Cellular Backhaul
A cable operator’s pervasive HFC network is often located near cellular base stations. That makes is natural to consider offering cellular backhaul services. However, plant facilities are not always within reach or cost-effective enough to connect to cellular base stations. In those cases, a simple solution is available, namely: point-to-point unlicensed wireless transmission using Ethernet-based radios.
Unlicensed wireless solutions enable reliable cellular backhaul transport, are scalable to meet growing throughput needs and can be operational within a day. They also enable non-line of sight deployments for difficult or otherwise cost prohibitive deployments where licensed microwave or fiber solutions could not operate.
Unlicensed wireless backhaul technology operates in a point-to-point manner. A wireless transmitter/receiver pair is co-located at the cellular base station transceiver and the other end of the link – that is, another transmitter/receiver pair – may be located at a base station switching controller or other optimal location within the cellular transport network. (See Figure 1.) For the majority of base stations currently in use, the dominant access technology is time division multiplexing (TDM) based, perhaps using T-1s. When TDM is required at the base station, a conversion is required to change the TDM signaling into an Internet protocol (IP) stream for transmission across the Ethernet radio link. This conversion is accomplished via pseudo-wire emulation solutions. Such solutions are widely available and in use across many currently deployed cellular backhaul technologies. They have been designed to support the requirements of a cellular network regarding timing, latency, jitter and packet loss.
Of particular importance for global system for mobile communications (GSM) and universal mobile telecommunications system (UMTS) networks is the ability to provide accurate timing. That is deployed by GSM/UMTS base stations for maintaining the appropriate cellular carrier frequency accuracy of 50 parts per billion. In the case of UMTS networks, this timing requirement is further tightened to 16 parts per billion. For code division multiple access (CDMA)-based networks, cellular towers utilize global positioning system (GPS) technology to generate timing. If there is a loss of accurate timing, then call sessions will be interrupted because of poor hand-offs from cell tower to cell tower. This timing can be accurately maintained by current pseudo-wire emulation technology that recreates the TDM transmission characteristics over IP streams.
A wireless radio must be able to overcome interference and offer carrier grade availability, that is, the often cited "four nines" of availability. Many innovations have taken place within the wireless radio Ethernet bridge solution area. These innovations enable a high reliability link within the unlicensed spectrum. Some of the most notable solutions utilized in wireless backhaul include the following:
Multiple-input multiple-output (MIMO): MIMO is a method of transmitting multiple data streams on multiple transmitters and antennas to multiple receivers and corresponding multiple antennas on the same frequency, a technique deployed to improve the fade margin of a wireless link and minimize the impact of multi-path fading. This is especially critical for non-line of sight wireless links.
MIMO is effective only when the data streams are not correlated with each other. This can be done in practice by using techniques such as spatial separation of antennas, separation of the waveforms via time, data sequence separation, polarization separation, frequency separation or modulation separation. When the multiple streams are sent, they can be recombined to negate the effects that a single carrier radio solution would incur because of the out-of-phase signals counteracting each other.
Orthogonal frequency division multiplexing (OFDM): OFDM involves the transmission of data on multiple frequencies for the duration of a symbol. By using multiple carriers, communication is maintained should one or both more carriers be affected by either narrowband or multipath interference. Individual carriers overlap to improve the efficiency of the available spectrum. In order to prevent these carriers from self-interfering with one another, the OFDM waveforms are positioned orthogonally to one another. In this manner, the carriers are not able to interfere with one another, and there is no cumulative signal loss or degradation.
Advanced spectrum management with dynamic frequency selection (DFS): DFS is a technique that allows a radio system to maintain the highest availability and resistance to interference automatically. Within a given band of spectrum, there are multiple channels on which to operate. When interference is detected, the radio automatically switches to the cleanest available channel for maximum performance. RF interference can be handled in a real-time manner to maintain link availability. It also can be overridden in situations where coexistence with multiple radios is required on the same mounting location, as in cellular backhaul applications where multiple backhauls are being aggregated at the base station switching controller.
Wireless radios can also support configurable bandwidth by limiting the size of the channel that is used for transmission. By changing the channel width size, such as from 15 MHz to 10 MHz, the optimal channel size within the unlicensed spectrum may be found, and a potential lower noise impact can be achieved.
Adaptive modulation: Adaptive modulation enables a wireless link to proactively adjust the modulation scheme employed according to the signal level received. With multiple modulation schemes available, such as quadrature phase shift keying (QPSK), binary phase shift keying (BPSK), and numerous quadrature amplitude modulation (QAM) formats, a radio link can maximize both data throughput and availability. The radio link will self-select which modulation scheme best meets the application requirements for a link and minimize the loss of packets. This decision is based upon existing RF conditions encountered by the wireless link such as carrier-to-noise ratio (CNR) and link loss estimates. Adaptive modulation is effective in counteracting temporary fade issues that naturally occur within the environment and for lessening the impact of potential obstructions along the radio link that may arise. Synchronization One key solution for cellular backhaul is the ability to have multiple links come together at one tower location. This is inherent in the architecture of a cellular network. To achieve this requirement, an enabling technology for wireless links is the synchronization of the transmit and receive cycles between multiple point-to-point links at a single location. A GPS antenna is provided at each radio location to establish the necessary timing for the synchronization. Through synchronization, there is limited impact from self-interference for adjacent radios.
The use of an element management system (EMS) is also required to enable operators to have visibility into the network. Typical EMSs will enable authentication for the radio links to ensure only the designated radios are communicating. They also support fault monitoring, radio performance and configuration of the radios as the network is in operation. The EMS can tie into a network management system (NMS) by providing the radios’ management information bases (MIBs) via standard northbound interfaces.
The use of these technologies can enable a cable operator to achieve a high level of reliability for cellular backhaul. Engineered properly, they will allow cellular carrier’s service requirements to be satisfied and meet service level agreements (SLAs) as required. Planning Cellular backhaul transmission links must be engineered to a carrier-grade level of availability. When problems arise, a carrier’s revenue is directly impacted. Proper link planning is critical.
Because the transmission medium is not controlled as it would be for a wireline deployment, software planning tools and site surveys are required to engineer a wireless link. With a proper understanding of the radio’s capabilities and a basic knowledge of RF, a highly reliable and suitable wireless link can be implemented for cellular backhaul deployments. Brent Patterson is a product manager for wireless and enterprise access networks at Motorola. Reach him at [email protected]. This article was drawn from a paper presented at the SCTE symposium on business services.
