Hello, everyone!
Today, I would like to share with you an article about Cellular Network.
While cellular telephone technology had its roots in analog voice communication, data services based on cellular standards are now the norm. Like Wi-Fi, cellular networks transmit data at certain bandwidths in the radio spectrum. Unlike Wi-Fi, which permits anyone to use a channel at either 2.4 or 5 GHz (all you have to do is set up a base station, as many of us do in our homes), exclusive use of various frequency bands have been auctioned off and licensed to service providers, who in turn sell mobile access service to their subscribers.
The frequency bands that are used for cellular networks vary around the world, and are complicated by the fact that ISPs often simultaneously support both old/legacy technologies and new/next-generation technologies, each of which occupies a different frequency band. The high-level summary is that traditional cellular technologies range from 700-MHz to 2400-MHz, with new mid-spectrum allocations now happening at 6-GHz and millimeter-wave (mmWave) allocations opening above 24-GHz.
Like 802.11, cellular technology relies on the use of base stations that are connected to a wired network. In the case of the cellular network, the base stations are often called Broadband Base Units (BBU), the mobile devices that connect to them are usually referred to as User Equipment (UE), and the set of BBUs are anchored at an Evolved Packet Core (EPC) hosted in a Central Office. The wireless network served by the EPC is often called a Radio Access Network (RAN).
BBUs officially go by another name—Evolved NodeB, often abbreviated eNodeB or eNB—where NodeB is what the radio unit was called in an early incarnation of cellular networks (and has since evolved). Given that the cellular world continues to evolve at a rapid pace and eNB’s are soon to be upgraded to gNB’s, we have decided to use the more generic and less cryptic BBU.
The below figure depicts one possible configuration of the end-to-end scenario, with a few additional bits of detail. The EPC has multiple subcomponents, including an MME (Mobility Management Entity), an HSS (Home Subscriber Server), and an S/PGW (Session/Packet Gateway) pair; the first tracks and manages the movement of UEs throughout the RAN, the second is a database that contains subscriber-related information, and the Gateway pair processes and forwards packets between the RAN and the Internet (it forms the EPC’s user plane). We say “one possible configuration” because the cellular standards allow wide variability in how many S/PGWs a given MME is responsible for, making is possible for a single MME to manage mobility across a wide geographic area that is served by multiple Central Offices. Finally, while not explicitly spelled out in the below figure, it is sometimes the case that the ISP’s PON network is used to connect the remote BBUs back to the Central Office.
A Radio Access Network (RAN) connecting a set of cellular devices (UEs) to an Evolved Packet Core (EPC) hosted in a Central Office
The geographic area served by a BBU’s antenna is called a cell. A BBU could serve a single cell or use multiple directional antennas to serve multiple cells. Cells don’t have crisp boundaries, and they overlap. Where they overlap, an UE could potentially communicate with multiple BBUs. At any time, however, the UE is in communication with, and under the control of, just one BBU. As the device begins to leave a cell, it moves into an area of overlap with one or more other cells. The current BBU senses the weakening signal from the phone and gives control of the device to whichever base station is receiving the strongest signal from it. If the device is involved in a call or other network session at the time, the session must be transferred to the new base station in what is called a handoff. The decision making process for handoffs is under the purview of the MME, which has historically been a proprietary aspect of the cellular equipment vendors (although open source MME implementations are now starting to be available).
There have been multiple generations of protocols implementing the cellular network, colloquially known as 1G, 2G, 3G, and so on. The first two generations supported only voice, with 3G defining the transition to broadband access, supporting data rates measured in hundreds of kilobits-per-second. Today, the industry is at 4G (supporting data rates typically measured in the few megabits-per-second) and is in the process of transitioning to 5G (with the promise of a tenfold increase in data rates).
As of 3G, the generational designation actually corresponds to a standard defined by the 3GPP (3rd Generation Partnership Project). Even though its name has “3G” in it, the 3GPP continues to define the standard for 4G and 5G, each of which corresponds to a release of the standard. Release 15, which is now published, is considered the demarcation point between 4G and 5G. By another name, this sequence of releases and generations is called LTE, which stands for Long-Term Evolution. The main takeaway is that while standards are published as a sequence of discrete releases, the industry as a whole has been on a fairly well-defined evolutionary path known as LTE. This section uses LTE terminology, but highlights the changes coming with 5G when appropriate.
The main innovation of LTE’s air interface is how it allocates the available radio spectrum to UEs. Unlike Wi-Fi, which is contention-based, LTE uses a reservation-based strategy. This difference is rooted in each system’s fundamental assumption about utilization: Wi-Fi assumes a lightly loaded network (and hence optimistically transmits when the wireless link is idle and backs off if contention is detected), while cellular networks assume (and strive for) high utilization (and hence explicitly assign different users to different “shares” of the available radio spectrum).
The state-of-the-art media access mechanism for LTE is called Orthogonal Frequency-Division Multiple Access (OFDMA). The idea is to multiplex data over a set of 12 orthogonal subcarrier frequencies, each of which is modulated independently. The “Multiple Access” in OFDMA implies that data can simultaneously be sent on behalf of multiple users, each on a different subcarrier frequency and for a different duration of time. The subbands are narrow (e.g., 15kHz), but the coding of user data into OFDMA symbols is designed to minimize the risk of data loss due to interference between adjacent bands.
The use of OFDMA naturally leads to conceptualizing the radio spectrum as a two-dimensional resource, as shown in the below figure. The minimal schedulable unit, called a Resource Element (RE), corresponds to a 15kHz-wide band around one subcarrier frequency and the time it takes to transmit one OFDMA symbol. The number of bits that can be encoded in each symbol depends on the modulation rate, so for example using Quadrature Amplitude Modulation (QAM), 16-QAM yields 4 bits per symbol and 64-QAM yields 6 bits per symbol.
The available radio spectrum is abstractly represented by a 2-D grid of schedulable Resource Elements
A scheduler makes allocation decisions at the granularity of blocks of 7x12=84 resource elements, called a Physical Resource Block (PRB). The above figure shows two back-to-back PRBs, where UEs are depicted by different colored blocks. Of course time continues to flow along one axis, and depending on the size of the licensed frequency band, there may be many more subcarrier slots (and hence PRBs) available along the other axis, so the scheduler is essentially scheduling a sequence of PRBs for transmission.
The 1ms Transmission Time Interval (TTI) shown in the above figure corresponds to the time frame in which the BBU receives feedback from UEs about the quality of the signal they are experiencing. This feedback, called a Channel Quality Indicator (CQI), essentially reports the observed signal-to-noise ratio, which impacts the UE’s ability to recover the data bits. The base station then uses this information to adapt how it allocates the available radio spectrum to the UEs it is serving.
Up to this point, the description of how we schedule the radio spectrum is specific to 4G. The transition from 4G to 5G introduces additional degrees-of-freedom in how the radio spectrum is scheduled, making it possible to adapt the cellular network to a more diverse set of devices and applications domains.
Fundamentally, 5G defines a family of waveforms—unlike 4G, which specified only one waveform—each optimized for a different band in the radio spectrum. The bands with carrier frequencies below 1GHz are designed to deliver mobile broadband and massive IoT services with a primary focus on range. Carrier frequencies between 1GHz-6GHz are designed to offer wider bandwidths, focusing on mobile broadband and mission-critical applications. Carrier frequencies above 24GHz (mmWaves) are designed to provide super wide bandwidths over short, line-of-sight coverage.
These different waveforms affect the scheduling and subcarrier intervals (i.e., the “size” of the Resource Elements just described).
• For sub-1GHz bands, 5G allows maximum 50MHz bandwidths. In this case, there are two waveforms: one with subcarrier spacing of 15kHz and another of 30kHz. (We used 15kHz in the example shown in the above figure. The corresponding scheduling intervals are 0.5ms and 0.25ms, respectively. (We used 0.5ms in the example shown in the above figure.)
• For 1GHz-6GHz bands, maximum bandwidths go up to 100MHz. Correspondingly, there are three waveforms with subcarrier spacings of 15kHz, 30kHz and 60kHz, corresponding to scheduling intervals of 0.5ms, 0.25ms and 0.125ms, respectively.
• For millimeter bands, bandwidths may go up to 400MHz. There are two waveforms, with subcarrier spacings of 60kHz and 120kHz. Both have scheduling intervals of 0.125ms.
This range of options is important because it adds another degree of freedom to the scheduler. In addition to allocating resource blocks to users, it has the ability to dynamically adjust the size of the resource blocks by changing the waveform being used in the band it is responsible for scheduling.
Whether 4G or 5G, the scheduling algorithm is a challenging optimization problem, with the objective of simultaneously (a) maximizing utilization of the available frequency band, and (b) ensuring that every UE receives the level of service it requires. This algorithm is not specified by 3GPP, but rather, is the proprietary intellectual property of the BBU vendors.
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