Design of a Dynamic PCM Selector for Non-deterministic Environment Liang-Bi Chen and Ing-Jer Huang
Yuan-Long Jeang
Department of Computer Science and Engineering National Sun Yat-Sen University Kaohsiung, Taiwan, R.O.C. {liangbi, ijhunag}@eslab.cse.nsysu.edu.tw
Department of Information Engineering Kun Shan University Tainan, Taiwan, R.O.C.
[email protected]
Abstract—The quality of transmission is very important in digital communication. However, in non-deterministic environment or different transmission message signal, bit error rate of PCM encoding methodologies maybe be varied according to different working condition or different signal source. Theoretically, the bit error rate of PCM encoding methodology can be computed by either mathematical or statistical methods. The bit error rate is generally assumed as a constant value. The bit error rate is not deterministic in the real world. This paper proposes a dynamic PCM selector that can dynamically select an optimal PCM codec module. The system can dynamically switch to the encoding mode with possible lowest bit error rate, according to the current type of the transmission. By way of this design, it can greatly reduce the overheads of the error correction process. Keywords—PCM, Pusle Code Mudulation, BER, Bit Error Rate, PCM codec optimization, digital communication, speech encoding, soft IP generator
I.
INTRODUCTION
PCM (Pulse Code Modulation) has been widely used in digital commutation applications. The PCM has become as the most popular modulation scheme for the transmission of analog information-bearing signal such as voice and video signals. However, the noise power may be varied due to the quality of transmission channel, external noise, and other interference factors. In most codec design, the BER (bit error rate) is assumed as a constant value. However, in real world the BER 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. 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, No is average noise power, and erfc is complementary error function. Pe =
1 e rfc 2
Eb No
(1)
Table I shows the influence of Eb/N0 on the probability of error using NRZ signaling. The error rates presented in the
1-4244-0387-1/06/$20.00 ©2006 IEEE
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 No 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
4.3dB 8.4dB 10.6dB 12.0dB 13.0dB 14.0dB
Probability of Error Pe −2
−3 10 second
−4
−1 10 second
10 10
−6
10
−8
10
−10
10
−12
10
For a Bit Rate of 105 b/s This is About One Error Every
10 20 1 3
second second day months
In this paper, the PCM codec with 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 dynamic PCM codec soft IP generator that is described later in this paper.
APCCAS 2006
Fig. 1 The basic element of a PCM system: (a) Transmitter (b) Receiver
II.
DESCRIPTION OF DYNAMIC PCM CODING SELECTOR FOR DYNAMIC PCM CODEC The proposed dynamic PCM codec module includes a PCM encoder, a dynamic decoder, and a DPS (dynamic PCM coding selector). With DPS module it selects the PCM encoding that has lowest BER to transmit signals. The proposed dynamic PCM codec module supports the following features: z
User can easily add or move the provided PCM encoding/decoding methods and their own PCM encoding/decoding methods by following the standard interface. When users remove the unwanted codec, the related internal modules will be removed automatically.
z
In real working environment, it can successfully eliminate combinational logic’s glitch [1], with realtime optimal PCM selection. The system can switch to current optimal PCM encoding method, instead of waiting for a couple of cycles.
z
The circuit shown in Fig.2 is the circuit of AMI encoder that it supports bipolar signal transmission mode. It consists of AMI encoder’s backend, decoder’s front end and related OP-AMP apologue circuit that will produce the needed signals.
Fig.2 AMI Encoder
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 or methods are described in [1, 3, 6, 7]. Several general PCM encoding methods 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. Fig.3 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.
Fig.3 PCM Encoders with DPS
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. B. Dynamic PCM Decoder Fig.4 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 bit stream digital signal from TX end.
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
