與IEEE 802.11ac公平共存之LTE MAC協定設計

Abstract

The rapid traffic growth of LTE cellular network is expected to result in the shortage of licensed spectrum. To satisfy the tremendous traffic demand, more spectrum and widely spreading LTE small cells are required, especially in metropolitan area with high population density. A proposed concept of LTE-U is that enabling LTE to operate in unlicensed band in order to offloads data traffic from licensed band to shared access of unlicensed band. However, it raises the coexistence issue with WiFi, which is the major user of unlicensed band and holds almost a half of all internet traffic. Two main approaches are duty cycle and listen before talk. The former shares the channel time equally without flexibly utilize the shared channel when it is idle, while the latter makes better use of shared channel, yet, it is hard to achieve fair sharing. Therefore, to design a listen-before-talk based LTE-U protocol and enable fair coexistence with WiFi in unlicensed band becomes a significant issue. In this research, we consider following questions: P1. How to define fairness in LTE-U & WiFi coexistence wireless network, and can be quantified? P2. How to design the LBT-based LTE-U protocol to satisfy our fairness definition? P3. How to fully exploit existing LTE protocol to realize out LBT-based LTE-U protocol design? P4. How is the performance of the design in fairness and total throughput compared with other LBT scheme? Challenges are: 1. How to define an objective and easy-to-measure fairness definition between LTE-U and WiFi. 2. How to achieve fairness in LBT-based LTE-U to coexist with WiFi 3. How to realize the protocol in the framework of existing LTE protocol? 4. How to achieve fairness while the throughput of WiFi can be no worse than WiFi-only environment? Our contributions are: C1) Give a fairness definition in term of the ratio of throughput to available bit rate of the technology, which is a measurable instance of Jain’s Fairness Index [JCH84]. The concept of fairness definition is based on equal opportunity that the ratio of outcome to input is equal among each user. C2) Prove that the fairness definition is equivalent to equal time of successful packet transmission, which enables listen before talk mechanism to achieve fairness by adjusting TXOP limit. The TXOP limit is a look-up table stored in HeNB, which has O(1) time complexity. C3) FaM-CSMA/CA is designed to enable listen-before-talk by adding a new module ‘Spectrum Resource Seeker’ in MAC scheduler. Spectrum Resource Seeker performs multiple CSMA/CA processes with RTS-like signal and designed HARQ ACK. C4) Numerical performance analysis shows that fair coexistence LTE-U protocol achieves almost extreme fairness between LTE-U and WiFi, while throughput of coexisting WiFi users slightly increase 0.7% from that of WiFi only network. Comparing to the existing LTE-U designs, FaM-CSMA/CA reaches totally time fairness between LTE-U and WiFi while compromising on high throughput increase.