Long Term Evolution ( LTE ) and 4G Cellular Systems
Long Term Evolution (LTE) is the wireless cellular technology that is being adopted around the world as the primary cell phone communications service. Multiple 2G and 3G cellular radio methods are heading toward phase-out as carriers build their new LTE networks. It will be years before this expansion is complete, and the older radio technologies like GSM and CDMA will coexist with LTE for a while. In the meantime, the next phase of the LTE standards as put forth by the Third Generation Partnership Project (3GPP) is ready to be deployed. Called LTE-Advanced, it is a significant upgrade to the LTE standard that will provide more speed and greater reliability. Even though LTE-A is still being developed, it is already in service in some areas. This section is a review
of LTE basics, a look at the features and benefits of LTE-A, and a glimpse of what 5G systems may be like.
The Road to LTE
LTE is a standard developed by the Third Generation Partnership Project (3GPP). This is the international organization that developed the widely used 3G standards UMTS WCDMA/HSPA. LTE has been in development for years, and different phases of it have been released sequentially over the years. The final LTE standard designated Release 8 was completed in 2010. Release 9 was an update to that. Release 10 defines LTEAdvanced and is available now.
Over the years, multiple cell phone technologies have been developed. The first generation was analog (FM) technology, which is no longer available. The second generation (2G) brought digital technology with its benefits to the industry. Multiple incompatible 2G standards were developed. Only two, GSM and IS-95A CDMA, have survived.
The third-generation (3G) standards were created next. Again multiple standards were developed. Notably, the major ones were WCDMA by the 3GPP and CDMA2000 by Qualcomm. Both have survived and are still in use today. The 3G standards were continually updated into what is known as 3.5G. WCDMA was upgraded to HSPA and CDMA2000 was expanded with 13RTT EV-DO Releases A and B. Both are still widely deployed. In fact, in many places around the world, carriers are still adding 3G or upgrading their 3G systems. In the United States, AT&T and T-Mobile use GSM/WCDMA/ HSPA, whereas Verizon and Sprint use CDMA2000/EV-DO. Sprint implemented a network based on WiMAX, a technology similar to LTE, but is phasing it out. All of these carriers are now offering LTE.
LTE came into being as an upgrade to the 3G standards. Its major benefits were recognized by the cellular industry and were embraced by virtually all mobile carriers as the next generation. All cellular operators are now on the path to implementing LTE. While 3GPP still defines LTE as a 3.9G technology, all of the current LTE networks are marketed at 4G. The real 4G as designated by 3GPP is LTE-Advanced.
Currently LTE is alive and functioning in many U.S. cellular companies and in others worldwide. The networks are not fully built out, and most of the older 2G and 3G systems are still functioning in parallel. Because LTE coverage is not universal, most cell phones still incorporate 2G and 3G systems for voice in areas where LTE is not yet fully deployed. LTE-A deployment is a future rollout.
Why LTE? LTE brings amazing new capabilities to the cellular business. First, it expands the capacity of the carrier, meaning that more subscribers can be added for a given spectrum assignment. Second, it provides the high data rates that are needed by the growing new applications, mainly video downloads to smartphones and other Internet access. And third, it makes cellular connectivity more reliable. All of these needs are important to maintaining growth and profitability in the wireless business.
LTE is likely the most complex wireless system ever developed. It incorporates features that could not have been economically implemented even a decade ago. Today, with large-scale ICs, LTE can easily be accommodated not only in a base station but in a battery-powered handset. The complexity is a function of the advanced wireless methods used as well as the many options and features that can be implemented. This section examines mainly the physical layer of LTE, including modulation, access, duplexing, and the use of MIMO.
LTE operates in some of the existing cellular bands but also newer bands. Specific bands have been designated for LTE. These are shown in Table 20-1. Different carriers use different bands depending upon the country of operation and the nature of their spectrum holdings. Most LTE phones use two of these bands, and they are not the same from carrier to carrier. For instance, the iPhone 5 for Verizon uses different bands than the iPhone 5 from AT&T. Most of the bands are set up for frequency-division duplexing (FDD), which uses two separate bands for uplink and downlink. Note in Table 20-1 that bands 33 through 44 are used for time-division duplexing (TDD), and therefore the same frequencies are used for both uplink and downlink.
LTE is a broadband wireless technology that uses wide channels to achieve high data rates and accommodate lots of users. The standard is set up to permit bandwidths of 1.4, 3, 5, 10, 15, and 20 MHz. The carrier selects the bandwidth depending on spectrum holdings as well as the type of service to be offered. The 5- and 10-MHz widths are the most common. Some bandwidths cannot be used in different bands.
LTE uses the popular orthogonal frequency-division multiplex (OFDM) modulation scheme. It provides the essential spectral effi ciency to achieve the high data rates but also permits multiple users to share a common channel. The OFDM technique divides a given channel into many narrower subcarriers. The spacing is such that the subcarriers are orthogonal—that is, they will not interfere with one another despite the lack of guard bands between them. This comes about by having the subcarrier spacing equal to the reciprocal of symbol time. All subcarriers have a complete number of sine wave cycles that upon demodulation will sum to zero.
In LTE the channel spacing is 15 kHz. The symbol period, therefore, is 1/15 kHz = 66.7 μs. The high-speed serial data to be transmitted is divided up into multiple slower streams, and each is used to modulate one of the subcarriers. For example, in a 5-MHz channel, up to 333 subcarriers could be used but the actual number is less like 300. A 20-MHz channel might use 1024 subcarriers. The modulation on each can be QPSK, 16QAM, or 64QAM, depending on the speed needs.
Fig. 20-14 shows how OFDM uses both frequency and time to spread the data, providing not only high speeds but also greater signal reliability. For each subcarrier, the data is sent in sequential symbols where each symbol represents multiple bits (e.g., QPSK, 2 bits; 16QAM, 4 bits; and 64QAM, 6 bits.) The basic data rate through a 15-kHz subcarrier channel is 15 kb/s. With higher-level modulation, higher data rates are possible.
Data to be transmitted is allocated to one or more resource blocks. A resource block (RB) is a segment of the OFDM spectrum that is 12 subcarriers wide for a total of 180 kHz. There are seven-time segments per subcarrier for a duration of 0.5 ms. Data is then transmitted in packets or frames, and a standard frame contains twenty 0.5-ms time slots. A resource block is the minimum basic building block of transmission, and most transmissions require many RBs.
Keep in mind the fact that the only practical way to implement OFDM is to do it in software. The basic process is handled with the fast Fourier transform (FFT). The transmitter uses the inverse FFT, while the receiver uses the FFT. The algorithms are implemented in a digital signal processor (DSP), an FPGA, or an ASIC designed for the process. The usual techniques of scrambling and adding forward error correcting codes are implemented as well.
The choice of OFDM for LTE is primarily due to its lesser sensitivity to multipath effects. At the higher microwave frequencies, transmitted signals can take multiple paths to the receiver. The direct path is the best and preferred, but signals may be reflected by multiple objects, creating new signals that reach the receiver somewhat later in time. Depending upon the number of refl ected signals, and their strengths, ranges, and other factors, the signals at the receiver may add in a destructive way, creating fading or signal dropout.
The multipath effects occur when the signals reach the receiver all within the time for one symbol period. Remember, a symbol is a modulation state that is either an amplitude, a phase, or amplitude-phase combination that representing two or more bits.
If the multipath effects are such that the signals arrive at the receiver spread over several symbol periods, the outcome is called intersymbol interference (ISI). The result is bit errors. This can be overcome with error detecting and correcting codes, but these codes add to the complexity of the system. An equalizer at the receiver that collects all the received signals and delays them such that they all add can also correct for this problem but only further complicates the process.
Spreading the signals in the form of multiple subcarriers over a wide bandwidth lessens these effects. This is especially true if the symbol rate of each subcarrier is longer, as it is in OFDM. If the multipath effects occur in less than one symbol period, then no equalizer is needed. Another effect is frequency variation of the subcarriers at the receiver caused by time or frequency shifts, such as that produced by the Doppler effect in a moving vehicle. This shift in frequency results in the loss of orthogonality and subsequently in bit errors. This problem is mitigated in LTE by adding a cyclical prefix (CP) to each transmitted bit sequence.
The CP is a portion of an OFDM symbol created during the DSP process that is copied and added back to the front of the symbol. This bit of redundancy allows the receiver to recover the symbol if the time dispersion is shorter than the cyclical prefix. This allows OFDM to be implemented without the complex equalization that can also correct this problem.
While the downlink of LTE uses OFDM, the uplink uses a different modulation scheme, known as single carrier-frequency division multiplexing (SC-FDMA). OFDM signals have a high peak-to-average-power ratio (PAPR), requiring a linear power amplifier with overall low efficiency. This is a poor quality for a handset that is battery-operated. SC-FDMA has a lower PAPR and is better suited to portable implementation. The SC-FDMA process is complex, and a detailed discussion of it is not included here.
A key feature of LTE is the incorporation multiple input multiple output (MIMO), a method of using two or more antennas and related receive and transmit circuitry to achieve higher speeds within a given channel. One common arrangement is 2 x 2 MIMO, where the first number indicates the number of transmit antennas and the second number is the number of receive antennas. Standard LTE can accommodate up to a 4 x 4 arrangement.
The MIMO technique divides the serial data to be transmitted into separate data streams that are then transmitted simultaneously over the same channel. Because all signal path are different, with special processing they can be recognized and separated at the receiver. The result is an increase in the overall data rate by a factor related to the number of antennas. This technique also mitigates the multipath problem and adds to the signal reliability because of the diversity of reception.
A special version of MIMO known as multi-user MIMO is being implemented in some cellular systems as well as in some Wi-Fi systems. It allows multiple users to share a common channel. MU-MIMO relies upon many antennas implementing spatial division multiple access (SDMA) as well as unique channel coding to keep the signals separate. Channel bandwidth is divided amongst the users as needed.
The difficultly in implementing MIMO arises because of the small size of the handset and its limited space for antennas. Already most smartphones contain five antennas, including those for all the different cellular bands plus Wi-Fi, Bluetooth, GPS, and perhaps NFC. It is not likely that most phones will contain more than two LTE MIMO antennas, and their inclusion depends on being able to space them far enough apart so that spatial diversity is preserved with sufficient isolation between them. Of course, it is easier to use more base station antennas. A typical LTE arrangement appears to be 4 x 2 to provide optimal coverage with the space available.
The data rate actually used or achieved with LTE is dependent upon several features, as you have seen. It depends on channel bandwidth, modulation type, MIMO configuration, and of course the quality of the wireless path. In the worst-case situation, data rate could be only a few Mbps, but under good conditions data rate can rise to over 300 Mbps. On average, most practical LTE downlink rates are in the 5- to 15-Mbps range. This is faster than some fixed Internet access services using cable or DSL.
Access refers to using the same channel to accommodate more than one user. This is effectively a multiplexing method. Standard methods include frequency-division multiple access (FDMA), time-division multiple access (TDMA), and code-division multiple access (CDMA). OFDMA makes use of some of the available subcarriers and time slots within those subcarriers for each user. The number of subcarriers and time slots used depends on multiple factors. In any case, it is usually possible to accommodate up to hundreds of users per channel bandwidth.
Most LTE will be of the FDD variety, at least in the United States, Europe, and parts of Asia. Paired-spectrum groups are required for the separate uplink and downlink transmissions. TD-LTE requires only a single spectrum segment. TD-LTE is being widely implemented in China and India because of the more limited spectrum availability. This conserves the spectrum and provides for more users per MHz. The LTE standards include a definition for TD-LTE. It is expected that some U.S. carriers, including Sprint, will use TD-LTE.
LTE-A builds on the LTE OFDM/MIMO architecture to further increase data rate. It is defined in 3GPP Releases 10 and 11. There are five major features: carrier aggregation, increased MIMO, coordinated multipoint transmission, Hetnet (small cells) support, and relays. Small cells are covered in the next section.
Carrier aggregation refers to the use of two or more 20-MHz channels combined into one to increase data speed. Up to five 20-MHz channels can be combined. These channels can be contiguous or noncontiguous as defined by the carrier’s spectrum assignments. With maximum MIMO assignments, 64QAM, and 100 MHz bandwidth, a peak downlink data rate of 1 Gbps can be achieved.
LTE defines MIMO configurations up to 4 x 4. With LTE-A that is extended to 8 x 8 with support for two transmit antennas in the handset. Most LTE handsets use two receive antennas and one transmits antenna. These MIMO additions will provide future data-speed increases if adopted.
HetNet support refers to support for small cells in a larger overall heterogeneous network. The HetNet is an amalgamation of standard macrocell base stations plus microcells, microcells, picocells, femtocells, and even Wi-Fi hot spots. This network provides increased coverage in a given area to improve connection reliability and increased data rates. Small cells are covered in the next section.
Coordinated multipoint transmission is also known as co-operative MIMO. It is a set of techniques using different forms of MIMO and beamforming to improve the performance at cell edges. It makes use of coordinated scheduling and transmitters and antennas that are not collocated to provide greater spatial diversity that can improve link reliability and data rate.
Relays is the use of repeater stations to help coverage in selected areas, especially indoors where most calls are initiated. LTE-A defines another base station type called a relay station. It is not a complete base station but a type of small cell that will fit in the HetNet infrastructure and provide a way to boost data rates and improve the dependability of a wireless link.
Some deployment of LTE-A occurred in late 2013 with increasing adoption in 2014 and beyond. LTE-A is forward and backward compatible with basic LTE, meaning that an LTE handset will work on an LTE-A network and an LTE-A handset will work on a standard LTE network.
LTE solves many problems in providing high-speed wireless service. There is no better method, at least for now. However, it does pose multiple serious design and deployment issues. The greatest problem to overcome is the necessity of having to use multiple bands that often are widely spaced from one another. As a result, this requires multiple antennas, multiple power amplifiers, multiple filters, switching circuits, and sometimes complex impedance matching solutions in the base stations as well as the phones themselves. Each cellular operator specifies cellphones for their spectrum.
In addition, the power amplifiers (PAs) must be very linear if error vector magnitude (EVM) is to be within specifications for the various multilevel modulation methods used. Linear amplifiers are inefficient, and because of this, they are the biggest consumer of power in the phone except for the touch screen. The need to cover multiple bands necessitates the use of multiple PAs. Battery life in an LTE phone is typically shorter as a result. The need to include MIMO also means additional antennas and PAs.
Solutions to these problems lie in more-efficient power amplifiers and fewer of them. Also, wider-bandwidth antennas solve the multiband problem. Tunable antennas are also being designed by several companies to cover multiple bands with a single structure. Another challenge is testing. Testing LTE systems with MIMO is a particularly complex process. Luckily a number of test equipment companies have created systems for this purpose. One of the greatest challenges is testing the higher-level MIMO configurations. LTE-A permits up to 8 x 8 MIMO.
Voice over LTE
LTE is a packet-based IP data network. Initially, LTE did not include a voice service. However, LTE voice is now being implemented. Today, if you are using an LTE smartphone, you may still be using the existing 2G or 3G network for what is called circuit-switched voice service. Eventually, voice-over LTE (VoLTE) will be fully implemented in the base stations and in the handsets. VoLTE is just VoIP over LTE, and it will operate simply as a data application on the IP network.
Although a VoLTE protocol has been defined, implementing it requires major engineering decisions and network changes. Most of these concerns maintaining voice connections for older non-LTE phones for some extended period. Particularly tricky are the changes that will allow LTE phone users to get voice service if they move out of an area with no LTE. When VoLTE is available, a subscriber could initiate a call using the LTE system but drive out of the LTE coverage area. Systems must be able to hand that call off to a traditional voice network. The mechanism for this is network software called circuit-switched fallback (CSFB). It is now available on most networks. Another issue is getting VoLTE into the handsets. VoLTE requires a separate chip in the phone, and few have such a capability today.
Implementing VoIP requires a vocoder, a circuit that is essentially an A-to-D converter to digitize the voice signal and a D-to-A converter to convert the digital voice back into analog voice for the user. A vocoder also incorporates voice compression, a technique that effectively minimizes the number of bits used to represent voice. This allows voice to be transmitted faster but at lower data rates so it does not occupy much bandwidth.
LTE uses what is called the Adaptive Multi-Rate (AMR) vocoder. This is also used in GSM systems and other 3GPP standards. It has a variable bit rate capability from 1.8 kbps to 12.2 kbps. Digitized voice is then assembled into AMR packets and then into IP packets, which are then scheduled into transmission sequence. A call is allocated to some of the OFDMA subcarriers and some of the time slots within the bitstreams of each subcarrier.
The most sophisticated cell phone available today is called a smartphone. It incorporates LTE and a wide range of other wireless technologies. Chances are that you already own a smartphone. Here is a brief analysis of what’s inside the typical modern smartphone.
Refer to the block diagram of a typical smartphone as shown in Fig. 20-15. Most cell phones today are no longer just two-way radios for making phone calls. Instead, at the core of the smartphone is a dual processor or quad processor microcomputer that implements all of the various functions. Today’s smartphone is in reality a general-purpose microcomputer that just happens to have cell phones with other communications capabilities built-in. The available computing power available has enabled the development of a wide range of software applications (apps). These apps give the smartphone capability far beyond its original function.
Color Touch Screens
The prominent feature of every smartphone today is its large color screen. These high-resolution displays are made with different technologies, such as LCD or OLED. All of them today are touch screens, meaning that all controls such as buttons, keyboards, and other operational tools are operated by pressing or swiping the screen.
Most cell phones and most smartphones contain a digital camera. These cameras use a CCD or CMOS light sensor and a lens to capture a scene and store it in fl ash memory. These cameras have resolutions from 5 to 40 megapixels, therefore rivaling stand-alone cameras. Many of the cameras are accompanied by an LED flash. While these cameras are primarily for still photos, most are capable of recording real-time video. Many smartphones also have a second, front-facing camera for making video phone calls and taking selfies.
Virtually all smartphones today contain LTE 4G technology. The newer phones use a two-antenna MIMO arrangement. The phone also usually contains a 2G and/or 3G radio for voice calls as well as the backward capability for calls in a region without LTE service.
Bluetooth is a short-range radio technology to be discussed in the next section. However, its primary function in most cell phones is to implement a wireless headset and microphone. The Bluetooth radio can also be used with iBeacon technology that allows cell phones to read nearby information tags and Bluetooth location devices. It could also possibly be used with cashless payment systems that replace credit cards. Bluetooth is also used as a connecting device for smartwatches, a wearable accessory to many smartphones. The smartwatch provides call, text, or e-mail alerts, time, and other information without taking the smartphone out of your pocket or purse. See Fig. 20-16.
Wi-Fi is the wireless local area network (WLAN) technology widely used throughout the world to implement Internet connections. Virtually all smartphones contain a Wi-Fi transceiver and may include two antennas for MIMO. The Wi-Fi capability permits smartphones to connect to any available access point or hot spot, including wireless routers that are part of many home broadband Internet connections today. Wi-Fi is covered.
another radio inside of most smartphones is a GPS receiver. This gives the smartphone navigation capability. The smartphone is usually accompanied by mapping software that provides vivid visual displays of maps tied to the GPS navigation satellites.
NFC is near field communications, another short-range radio technology built into some smartphones. It is used in payment systems that eliminate cash and credit cards. NFC is not on all cell phones.
In addition to being able to make a phone call to any other phone anywhere in the world, all smartphones have Internet access. The operating system software includes a browser to allows you to access the Internet just as you would from a desktop PC or laptop. The only disadvantage is the small screen.
Another common feature is e-mail. Since you can access the Internet, you can easily access your regular e-mail account from any location. Texting is another common feature that is used more often than e-mail.
Music playback is a part of all smartphones. Virtually all smartphones have the ability to store songs and music videos in compressed form for playback on built-in speakers or through headphones.
One of the most popular features of modern smartphones is the ability to play videos. Thanks to digital compression technology, the video presentation is very common. By way of an Internet connection, you can watch videos from multiple sources, such as Google, YouTube, Netflix, Hulu, and other video sources for movies and other short subjects.
Another application that is possible thanks to video a and high-resolution display is games. There are literally thousands of games available for smartphones today, and these are widely used.
All of these features are possible because of the internal processing power. Most smartphones contain a processor that contains two cores, each core is a full 32-bit microcontroller. Some phones also have larger quad-core processors, and many incorporate 64-bit processing power as well as 32-bit processing power. These multiple processors implement the DSP required of most phones today. All cell phones are software-defined radios, and virtually all of the modulation, demodulation, encoding, decoding, voice, and video compression and other functions are handled by the processors. The processors also run the operating system and any application software that is added later. The processors are usually accompanied by standard DRAM and flash memory. A variety of special serial and parallel interfaces are provided to connect to the display, cameras, and
Battery and Charger
A key part of every cell phone is the battery power supply and its power management hardware. The lithium-ion battery operates a whole array of multiple voltage regulators and DC-DC converters that supply all of the different operating voltages to the ICs, interfaces, and peripheral devices. The power management system also shuts down unneeded circuits, such as the display, when they are not in use.
In addition, all phones include a battery charger. The battery charger operates from an external AC power source. Some newer phones include wireless battery charging. This system uses magnetic induction to connect the cell phone to its charger. Both the charger and the cell phone have built-in coils that serve as windings on a transformer. By putting the coils close together, the transformer action transfers the AC power from the charger to the charging circuitry inside the cell phone by magnetic induction, so that no wire connection is needed.
What Is 5G?
It will take a decade or more for LTE and especially LTE-A to dominate in cellular coverage. Furthermore, new LTE releases from 3GPP are yet to come. In addition, some provisions of current LTE releases have yet to be implemented. An example is self-organizing networks (SONs), a feature that makes networks easier to plan, configure, optimize, and manage. With a SON, all base stations would be self-configuring, taking into account nearby base stations and using internal algorithms to heal, self-optimize, and adapt to new nearby stations and other conditions. The small-cell movement to be discussed in the next section is definitely LTE-based and extensive deployment with SON is yet to come.
In the meantime, research continues with what is the fifth generation (5G) of cellular wireless. It is possible that 5G will simply stay on the same path as 4G and LTE— that is, that 5G will use higher frequencies and wider bandwidths to achieve even higher data rates. With semiconductor technology still viable at ever smaller IC feature sizes, operation well into the hundreds of GHz is possible. Already, advanced millimeter-wave (30- to 300-GHz) systems are functioning with advanced chipsets in short-range personal area networks (PANs) for home video transfer (60 GHz), automotive radar (77 GHz), and cellular/hot spot backhaul (80 GHz). Some think the 28-GHz and 38-GHz spectrum segments offer good opportunities for cellular. Because of the higher frequencies, the range is shorter, meaning that there are more but smaller cell sites. However, by using higher-level MIMO, higher-gain antenna arrays and beamforming, coverage will be reliable and the available bandwidth will permit download data rates as high as 10 Gbps. In summary, 5G will most likely be many small cells operating in one or more millimeter-wave bands using smart, steerable, high-gain antennas.
Base Stations and Small Cells
Base stations, also called cell sites or macro cells, form the heart of the cellular network. They implement all radio connectivity and operational and control functions and connect to the carrier’s telephone network as well as the Internet by way of backhaul equipment. A key trend today is the incorporation of small cells to complement the existing base stations to improve coverage and data speeds.
The most complex and expensive part of any cellular telephone system is the network of base stations that carriers must have to make it all work. Over the years, carriers have added many more base stations to handle the constantly growing number of subscribers. In addition, each base station has expanded and become more complex because of the growing number and variety of radio standards it must handle. Base stations must continue to support 2G technologies, 2.5G enhancements, and now 3G systems. Support for multiple standards has led to considerably more equipment. An effort has been made by base station manufacturers to consolidate the equipment by using software-defined radio techniques and DSP. These methods permit the base station receivers to accommodate existing multiple standards and to be able to work with new standards by reprogramming rather than replacing equipment.
Base stations consist of multiple receivers and transmitters so that many calls can be handled on many different channels simultaneously. The transmitters in the cell site are much more powerful than those in the handsets. Power levels up to 40 W are typical. These power levels are achieved with highly linear broadband class A or class AB power amplifiers. Since LTE and CDMA systems require linear amplification, base stations are using digital pre-distortion (DPD) and envelope tracking (ET) to improve efficiency.
Superior linearity is critical, especially in CDMA and LTE systems that cover a broad spectrum. Nonlinearities produce intermodulation and spurious signals that can make a system inoperable.
The most visible feature of a base station, of course, is its antenna on a tower. The antennas used by base stations must serve many transmitters and receivers by means of isolators, combiners, and splitters. Base station antennas have become directional rather omnidirectional, as the cell patterns suggest. This “sectorization” of the cell site has helped to increase subscriber capacity with minimal cost. Most base stations use a triangular antenna array that looks like the one shown in Fig. 20-17. On each side of the triangular frame is an array of three vertical antennas forming broadside or collinear array that may also use reflectors. Each of the three arrays produces a gain of about 8 dB and an antenna pattern with a beamwidth of 120°. This divides the cell coverage into three equal sectors. See Fig. 20-18. This directional capability provides excellent isolation of the three sectors, which in turn permits the same channel frequencies to be used in each of three 120° sectors. Some sites also use two supplementary antennas that provide spatial diversity, which greatly improves the reception of weak signals from the handset. Carriers are beginning to use more sophisticated “smart” beam-forming antennas with more elements, arrays, and sectors to further improve capacity.
A key trend is packaging all the RF transmit and receive circuitry, including power amplifiers, into a small housing that is mounted at the antennas at the top of the tower. These are called remote radio heads (RRHs). They eliminate the long, expensive, and high-loss transmission lines, thereby greatly improving base station power efficiency. The base station equipment is connected to the RRH by way of a digital fi ber interface called the Common Public Radio Interface (CPRI).
Small Cells and HetNet Rationale
A small cell is a miniature cellular basestation with limited power and range. Its function is to complement the larger macro basestations now the most common form of cell site. This permits greater subscriber capacity as well as higher data speeds for all concerned. Small cells are a growing trend that is expected to have its greatest impact in the future. Currently, cell phone traffic is handled by a huge network of cell sites called macro basestations (BS). With their high power, tall towers with multiple antenna arrays, long-range, and backup power sources, these macro BS cover most of the United States except for some very rural and geographically challenged areas. There are nearly 300,000 such basestations in the United States. It is getting increasingly more difficult to find and secure suitable locations for such BS. The solution is the small cell.
In addition, the success of the smartphone and the growing subscriber demand for more and faster service are putting the pressure on cellular carriers to expand and upgrade their BS deployments. The carriers’ response has been to upgrade their systems with the 4G LTE technology that offers signifi cantly faster download speeds demanded by the major increase in video consumption. LTE is still being installed with full coverage not expected for several more years. One clear solution is the LTE small cell.
While the adoption of LTE will boost capacity and speed, limits are being reached. The OFDM of LTE along with advanced modulation methods and MIMO have pushed the spectral efficiency (bits/Hz/Hz) of the cellular system to the Shannon limit. LTE Advanced will improve speeds by providing more bandwidth through carrier aggregation. The ultimate limit is the spectrum available to the carrier. Again small cells and frequency reuse can provide an interim solution until more spectrum is freed up.
Another major issue is the indoor performance of cell phones. It has been shown that over 80 percent or all cell phone calls are carried out indoors, in homes, offices, shopping malls, hotels, and other venues. Indoor performance is significantly poorer than outdoor performance as the radio signals are seriously attenuated, distorted, and redirected by walls, ceilings, floors, furniture, and other obstacles. Indoor situations limit the range of the radio and furthermore greatly curtails data speeds. LTE is helpful in overcoming this problem, but the real solution is the small cell.
As it turns out, public and private Wi-Fi hotspots and access points fit the basic definition of a small cell. They can connect to a user’s smartphone, tablet, or laptop and provide access to the video and other information and media demanded by the user. You don’t have to use the cellular network to download video or access other big data applications if a Wi-Fi hotspot is nearby. Most cellular small cells will include a Wi-Fi access point.
Groups of physically small cells can be installed anywhere, indoors or out. They can sit on a desk or be mounted on a wall, roof, lamp post, or light pole. The small cells fill in the gaps in coverage and provide service where macro cell coverage is poor. In a high-density population, cities with tall buildings are examples.
It is estimated there will be from 5 to 25 small cells per macrocell in most networks. Networks of small cells overlay, or as some say underlay, the macro network to provide an overall boost not only in data speeds but also subscriber capacity. The general customer performance is greatly improved with more reliable connections and significantly higher download speeds.
Another piece of the small cell trend is distributed antenna systems (DAS). DAS uses fiber optic cable from a macro BS to an array of antennas spread over a wide area to extend the reach and improve connection reliability. It is used in large buildings, airports, convention centers, sports complexes, and other large public venues. The collection of macro BS, small cells, Wi-Fi hotspots, and DAS is now referred to as heterogeneous network or HetNet.
All small cells with use the existing licensed spectrum assigned to the carrier’s networks. The limited spectrum is shared by the method of frequency reuse and spatial diversity. Frequency reuse refers to the use of the same band by multiple cell sites. Spatial diversity means that these sites are spaced from one another so that coverage areas do not overlap and power levels are controlled to eliminate or minimize interference to adjacent cells and those on the same frequency.
Small Cells Defined
There are several different sizes and versions of small cells. They vary in the number of users they can handle, their power and range. In virtually all cases, they all include the essential 3G technologies of the carrier, LTE and Wi-Fi. They have a power source and a backhaul connection to the cellular network. Table 20-2 shows the general names and capabilities of each major classification.
The smallest is the femtocell. A femtocell is a single box BS used by the consumer or small offices to improve local cellular service. Femtos have been around for years, and millions have been installed by most of the larger carriers. Backhaul is by way of the customer’s high-speed Internet connection via a cable TV or DSL telecom provider.
There are also enterprise femtos that handle more users and provide a significant boost in indoor accessibility.
There are progressively larger small cells such as the picocell, microcell, and metro cell, each with increasing capacity, power and range. Virtually all handle legacy 3G, LTE and include Wi-Fi. Many of the future small cells will also feature LTE-Advanced, the faster version of this 4G technology. See Table 20-2 for details.
Inside the Small Cell
A small cell is still a cellular base station but boiled down to only a few key chips and circuits. Thanks to super fast multicore processors, most 3G and LTE baseband operations are easily handled by a single IC. This baseband IC is then connected to the RF circuitry making up the radio transceiver. A general block diagram is shown in Fig. 20-19.
The RF transceiver called the analog front-end consists of the receiver (RX) and transmitter (TX). The receiver gets its signal from the antenna amplifies it in a low noise amplifier (LNA) and sends the signal to I/Q mixers forming a demodulator that recovers the signal. The signals are passed to analog-to-digital converters (ADCs) that create the input to the digital front-end processor.
Between the RF front-end and the baseband processor is additional circuitry that performs decimation, digital upconversion (DUC), and digital downconversion (DDC). Other digital processing includes crest factor reduction (CFR), digital pre-distortion (DPD), and envelope tracking (ET). DPD and ET are used for the linearization of the RF power amplifiers to improve efficiency. All this circuitry may be in a separate ASIC or FPGA or may be included in the baseband chip or front-end RF circuitry.
The baseband processor creates the digital I/Q signals for the transmitter. These go to digital-to-analog converters (DACs) in the analog front-end that produce the analog equivalent signals. These are sent to I/Q mixers that form a modulator. The modulator output is then sent to one or more power amplifiers (PAs) and then to the antenna.
The baseband processor has both multiple standard CPUs and digital signal processors (DSP) and handles all the modulation and demodulation and other process involved with the various cellular standards. The I/O to the backhaul is typically by Ethernet. Power comes from a power over Ethernet (PoE) connection if available or by some other source.
Fig. 20-20 shows an example of single chip analog front-end. This is the MAX2580 by Maxim Integrated. It contains the I/Q modulator and demodulator as well as their fractional-N frequency synthesizers for channel selection. The multiple circuits support 2 x 2 MIMO. The synthesizers cover all LTE bands 1 to 41 and provide for bandwidth selection from 1.4 MHz to 20 MHz. The ADCs and DACs are included on chip. The digital interfaces to the baseband processor are the JESD207.
The receiver section contains the LNAs however additional external LNAs could be added if necessary. The transmitter output amplifiers provide 0 dBm. If more power is needed external power amplifiers can be added. Maxim makes a wide range of other RF circuits including the MAX2550-MAX2553 3G femtocell transceivers for CDMA systems.
The baseband functions in a base station or small cell is implemented by a single chip containing multiple processors, memory, logic and interface circuits. An example from Texas Instruments is the KeyStone line these processors contain a mix of ARM A15 RISC processors as well as TIs C66x DSPs. One typical unit is the TCI6630K2L shown in Fig. 20-21. It contains two ARM cores and four C66x DSPs. It also contains multiple logic accelerators to speed up operations, minimize the number of cores and reduce power consumption. This chip also includes digital front-end circuits like DUC/DDC/DPD/CFR. Multiple interfaces include the JESD240B, PCIe, SPI, USB, and a gigabit Ethernet switch.
Finally, most small cells will generally contain Wi-Fi and GPS. Power will come from Power over Ethernet (PoE) plus a mix of DC-DC converters and regulators. Most power is consumed by the RF power amplifiers and the baseband processor SoC.
Timing and Synchronization
A key requirement of all LTE base stations, macro or small cell, is timing and synchronization of all the radios in the network. Timing and synchronization are essential to achieve the specifications of the Third Generation Partnership Project (3GPP), the organization establishing the LTE standards. The timing and synchronizations are implemented by delivering a formatted clock signal to the radio circuits of the base station. These signals are then used to create the phase and frequency components of the LTE modulation. Timing and synchronization are also essential for proper handoff and backhaul coordination.
Several timing and synchronization methods have been developed including synchronous Ethernet (SyncE) or G.8262 by the ITU and the IEEE’s Precision Time Protocol (PTP) 1588–2008. The latter seems to be the preferred method, but both are used. The PTP can be delivered with a grandmaster clock in the form of a timestamp over the packet network. An alternative is Network Time Protocol (NTP) that is designed to synchronize the clock to some time reference over a variable latency data network.
The timing requirements for implementing LTE in the network are severe. The typical clock precision required is 16 parts per billion (ppb) in the transport network and 50 ppb in the air interface. For TDD and Advanced versions of LTE, the phase requirements are also critical. In addition, the method of backhaul will vary with different types of small cells. Many will use microwave, others will use fiber and in residential femtos DSL or cable TV provide the backhaul. Different timing schemes are needed to optimize the performance.
The LTE Advanced small cells to come also have strict timing and synchronization requirements to implement the key features of enhanced inter-cell interference coordination (eICIC) and coordinated multipoint transmission/reception (CoMP) both a type of interference management technology required for self-organizing networks (SON).
The concept of Wi-Fi offload is simple. It is the formal use of available hotspots and access points to carry the high-speed data relieving the cellular network of that burden. Since all smartphones have Wi-Fi, it is possible to create a system that automatically selects Wi-Fi for a fast download if a hot spot is nearby. While a subscriber could voluntarily access the data with Wi-Fi, he or she may not be aware of a useable hotspot. By offloading the cellular system, that network can handle more users with high-speed data needs that cannot be addressed with Wi-Fi. Today, the users can automatically offload the network themselves by actively seeking and available hotspot to avoid the cost of using the cellular network. Otherwise, an automatic carrier-driven approach can be implemented to make the offloaded work seamlessly when a user accesses a high-volume download.
While the ultimate solution is to roll out a small cell underlay to increase capacity and coverage, Wi-Fi offers an immediate solution to the demand for faster downloads. Since high-speed traffic like video is growing faster than the carriers can implement a full small cell system, Wi-Fi offers a fast and inexpensive way to deal with the problem.
To make this work, several things must happen. First, cellular operators will have to partner with existing Wi-Fi providers in their coverage areas. Alternately, the cellular operators will need to build out their own Wi-Fi networks. Many have already constructed their own Wi-Fi networks to ensure the desired coverage. Wi-Fi networks are significantly less expensive to install than cellular base stations, including small cells. And they are usually faster than most even 4G networks. These cellular operator Wi-Fi networks are referred to as carrier-grade networks
Second, some mechanism is needed to initiate an automatic selection of Wi-Fi vs. cellular network when a subscriber attempts to access some source of video or other big data. A subscriber’s smartphone or tablet will seek out the available networks then select the best option mostly favoring Wi-Fi if it is available. It has been estimated that up to 50 percent of cellular data traffic will eventually be offloaded to Wi-Fi giving carriers time to roll out more small cells, expand their LTE networks, or add new spectrum while minimizing capital expenditures (CAPEX).
The mechanism for this is now available in the form of the Wi-Fi Alliance’s Hotspot 2.0 and the IEEE 802.11u standard. First, 802.11u is a relatively recent enhancement to the 802.11 WLAN standards. It enables Wi-Fi to work with other networks including cellular networks. The enhancement essentially automates the connection between a smartphone, tablet or laptop to different Wi-Fi networks. It replaces the process of discovering nearby hotspots, entering passwords, performing authentication, and connecting. Next, Hotspot 2.0 is an addition to the basic standard that uses 802.11u to automate access point discovery, registration, and provisioning and connection. This not
only enables roaming between hotspots but also provides a mechanism to link to cellular networks to perform automated handoff between the cellular network and available Wi-Fi hotspots.
Virtually all 3G/4G small cells will also include carrier-grade Wi-Fi either in the same enclosure or in an adjacent box. Furthermore, the newer cell phone models will incorporate Hotspot 2.0/802.11u to make the offload option function. And no doubt the cellular operators see the offload strategy as a way to buy time until more wireless spectrum is available or as funds are available to acquire it.
Backhaul is the name of the connection of a cell site to the core network. Older original base stations used T1 or T2 lines, but today most macro BS in the United States use fiber optic cable. Some in harder to reach areas use a microwave link. Fiber is preferred, of course, as it is fast and reliable. However, it is costly to install as it requires access to property, digging in the ground, or permission to use power poles. Microwave is simply line of sight (LOS) point and shoot wireless. It is not as fast but that limitation is gradually going away with the new systems. With most microwave links, a data capacity up to 1 Gb/s is usually available. Small cell backhaul will most likely be a mix of fi ber and wireless. If fiber is available and affordable it will be used. Otherwise, a wireless link will be the backhaul of choice.
Small cell backhaul will sometimes be tricky. With small cells on lamp posts, sides of buildings and in other odd locations, fiber or even AC power may be hard to come by. A wireless link may be the only choice. And even that could be a challenge in large cities with tall buildings and other structures blocking most paths back to the core network. Multiple hop links may be used in some instances.
The most popular wireless backhaul frequencies are 6, 11, 18, and 23 GHz. These frequencies require a license to use, and equipment is generally expensive. However, other potential bands are the 60 GHz band and 70/80 GHz E-band. The 80 GHz band does also require a license, but the 60 GHz band does not. The 60 GHz band (57-64 GHz) is an industrial-scientific-medical (ISM) band that is open to any service.
The millimeter-wave bands above 30 GHz offer lots of bandwidth to support higher data rates; their range is severely limited by the physics of their short wavelength. However, with high gain directional antennas and higher power, ranges can extend to several kilometers. Just recently the FCC modified the Part 15 rules and regulations to permit higher power and antenna gains in the 60 GHz band to make it more useful for small cell backhaul. Fig. 20-22 shows one kind of small cell backhaul unit. It mounts next to the small cell enclosure.
Self-organizing networks (SON) are a software solution to managing a HetNet. While the interaction between macrocells is usually managed manually, with multiple small cells such a manual task is overwhelming. With SON, the HetNet will essentially manage itself. SON can automate confi guration and dynamically optimize the network based on the traffic loads.
SONs can be categorized by their three basic functions: self-configuration, selfoptimization, and self-healing. Self-configuration refers to the system as one that adjusts the small cell frequency, power level, interfaces automatically as it is added into the system. It works with the automatic neighbor relations (ANR) software that builds and maintains a list of all cells in the network and the location and physical characteristics of each. If any new cell is added, the configuration is automatic and the list is updated. The same occurs if a cell is removed.
Self-optimization refers to the ability of the network to adapt itself to surrounding conditions and optimize its performance based on coverage, capacity, handover between cells, and interference. Two key functions are load balancing and interference mitigation. Load balancing is dividing the traffic between the cells so that no one cell becomes too overloaded if adjacent cells are within range and have available capacity. Load balancing occurs automatically. And this ability also helps balance the backhaul traffic load.
Interference management is essential in a HetNet as the small cells are generally closely spaced and could potentially interfere with one another. SON software uses the cells to measure the characteristics of nearby cells to make a determination if interference is a possibility the makes adjustments dynamically to change frequency or power level as necessary to minimize interference.
Self-healing refers to SONs ability to adjust to changing conditions such as cell failure. SON is a key part of a HetNet, and LTE makes provision for it in the standard. Tests have shown that SON can monitor and update a network within milliseconds in some cases and dynamically adapt. Overall throughput can be improved by 10 to 45 percent in many cases.
Distributed Antenna Systems
A key part of the HetNet movement is distributed antenna systems (DAS). DAS is not exactly a small cell, but its effect is similar in that it provides improved coverage and performance in a given region. What a DAS does is expand the coverage on a given basestation by distributing the signal over a wider area using a network of antennas. DAS is useful for improving coverage in multi-floor office buildings, stadiums, hotels, malls, airports, subways, as well as tunnels and roadways. They can be used indoors or outdoors, although indoor coverage is more common.
A DAS system essentially makes a connection to one or more existing macro base stations either by direct connection or by a wireless link. It then distributes this service over fiber optic cable or coaxes cable or some combination. Typically the unit connected to the base station involves a repeater or controller that amplifies the RF signal and converts it to an optical signal and sends it to various regions in the coverage area by way of fiber optic cables. The fiber connects to distribution boxes that convert the optical signals to RF for coax cable distribution to an array of antennas. These antennas must be separated from one another by several wavelengths to be effective. The array essentially divides the transmitted power among the antennas. DAS effectively eliminates dead zones caused by the huge attenuation with distance and through walls and ceiling and other obstructions. It provides a more direct line of sight connection to the cell phone or other user device.
DAS may be either passive or active. Passive systems are simplest and use a mix of filters, splitters and couplers to distribute the signals. Active system uses amplifiers and repeaters to boost signal levels.
The DAS is usually owned by the facility owner instead of the cellular carrier as are all other small cells. A distribution agreement with the carrier is necessary. DAS may be carrier-specific or generic to handle any 2G/3G/4G signals. Some DAS works with Wi-Fi as well.