Performance Analysis of the D-STTD Communication System with AMC Scheme ()
1. Introduction
With the rapidly increasing demand for high-speed data transmission, next-generation mobile communication is expected to adopt a number of new technologies, including AMC, Orthogonal Frequency Division Multiplexing (OFDM), Multiple Input Multiple Output (MIMO), and Hybrid-Automatic Repeat Request (H-ARQ).
The AMC scheme varies the coding rate and modulation scheme according to the channel status to improve its transmission rate and has been employed in various high-speed wireless communication applications including Wireless Broadband (Wibro) Internet and High Speed Downlink Packet Access (HSDPA). And through continued technical developments, it is expected to be adopted by numerous wireless communication standards of the future. The D-STTD scheme complements the conventional STTD scheme in terms of throughput and its use is being widely discussed regarding various wireless communication standards.
2. AMC Scheme and D-STTD Scheme
The AMC scheme selects an optimal channel coding rate and modulation scheme based on the channel response data for signal transmission, creating a balance between error rate and throughput to improve the overall system throughput and transmission quality. Channel status has a significant effect on signal transmission and reception in the wireless mobile communication environment. Accordingly, a process referred to as link adaptation modifies transmission parameters to compensate for the losses caused by the changes in channel status. The AMC scheme is a type of link adaptation, and its structure is shown in Figure 1 [1-3].
Under a typical wireless communication environment, data is transmitted based on the processes of channel coding, interleaving, and modulation. Signal received via the communication channel is subject to the process of estimating the channel status according to SNR, and the original data is recovered through the reverse process in
Figure 1. Structure of AMC scheme.
Figure 2. Structure of D-STTD scheme.
the transmitter. The estimated channel information is returned to the transmitter, which determines the MCS level based on this information, adjusts channel coding, interleaving, and the modulation scheme suitable for the channel status to carry out transmission. Like above, the AMC scheme selects the MCS level according to the channel status to find a balance between error rate and throughput to help improve the overall system throughput and transmission quality.
The D-STTD scheme is a type of MIMO scheme and expands from the STTD scheme proposed by Alamouti [4-6]. Whereas conventional STTD scheme only provides diversity gain, D-STTD scheme offers multiplexing gain in addition to diversity gain, making it a very effective MIMO scheme. The overall transmission pattern of the D-STTD scheme is shown in Figure 2. The received signal of the D-STTD scheme can be expressed in a simple matrix equation as follows:
(1)
where r denotes the received signal and h, s, and n represent channel response, transmitted signal, and noise, respectively. Channel response hij denotes a channel response between j-th transmit antenna and i-th receive antenna. Each channel response is independent and identically distributed (i.i.d.) and displays a complex Gaussian distribution with an average of zero. Noise n is an Additive White Gaussian Noise (AWGN) with an average of zero and a distribution of σ2I.
From Equation (1) and Figure 2, it can be verified that the D-STTD scheme transmits four symbols during two time slots, which corresponds to twice the throughput and the multiplexing gain of the STTD scheme.
3. D-STTD Communication System with AMC Scheme
As a conventional D-STTD communication system uses a fixed channel coding rate and modulation scheme regardless of the channel response status, it can only select and use a low channel coding rate and a low-level modulation scheme in an environment that does not offer a certain level of SNR. On the other hand, a D-STTD communication system with AMC scheme selects a MCS level based on SNR information to flexibly apply the channel coding rate and the modulation scheme according to the channel response status, securing an optimal throughput at any given SNR.
Figure 3 shows the structure of a D-STTD communication system with AMC scheme. It can be seen that AMC scheme is incorporated into the D-STTD scheme explained in Chapter 2 to complement the system structure. The initially created data undergoes coding and interleaving processes based on the channel selected by the MCS level selector and is modulated with the modulation scheme also selected by the MCS level selector. The modulated signal is encoded by the D-STTD encoder for transmission over the channel and converted to an estimation of the original signal by the D-STTD decoder at the receiver. The signal then goes through the reverse process of the transmitter, and the original signal is restored.
4. Simulation Results
This chapter comparatively analyzes throughput of a conventional system and the D-STTD communication system with AMC scheme. Tables 1 and 2 list the MCS level selection criteria and the environmental parameters for the simulation experiment, which were prepared according to HSDPA and 3G Long Term Evolution (LTE) standards [7-9].
Figure 3. Structure of D-STTD communication system with AMC scheme.
Table 1. MCS levels.
Table 2. Simulation Parameters.
Figure 4 shows the throughput of a D-STTD 4×2 communication system for a fixed MCS level in the Rayleigh Flat Fading channel. As mentioned in Chapter 3, data is transmitted without loss for MCS level 1 (QPSK, Turbo Code 1/3) with the low SNR of roughly 2dB. However, at MCS level 4 (64QAM, Turbo Code 1/2), the data is transmitted without loss when SNR is 18dB or higher. If we assume that AMC scheme is not applied, a single coding rate and a specific modulation scheme must be selected, the coding rate and the modulation scheme that correspond to MCS levels 3 or 4 are inevitably excluded from the outset because they both have high probabilities of frame errors in low SNR range.
Figure 5 shows the MCS level selection probability of a D-STTD 4×2 communication system with the AMC scheme under the Rayleigh Flat Fading channel converted to an overall probability of 1. A low MCS level is selected for a low SNR range, and as the channel status improved along with the SNR, the probability of selecting a higher MCS level increased.