By: Peter Croy, Sr. Network Architect, Aviat Networks INTRODUCTION LTE mobile broadband technology is now being launched across the world with more than 140 service providers committed to implement it within the next two years. 3GPP, the LTE standards defining body, has detailed the requirements of the new radio and core network domain, but not for the packet backhaul network linking the two domains together. Therefore, the industry has been left to extrapolate requirements for the backhaul network based on the best-case radio interface capabilities. This leads to exaggerated predictions and is insufficient to plan backhaul networks. This paper estimates backhaul capacity more realistically by combining factors of LTE channel capacity, radio propagation, cell site design and traffic aggregation. LTE USER DOWNLOAD SPEEDS Base stations transmit and receive user traffic over the assigned LTE radio or channel. Traffic volume is directly proportional to the LTE radio channel size. 3GPP defined channels of 5MHz, 10MHz, 20MHz and others in the R8 standard. The standard also defines radio interface spectral efficiency targets for download (5 bits/hz) and for upload (2.5 bits/hz) of this radio channel. To estimate the available over-the-air LTE radio peak rate, we need to look at how an LTE radio base station maximizes the available radio channel. The main parameter is the received signal quality measured at the handset. The closer a handset is to the base station, the better the radio signal quality and the higher the radio peak rate. LTE defines three modulation settings: QPSK, 16QAM and 64QAM. The best results are achieved using 64QAM, but this is only available very close to the cell centre and represents the maximum peak rate (100%). 16QAM corresponds to 66% and QPSK to 33% of this maximum. This reduced peak rate is available throughout most of the cell coverage area, with QPSK right up to the cell edge. All LTE peak rate figures assume the use of the entire LTE cell by a single user handset and include the radio layer 1 and layer 2 overhead of 10-25% (e.g., error correction, MAC control). In reality, multiple handsets will share the available radio resources of the LTE cell, resulting in a lower peak rate as well as average throughput per user. Figure 1 below provides an overview of realistic user handset peak data rates. As an example, the calculation for a 10MHz LTE spectrum at medium cell centre distance is as follows: Theoretical peak data rate (50Mbps) x 66% (16QAM) x 85% (minus L1&2 overhead) = 28 Mbps. Figure 1: Estimated net LTE user data peak rates 1 AVIAT NETWORKS FEBRUARY 2011
LTE CELL SITE BACKHAUL REQUIREMENTS Having estimated the LTE radio cell or sector peak data throughput, we can look at realistic cell site backhaul requirements. Each LTE cell site typically combines three LTE radio sectors into one macro site. The backhaul network connects to it via the S1 interface as specified in R8 (Figure 2). This is a logical IP interface split into a user session (S1-U) and control traffic part (S1-MME). Figure 2: LTE backhaul network interfaces Because LTE handsets are statistically distributed within the three radio sectors and not downloading at maximum peak rates all the time, backhaul capacity can be split and overbooked among individual sectors. The busier a macro cell is, the lower this overbooking factor needs to be. For low usage sites, the factor can be safely increased. Existing backhaul transmission networks for 3G, CDMA and WiMAX have shown that factors between two to five work quite well. Dense urban sites certainly have higher needs than rural sites. The size of the cell coverage area is also an important factor as it determines the mean peak rates of the handsets served by the site. Rural sites cover a much larger area and the majority of handsets transmit at lower peak rates, as they are farther away from the cell center. Figure 3 provides an overview of backhaul capacity vs. cell site type. Figure 3: LTE cell backhaul requirements overview In the following example, the calculation for a suburban cell with 10MHz LTE channel would be: 28Mbit/s (medium range peak of Figure 1) x 3 sectors / 3 (overbooking factor OBF) x 100/85 (15% QoS margin) = 33Mbit/s. The QoS margin ensures that the backhaul network transmission buffers can deal with highpriority data bursts without dropping frames. 2 AVIAT NETWORKS FEBRUARY 2011
These values are a realistic starting point for designing the packet backhaul network link capacities. Of course, these figures must be viewed adjusted to the backhaul network traffic reports of regular operational capacity reviews. CAPACITY INCREASE THROUGH LTE TECHNOLOGY EVOLUTION The LTE standard is of course evolving and many of its aspects are still theoretical. For example, a major factor is the available antenna technology in use between the base station and the handset. Figure 4 shows an extrapolation of the backhaul capacity requirements trend for a suburban radio cell site with 10MHz LTE radio channel capacity. Service uptake, cell site rollout and antenna technology progress drives the requirements for backhaul capacity. This capacity is not rising exponentially but is rather linear and likely to level off somewhere between 75-90Mbps. Figure 4: 10MHz LTE channel evolution with MIMO Although MIMO (multiple input, multiple output) antenna technology will enhance the peak data rates of handsets, the radio signal conditions of a noisy and interfering LTE environment are still forming a fundamental ceiling of how much data throughput improvement is achievable. Figure 4 is showing this improvement through increased backhaul requirements. But even for a dense urban area with large numbers of users, a three-sector macro cell using a 20MHz radio channel plus 4x4 MIMO technology is unlikely to require more than 150-200Mbps dedicated backhaul capacity. IMPROVING LTE THROUGHPUT AND SERVICE QUALITY Adding LTE radio capacity will improve user data throughput and user experience. Given that the licensed LTE radio channel size cannot be easily increased, the alternative is to deploy more LTE cell sites covering smaller radio areas. Many smaller cells, e.g. picocells, can supplement three-sector macro cells. Picocells host fewer concurrent user handsets, but typically provide better radio signal quality and throughput rates to each user. This will require separate, dedicated backhaul networks to high user demand environments like sports stadiums, business parks, conference centers or public areas. The deployment of smaller cells within the coverage area of macro cells also adds radio interference and increases handovers. Advanced LTE radio network analysis and design 3 AVIAT NETWORKS FEBRUARY 2011
adaptation is required to minimize side effects of deploying many small radio cells to enhance LTE throughput. TOPOLOGIES FOR LTE BACKHAUL Link capacity alone is not enough for a successful LTE backhaul network. Several factors need to be considered when designing the network. DAISY CHAINS Existing 2G/3G backhaul networks for TDM circuits have often relied on daisy-chained links to carry the individual circuits. For Ethernet packet networks that connect individual cell sites to each other and to the core network, daisy chains are not a good choice as traffic needs to pass links multiple times to provide the desired connectivity. It is also quite difficult to insert new cell sites because it makes the chains even longer and exacerbates associated problems. HUB & SPOKE Individual links spreading out from a single hub site is improving the connectivity and capacity requirements of LTE compared to daisy chains. The overall packet transport capacity is still concentrated in the hub site, but network availability and upgradeability factors are good. TREE/TIERED NETWORKS Splitting the network into multiple, smaller hub and branch sites will result in more flexible traffic routing and capacity distribution. Site-to-site connections (LTE X2 interface) are much shorter and link upgrades increase and provide capacity exactly where needed. The topology also results in very few hops from the core edge to the cell site, which is critical to the strict low packet delay and latency requirements of LTE. MESH AND RING NETWORKS This topology incorporates the best of all networking features in terms of capacity, availability and upgradeability. Usually, it is combined with fast link failure detection and recovery to ensure carrier-class transport operation. A ring network topology dimensioned for backhauling six LTE cell sites is pictured in Figure 6: Figure 6: Ring network topology BACKHAUL NETWORK DIMENSIONING We have already looked at LTE radio throughput and how to combine three sectors into a single macro cell site. Each site or network node requires transport capacity from the network. Similarly, we can connect multiple cell sites into a single backhaul network that we need to dimension. Once we have chosen our desired network topology, we can estimate the capacity of each link that connects the sites. 4 AVIAT NETWORKS FEBRUARY 2011
Statistical traffic distribution LTE backhaul traffic consists of individual IP/Ethernet packet data streams that can be described best by statistical methods. Interface buffers in each base station and network transport equipment help to adjust to extremely random periods of high and low packet rates sent through the network. LTE users are also spread across the cell sites and moving among them. Not all base stations are operating at the theoretical maximum peak data rate at the same time, resulting in a significantly lower backhaul data rate during a given time interval. The higher the peak packet rates are compared to the averaged packet rate, the higher the overbooking factor (OBF) for this base station can be. We have already discussed an OBF of between 3 and 5 for a typical macro base station of three sectors. Combining multiple base stations works in a similar manner as it exploits the statistical distribution of packet traffic. OBF factors of 1.5 to 3 have proved realistic in HSPA backhaul networks, which show similar packet behavior. EXAMPLE DIMENSIONING: RING TOPOLOGY The ring network example in Figure 6 has been dimensioned with an OBF of 1.5 within the transport ring and with an OBF of 2 for the link of all six sites towards the core network. Each cell is located in a dense urban area and uses a 10MHz LTE channel size. The link connecting the ring to the core network for example, requires a capacity of 6 x 50Mbps / 2 = 150Mbps. NETWORK DIMENSIONING GUIDELINES It is difficult to provide dimensioning rules and overbooking factors that suit all network scenarios and topologies. But as a general principle, OBF factors can be applied at every point in the network that aggregates and combines packet data streams from multiple sources. The closer the aggregation point is to the core domain of the network, the smaller the OBF factor needs to be. Figure 7 provides an overview of OBF factors relative to the location of the aggregation point in the backhaul network domain: Figure 7: Overbooking factors of backhaul links TRAFFIC ASYMMETRY We need to remember that LTE traffic volume is asymmetrically skewed toward user downloads. Packet rates from the core network toward LTE base stations are much higher than in the other direction. This is important for implementing QoS policies as it s more likely download traffic may overload links from the core network toward cell sites. Backhaul network links are typically symmetrical, so uploads play no role in network dimensioning. 5 AVIAT NETWORKS FEBRUARY 2011
CONCLUSION As in early phases of every technology, LTE backhaul capacity needs are being overstated. This will change when the technology is more established. The introduction of 3G in the early 2000s went through similar hype. LTE is likely to have an accelerated cycle, but the mobile industry might still suffer from initial frustrations. Usable LTE data capacity is fixed per site independent of number of users or handsets determined by base station technology and channel bandwidth. This can be increased by adding more cell sites or increasing LTE channel bandwidth, which is highly improbable due to LTE licensing requirements. Adding more macro cell sites would significantly increase network operations cost. Smaller picocells with more cost-effective backhaul options are a good compromise for LTE. Operators will have to provide these rather than try to achieve theoretical maximum coverage of macro cells especially in high user demand urban areas. While a backhaul capacity limit of 150 Mbit/s for a three-sector site with 10MHz LTE channels is below some LTE backhaul claims, it takes into account radio propagation limits. LTE backhaul needs to fulfill several requirements of an evolving mobile network. High availability, low latency, low packet loss, QoS, direct site-to-site connectivity and high network capacity are the most important. Ring or mesh topologies with multiple paths from access to the core will fit better than long chains of single-path links. They can also accommodate new network nodes more easily without disturbing the existing network capacity distribution and connectivity relationships. WWW.AVIATNETWORKS.COM Aviat, Aviat Networks, and Aviat logo are trademarks or registered trademarks of Aviat Networks, Inc. Aviat Networks, Inc. 2011. All Rights Reserved 6 AVIAT NETWORKS FEBRUARY 2011