How small cells are becoming an integral part of futuristic mobile networks?

LTE as a technology and air interface has been hogging the bulk of limelight in the world of wireless communications. But another strategically crucial technology that many major mobile operators globally are going after is the small cell. In simple terms, small cell is a miniature version of the traditional macrocell. It compresses the attributes of a cell tower like radios and antennas into a low power, portable and easy to deploy radio device. Small cells typically have a range varying from 10 meters to a few hundred meters and are used by operators to either offload traffic from the macro network in a high density short range environment or to strengthen the range and efficiency of a mobile network. Before going into further details about small cells, have a look at the following diagram that illustrates how they fit into an operator’s network and strategy.

As seen in the image above, small cells provide enhanced coverage and capacity both indoors and outdoors. Umbrella coverage is provided by the macrocell. Microcells and picocells are designed to support hundreds of users and can be used in smaller networks that are not necessarily inside the range of a macrocell. Residential areas that are located outside the range of a cell network can deploy femtocells for better signal and bandwidth indoors. WiFi can be utilized for traffic offload or can serve as a standalone high speed short range network. Following are some of the advantages that small cells bring to the table –

• Augmented coverage and capacity – The quality of signal on a device and whether that signal is good enough for multimedia data browsing are two factors that decide a customer’s experience in the mobile world nowadays. Small cells bring ubiquity to this idea, along with the added advantage of low latency. So whether you are in a packed stadium or office basement, you will be covered.
• Superior in-building and cell edge performance – Contemporary wireless networks regularly face issues of poor coverage inside buildings and in areas far away from the cell tower. Small cells significantly improve the overall experience in such circumstances.
• Support for various environments – The main conclusion to be drawn from the diagram above is that these tiny base stations find utility in multiple scenarios. Femtocells inside a house not only provide 3G or higher level speeds, they also reduce strain on the user devices’ battery. And all this is achieved by using the Internet service provider’s backhaul. At an enterprise level, microcells enhance service quality in the highly dense office environment. Similarly, they can be equally effective in remote rural or dense urban spaces.
• Easier technology integration – Small cells can be integrated with all flavors of 3G, LTE, LTE-Advanced and WiFi technologies. An operator’s small cell strategy could be influenced by the type of wireless technologies it has deployed, the area of service and regional demand. Microcells, picocells and femtocells are fortunately compatible with all major types of wireless networks.
• Higher spectrum bands are welcome – Recently, the mobile network providers have been fighting a battle for the lower band spectrum below 1 GHz. But since limited propagation characteristics are not an issue for these miniscule networks, and more bits/Hz are required, spectrum over 2 GHz is considered good. The FCC in US has been pushing for 3.5 GHz spectrum for small cell networks. Some stakeholders have asked for unlicensed spectrum for such networks. Europe is said to be discussing the 2.3 GHz TD-LTE spectrum this purpose.
• Long term solution for the operator – Even though more base stations and state-of-the-art technologies can be deployed to temporarily resolve network congestion issues, the demand will generally exceed the supply. However, small cells are designed to offer adequate network resources to handle growing data demand for a few years within a specific environment.
• Attractive business case – The reduced capital and operational expenditure (CAPEX/OPEX) involved in the small cell ecosystem has made them a tempting business proposition for the mobile service provider. Studies have shown that the cost of radio equipment for small cells could be just one-tenth of the corresponding costs for a macrocell. The ease, flexibility and swiftness of deployment make such networks even more appealing.

Many operators and vendors around the globe showcased their small cell strategy and progress at the Mobile World Congress (MWC) in Barcelona earlier this year. Vodafone emphasized that this technology is vital to their network portfolio. The telco plans to deploy about 70,000 small cells within the next 2 years. Korea Telecom announced that they have 18,000 such cells already active in urban areas of the country. Samsung Mobile was tapped by Verizon as a vendor for indoor LTE small cell solutions. Verizon already had similar partnerships with both Alcatel-Lucent and Ericsson for indoor enterprise and outdoor environments. TIM Brazil, the country’s second largest operator, shared details about a deal with Alcatel-Lucent at MWC that will integrate femtocells into the carrier’s 3G network. SingTel from Singapore has been investing in these tiny networks too and has contracted Ericsson for the deployment. Many other small cell related developments have been picking up in the last year or so. AT&T’s 3G small cells are available in 18 states across the US. The operator has committed to deploying 40,000 multimode little base stations by the end of 2015. Sprint has been testing indoor and outdoor small cells for many months and intends a commercial launch later this year. The telco has also been running trials with Qualcomm’s network equipment. World’s biggest wireless service provider by subscribers, China Mobile, recently showed off a self-organizing outdoor small cell backhaul system as part of its TD-LTE network. Japan’s NTT Docomo has been using multiband small cell base stations for more than a year in some of its major markets. Note that as of now, most small cell networks operate on service provider’s existing spectrum holdings. But in the near future, dedicated airwaves could be allocated for these networks.

Multiple recent studies and analyses have predicted a ramp up in the small cell market. Infonetics Research has reported that small cell revenue was a modest $771 million last year but will grow by 65% to $1.3 billion this year. According to their report, 642,000 small cell units were shipped last year and about half of them were 3G, although LTE is projected to take the lead this year. ABI Research forecasted $1.8 billion market for outdoor small cells in 2014. The Asia-Pacific region will represent half of the small cell market by 2019. Allied Market Research put the global femtocell market size at $305 million in 2013 and predicted that this could grow more than ten-fold to $3.7 billion by 2020.

Although the predictions are upbeat, challenges remain for the small cell ecosystem. The cost and availability of backhaul for such stations is an issue. Because of municipal regulations, outdoor site acquisition can be a problematic process. The coordination and synchronization of these cells with local WiFi and the macro network is not as easy as it sounds. In urban scenarios, achieving line-of-sight may be technically difficult for low height in-building base stations. Despite these challenges, the overall small cell industry outlook is favorable. All major telcos and equipment providers have been evolving a small cell strategy. With consumers becoming increasingly intolerable towards bad wireless service, these tiny towers and stations are set to establish a niche but substantial market for themselves.

Are HetNets the solution for data hogging wireless networks?

In wireless communications, Heterogeneous Network or HetNet is a system comprising of wireless networks of different cell sizes with varying output power, various radio access technologies and multiple architectures. The primary objectives of a HetNet are achieving greater coverage and higher capacity while offering seamless handoff and connectivity. Data traffic has shown exponential growth and every operator is looking for a cost-effective, spectrally efficient solution to offer higher data rates. Poor coverage and blind spots have long challenged the wireless industry. The top approach to solve these issues is the upgradation of radio access technology at the macrocell sites. LTE is considered a very efficient radio technology and provides users with data access generally in the range of 5 to 25 Mbps (depending on the service provider and environment) while reducing the cost per bit for the service provider. More spectrum and multiple antennas are also popular options to increase the data rate. Increasing the number of cell sites in a given area is another proven method of augmenting network performance in high traffic urban areas. Imagine these scenarios and add low power picocells and femtocells. Finally append a WiFi network to this environment and the resulting system would be a multilayer large capacity heterogeneous network. The varying power levels and multiple cell sizes make this network heterogeneous. It effectively brings the network closer to the customer.

Let us discuss the various cell types in a HetNet. Macrocells are the common cells sites supporting technologies like HSPA+ and LTE. The normal range may vary from a few hundred meters to a few kilometers. Output power is of the order of tens of watts. A microcell typically covers a smaller area maybe up to a kilometer. They usually transmit within a range of milliwatts to a few watts. Microcells are deployed for providing temporary cellular coverage and capacity in sports stadiums, convention centers etc. Sometimes, microcells may use distributed antenna systems (DAS) to improve bandwidth and reliability. Picocells are generally deployed by the network operator within limited indoor areas such as office buildings and shopping malls. Femtocells cover an even smaller space like a house or small business. They are sold by the operator but self-installed by the customer. Some femtocells may be autonomous implying that they have the ability to determine the best frequency and power levels to operate.

Two arguments in favor of small cells are the reduced capital and operational expenditure (CAPEX and OPEX) as compared to techniques like cell splitting and ability of small cells to provide adequate coverage in extremely densely populated areas. The idea of merging small cells with the macrocell network has the advantage of offloading traffic from the bigger cell sites to the smaller cells. This permits the macrocells to operate at their normal capacity. Such consolidation is transparent to the customer. It involves seamless handoff between the multiple sized cells and facilitates uninterrupted data services for the user. A key component of HetNet which will help in meeting the above requirements is network intelligence. Macro, micro, pico and femtocells along with Wi-Fi have to be integrated into a single framework in a manner which avoids interference. As far as frequencies are concerned, all cells in heterogeneous network need not be using the same spectrum. Macrocells can operate at a lower frequency to boost penetration. Higher bandwidth (bits/Hz) is desired for smaller cells, so it would be best to allocate higher frequencies to such cells. The figure below shows an example of a HetNet in a LTE-Advanced Network. Note that femtocells are indoors, but are shown around the building in this diagram for depiction purposes.

HetNets, like all other wireless networks face some challenges. In order to raise the spectral efficiency, minimizing harmful interference between the neighboring cells is necessary. In the 3GPP Release 10 for LTE Advanced technology, a procedure called Enhanced Inter-Cell Interference Coordination (eICIC) has been defined to mitigate interference. EICIC uses time domain to avoid interference between the macrocell and small (micro, pico) cells in a heterogeneous network. In this technique, some subframes which are sent out by the macrocell are blanked out. These blank subframes do not contain any data. They are low power control channels. Once such blank frames are configured by the macrocell, the users connected to a small cell can send and receive data during such subframes, thus minimizing interference from the macrocell. Another fundamental requirement of a heterogeneous network is adequate backhaul link. A powerful backhaul enhances the data throughput and strengthens coordination between the nodes. Harmonization and optimization between asymmetrically sized cells would be an additional area of focus in a heterogeneous network.

HetNets are in very early stages of deployments in the wireless world. Vodafone, Telefonica, France Telecom and Telecom Italia in Europe have been running HetNet trials. NTT DoCoMo in Japan is expected to launch a HSPA/femtocell heterogeneous network by next year. SK Telecom is operating around 44,000 femtocells in South Korea in addition to their 3G and LTE network.  In the US, FCC has recently identified 3.5 GHz as the potential band for small cell expansion. AT&T has announced plans to rollout HetNets in the first quarter of 2013. Sprint Nextel terms HetNet as the second phase of their Network Vision project. Their small cell strategy is considered to be one of the motives behind the company’s recent bid for Clearwire. Clearwire’s rich spectrum holdings in 2.5 GHz fit well in the small cell strategy. With MetroPCS’s already deployed DAS nodes, T-Mobile also hopes to get into HetNet play. In India, Bharti Airtel, Tata Teleservices and Idea are conducting small cell trials.

Clearly, a number of issues need to be sorted out before heterogeneous networks become a reality. Many telecom industry experts are calling HetNets as important components of futuristic wireless networks, but their real potential can only be gauged once they are commercially launched and adopted.