Evolve Random Access Channels for IoT: II Existing Access Probe Designs


Access Channel Enhancements for 1x Rel. F
One Eighth Rate Access Probes for Smart Terminals
Evolve Random Access Channels for IoT: I Introduction
Evolve Random Access Channels for IoT: III Slotted ALOHA Models
Evolving Random Access Channel for IoT: IV Dumb Access Probes and Smart Access Probes


The IoT traffic characteristics are known to be substantially different from those of smartphones. Mobile networks were traditionally designed and optimized to transport connection-oriented traffic, where each connection is expected to be continuous with low latency. On the other hand, many IoT services are intended to be of low throughput, short duration and delay tolerant and characterized to be “connectionless.” As such while the existing mobile network operators start studying and optimizing their networks to accommodate the dynamics of growing IoT traffic, some companies start developing possible alternatives for IoT services.



LoRaWAN

LoRaWAN (low power wide area network) is a connectivity technology proposed by LoRa Alliance for battery-operated applications. It is designed to support only low- rate burst data packets between servers and end-devices. As such, it doesn’t including the connection-oriented signaling to save power. For example, there is no synchronization mechanism between receivers and transmitters. The communications between receiver and transmitter are completely ad hoc.  Further in order to increase its link sensitivity for extended coverage, LoRaWAN employed spread spectrum and coding techniques to reduce the minimum demodulation and detection SINR (Signal to Interference and Noise Ratio) at the expense of increasing signal transmission duration and collision probability.  LoRaWAN uses a pure ALOHA protocol to connect end-devices with their server. It allows an end-device to start a data exchange immediately when the data arrives. For example, its low power Class A device starts the communication from sending a uplink access probe embedded with a data payload and followed by two short downlink receive windows for waiting and receiving corresponding downlink data from the server.

LTE RACH (Random Access Channel)

LTE can support the IoT data traffic with extreme low duty cycles through its slotted ALOHA access probes, in which active user equipments (UEs) start their transmission synchronously on predefined access boundaries. In LTE, UEs use an uplink channel, Physical Random Access Channel (PRACH), to request a radio resource assignment for sequential data exchange during the initial access to the system. The PRACH is allocated on each frame with up to 16 different RACH configurations and up to 10 PRACH resources per frame per access cycle. The case of 10, with one PRACH resource in each subframe, is designed for situations in which the access load is high.  For the network of a large coverage, each frame supports 3 PRACH resource, each including 3 consecutive subframes. Similar to a slotted-ALOHA protocol, the transmission on PRACH is shared by all active UEs within the same sector. Firstly, a UE randomly choose one of the maximum of 64 preamble sequence (Msg 1) and sends it to the eNodeB.  A collision can occur at the eNodeB when two or more UEs choose identical preamble sequences and send them at the same time. Preamble transmission may also fail due to insufficient transmission power. Depending on the configuration of a eNodeB, 3GPP defines the minimum PRACH detection performance of each eNodeB. For example, the PRACH missed detection requirement for a eNodeB with 8 Rx antennas should be no less than -21 dB in additive white Gaussian noise (AWGN) channel with the false alarm rate less than 0.1% and the successful rate not less than 99%.

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