paper proposes two methods to solve the modulation identification error problem. One of the methods is the Sub-Phase Lock Detector(S-PLD) which notices the differences of the signal positions in I-Q plane among the modulation schemes. The another proposed method is the State Transition Model (STM) for the carrier recovery and the modulation identification. In the conventional carrier recovery process, there are two states which are “Initial Lock Acquisition” and “Tracking”. In the multimode PLL, not only the carrier lock detection but also the modulation identification have to be considered. In addition, the both detection and identification are performed at the same instant. Improving the success rate of the modulation identification, observation period denoting window size has to be longer, however, when window size is longer, the robustness for carrier offset deteriorates. On the other hand, in the proposed STM, there is a new state denoted as ”Tentative Lock” where the first lock detection is confirmed again. This can be considered to be equivalent to the case when longer the window size. The results show these proposed methods ”S-PLD” and ”STM” are efficient to improve the performance of the multimode PLL. The rest of this paper is organized as follows. In section 2 the system layout and assumptions made are shown. The configuration of the multi-mode PLL and the modulation identification logic is discussed in detail. Next, in section 3, the proposed methods ”S-PLD” and ”STM” are explained, and evaluated in an adaptive modulation scheme with computer simulations. Finally, the conclusions are drawn in section 4.
2. System block diagram and assumptions 2.1. Carrier recovery aided by modulation identification in mulitimode PLL A simplified block diagram of the proposed multi-mode PLL is shown in Fig.1, where the two main functions, modulation identification and carrier recovery are illustrated. SW denotes a switch opening or blocking the carrier recovery loop. NCO is a Numerically Controlled Oscillator that outputting the signal sl (n) used to cancel the frequency offset. The received complex baseband signal R(t) is shown with continuous-time value as follows, R(t)
=
exp[−j(φ + 2π∆f t)] ∞
s(i)H(t − iTs ) + g(t),
(1)
i=−∞
where φ and ∆f are the phase offset and the frequency offset, respectively, φ is the initial phase offset which is the differ-
Q
Digital PLL From matched filter sin/cos table
π/8 π/8
BPSK PLD
π/16
8PSK PLD
QPSK PLD
16QAM PLD
I
BPSK PLD
8PSK PLD
QPSK PLD
NCO
π/8
π/4 I
r’(n)
SW
Q
π/8 π/2
Bank of phase lock detector
slo(n) r(n)
Q
π/4
I/Q symbols
I
Q
Modulation identification
Lock Area
LPF
Modulation ID (LOCK)
Bank of PED
Non-lock Area
a
Control Signal
2a
2a
I
16QAM PLD
Figure 1: Multimode PLL Figure 2: Each Phase Lock Detectors (PLD) configuration
r(n) = exp[−j(φ + 2π∆f Ts n)]s(n) + g(n)
(2)
Let ∆f be small compared to the symbol rate Rs , so that the received signal lies within the capture range of the digital PLL. This condition is equivalent to ∆f Ts T HS−P LD1,2 the counter is increased by 1. T HS−P LD1,i is denoted as the amplitude thresholds for S − P LD1, the T HS−P LD1,i are set to 16QAM signal points, these are shown Fig. 6. The counting result during a window is denoted as NCS−P LD1 . In the M-ary PSK, modulation iden-
tification rule or Lock rule is changed when: ”NC > NT and NCS−P LD1 < NTS−P LD1 ” where NTS−P LD1 = 2.
#1 #2 Tentative Lock
Initial Lock Acquisition
Q #3
#5
Tracking
Lock Area #4
Figure 8: StateTransiton Model
Non-Lock Area 1. Initial lock Acquisition state (a) Function; Modulations identification block is working to identify the modulation scheme. SW is open.
Lock Area I THS-PLD1,1
THS-PLD1,2
Figure 6: S-PLD for ER − M ODEL1 Fig. 7 shows Es/No vs Modulation Identification Error Rate characteristic, when S-PLD1 is used. Comparing Fig. 7 and ??, the ER characteristic is clearly improved due to S-PLD1. Correspondingly, S-PLDs are designed for each error models ER − M ODELi . By using S-PLD, the modulation identification error characteristic is improved effectively. 1
’file2_1_4.dat’ u 1:14 ’file2_1_4.dat’ u 1:15 ’file2_1_4.dat’ u 1:16
ER
QPSK BPSK 8PSK
(a) Function; SW is closed, and the multimode PLL is working by using the modulation identification result MOD1 which is obtained in state ”1: Initial lock state”. During a window, the modulation identification in this state is reconfirmed that it is correspond to the previous result MOD1.
3. Tracking state
0.001
0.0001
2. Tentative lock state
(b) Rule to change state; During a windows the modulation identification result is MOD1, the state is changed to ”3: Settlement lock state”. Otherwise, the state is returned to ”1” again.
0.1
0.01
(b) Rule to change state; If any modulations scheme is identified in a window, flag signal has to be ”1” and state is changed to ”2: Tentative lock state”. If result of the modulation identification is ”non”, flag signal is ”0”, and the state is not changed.
0
10
20
30 40 50 60 Initial Phase offset [degree]
70
80
90
Figure 7: Initial Phase offset vs Modulation Identification Error Rate, Modulation: 16QAM, In multimode PLL, S-PLD1 is added. normalized frequency offsets ∆f Ts = 0.001
3.2. State Transition Model for Modulation Identification The simple method to decrease the modulation identification error is to increase the window size. However, if the window size is increased, the maximum frequency offset which is shown in expression (2) has to be small. Therefore, we propose the ”State Transition Model (STM)” which is shown in Fig. 8. In STM, unlike the conventional design, there exists a ”Tentative Lock” state between the ”Initial Lock Acquisition” and the ”Tracking” states. The effect of this unique state is to increase the window size while keeping the maximum frequency offset. The rules of the state transition are stated as follows;
(a) Function; SW is closed, and the PLL is designed according to the modulation identification result. In each window, the modulation scheme is checked. In this case the modulation scheme is assumed to be MOD1. (b) Rule to change state; If the modulation identification result is changed from MOD1, the state is changed to ”1”. The modulation identification result is kept as MOD1, the state is kept as ”3”. In Fig. 9 and 10, two important evaluations, i.e. ER and TN onLOCK are shown. In both figures the x-axis is the carrier offset ∆f Ts , and the y-axis is ER and TN onLOCK , respectively. A dot line means that STM is not used in the multimode PLL, a solid line means that STM is used which is the proposed scheme. In both figures, the region where the carrier offset is less than 0.0016, the characteristics of the proposed scheme are better than the conventional scheme. It can be said that STM is effective in the multimode PLL for AMS.
0.01
0.001
ER
Multimode PLL without proposed state transition
0.0001 Multimode PLL with proposed state transition 1e-05
1e-06
0
0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014 0.0016 0.0018 0.002 Carrier Offset (Normarized by Symbol Rate)[Hz]
Figure 9: Carrier offset ∆f Ts vs ER characteristic 0.3
Non detection rate
0.25
Multimode PLL without proposed state transition
0.2
0.15 Multimode PLL with proposed state transition
0.1
0.05
0
0.0002 0.0004 0.0006 0.0008 0.001 0.0012 0.0014 0.0016 0.0018 0.002 Carrier Offset (Normarized by Symbol Rate)[Hz]
Figure 10: Carrier offset ∆f Ts vs TN onLOCK characteristic 1
4. Conclusion This paper showed the problem of the modulation identification error by initial phase offset and the positions of the transmitted signals bias. To solve the problem, the models of the modulation identification error were categorized. S-PLD and STM are proposed using the models of the modulation identification error with the aid of the error models. Simulation results also show that these proposed methods can improve th emodulation idenfication error characteristic.
5. References [1] J. Mitola, ”The software radio architecture,” IEEE Commun. Mag., vol.33, no.5, pp.26-38, May 1995. [1]
R.H. Morelos-Zaragoza, “Joint Phase-Lock Detection and Identification of M-PSK/M-QAM Modulation, ” Proc. 2000 IEEE Third Generation Wireless Communications Conference, pp. 272-279, San Francisco, CA, June 15, 2000
[2]
R.H. Morelos-Zaragoza, Kenta Umebayashi, Ryuji Kohno “A Method of Non-Data-Aided Carrier Recovery with Modulation Identification, ”Proc. 2001 IEEE Global Telecommunication Conference, pp. 3375-3379, San Antonio, Texas, November 28, 2001
[3]
Kenta Umebayashi, R.H. Morelos-Zaragoza, Ryuji Kohno 1
“Considering the PLD of a Multi Mode PLL for Modulation Identification ”Proc. 2002 International Symposium on Information Theory and Its Applications, pp. 595-598, Xi’ October 29, 2002
