5G wireless is the fifth generation cellular mobile technology based on specifications defined in Release 15 of 3GPP standards .
5G wireless is fundamentally transforming a radio network from pure wireless connectivity to the network for services. Mobile wireless access technologies have gone through several generations of evolutions to increase radio access capacity. The spectral efficiency has approaching Shannon capacity. However, there is enormous opportunity on support of various services. 5G wireless (3GPP TS 38.201-36.215 2017; 3G3GPP TR 38.801 2017; 3GPP TR 38.802 2017; 3GPP TR 38.803 2017; 3GPP TR 38.804 2017) is expected to support a variety of services more efficiently.
5G applications can range from Gigabit Society with personalized ultra-broadband TV and smartphone to autonomous car, connected home and cities, as well as remote sensors for agriculture and smart utility grid. It will dramatically improve our daily life. One example is healthcare. It could shift from curing to prevention. An ambulance could stop by your side at the moment you have a stroke or life-threatening event. Verizon State of the IoT Market 2017 report has confirmed the double digital growth and expected game change from 5G services (Verizon 2017; Yang et al. 2016).
This network transformation can support network slicing and differentiate services at access and transport nodes to ensure end-to-end performance, enabling fast service delivery. It gives us the ability to steer traffic in fine granularity to enforce policy-based routing and meet end-to-end Quality of Service (QoS) requirements of various applications.
5G wireless consists of an evolution of today’s 4G LTE (Long-Term Evolution) network as well as a new standardized radio access technology known as 5G New Radio (NR). The first standardization phase of NR is expected to be completed in 2018, with a sequence of releases following after. 5G wireless brings us opportunities for more innovative services and revenue streams and also more challenges to support those services efficiently and reliably.
5G wireless demands large bandwidth to achieve the gigabit per second (Gbps) user data rate. 5G NR can operate in the frequency ranging from hundreds of MHz to 100 GHz with paired and unpaired spectrum at more than 100 MHz bandwidth to achieve 10 Gbps targeted peak data rate. Field trials for 5G NR critical technologies are conducted at 4 GHz, as well as 27 GHz and 37 GHz. Thus, spectrum and resource management are extremely important.
The low-band in sub-6 GHz can be utilized for wide area services, while high-band for ultra-dense small cells or fixed wireless applications. 5G wireless needs to support licensed spectrum and leverage unlicensed and shared spectrum to increase throughputs and complement user performance.
Millimeter wave (mmWave) with extensive hundreds of MHz is critical for average user throughput larger than 1 Gbps in a loaded dense network with cell radius less than 100 m. mmWave and relies on massive MIMO (multiple inputs multiple outputs) to mitigate sensitivity to rain and fogs. Massive antenna system can form and track user-specific narrow beam to ensure service quality and reliability. This can mitigate a relatively large propagation loss and recover from blockages.
Dual connectivity will gracefully extend initial 5G to existing LTE wide area coverage. 5G could use licensed assisted access scheme similar to LTE to leverage more than 500 MHz unlicensed spectrum under 6 GHz and even more in mmWave.
Idle and connected mode mobility management is essential to ensure consistent user experience across the network. Those are challenging in a heterogeneous network with large range of inter-site distances and frequency bands. NR supports mobility without reconfiguration and make-before-break handover with multiple connections. Thus NR reduces the handover interruption time to zero from current LTE’s minimal 25 ms.
5G will bring in more flexibility on frequency and time allocation and allow flexible uplink and downlink multiplex to improve spectrum utilization. The 5G NR in Rel.15 will support more flexible Time Division Duplex (TDD) resource allocations; hence each user can have individually specified downlink and uplink time slots. In later releases, we are expecting even more flexible full dynamic Frequency Division Duplex (FDD) and TDD allocation, as well as full duplex radios to improve Integrated Access Backhaul.
Fundamental of 5G Wireless
5G NR specification is still under development in 3GPP (3GPP TS 38.201-36.215 2017). The technical studies (3G3GPP TR 38.801 2017; 3GPP TR 38.802 2017; 3GPP TR 38.803 2017; 3GPP TR 38.804 2017; Ericsson 2017) completed in March 2017 have summarized very well the agreed NR framework. In this section, we will provide high-level agreement of 5G wireless from access scheme, massive MIMO, and signaling aspects.
5G NR has a dynamic frame structure with flexible numerology that is scalable for a wide range of spectrums and applications. The key components consist of waveform, frame structure, modulation, and coding schemes.
5G NR waveform is Orthogonal Frequency Division Multiplexing (OFDM) with a cyclic prefix (CP) in both downlink and uplink up to at least 52.6 GHz. The same waveform on downlink and uplink has simplified the overall design, in particular, inclusive of wireless backhauling, device-to-device, and vehicle-to-X applications. Additionally, discrete Fourier transform spread OFDM (DFT-s-OFDM) is also supported in uplink to match LTE coverage. Windowing or filtering can be applied on top of OFDM to improve spectrum confinement and enable spectrum utilization higher than 90% of current LTE.
The NR frame structure supports a self-contained transmission; thus the reference signals required for data demodulation are included in the given slot or beam. It also supports well-confined transmissions in time and frequency, avoiding control channels mapping across full system bandwidth. It avoids static timing relation across slots and different transmission directions and removes the resource allocation limitation of predefined transmission time.
NR supports the same modulation schemes as that of LTE, which includes QPSK, 16-QAM, 64-QAM, and 256-QAM. It supports π/4-BPSK for DFT-s-OFDM for extended coverage. It will define 1024-QAM for fixed wireless applications. The final NR specification will cover a more extensive modulation scheme with different user equipment (UE) categories for diverse use cases.
NR supports low-density parity-check (LDPC) codes for the data channel and polar codes for the control channel. Those have evolved from traditional convolutional and turbo codes used in LTE. The LDPC has showed much superior performance at a wide range of coding rates. It supports high peak rate and low latency at high coding rate and high coding gain with high reliability at low coding rate. Polar codes are the first class of codes shown to achieve the Shannon capacity. It will be used for Physical Broadcast Channel (PBCH) and control information with shorter block length at a reasonable decoding complexity.
The massive MIMO provides a means to support high reliable services and achieve the three times spectral efficiency enhancement required by IMT-2020. NR beamforming can be applied to both data transmissions and initial access.
NR will define various antenna solutions depending on spectrum. For sub-6 GHz, most likely up to 32 transmitter chains with FDD spectrum are used. The spectral efficiency is improved by high-resolution channel state information (CSI) reporting and multiuser MIMO (MU-MIMO). For higher frequencies, a larger number of antenna (>128) can be supported in a given aperture. TDD spectrum- and reciprocity-based schemes are assumed. mmWave with an analog beamforming is typically limited to a single beam per slot per radio chain and thus requires beamforming on both transmitter and receiver. To support those diverse user cases across 100 GHz, NR defined a highly flexible and unified CSI framework to reduce the coupling among CSI measurement, CSI reporting, and the actual downlink transmissions. This framework also supports multipoint transmissions and coordination. The self-contained frame structure allows network to seamlessly change the transmission point or beam as devices moving across radio nodes.
Massive MIMO also brings challenges as the beamforming algorithms are complicated. The device and eNB complexity does not scale well with the number of increased antenna elements. RF and baseband design for massive MIMO at network, as well as at user equipment, are still at development stages. Mobility and connectivity over very narrow beams require fast tracking and switching. Front-haul bandwidth in tens of Gbps is required for the Gbps data transmission among radio, baseband, and network. Thus a lower-layer splitting with compression is necessary to enable advanced receiver at base station while maintaining reasonable front-haul throughput and latency requirement. The standardized lower-layer splitting will enable operator to mix and match radio units and base station processors from different vendors.
Dynamical antenna array configurations with flexible number of beams on vertical and horizontal planes are required to support various deployments as showed in Fig. 3. Simulations in 3GPP have shown that for a dense urban skyscraper with vertical traffic distribution, the vertical dominant antenna ports provided more gain, while for typical suburban low-rise deployment, more horizontal ports will provide high capacity. Those will serve as a stepping stone toward 5G by gradually increasing the number of antenna ports from current 4 to more than 32 transmission ports. This will prepare us for 5G massive number of antenna ports.
Reference Signals and Signaling Efficiency
5G NR has much lean reference signals and signaling overhead that translates directly to a better spectral efficient. The always-on broadcast signals are minimized. Common reference signals are replaced by UE-specific reference signals with improved feedback accuracy and less overhead.
The four main reference signals in NR are demodulation reference signal (DM-RS), phase-tracking reference signal (PTRS), sounding reference signal (SRS), and channel state information reference signal (CSI-RS). DM-RS is used to estimate the radio channel for demodulation. It is UE-specific, only transmitted on an as-needed basis, for example, the transmit interval could be long for low-speed user, while short for high-speed mobile users. PTRS is introduced to compensate UE-specific oscillator phase noise for mmWave. The SRS on uplink and CSI-RS on downlink are used to conduct CSI measurement for a scheduler and link adaptation. They will also be utilized for massive MIMO precoder design and beam management on uplink and downlink, respectively. It is expected 5G NR will improve spectral efficiency by at least 10% from those UE-specific reference signals.
5G NR also reduces signaling overhead by introducing a new RRC inactive state. Thus it can save control signaling by 15% and decrease signaling latency. It also replaces the always-on broadcast signals with on-demand system information.
This reduced signaling increases the system stability, particularly in an overloaded scenario (Yang et al. 2016). Thus, 5G NR is a more efficient, reliable, and robust communication means.
One Network for a Variety of Services
One operational network is a cost-efficient way to engineer, maintain, and operate radio networks from the perspective of capital and operational expense. The 5G wireless network can be used for eMBB, mMTC, and URLLC applications in a wide area radio access, D2D, V2X, and wireless backhaul use cases.
Wireless networks are evolved toward both centralized resource sharing and distributed edge mobile computing. Artificial intelligence is introduced to enable self-organizing at radio accesses, software-defined networking, and network function virtualization at core networks to allow the network to be more dynamic and intelligent with service and content awareness.
5G wireless will be a combination of evolved LTE network with new radio access technologies targeting for wide bandwidth, expanded spectrum, enhanced mobile broadband, as well as mission critical connections. This access network with intelligence at both edge and cloud can serve entire communities and industries, supporting both Gigabit Society and the Internet of Everything.
5G wireless will support varieties of applications from ultra-broadband services, to mission critical applications, as well as massive Internet of Things. It will support above 100 MHz instantaneous bandwidth from hundreds of MHz to GHz radio frequency ranges. 5G will profoundly enrich our daily life.
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