CPLink Interference-Free Reuse of Cyclic-Prefix Intervals in OFDM-Based Networks

CP Link Interference-Free Reuse of Cyclic-Prefix Intervals in OFDM-Based Networks

Abstract: In this paper, we propose a method to reuse the cyclic-prefix (CP) intervals of an ongoing OFDM link, by another link, without degrading the OFDM link performance. We label the link that reuses the CP-intervals as CP Link as they use only the CP-intervals of the ongoing link labeled Main Link. We leverage the fact that the most commonly used OFDM receivers discard the samples in CP-intervals to design CP Link that ensures below-noise-floor interference at every Main Linkreceiver. The key contribution in the paper is the design and study of zero knowledge CP Link, in which the CP Link-transmitter ensures below-noise-floor interference at every Main Link-receiver with no knowledge about the locations or the number of Main Link receivers. We analytically show that the zero-knowledge CP Link capacity is positive. For LTE frame structure with 20 MHz bandwidth and 2 km cell-radius, we evaluate CP Link for two kinds of CP Link-receivers: fullduplex base-station and a half duplex device. Even for the cell-edge CP Linktransmitters, full duplex zero-knowledge CP Link data rates can be up to 20 Mbps (60 Mbps) when the CP-duration is _7% (25%) of data symbol duration. The halfduplex CP Link rate is 5-40 Mbps when CP Link-transmitter is within 250 m of CP

Link-receiver. CP Link capacity is found to increase near-linearly with the CPduration, thus mitigating CP overhead.

Existing system: The above discussion highlights that the content of CP interval, as seen at the receiver, is non-consequential, since the CP is discarded at the receiver. Hence, ideally, OFDM link would not be affected if any co-channel interference lies only in the CP intervals at the OFDM receiver.2 In this paper, we build upon this simple idea of aligning co-channel interference with the CP intervals and devise a method to facilitate the reuse of the channel during the CP intervals of an active OFDM link with minimal impact on the performance of OFDM link. We label this reuse as CP Link, short for Cyclic-Prefix Link. The main idea is to activate CP Link in the CP intervals of an ongoing OFDM link, which we will label as Main Link. Thus, CP Link is an on-off channel.

Proposed system: Thus, CP Link is an on-off channel that is active only during the CP intervals of the Main Link. In summary, two links operate simultaneously on the same spectrum – Main Link sending data like a regular OFDM link, and CP Link sending data only during CP intervals. Figure 1 depicts the OFDM signal, CP Link signal and the CP Link interference at the receiver of the Main Link.The proposed CP Link is only useful if (i) Main Link continue to enjoy the benefits of the cyclic prefix, as discussed above, and (ii) CP Link achieves a non-zero capacity while ensuring that interference on Main Link data phase is below noise-floor. We show that the first requirement is easy to achieve, and the main challenge comes from the second requirement, which we discuss in more detail next.

Advantages: Orthogonal frequency division multiplexing (OFDM) is now used in nearly all wireless standards. The most prevalent version of OFDM is the cyclic-prefix OFDM (CP-OFDM). In CP-OFDM, each transmitted symbol has a CP followed by the data symbol, where CP consists of the tail-end samples of the data symbol it precedes.

Most deployed OFDM receivers discard the samples in the CP interval and use only the samples in the data symbol interval for recovering the information.

Discarding the CP1 greatly simplifies the channel equalization.

Disadvantages: The cognitive radio techniques share a similar goal of reusing the airtime/spectrum in a wireless network. But, the key feature that sets CP Link apart from cognitive radio techniques is that the CP Link reuses the airtime during Main Link transmissions without harming Main Link performance, while cognitive radio techniques are often designed to reuse only the airtime that does not contain Main Link transmissions. To the best of authors’ knowledge, this is the first paper that suggests a way of reusing the CP airtime for setting up communication links. The CP Link concept (reusing CP intervals) can be employed in any network to reuse the airtime unused at the Main Link receivers, specifically the guard intervals.

Modules: CP Link: We next study applications of CP Link in two use cases. The first use case is the full-duplex CP Link, where the CP Link receiver is a full-duplex base-station, as depicted. The second use case is the half-duplex CP Link, depicted in Figure 2b, where any node other than the base-station is the CP Link receiver. In both full and half-duplex CP Link use cases, all the user devices in the cell can be half-duplex.

An example of half-duplex CP Link is a link between two nodes within the network, e.g., device-to-device or peer-to-peer link. We show that the spatial reuse of CP intervals for data communication, in both full-duplex and half-duplex use cases could increase the network throughput significantly. In TDD networks, the full-duplex CP Link would carry uplink signals during the downlink mode. Hence, using full-duplex CP Link as uplink control channel can potentially result in FDDlike latency in TDD networks. To evaluate the utility of CP Link, we numerically study CP Link in a setting similar to that of an LTE cellular network with 20 MHz bandwidth, in which the CP duration can be either 7.13% or 25% of that of the data symbol.

Main link receiver:

CP Link interference at the Main Link receiver depicts the timing of CP Link and Main Link signals. In our system, CP Link transmitter begins the transmission exactly at the start of CP, where the start of CP is as observed at the CP Link transmitter. For a fixed CPLink transmitter location, CP Link interference at the Main Link receiver depends on the receive power of the CP Link signal, and the amount of overlap between the Main Link data symbol and the CP Link symbol.

The receive power depends on rcm, the distance between CP Link transmitter and the Main Link receiver. The amount of overlap depends additionally on rbm, the distance between base-station and Main Link receiver. Let Tov be the duration for which CP Link symbol overlaps with Main Link data symbol at the Main Link receiver.

CPLink rate: For the CPLink receiver, the CPLink signal is the desired signal and the Main Link signal is the interference. CPLink receiver can improve the receive SINR by canceling the Main Link signal from the total received signal. We assume that the CPLink receiver is capable of canceling the Main Link interference from its received signal3 and that the coding used on CPLink transmissions is capacity achieving. Let be the noise and the residual Main Link-interference floor at the CPLink receiver. For a given pair of CPLink transmitter and receiver, the CPLink rate is a function of on-time and transmits power, which is given.

Impact of CPLink on the design objectives of cyclic prefix of Main Link: The design of cyclic prefix achieves two objectives. First, by discarding the CP samples, inter-symbol-interference is automatically mitigated at the receiver.

Second, the presence of CP before the data symbol simplifies the equalization at the receiver by circularizing the linear convolution between the OFDM signal and the channel. Thus, the reuse of CP intervals by CPLink raises the question whether CPLink disrupts the design objectives of CP in the Main Link? We show that the CPLink does not affect the design objectives of CP, first with the help of a simple example before we provide a general proof.