In this paper, we propose a framework for cross-layer optimization to ensure ultra-high reliability and ultra-low latency in radio access networks, where both transmission delay and queueing delay are considered. With short transmission time, the blocklength of channel codes is finite, and the Shannon Capacity cannot be used to characterize the maximal achievable rate with given transmission error probability. With randomly arrived packets, some packets may violate the queueing delay. Moreover, since the queueing delay is shorter than the channel coherence time in typical scenarios, the required transmit power to guarantee the queueing delay and transmission error probability will become unbounded even with spatial diversity. To ensure the required quality-of-service (QoS) with finite transmit power, a proactive packet dropping mechanism is introduced. Then, the overall packet loss probability includes transmission error probability, queueing delay violation probability, and packet dropping probability. We optimize the packet dropping policy, power allocation policy, and bandwidth allocation policy to minimize the transmit power under the QoS constraint. The optimal solution is obtained, which depends on both channel and queue state information. Simulation and numerical results validate our analysis, and show that setting packet loss probabilities equal is a near optimal solution.
While reducing latency in wireless networks is challenging, further ensuring high reliability makes the problem more intricate. To reduce the delay caused by transmission and signalling @cite_15 , a short frame structure was introduced in @cite_16 , and the TTI was set identical to the frame duration. To ensure high reliability of transmission with short frame, proper channel coding with finite blocklength is important. Fortunately, the results in @cite_26 indicate that it is possible to guarantee very low transmission error probability with short blocklength channel codes, at the expense of achievable rate reduction. By using practical coding schemes like Polar codes @cite_11 , the delays caused by transmission, signal processing and coding can be reduced.
@cite_15 @cite_16 @cite_26 @cite_11
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Future generation of wireless networks, i.e. 5G, is envisioned to support several new use-cases demanding transmission reliability and latency that cannot be achieved by the current cellular networks such as long-term evolution (LTE). This paper looks at different design aspects of the control channel(s) to support ultra-reliable low-latency communication considering factory automation as an example scenario. In particular, we show that a fairly balanced design for both the uplink and the downlink control channels can be made given an appropriate selection of modulation, coding, diversity scheme, and time frequency resources. By means of link-level simulations, we also show that the proposed control channel design supports a block-error rate of 10-9 under Rayleigh fading conditions at a signal-to-interference-plus-noise ratio comparable to that supported by current 4G systems (e.g. LTE). Furthermore, a radio frame structure is proposed to support the user plane end-to-end latency of 1 ms. This paper proposes a novel frame structure for the radio access interface of the next generation of mobile networks. The proposed frame structure has been designed to support multiuser spatial multiplexing, short latencies on the radio access interface, as well as mobility and small packet transmissions. The focus is on ultra dense small cell networks deployed in outdoor environments. This paper also highlights the various prospects and constraints of the proposed dense outdoor system in comparison with alternative system designs. Numerical results are included and a comparison to the Long Term Evolution (LTE) system is provided. Results show that the proposed radio frame structure leads to an improvement of the area spectral efficiency by a factor of &#126;2.4 as well as a reduction of the average air interface latency by a factor of 5, thus remaining shorter than 1 millisecond. This paper investigates the maximal channel coding rate achievable at a given blocklength and error probability. For general classes of channels new achievability and converse bounds are given, which are tighter than existing bounds for wide ranges of parameters of interest, and lead to tight approximations of the maximal achievable rate for blocklengths n as short as 100. It is also shown analytically that the maximal rate achievable with error probability ? isclosely approximated by C - ?(V n) Q-1(?) where C is the capacity, V is a characteristic of the channel referred to as channel dispersion , and Q is the complementary Gaussian cumulative distribution function. Polar codes represent an emerging class of error-correcting codes with power to approach the capacity of a discrete memoryless channel. This overview article aims to illustrate its principle, generation and decoding techniques. Unlike the traditional capacity-approaching coding strategy that tries to make codes as random as possible, the polar codes follow a different philosophy, also originated by Shannon, by creating a jointly typical set. Channel polarization, a concept central to polar codes, is intuitively elaborated by a Matthew effect in the digital world, followed by a detailed overview of construction methods for polar encoding. The butterfly structure of polar codes introduces correlation among source bits, justifying the use of the SC algorithm for efficient decoding. The SC decoding technique is investigated from the conceptual and practical viewpoints. State-of-the-art decoding algorithms, such as the BP and some generalized SC decoding, are also explained in a broad framework. Simulation results show that the performance of polar codes concatenated with CRC codes can outperform that of turbo or LDPC codes. Some promising research directions in practical scenarios are also discussed in the end.