Mobile Broadband

Editor's Notes

• #10: 3GPP has defined EPS in Release 8 as a framework for an evolution or migration of the3GPP system to a higher-data-rate, lower-latency packet-optimized system that supports multiple radio-access technologies. The focus of this work is on the packet switcheddomain, with the assumption that the system will support all services—including voice—in this domain. (EPS was previously called System ArchitectureEvolution.)Although it will most likely be deployed in conjunction with LTE, EPS could also be deployed for use with HSPA+, where it could provide a stepping-stone to LTE. EPS willbe optimized for all services to be delivered via IP in a manner that is as efficient as possible—through minimization of latency within the system, for example. It will supportservice continuity across heterogeneous networks, which will be important for LTE operators that must simultaneously support GSM/GPRS/EDGE/UMTS/HSPA customers.One important performance aspect of EPS is a flatter architecture. For packet flow, EPS includes two network elements, called Evolved Node B (eNodeB) and the AccessGateway (AGW). The eNodeB (base station) integrates the functions traditionally performed by the radio-network controller, which previously was a separate nodecontrolling multiple Node Bs. Meanwhile, the AGW integrates the functions traditionally performed by the SGSN. The AGW has both control functions, handled through theMobile Management Entity (MME), and user plane (data communications) functions. The user plane functions consist of two elements: a serving gateway that addresses 3GPPmobility and terminates eNodeB connections, and a Packet Data Network (PDN) gateway that addresses service requirements and also terminates access by non-3GPP networks.The MME, serving gateway, and PDN gateways can be collocated in the same physicalnode or distributed, based on vendor implementations and deployment scenarios.The EPS architecture is similar to the HSPA One-Tunnel Architecture, discussed in the“HSPA+” section, which allows for easy integration of HSPA networks to the EPS. EPSalso allows integration of non-3GPP networks such as WiMAX.EPS will use IMS as a component. It will also manage QoS across the whole system,which will be essential for enabling a rich set of multimedia-based services.The MME, serving gateway, and PDN gateways can be collocated in the same physicalnode or distributed, based on vendor implementations and deployment scenarios.The EPS architecture is similar to the HSPA One-Tunnel Architecture, discussed in the“HSPA+” section, which allows for easy integration of HSPA networks to the EPS. EPSalso allows integration of non-3GPP networks such as WiMAX.EPS will use IMS as a component. It will also manage QoS across the whole system,which will be essential for enabling a rich set of multimedia-based services.Elements of the EPS architecture include:- Support for legacy GERAN and UTRAN networks connected via SGSN.- Support for new radio-access networks such as LTE.- The Serving Gateway that terminates the interface toward the 3GPP radio-accessnetworks.- The PDN gateway that controls IP data services, does routing, allocates IPaddresses, enforces policy, and provides access for non-3GPP access networks.- The MME that supports user equipment context and identity as well asauthenticates and authorizes users.- The Policy Control and Charging Rules Function (PCRF) that manages QoSaspects.
• #15: The basic principle of OFDM is to split a high-rate data stream into a number of parallel low-rate data streams, each a narrowband signal carried by a subcarrier. The different narrowband streams are generated in the frequency domain and then combined to form the broadband stream using a mathematical algorithm called an Inverse Fast Fourier Transform (IFFT) that is implemented in digital-signal processors.The system is called orthogonal, because the subcarriers are generated as orthogonal in the frequency domain and the IFFT conserves that characteristic. OFDMsystems may lose their orthogonal nature as a result of the Doppler shift induced by the speed of the transmitter or the receiver.
• #16: The composite signal is obtained after the IFFT is extended by repeating the initial part of the signal (called the Cyclic Prefix [CP]). This extended signal represents an OFDMsymbol. The CP is basically a guard time during which reflected signals will reach the receiver. It results in an almost complete elimination of Intersymbol Interference (ISI),which otherwise makes extremely high data rate transmissions problematic.
• #18: Most WCDMA and HSDPA deployments are based on FDD, where the operator uses different radio bands for transmit and receive. An alternate approach is TDD, in whichboth transmit and receive functions alternate in time on the same radio channel.Many data applications are asymmetric, with the downlink consuming more bandwidth than the uplink, especially for applications like Web browsing or multimedia downloads. A TDD radio interface can dynamically adjust the downlink-to-uplink ratio accordingly, hence balancing both forward-link and reverse-link capacity.TDD systems require network synchronization and careful coordination between operators or guard bands.
• #23: The multiple-access aspect of OFDMA comes from being able to assign different usersdifferent subcarriers over time. A minimum resource block that the system can assign toa user transmission consists of 12 subcarriers over 14 symbols (approx 1.0 msec.)
• #24: By having control over which subcarriers are assigned in which sectors, LTE can control frequency reuse. By using all the subcarriers in each sector, the system wouldoperate at a frequency reuse of 1; but by using a different one third of the subcarriers in each sector, the system achieves a looser frequency reuse of 1/3. The looser frequency reduces overall spectral efficiency but delivers high peak rates to users.
• #28: LTE uses a variety of multiple antenna techniques. Sometimes we loosely refer to these as MIMO (Multiple Input Multiple Output, but we have to use the term MIMO carefully. MIMO typically refers to spatial multiplexing whereby multiple streams of data (called layers in LTE) are transmitted in the same channel simultaneously. Spatial Multiplexing is only possible in a decorrelated channel and with multiple transmitter and receivers.In addition to Spatial Multiplexing (classical MIMO), Multiple antenna techniques include transmit and receive diversity in MISO, SIMO and MIMO configurations. Spatial Multiplexing typically requires high signal to noise ratio (SNR) conditions. In the presence of low SNR or excessive doppler, multiple transmitters can be used for transmit diversity such as Cyclic Delay Diversity CDD and multiple receivers can be used for receive diversity techniques such ash MRC maximal ratio combining. Both transmit and receive diversity can be used simultaneously further improving the robustness of the channel. While spatial multiplexing of 2 layers has the potential of doubling the data rate, diversity techniques use multiple radios for redundant transmission of a single stream and hence have lower theoretical throughpout. LTE MIMO radios can dynamically select Spatial Multiplexing in channel conditions that are suitable for this and then switch to transmit and receive diversity when channel conditions deteriorate.