You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: Survey of RF Communications and Sensing Convergence Research 
Chapter Title:  D. SPREAD SPECTRUM METHODS 
1. Abstract of Cited Paper (BIB001):  With the rapidly incremental demand in electronic devices, there is something in exploring multifunctional RF systems for reducing cost, minimizing the radar cross and probability of intercept, etc. This paper presents an integration system of radar and communication based on DSSS (direct sequence spread spectrum) technique, an attractive technique which is highly secure and robust. It utilizes different PN code to spread the spectrum of radar and communication data to avoid jamming mutually. Additionally, it uses BPM to gain better performance. The simulation over an ideal AWGN channel shows that the system can work perfectly 
2. Abstract of Cited Paper (BIB002):  Multiple-input multiple-output (MIMO) extensions to radar systems enable a number of advantages compared to traditional approaches. These advantages include improved angle estimation and target detection. In this paper, MIMO ground moving target indication (GMTI) radar is addressed. The concept of coherent MIMO radar is introduced. Comparisons are presented comparing MIMO GMTI and traditional radar performance. Simulations and theoretical bounds for MIMO GMTI angle estimation and minimum detectable velocity are presented. The simulations are evaluated in the time domain, enabling waveform design studies. For some applications, these results indicate significant potential improvements in clutter-mitigation SINR loss and reduction in angle-estimation error for slow-moving targets. 
3. Abstract of Cited Paper (BIB003):  Radar and communication integrated system has been a development trend of technologies. To resolve existing problems of integrated systems, this paper presents a novel method of waveform design for radar and communication integrated system based on chirped spread spectrum (CSS) and proposes the structure of the integrated system. The advantage of this system is that different functions can be realized in a same platform but without jamming between each other. Performance analysis and simulation results show that the proposed system can be efficiently implemented and meet practical demands.

Literature Review:
Similar to OFDM, spread spectrum waveforms have been proposed for their attractive, noise-like autocorrelation properties. Some work focused on orthogonal spreading codes between the radar and communications users employing direct-sequence spread spectrum (DSSS) BIB002 , while others looked at chirped spread spectrum (CSS) to avoid jamming between the two users BIB004 , BIB001 . Ultimately, the performance of these systems is limited by the degree of orthogonality that can be obtained in the joint system, theoretically limited to the inverse of the time bandwidth product BIB003 .