Efficient MAC Protocol for Subcarrier-Wise Rate Adaptation over WLAN
© SungWon Kim et al. 2010
Received: 1 October 2009
Accepted: 18 May 2010
Published: 16 June 2010
While bit-loading algorithms over wireless systems have been extensively studied, the development of a protocol which implements bit-loading-based rate adaptation over wireless systems has not been highlighted. The design of such a protocol is not a trivial problem, due to the overhead associated with the feedback information. In this paper, a novel protocol is proposed to provide an efficient way to implement subcarrier-wise rate adaptation in OFDM-based wireless systems. When receiving a Ready-To-Send (RTS) packet, the receiver determines the number of bits to be allocated on each subcarrier through channel estimation. This decision is delivered to the sender using an additional OFDM symbol in the Clear-To-Send (CTS) packet. That is, bit-allocation over subcarriers is achieved using only one additional OFDM symbol. The protocol enhances the channel efficiency in spite of the overhead of one additional OFDM symbol.
Wireless communication is experiencing an explosive growth of rate demand. The high demand for wireless communication services requires increased system capacity. Orthogonal Frequency Division Multiplexing (OFDM) is a promising technology allowing wireless networks to provide high spectral efficiency into relatively small spectrum bandwidths. The attention has been focused on the application of the OFDM in the wireless local area network (WLAN). The existing examples of these systems are IEEE802.11a and HIPERLAN-2 standards which have chosen OFDM as a modulation scheme due to its good performance in multipath fading environment and its robustness against intersymbol interference.
Conventional rate-adaptive OFDM-based wireless systems use a fixed constellation size and power level over all subcarriers, as in the case of the IEEE802.11a standard [1–3]. However, all or some of the subcarriers experience different channel conditions due to multipath fading. As a consequence, applying a different constellation size (or number of bits or data rate) to each subcarrier according to its channel condition provides more reliable and efficient data transmission than applying the same constellation size to all subcarriers.
The allocation of a unique number of bits to all subcarriers in an OFDM symbol, called bit-loading, has been used in wired communication systems, such as digital subscriber lines (DSLs). Since the wired channel is slowly time-varying, the receiver can provide reliable channel state information to the transmitter using robust feedback channel. Therefore, adaptively loading the carriers seems to be an interesting approach for increasing the channel utilization. The theoretical channel capacity can be achieved by distributing the total transmitted energy according to the water-filling principle . However, this distribution has computational complexity and assumes infinite granularity in constellation size. In the realistic case where a finite granularity in constellation size is required, the rounded bit distribution obtained starting from the water-filling solution could still not be the optimum. Some suboptimum algorithms to reduce the complexity have been proposed in [5–7].
The performance of wireless networks is degraded by the adoption of the bit-loading scheme, since, unlike wired channels, wireless channels have fast time-variant property. The fast time-variant nature of wireless channels requires more frequent changes in the number of bits allocated in the subcarriers. Furthermore, since the sender and receiver have to share this allocation information, frequent exchanges of the bit-loading information between them are required. This increases the overhead and, as a consequence, degrades the network performance.
The problem of increased feedback overhead in centralized networks such as cellular systems may be less severe than that in distributed networks, since all of the terminals communicate with an Access Point (AP) and the AP can control the feedback information without disturbing the ongoing traffic. In addition to this, terminals in centralized networks can periodically send Channel State Information (CSI). However, in distributed networks such as ad hoc networks, the communication occurs in a peer-to-peer way and there is no such arbitrator. Therefore, since each communication pair has to exchange its CSI, more feedback packets are generated than in centralized networks. In addition to this, since the feedback packets are not controlled by an AP, collisions can occur with ongoing packets. Furthermore, the volume of the feedback information, including the subcarrier condition, increases as the number of subcarriers increases. Therefore, in order to utilize subcarrier-wise bit allocation in wireless ad hoc networks, an efficient protocol with minimum feedback information is required.
However, researches in this area have focused on allocating the optimal energy and rate and reducing the complexity of the bit-loading calculation itself [8–13] with the assumption of the availability of the feedback channel information. On the other hand, little effort has been made to design efficient protocols for bit loading in wireless networks. Designing an efficient protocol is not a trivial problem, because not only is the feedback information quite voluminous but also the time-varying wireless channel requires frequent transmissions of such large amounts of information. The research in  targeted centralized networks; so it assumed the existence of a central node which knows all the channel conditions of all member nodes. Moreover, due to the slow fading channel model used in this research, it does not suffer from feedback overhead. In the method proposed in , only the strongest subcarriers are used with high-order constellation to meet the target total data rate. The receiver informs the sender of the identification numbers (IDs) of the subcarriers which will be used for the next data transmission. Even though the feedback overhead is reduced to two OFDM symbols, the method in  cannot fully utilize all of the subcarriers. In effect, some of the chosen subcarriers may not be strong enough to deal with the high-order constellation. In addition, a separate feedback channel is used in . Thus, such voluminous and frequent feedback information does not deteriorate the performance of wireless networks. However, all the previous works do not propose the feedback method how the transmitter and receiver estimate and share the dynamic channel status. To the best of our knowledge, no work has been published about bit-loading in distributed wireless networks with practical feedback overhead.
An efficient method of implementing subcarrier-wise rate adaptation with minimum overhead over a wireless system is proposed in this paper. The proposed method requires only one additional OFDM symbol. In spite of the overhead engendered by this additional OFDM symbol, the network throughput and delay performances are improved. In Section 2, the motivation for the development of the proposed method is presented, followed by a detailed description. In Section 3, the proposed method is evaluated through simulations and the resultant performance improvements are demonstrated. Finally, the conclusion is given in Section 4.
2. Subcarrier-Wise Rate Adaptation with Minimum Overhead over WLANs
The proposed protocol is designed for WLANs with heavy traffic such as those including the download of large size files (music, video, documents, etc.). It is assumed that the wireless stations move at pedestrian speed so that the wireless channel changes slowly. The method proposed in this paper is based on the 4-way handshaking mechanism composed of RTS/CTS/DATA/ACK sequences specified in IEEE 802.11 .
The proposed method is based on the use of a Bit Map. The Bit Map is a table recording and indicating how many bits were or are allocated on each subcarrier of OFDM symbols in a previous or current packet, respectively. In fact, the number of bits is directly proportional to the data rate of each subcarrier (data rate = the number of bits/OFDM symbol duration). In order to avoid confusion, hereinafter we use only the data rate. The overall operation of the protocol is as follows. The sender sends an RTS to the receiver. When it receives the RTS, the receiver estimates the condition of all subcarriers and determines the data rate that can be sent on each of them. After updating its Bit Map, the receiver sends a CTS packet, which uses a modified packet format derived from the IEEE 802.11 standard. The detailed CTS packet format is described in Section 2.2. The sender updates its Bit Map according to the information embedded in the CTS. The details of the method are illustrated in the following subsections.
2.1. Bit Map
2.2. Revised Formats of CTS and DATA Packets
This additional single OFDM symbol is composed of 48 data subcarriers and 4 parity subcarriers. Each parity bit covers 12 subcarriers. Each subcarrier is set to 1/−1 (BPSK) or one of the BPSK symbols. The objective of this additional OFDM symbol is to adjust the data rate allocated on the subcarriers for the subsequent data transmission. The method employed to allocate the data rate to each subcarrier is described in the following subsection. For the DATA packet, only one subfield is changed, as shown in Figure 3. The "Reserved" subfield in the DATA PLCP header in IEEE 802.11a is used as a "Confirmation" subfield. This subfield is used as an Acknowledgment for the Bit Map in the CTS packet. If the sender agrees with the Bit Map, the bit is set to 1. Otherwise, it is set to 0. The "Confirmation" subfield is used for the error recovery process, which is described in detail in Section 2.4.
2.3. Rate Selection and Rate Change Procedure in Receiver
The process used to update the data rate of each subcarrier for a subsequent data transmission is as follows.
Step 1 (Negotiation of using the proposed subcarrier-wise rate adaptation).
A sender sends an RTS packet setting "Rate" subfield in PLCP header to 1111. This informs a receiver the use of the subcarrier-wise rate adaptation method. If the receiver can process the proposed method, it goes to the next steps. Otherwise, the receiver sends a legacy CTS packet back to the sender and then the conventional procedure as defined in IEEE 802.11 standard is processed.
Step 2 (Estimate the channel condition of each subcarrier and find the optimal data rate).
The channel condition of a subcarrier (e.g., Signal-to-Noise Ratio (SNR)) is estimated from the received RTS packet. The receiver chooses a data rate suitable for the channel condition. We assume that the data rate is selected based on the predetermined threshold value [1–3].
Step 3 (Compare the chosen data rate with the data rate in the Bit-Map).
The chosen data rate for the subcarrier is compared to the data rate in the current Bit-Map. After comparison, the receiver chooses one of three actions: to increase, decrease, or not to change the data rate on each subcarrier. For example, if the data rate in the current Bit Map is smaller than the chosen data rate, the receiver chooses to increase the data rate for that subcarrier.
Step 4 (Set the value of the Bit-Map-Adjustment symbol of the CTS).
According to the decision in Step 2, the receiver sets the Bit-Map-Adjustment symbol to 1 to increase or -1 to decrease the data rate on each subcarrier. If the decision of a subcarrier is not to change the data rate, the receiver sets the Bit-Map-Adjustment symbol to a different value from the one used in the same subcarrier of the previous CTS.
Step 5 (Update the Bit-Map).
Once the values of the Bit-Map-Adjustment symbol are decided, the actual data rate for the upcoming data transmission is selected according to Table 1. Table 1 illustrates how to choose the data rate based on the values on both the current and previous CTS Bit-Map-Adjustment OFDM symbols. As noted in Step 3, if the current value is different from the value used in the previous CTS, the data rate is not changed. The Bit-Map is updated with the currently chosen data rates.
Data rate adjustment on each subcarrier according to symbols assigned in bit-map-adjustment ofdm symbols.
Symbol on a subcarrier in previous Bit-Map-Adjustment OFDM symbol
Symbol on a subcarrier in current Bit-Map Adjustment OFDM symbol
Data rate adjustment in Bit Map
Decrease one level from the previous data rate
Do not change
Do not change
Increase one level from the previous data rate
2.4. Rate Change Procedure at a Sender and Error Recovery
After the process illustrated in Section 2.3 is completed, the receiver (i.e., the destination of the RTS packet) sends a CTS packet to the sender (i.e., the source of the RTS packet). The CTS packet includes the Bit-Map-Adjustment OFDM symbol updated by the receiver. After receiving the CTS packet, the sender also updates its own Bit Map following the rule shown in Table 1. By using the data rates represented in the updated Bit-Map, the sender generates and sends a DATA packet with the "Confirmation" subfield set to 1. When the receiver receives this DATA packet, it demodulates the packet based on the Bit Map information and sends an ACK to the sender.
If the transmission of the CTS packet fails, the receiver goes back to the previous bit allocation information contained in the Bit Map, as in the case of DATA packet loss. This case is shown in Figure 7(b).
3. Performance Evaluation
For the purpose of the evaluation, the proposed method, which is referred to as "Adaptive", is compared with two prior art methods. The first one is a method proposed in  that is also briefly described in Section 1. We name this method "OSS". The other one is a packet-based rate adaptation method described in [2, 3, 17, 18], which changes the data rate based on the channel condition, but uses the same PHY mode over all subcarriers. Since this method uses a fixed PHY mode over all subcarriers in an OFDM symbol, we name it "Fixed". "Fixed" chooses a data rate, which is appropriate for a subcarrier having the worst channel condition over all subcarriers. All three methods use the 4-way handshaking procedure (RTS/CTS/DATA/ACK) defined in the IEEE 802.11 standard. In terms of the overhead in the control packets, such as RTS, CTS, and ACK, our method has one more OFDM symbol compared to "Fixed" due to the addition of the "Bit-Map-Adjustment" OFDM symbol. Although it might need two or more OFDM symbols, we assume that OSS also uses one OFDM symbol for the feedback of the subcarrier information. In OSS, the threshold level used to select the strongest subcarriers is set to 54 Mbps, which corresponds to −65 dBm in Table 2. Only the selected strongest subcarriers are used for packet transmission, as described in . The value of −65 dBm is selected as the threshold level for OSS, because this level provides the best throughput performance among the 8 levels in Table 2.
An efficient protocol which realizes subcarrier-wise rate adaptation over wireless channels has not previously been proposed, due to the large overhead caused by the frequent transmission of the feedback information, which is not small. In this paper, we propose a novel rate-adaptive MAC protocol for OFDM-based wireless communication systems. The proposed method provides an efficient way to implement subcarrier-wise rate adaptation by designing a protocol which has a relatively small feedback overhead associated with the subcarrier state information. The proposed protocol plugs one OFDM symbol into a CTS packet. By utilizing the OFDM symbol and synchronously maintaining the bit allocation maps at both the sender and receiver, it can adaptively change the data rate allocated to each subcarrier. To synchronize the bit allocation maps at both the sender and receiver over an error-prone wireless channel, a detailed error recovery procedure is also proposed.
The simulation results show that the proposed method increases the network performances, because it utilizes the entire set of subcarriers more efficiently than the prior art methods. Even though we add one OFDM symbol to the CTS packet, the overhead caused by this extension is relatively small in terms of the overall packet size. As a result, the performance improvement due to the subcarrier-wise rate adaptation surpasses the performance degradation that results from the feedback overhead associated with the subcarrier state information.
This research was supported in part by the MKE (The Ministry of Knowledge Economy), Republic of Korea, under the ITRC (Information Technology Research Center) support program supervised by the NIPA (National IT Industry Promotion Agency (NIPA-2010-(C1090-1021-0011)) and in part by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2010-0015236) (2009-0089304).
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