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Abstract—



Index Terms — Wireless LAN, OFDM modulation,

Interference elimination.

I. INTRODUCTION

ireless LAN radio interfaces based on IEEE 802.11b/g can experience severe adjacent channel interference (ACI) if channels are not carefully selected. The ACI effects are also present in 802.11a and n. The issue is discussed in [1]. We show by means of the simulation that ACI can be rejected from the two interfering adjacent OFDM signals. A receiver with one RF block and 40-MHz filter is necessary. To reject ACI and detect data from both interfering OFDM signals the receiver has to be provided with a new functionality [2].

In this paper, we remind some key features of 802.11n. Next, the new functionality, necessary to reject the ACI, is described. The functionality realizes the iterative procedure in which subsequent operations of demodulation, detection and remodulation are alternately done (synchronously with the symbol timing of each interfering OFDM signal). It is assumed that the receiver knows the symbol timing shift between the two 20-MHz OFDM components of the 40-MHz received signal. The simulation results present BER(SNR) for different pairs of OFDM signals received by a 40-MHz one RF-block receiver. Some applications of the new 40-MHz receiver are discussed.

II. OVERVIEW OF SOME IEEE 802.11N FEATURES In the 802.11n network both 20-MHz and 40-MHz channels can be used. Each 40-MHz channel consists of the primary and the secondary 20-MHz channels. The 40-MHz channels This work was supported by the Polish Ministry of Science and Higher Education under research grant 5545/B/T02/2010/39.

are allocated in such a way that one 40-MHz channel does not overlap with its adjacent 40-MHz channel.



The start time of a frame transmission in 802.11n 20- or 40-MHz channel is solely based on the clear channel assessment (CCA) result for the primary channel. A station may start a frame transmission in the 40-MHz channel, if the 20-MHz primary channel is idle for DIFS plus the backoff counter time, and the secondary channel is idle within PIFS before the scheduled transmission start time over the primary channel. If the secondary channel is not idle, the station can adopt one of the following policies: static 40-MHz bandwidth channel access or dynamic 20/40-MHz bandwidth channel access. In the first case the station reattempts to access the 40-MHz channel by restarting the channel access attempt procedure. In the second case it transmits only in the primary 20-MHz channel. A significant part of transmission band or time is wasted in both above scenarios.

In the paper it is assumed that an 802.11 access point (AP), equipped with one RF block operating in 5 GHz band, is to receive data in 40-MHz channel. The channel can be occupied by a 40-MHz 802.11n OFDM signal, two adjacent 20-MHz 802.11n OFDM signals, two adjacent 802.11a OFDM signals, or two adjacent 20-MHz 802.11a and n signals. The adjacent 20-MHz signals are transmitted asynchronously by the stations, but the symbol timing shift between them is known to the receiver.

The IEEE 802.11 standard specifies four types of acknowledgment polices (Normal Ack, No Ack, No Explicit Acknowledgment, Block Ack). When the Normal Ack policy is used the addressed recipient after SIFS returns an ACK frame for each received frame. In the No Ack policy the addressed recipient does not send ACK frame. The policy No Explicit Acknowledgment denotes that an ACK frame is not required. In case of the Block Ack policy a block of frames is acknowledged by a single Block-ACK, instead of using several ACKs, one for each frame. This mechanism improves system throughput by reducing the amount of overhead required by a station to acknowledge a burst of received frames.

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Robert Kotrys, Maciej Krasicki, Piotr Remlein, Andrzej Stelter, Paweł Szulakiewicz

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Fig. 1. 20- and 40-MHz channels in the 802.11n

2 When an ACK frame has to be sent immediately after a data

frame (or block of frames) is received in any of the adjacent 20-MHz channels, the proposed functionality cannot be used because 40-MHz RF block cannot receive data in one half of its band, and simultaneously transmit data in the second half of it. This is the reason why in the paper it is considered the scenario in which the stations do not require immediate acknowledgment from AP for each transmitted frame, i.e. either the stations do not require acknowledgment at all (No Ack policy) or they use a nonstandard Block Ack policy with a flexible delay between Block-ACK request and Block-ACK response.

No Ack policy could be used in case of real time traffic, like video streaming in witch retransmission delay is not allowed. The IEEE 802.11a/n WLAN monitoring device is another example in which the usage of the proposed functionality can be considered. The network monitoring receiver (one RF block) provided with the proposed functionality, which operates in 40-MHz channel and monitors two 20-MHz channels at the same time or one 40-MHz channel, is possible. The 80-MHz or 160-MHz receiver monitoring four or eight adjacent 20-MHz channels, respectively, is also possible.

III. A NEW AP RECEIVER FUNCTIONALITY

Š‡•‹‰ƒŽ›ሺ–ሻ”‡…‡‹˜‡†ˆ”‘–Š‡…Šƒ‡Ž‹•ƒ•—‘ˆ–™‘  •‹‰ƒŽ•ƒ†‘‹•‡ǡ‹Ǥ‡Ǥ



y(t) = s

(t) + s

(t) + n(t) (1)



To cancel the interference between two components s0(t) and s1(t) of the signal y(t) received from the channel, the iterative procedure is applicable. It consists in subsequent regeneration of each component from its noisy estimate ݏǁ, computed previously as a difference between y(t) and the regenerated remaining signal component:



ݏǁ଴Ȁଵൌ ݕሺݐሻ െ ݏƸଵȀ଴. (2)



The key feature is that regeneration of particular components is done with respect to their timing shifts.

The block diagram of the interference canceller proposed‹ –Š‹•’ƒ’‡”‹••Š‘™‹ ‹‰ǤʹǤ It is assumed that the timing of s1(t) is shifted by τ with respect to the timing of s0(t). Let

[א ൛Ͳǡ ͳൟ be an index†‡‘–‹‰‘‡of the components to be regenerated at a given time. Note–Šƒ–ξ is fed into the address inputs of demultiplexer and multiplexer, shown in Fig. 2. To regenerate s1(t) component, the previously regenerated remaining component ݏƸ_଴ሺݐሻ is subtracted from the signal y(t) received from the channel. (While [ൌ ͳ the multiplexer passes ݏƸ_଴ሺݐሻ to its output.) A resultant noisy estimate ݏǁ_ଵሺݐሻ is then passed through the demultiplexer and then shifted forward by τ to match the timing shift of ݏ_ଵሺݐሻ. Then decisions on the data carried by ݏ_ଵሺݐሻ are made. Afterwards the subcarriers occupied by data in ݏ_ଵሺݐሻ are remodulated by decided data. The outcoming signal ݏƸ_ଵሺݐ ൅Wሻ is delayed by τ to simulate the immanent timing shift between components

ݏ଴ሺݐሻ and ݏଵሺݐሻ of y(t). Now both demultiplexer and

multiplexer switch (ξ:=0) and the regenerated ݏƸ_ଵሺݐሻsignal is subtracted from y(t) in order to ‰‡– the noisy estimate ݏǁ_଴, essential to regeneration of s0(t). The regeneration performs in the same manner as for s1(t) with the exception of timing shifts. After several iterations, the decided data, carried by both component signals, are outputted.

 IV. RESULTS OF THE SIMULATION

In the simulation the systems conformant with either IEEE 802.11a or 802.11n specification were considered. The ACI cancelation method [2] was used in the simulations to show that a single radio 40-MHz AP receiver is able to simultaneously receive two adjacent OFDM signals compatible with the IEEE 802.11a and 802.11n standards. The signals are transmitted asynchronously and their symbol timing shift equal IJ is known to the AP. The performance of the new 40-MHz AP receiver was also tested for two adjacent 20-MHz 256-QAM OFDM signals which are allowed in an 802.11ac draft [3]. In the simulations, both adjacent OFDM signals have equal power and the same modulation scheme. We used an arbitrarily selected symbol timing shift IJ = 0.1328T. The value of τ does not have a significant influence on the simulation results.

In Fig. 3 and Fig. 4 there are results of BER(SNR) for the 16-QAM OFDM signals transmitted in both adjacent channels. The results show that the presence of ACI (dashed line with ‘+’ marks) causes significant BER degradation. If the described ACI cancelation method is used, the BER(SNR) curves for the receiver performing one or two iterations are very close to each other and to the BER(SNR) curve for no-ACI case. Thus, it is clear that one iteration is sufficient to achieve the complete ACI removal for 16-QAM signals.

3

Fig. 3. BER vs. SNR for OFDM 802.11a signals, 16-QAM on subcarriers, IJ = 0.1328 T

Fig. 4. BER vs. SNR for OFDM 802.11n signals, 16-QAM on subcarriers, IJ = 0.1328 T

Fig. 6. BER vs. SNR for OFDM 802.11n signals, 64-QAM on subcarriers, IJ = 0.1328 T Fig. 5. BER vs. SNR for OFDM 802.11a signals,

64-QAM on subcarriers, IJ = 0.1328 T

Fig. 7. BER vs. SNR for OFDM 802.11a signals,

4 The simulation results for two 64-QAM OFDM adjacent

signals are shown in Fig. 5 and Fig. 6. In case of 64-QAM a complete ACI cancelation is achieved after the first iteration for the 802.11a signals, and after the second iteration for 802.11n signals. Results for two 256-QAM OFDM adjacent signals are shown in Fig. 7 and Fig. 8. In case of 256-QAM two iterations are needed to complete the ACI cancelation for the 802.11a and 802.11n signals.

V. CONCLUSION

In the considered scenario, the 802.11n compatible AP is able to receive data sent by an 802.11n station in 40-MHz channel or data sent by 802.11n or 802.11a station in the primary 20-MHz channel. The AP is not able to simultaneously receive two different asynchronous OFDM signals from both 20-MHz channels.

The new functionality, described in this article and proposed in [2], when applied to the AP allows it to receive

OFDM signals at the same time from both primary and secondary channels irrespective of the transmitting stations types. The AP is able to receive data transmitted by an 802.11n station occupying 40-MHz channel, as well as any combination of two 802.11n and 802.11a stations occupying 20-MHz adjacent channel each.

REFERENCES

[1] V. Angelakis et al., “Adjacent Channel Interference in 802.11a is Harmful: Testbed Validation of a Simple Quantification Model,” in

IEEE Communication Magazine , March 2011, Vol. 49, No. 3, pp.

160-166

[2] P. Szulakiewicz et al., “Iterative rejection of the adjacent channel interference (ACI) in the OFDM transmission scheme,” - paper submitted to IEEE Communications Letters

[3] IEEE 802.11 TGac, “802.11ac - Technical Specification Draft 0.1", January 20, 2011