CHAPTER 6: DEVELOPMENT OF ENHANCED ADAPTIVE ARTIFICIAL
6.2. Proposed Adaptive Neural Predictor
In this part of research work, a hybrid neural predictor which combines the ADALINE with MLP ANN is employed for optimal signal power prediction in micro-cellular LTE radio networks. Related approach has been employed in [191], but for signal noise filtering, reduction and prediction in Rayleigh fading channel. Particularly, while Gao et.al. [191]
concentrated on effective noise reduction in simulated temporal signal data, our focus is combining the faster learning capability of ADALINE with MLP for optimal training and improved prediction accuracy of realistic spatial signal data. Unlike in [191], the Minimum Mean Square Error (MMSE) is considered to solve the neural network structure complexity for optimal prediction.
This research drive is to find an efficient predictive model that combines a linear predictor and a non-linear predictor for adaptive prediction of large scale signal power. Also,
120 to find equivalently signal propagation loss with minimal error in any radio propagation environment.
6.2.1. Proposed Artificial Neural Network Architecture
The respective feedforward neural network architecture of ADALINE (the linear predictor) and MLP (the non-linear predictor) are shown in figures 6.1 and 6.2 respectively.
The z−1 designates the structural delay element to enable the afore-mentioned scale and re- sample the reference input data at different rate. Assuming that the ADALINE has n input nodes x1, x2, x3,.xn which are the signal power sample number to be predicted, the ADALINE prediction output can be expressed as:
Figure 6.1. ADALINE neural-network [73].
1 n
j j
y w x
(6.1) where xn and wjx are the expected value and weights sum of the linear neural predictor.To enhance the neural training structure, the Minimum Mean Square Error (MMSE) technique is employed at the input nodes as [192]:
n 2
2
j 0 0
n
n n
j
E y y e
(6.2) where y
is the nth desired output prediction response.
121 Figure 6.2. Multi-layer perceptron neural-network with one Hidden Layer [65].
The input data are transmitted through the layers in a forward direction. Figure 6.2 is a single (i.e. one) hidden layer MLP neural network architecture and output can be expressed as:
0 0
n n
i o jq h iq i
j i
y f w f w x
for q=1, 2, ... n (6.3)where wiq and wjq indicate the respective connection weight and synaptic weight vectors in relation to hidden layer neuron-inputs, and from hidden layer-output neuron. xi stands for the input vector elements, i =1…. n, fo and fh are the respective output layer and hidden layer activation function. The MLP learning and training process entails minimizing the error function (i.e. cost function), which can be expressed as:
N
q q
q d eq
y w
E
1 2
2 1 2
) 1
( (6.4)
where eq = yq – dq, yq and dq indicate desired target output value and actual network value, respectively.
122 The set-up of the proposed adaptive hybrid predictor (figure 6.3) is such that the measured signal power values are first trained with ADALINE system predictor and the resultant output fed into the MLP predictor for further training to deliver optimal prediction accuracy. The adaptive hybrid predictor is trained using three different fast backpropagation algorithms discussed in chapter 4: (i) Resilient propagation (RP), (ii) Leverberg-Marquardt (LM), and (iii) Bayesian regularization algorithms to establish the algorithm that best train the hybrid predictor and predict the behaviour of the signal strength with most favourable convergence results and performance.
Figure 6.3. The Proposed Hybrid Adaptive Neural structure.
The field measurements are piloted using LTE cellular networks that operate at 1900 MHz frequency band between JAN-DEC 2017. A drive-test tools and measurement technique as described in chapter 5 are used and some key computer software such as MapInfo, MATLAB 2013a and excel spread utilized for post-processing of acquired signal testing log files and data analysis. The LTE mobile phones and the HP laptop are both engrained with Test Mobile Software (TEMS) which enables it to access, record and extract the signal data along the measurement test routes. The Global Positioning System (GPS) and compass are used for matching up the Mobile Station (MS) (i.e. user equipment) measurement locations in correspondent to field test environment and the Base Station (BS) transmitter. The main LTE signal parameter extracted from the drive test log file for ANN prediction analysis is the Reference Signal Receive Power (RSRP dB m). This provides signal strength information at the UE receiver terminal. The field signal power measurements are carried out in 5 selected
123 locations with special concentration on built-up busy urban streets, roads and open areas with mixed residential and commercial building structures. The first 4 locations, i.e. locations 1-4 are built-up urban terrains with mixed residential and cluttered building structures. The remaining 1 location is an uncluttered open urban terrain with many commercial buildings, few residential buildings, country motor parks, petrol filling stations and among others. The
All the written neural network program, simulations and computations are achieved using the MATLAB2013a software. To avoid over-fitting, which often reduces ANN predictive and modelling capability, the early-stopping technique has been employed. In view of that, the data-set are shared into three parts using ratio 75%: 15%: 15% for training, testing and validation respectively. The input data-sets are normalized to enhance the generalization of ANNs as expressed [193].
max min
min
V V
V V
nV
o
(6.6)where Vo, Vn, Vmax and Vmin are original value of the parameter, normalized value, maximum parameter value and minimum parameter value respectively.
The results of ANNs prediction accuracy are measured in terms of four statistical performance indicators: RMSE, MAE, SD and r. Both MAE and RMSE express the mean error magnitude between actual observation and prediction result. The SD articulates the measure of dispersion between actual observation and prediction result while r measures the strength of relationship between actual observation and prediction result.