Proposal of Adaptive Downlink Modulation Using OFDM and MC-CDMA for Future Mobile Communications System

To meet the strong demand for broadband multimedia services by both nomadic and mobile users, it is important to increase the bit rate of future mobile communications systems. To enhance system capacity, novel technologies or new concepts for improving bandwidth efficiency are indispensable. In this paper, an adaptive downlink modulation using OFDM and MC-CDMA is presented for maximizing the system capacity and actualizing QoS (Quality of Service) control of future mobile communications. The basic concept of the proposal method is that a time frame is divided into two sub-frames, one is for OFDM and the other is for MC-CDMA, and the base station (BS) allocates a preferable modulation scheme to each user per each time slot in accordance with their service requirements and link conditions such as the received signal strength indication (RSSI) level and interference signal strength [1].The detailed concept of the proposed adaptive downlink modulation technique and slot allocation algorithm is discussed in this paper. Computer simulation was also conducted to evaluate the throughput performance of the proposed system. The simulation results show that the throughput performance of the proposed system is better than that of the system using OFDM or MC-CDMA only. 1. Technology Concept Many modulation schemes such as Single Carrier (SC), CDMA, OFDM, and Multi-Carrier (MC)-CDMA have been proposed for mobile systems, nomadic wireless access and fixed wireless access [2]-[5]. The selection of radio interface depends on the specifications of the system. OFDM is an attractive modulation scheme because of its high immunity to multi-path fading and its capability of offering a high transmission rate. However, the link quality of the OFDM system could be degraded when the co-channel interference signal strength from adjacent cells is increased. OFDM and CDMA combined modulation schemes such as MC-CDMA and MC-DS/CDMA are attractive techniques that increase the process gains in the frequency domain and time domain, respectively [2]. In addition, the OFDM and CDMA combined scheme offers high transmission rate under multi-path fading environments and mitigates co-channel interference from adjacent cells. However, it is difficult to enhance its transmission rate per user by restricting the allocated bandwidth when the spreading factor (SF) is very large. As explained above, each modulation scheme has distinct physical features. Namely, schemes have advantages and disadvantages in accordance with channel conditions such as the Carrier to Noise Ratio (CNR), Carrier to Interference Ratio (CIR), delay spread, and other parameters. The time and frequency spreading technique proposed in [3] is one method for adjusting the spreading factors in accordance with channel conditions. However, this method requires both a time and frequency spreading function for the user terminal and the base station has to be able to manage the complicated spreading codes to maintain the orthogonality between users. The adaptive downlink modulation scheme using OFDM and MC-CDMA is a candidate for maximizing the system capacity of future mobile communications systems [1]. The basic concept of the method is that a time frame is divided into two sub-frames, one is for OFDM and the other is for MC-CDMA. In this scheme, the BS allocates a preferable modulation scheme and its parameters such as spreading factor, sub-carrier modulation and coding rate to each user per each time slot in accordance with the RSSI level and interference signal strength. As the hardware structures of OFDM and MC-CDMA are basically identical, the hardware complexity of the proposed method is much smaller than that of the time and frequency spreading techniques. In addition, the BS will be able to allocate the modulation scheme and its parameters depending on user request (QoS). Using the adaptive downlink modulation scheme, the same channel frequency will be reused in every radio cell by allocating the OFDM scheme to users located in optimum RSSI and CIR conditions and the MC-CDMA scheme to users in harsh conditions, respectively. This feature also offers different service quality to different users in accordance with their requirements and channel conditions. By allocating the modulation scheme and its parameters (mapping pattern, coding rate, spreading factor, etc.) adaptively, users will be able to maintain their communications even in harsh wireless environments. 2. System Configuration Figure 1 presents a frame structure of the adaptive downlink modulation scheme using OFDM and MCCDMA. In this figure, a frame is divided into multiple slots. Some slots are allocated to OFDM and others to MC-CDMA. The transmission power for OFDM slots and MC-CDMA slots is set to be identical to maintain the continuity of the signal level between two modulation schemes. Figure 2 presents a selection algorithm for the modulation scheme and its parameters. When the CNIR of the channel is high and the distance of the wireless link is short (RSSI level is high), the BS assigns an OFDM slot with high rate sub-carrier modulation such as 16QAM with a high coding rate. If the CINR is very low, the BS allocates an MC-CDMA slot with high spreading factor and low coding rate to maintain the communication link. The concept of this algorithm is based on the combination of adaptively allocating the radio interface and adaptive selection of its parameters. Moreover, the selection of the modulation scheme and its parameters will also be established with regard to the user’s QoS. Consequently, the adaptive downlink modulation scheme will maximize the system capacity for wireless communications systems and respond to a user request by allocating a preferable modulation scheme to each time slot per user. In this case, service areas of OFDM slots should be restricted around the BS and not overlap. Therefore, the same channel frequency can be allocated in every cell, which will enhance the efficiency of channel utilization. In contrast, the service area of MC-CDMA will be overlapped because co-channel interference between adjacent cells is mitigated using the spreading code in the frequency domain. The selection of the spreading code per user should consider the orthogonality between the other codes used in the same cell. As the same service areas of MC-CDMA signals are deployed as the current cellular systems, users will be able to establish their communication link in high mobility environments. 3. Computer Simulation Computer simulation was conducted to evaluate the throughput performance of a wireless communications system using adaptive down link modulation technology. The parameters used in the simulation are summarized in Table 1. In each frame, OFDM or MCCDMA slots are assigned to users independently in accordance with channel conditions under fast Rayleigh fading environments. Figure 3 shows the allocation diagram of the modulation scheme per each user. The users close to the BS (RSSI level is high) should be allocated OFDM to provide higher bit rate, and other users far from the BS (RSSI level is low) should be allocated MC-CDMA to enhance immunity to cochannel interference. If the number of OFDM slots is insufficient, MC-CDMA should be allocated to the users. At the same time, the modulation type for subcarriers, coding rate and spreading factor (MCCDMA), is selected at each slot by monitoring the RSSI and CIR level of the control channel transmitted from BS. These procedures actualize a QoS control that allocates high-speed data channels for the users located near the BS. Figure 4 shows a selection diagram of modulation type and coding rate in relation to the RSSI level and CIR. The RSSI threshold level that achieves the BER of 10 is derived by assuming the noise figure (NF) = 4 dB, absolute temperature = 290 K and bandwidth = 40 MHz. From Fig. 4, it is obvious that high throughput performance will be obtained when the RSSI level and CIR are high. Figures 5-7 show the selection diagrams of modulation type and coding rate in the same manner when the users are assigned the MC-CDMA system. For MC-CDMA, the spreading factor should be changed in accordance with the number of users, because the spreading factors should be greater than the number of users. Therefore, the modulation and coding type in relation to the RSSI level and CIR would be changed in accordance with the number of users assigned the MC-CDMA system. In these figures, the modulation schemes and coding rate OFDM MCCDMA OFDM MCCDMA MCCDMA 1 frame 1 frame 1 frame OFDM multiple slots MC-CDMA multiple slots Figure 1: Frame structure