A Dynamic PCM Codec Selector for Different Working Environments Liang-Bi Chen1, Yuan-Long Jeang2, Tsung-Yu Ho1, and Ing-Jer Huang1 1
Department of Computer Science and Engineering, National Sun Yat-Sen University Kaohsiung, Taiwan, R.O.C. E-mail: {liangbi, tyho, ijhuang}@eslab.cse.nsysu.edu.tw 2 Department of Electronic Engineering, National Kaohsiung University of Applied Sciences Kaohsiung, Taiwan, R.O.C. E-mail:
[email protected] Abstract—Quality of transmission is very important in digital communication. The BER (bit error rate) of a PCM encoding method may be varied according to different working condition or different signal source. Theoretically, the BER of encoding method can be estimated mathematically. The BER is generally assumed as a constant value. The BER, however, is not deterministic in real world. In this paper, a Dynamic PCM Selector that can dynamically select an optimal PCM codec module is proposed. The system can dynamically switch to the encoding mode with possible lowest BER, according to the current type of the transmission. By way of this design, it can greatly reduce the overheads of the error correction process.
I.
Pe =
1 e rfc 2
Eb No
(1)
INTRODUCTION
PCM (Pulse Code Modulation) has become as the most popular modulation scheme for the transmission of analog information-bearing signal such as voice and video signals. Noise power may be varied due to the quality of transmission channel, external noise, and other interference factors. In most design the BER is assumed as a constant value. However, in real world the BER (bit error rate) is not a constant that is different from the assumption of most designs. Theoretically, the probability of the encoding error can be estimated by mathematical methods. However, in the real world it is not always true. In this paper, the dynamic PCM encoding selector (DPS) is proposed. The dashed block in Fig.1 shows the prototype of efficient PCM codec depending on the DPS. Several general PCM encoding modes including RZ (Return to Zero), NRZ (Non-Return to Zero), Manchester, AMI (Alternate Mark Inversion), NRZ(L), NRZ(M), NRZ(S), and Miller are provided in the PCM codec module. Besides, user can add or remove encoder/decoder to optimal PCM codec design using our PCM codec soft IP generator that is described later in this paper. Traditional PCM model were described in [2, 3]. The Pe (average probability of an error) can be estimated by using formula (1). Where Eb is energy per bit; N0 is average noise power, and erfc is complementary error function.
Fig. 1 The basic element of a PCM system: (a) Transmitter (b) Receiver
Table I shows the influence of Eb/N0 on the probability of error using NRZ signaling. The error rates presented in the last column of the Table I is under the assumption of a 105 bps bit rate. From Table I it is clear that there is an error threshold (at about 11dB). For Eb/Eo below the error threshold the receiver performance involves significant numbers of errors, and above it the effect of channel noise is practically negligible. In other words, provided that the ratio Eb/E0 exceeds the error threshold, channel noise has virtually does not influence on the receiver performance, which is precisely the goal of PCM. Based on the excluding statistical theory, the proposed design sets the average noise power N0 to a deterministic signal mode. The goal of this design is to determine noise value using actual transmission environment instead of using a fixed theoretical value. The main contribution of this proposal is that it enhances the reliability of the PCM codec in various noise levels, instead of using mathematical Pe value as its parameters.
Additionally, by selecting the lowest BER to determine the optimal selection, the quality of transmission can also be increased.
TABLE I. Influence of Eb/N0 on the probability of error Eb/N0
Probability of Error Pe
4.3dB 8.4dB 10.6dB 12.0dB 13.0dB 14.0dB
II.
−3 10 second
−4
−1 10 second
10
−6
10
−8
10
−10
10
−12
10
NRZ(M): A level change is used to indicate a mark (that is, a 1) and no level change for a space (that is , a 0).
z
NRZ(S): It is similar but the level changes are used to indicate a space or zero.
z
Miller: The signal bit value 1 represents transition in the middle of bit. The signal bit value 0 represents no transition if followed by a bit value 1, transition at end of bit if followed by a bit value 0.
For a Bit Rate of 105 b/s This is About One Error Every
−2
10
z
10 20 1 3
second second day months
PCM CODEC WITH DYNAMIC CODING SELECTOR
The proposed PCM codec IP module includes a PCM encoder, a decoder, and a dynamic PCM coding selector (DPS). With DPS module it selects the PCM encoding that has lowest BER to transmit signals. A. PCM Encoder whih Dynamic PCM Coding Selector Since baseband PCM digital signal transmission has distance restriction, it has to be used with modulator. Before PCM sending to a modulator, signals can be encoded to several PCM Encodings by PCM encoder. However, the choice between the PCM Encodings shall depend on the modulation method, the demodulation method, and the bandwidth limit. Several encoding methods are provided. Based on its PCM encoding methods, the related circuits is designed and implemented. The PCM coding detail circuit design are described in [1, 3, 6, 7, 8]. The PCM coding model is as following: z RZ: The signal level representing bit value 1 lasts for the first half of the bit interval, after which the signal returns to the reference level (0) for the remaining half of the bit interval. A 0 is indicated by no change, with the signal remaining at the reference level. z
NRZ: Pulses of positive and negative amplitudes represent bit value 1 and 0.
z
Manchester: A positive pulse followed by a negative pulse of equal amplitude and half-symbol width represents the signal bit value 1.
z
AMI: It uses three amplitude levels. For the bit value 0, the polarities of these two pulses are reversed. Else, positive and negative pulses of equal amplitude are used alternately for bit value 1, and no pulse is always used for bit value 0.
z
NRZ(L): It is the most common mode of NRZ transmission, due to the simplicity of the transmitter and receiver circuitry.
Fig.2 shows a PCM encoder with DPS design. Some of their implementations need to reference other encoding methods, the related circuits is design and implemented. The DPS can be converted to different PCM Encodings depending on modulation methods, demodulation methods, and bandwidth limit. The DPS selects the encoder that can produce lower BER under current operational environment. It consists of an error detector, an error counter, an error comparator, and a feedback selection controller: z Error detector: the error detector detects the position of an error bit according to the encoding method. z
Error counter: the error counter concretizes and digitizes the system performance. It is very important for raising the confidence of the communication system. With error counter, the number of occurrences of error for each PCM encoding method can be clearly recorded.
z
Error comparator: the error comparator compares error rate under current environment can be identified. The feedback selection controller according to the result of error comparator.
z
Feedback selection controller: the feedback selection controller can be used to select a PCM encoding method of transmission. The main idea of DPS design is that the design enhances the reliability of the PCM codec according to different noise level, while the traditional design uses the mathematical Pe value as its parameters. Additionally, by using BER to determine the optimal selection, the quality of transmission can be increased.
Fig.2 PCM Encoders with DPS
B. Dynamic PCM Decoder Fig.3 shows a dynamic PCM decoder, the input signal of transmission channel end is a sequential signal. Therefore, a multiplexer/de-multiplexer pair is designed to select the encoding method with related signals as decoder’s input signal, and then decode it, followed by the RX end receive the digital signal from TX end. (a) PCM encoder generator
(b) PCM decoder generator
Fig.3 Dynamic PCM decoder
C. Optimal PCM Codec Soft IP Generator The optimal PCM codec IP generator is provided. The generator can help dynamic PCM codec design to quickly and precisely generate commonly used codec modules. It can quickly integrate IP generator with design parameter to achieve better demand. The parameters of the design are shown in Table II. The algorithm of an optimal PCM codec IP generator is shown in Fig.4. The GUI Interfaces are shown in Fig. 5.
(c) Optimal PCM codec Generator (d) PN code generator Fig. 5 GUI interface for Soft IP generate
III.
EXPERIMENTAL RESULTS
Fig. 6 shows the demo environment of the system verification. This environment has built in several experimental modules, control switches, LEDs, and 7-SEG DISPLAYs that help to conduct physical verification conveniently.
TABLE II. The design parameter of optimal PCM codec Design parameters
EN DE ER PNC CMD ASO
Explanations Selection of PCM encoder type, one or more Selection of PCM decoder type, one or more Output of error encoding detection. Used to observe error rate’s test vector. Produce needed PN code data width and working type. Use IP generator’s default value. Used to select the optimal codec. Record the relationships of varies PCM types.
Algorithm: An Optimal PCM Codec IP Generator Input: Several options of PCM codec methods Output: An Optimal PCM codec IP begin Check user’s selection of function names, and record them For (n=0;n
