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Convergence Technologies
Published in K.R. Rao, Zoran S. Bojkovic, Dragorad A. Milovanovic, Wireless Multimedia Communications, 2018
K.R. Rao, Zoran S. Bojkovic, Dragorad A. Milovanovic
Traditional cellular systems have typically allocated resources in a relatively static way, where the data rate for a user is changed slowly or not at all. This approach is efficient for applications with a relatively constant data rate such as voice. For data with a bursty nature and rapidly varying resource requirements, fast allocation of shared resources is more efficient. In WCDMA, the shared downlink resource consists of transmission power and channelization codes in node B (the base station), while in the uplink the shared radio resource is the interference at the base station. Fast scheduling is used to control allocation of the shared resource among users on a rapid basis. Additionally, fast hybrid ARQ with soft combining enables fast retransmission of erroneous data packets. A short transmission time interval (TTI) is also employed to reduce the delays and allow the other features to adapt rapidly. Similar principles are used for both HSDPA and enhanced uplink, although the fundamental differences between downlink and uplink must be accounted for.
4G/5G Radio Access Network
Published in Saad Z. Asif, 5G Mobile Communications Concepts and Technologies, 2018
The physical layer of NR supports multiple numerologies where numerology is defined by subcarrier spacing and cyclic prefix (CP) overhead. This is in contrast to LTE that supports a fixed numerology of 15 kHz. 5G NR introduces scalable OFDM numerology supporting subcarrier spacing from 15 kHz to 480 kHz. Similarly, the TTI, which refers to the duration of a transmission on the radio link, is still applicable in NR. As stated in [78], the TTI duration corresponds to a number of consecutive symbols in the time domain in one transmission direction. LTE supports a fixed TT1 duration of 1 ms and there is ongoing research to have TTIs of various durations to support lower latency requirements of 5G NR. Research is also underway to introduce service ware TTI. For example, a high QoS eMBB service may utilize a 500 μs TTI while some other latency sensitive service further shortens it to 140 μs.
From Machine-to-Machine Communications to Internet of Things: Enabling Communication Technologies
Published in Hongjian Sun, Chao Wang, Bashar I. Ahmad, From Internet of Things to Smart Cities, 2017
Hamidreza Shariatmadari, Sassan Iraji, Riku Jäntti
As mentioned earlier, some of the simplified functionalities that are defined for LTE UE categories for reducing the chip cost result in lower signal energy at the receiver. In addition, some IoT devices might be deployed in locations with high penetration loss. In order to overcome these challenges, coverage enhancement techniques can be applied. LTE Rel-12 and Rel-13 introduced some techniques to achieved this goal. Some of employed enhancements in these releases and other possible enhancements are as follows.Retransmission: Data retransmission, using automatic repeat request (ARQ) or hybrid ARQ (HARQ), can be utilized to ensure the receiver can decode the message correctly. In the ARQ scheme, the receiver tries to decode the message by utilizing received information in the last transmission round, while in the HARQ scheme, the receiver utilizes all the received information to retrieve the message.Transmission time interval bundling: Transmission time interval (TTI) is the time unit for scheduling uplink and downlink transmissions. In the TTI bundling, several consecutive TTIs are combined to transmit data over a longer period. Hence, data transmissions can be performed with a lower rate, improving the success rate of decoding data.Frequency hopping: Through the frequency hopping, data can be transmitted over different frequency bands. This scheme alleviates the effects of frequency fading and provides more robust data transmissions.Power boosting and power spectral density boosting: In the downlink, the base station can increase the transmission power for devices with poor channel conditions. In the uplink, a device can employ power spectral density (PSD) boosting by concentrating the transmission power on a decreased bandwidth, which results in higher power density over the bandwidth.Relaxed requirements: Some control channel performance requirements can be relaxed for IoT devices, such as the minimum probability of decoding the random access response.Increasing reference signal density: The number of resources allocated for reference signal (RS) can be increased to provide better channel estimations.
Towards Connected Living: 5G Enabled Internet of Things (IoT)
Published in IETE Technical Review, 2018
Mamta Agiwal, Navrati Saxena, Abhishek Roy
Figure 5(b) shows the comparison of latency for legacy standards to the proposed shorter TTIs. 3GPP standards define a TTI of 1 ms for LTE networks [27] and the communication system has been designed with the same reference. However, recent 3GPP research efforts are focusing on the feasibility of shorter TTIs (TTIs ranging from 0.5 ms and 1 OFDM symbol) to meet the low-latency requirements of 5G networks. We evaluate latency, based on the time required for scheduling at evolved Node B (eNB), scheduling request, average delay to next scheduling opportunity, data processing for downlink, transmission, etc. We also considered different losses to account for good (1% loss), poor (30% loss), and average (10% loss) channel conditions. HARQ retransmission with a maximum of four downlink retransmissions is considered. The shorter TTI of 0.2 ms achieves around 81% reduction in delay compared to 1 ms of TTI in legacy networks. Thus, smaller TTIs in 5G are far more conducive for delay-sensitive IoT services (pointed out in Section 2.4), compared to LTE networks.