Cyclic Prefix Direct Sequence Spread Spectrum Capacity Analysis

Abstract

Cyclic Prefix Direct Sequence Spread Spectrum (CP-DSSS) is a novel waveform that is proposed as a solution to massive machine type communications (mMTC) for 5G and beyond. This paper analyzes the capacity of CP-DSSS in comparison with Orthogonal Frequency-Division Multiplexing (OFDM). We show that CP-DSSS achieves the same capacity as OFDM and can be optimized with similar precoding methods (e.g., water-filling). Because of its spread spectrum nature, CP-DSSS can operate as secondary network using the same spectrum as the primary 4G or 5G network, but transmitting at much lower power. Accordingly, the combination of primary and secondary signals in the envisioned setup may be viewed as a power NOMA (non-orthogonal multiple access) technique where primary signals are detected and subtracted from the received signal first before detecting the secondary signals. In order to operate at a sufficiently low interference level to the primary network, details of CP-DSSS capacity for symbol rate reduction and multi-antenna operation are developed. The capacity limits established in this paper can be used as a baseline to evaluate the performance of future CP-DSSS receiver architectures for single-and multi-user scenarios. I. INTRODUCTIONCyclic prefix direct sequence spread spectrum (CP-DSSS) is a novel waveform that was originally proposed as a solution to ultra-reliable low latency communication (URLLC) [1]-[3]. These references introduced CP-DSSS as a signaling method for sending a small number of information/control bits within each OFDM symbol interval concurrent with other network communication signals.CP-DSSS can also be used as a multi-user data channel to achieve massive machine type communication (mMTC) network density objectives as explored in [4] and [5]. Unlike URLLC, which is concerned largely with latency, probability of detection, and supporting packet formats, mMTC endeavors to support a massive number of low-cost and lowpower terminals. As low-cost and low-power transceivers are designed, it is important to compare their performance to the theoretical bounds. This paper presents a capacity analysis of CP-DSSS for a point-to-point link and establishes the peak per-user capacity, assuming an ideal receiver. Although the developed results are applicable to any signal-to-noise (SNR) ratio, the emphasis of this paper is on the low SNR regime

Multi-user Capacity of Cyclic Prefix Direct Sequence Spread Spectrum with Linear Detection and Precoding

“…In order to keep the transmitted power constant, there is a constraint that tr(G H G) = N . Another constraint applied to facilitate analysis with precoding in [3] is to make G circulant. It should be noted that this last constraint does not limit the precoder's ability to scale power on a frequency bin basis.…”

“…It should be noted that this last constraint does not limit the precoder's ability to scale power on a frequency bin basis. Two precoding options were discussed in [3]. The first precoder used the water-filling (WF) result, which has been shown to be optimal in terms of capacity for OFDM.…”

“…For a given number of users, the signal to interference plus noise ratio (SINR) of each user is evaluated for a number of channel realizations, and the results are then converted to capacity. These results are presented along with the ideal-receiver single-user capacity calculated in [3] for comparison. CP-DSSS offers a very convenient method for channel estimation.…”

“…The data trends suggest that more simultaneous users would be possible. As discussed in [3], CP-DSSS is well-suited for a femtocell scenario, where the network density can be dramatically increased by supporting several user terminals in a small geographic area (e.g., 10s of meters), serviced by the femtocell gateway (FGW). This allows for several FGW's to be distributed throughout the traditional cell.…”

CP-DSSS: An OFDM Compatible Variable Rate Modulation for 5G and Beyond

“…Here, we quantify this performance degradation/improvement through some theoretical explanation and presentation of some numerical results. We further note that CP-DSSS operation in low SNR regime is of particular interest in many use cases of this waveform as discussed in [5] and [6]. Considering this point, the PAPR reduction method that is introduced below will be presented with emphasis on CP-DSSS waveform, even though it is equally extendable to OFDM as well.…”