3.8: WiMAX and LTE

WiMAX and LTE are both wireless network technologies suitable for data connections to mobile (and sometimes stationary) devices.

WiMAX is an IEEE standard, 802.16; its original name is WirelessMAN (for Metropolitan Area Network), and this name appears intermittently in the IEEE standards. In its earlier versions it was intended for stationary subscribers (802.16d), but was later expanded to support mobile subscribers (802.16e). The stationary-subscriber version is often used to provide residential Internet connectivity, in both urban and rural areas.

LTE (the acronym itself stands for Long Term Evolution) is a product of the mobile telecom world; it was designed for mobile subscribers from the beginning. Its official name – at least for its radio protocols – is Evolved UTRA, or E-UTRA, where UTRA in turn stands for UMTS Terrestrial Radio Access. UMTS stands for Universal Mobile Telecommunications System, a core mobile-device data-network mechanism with standards dating from the year 2000.

4G Capacity

A medium-level wireless data plan often comes with a 5 GB monthly cap. At the 100 Mbps 4G data rate, that allotment can be downloaded in under six minutes. Data rate isn’t everything.

Both LTE and the mobile version of WiMAX are often marketed as fourth generation (or 4G) networking technology. The ITU has a specific definition for 4G developed in 2008, officially named IMT-Advanced and including a 100 Mbps download rate to moving devices and a 1 Gbps download rate to more-or-less-stationary devices. Neither WiMAX nor LTE quite qualified technically, but to marketers that was no impediment. In any event, in December 2010 the ITU issued a statement in which it “recognized that [the term 4G], while undefined, may also be applied to the forerunners of these technologies, LTE and WiMax”. So-called Advanced LTE and WiMAX2 are true IMT-Advanced protocols.

As in 3.6.4 Band Width we will use the term “data rate” for what is commonly called “bandwidth” to avoid confusion with the radio-specific meaning of the latter term.

WiMAX can use unlicensed frequencies, like Wi-Fi, but its primary use is over licensed radio spectrum; LTE is used almost exclusively over licensed spectrum.

WiMAX and LTE both support a number of options for the width of the frequency band; the wider the band, the higher the data rate. Downlink (base station to subscriber) data rates can be well over 100 Mbps (uplink rates are usually smaller). Most LTE bands are either in the range 700-900 MHz or are above 1700 MHz; the lower frequencies tend to be better at penetrating trees and walls.

Like Wi-Fi, WiMAX and LTE subscriber stations connect to a central access point. The WiMAX standard prefers the term base station which we will use henceforth for both protocols; LTE officially prefers the term “evolved NodeB” or eNB.

The coverage radius for LTE and mobile-subscriber WiMAX might be one to ten kilometers, versus less (sometimes much less) than 100 meters for Wi-Fi. Stationary-subscriber WiMAX can operate on a larger scale; the coverage radius can be several tens of kilometers. As distances increase, the data rate is reduced.

Large-radius base stations are typically mounted in towers; smaller-radius base-stations, generally used only in areas densely populated with subscribers, may use lower antennas integrated discretely into the local architecture. Subscriber stations are not expected to be able to hear other stations; they interact only with the base station.

3.8.1 Uplink Scheduling

As distances increase, the subscriber-to-base RTT becomes non-negligible. At 10 kilometers, this RTT is 66 µsec, based on the speed of light of about 300 m/µsec. At 100 Mbps this is enough time to send 800 bytes, making it a priority to reduce the number of RTTs. To this end, it is no longer practical to use Wi-Fi-style collisions to resolve access contention; it is not even practical to use the Wi-Fi PCF mode of 3.7.7 Wi-Fi Polling Mode because polling requires additional RTTs. Instead, WiMAX and LTE rely on base-station-regulated scheduling of transmissions.

The base station has no difficulty scheduling downlink transmissions, from base to subscriber: the base station simply sends the packets sequentially (or in parallel on different sets of subcarriers if OFDM is used). If beamforming MISO antennas are used, or multiple physically directional antennas, the base station will take this into account.

It is the uplink transmissions – from subscriber to base – that are more complicated to coordinate. Once a subscriber station completes the network entry process to connect to a base station (3.8.3 Network Entry), it is assigned regular transmission slots, including times and frequencies. These transmission slots may vary in size over time; the base station may regularly issue new transmission schedules. Each subscriber station is told in effect that it may transmit on its assigned frequencies starting at an assigned time and for an assigned length; LTE lengths start at 1 ms and WiMAX lengths at 2 ms. The station synchronizes its clock with that of the base station as part of the network entry process.

Each subscriber station is scheduled to transmit so that one transmission finishes arriving at the base station just before the next station’s same-frequency transmission begins arriving. Only minimal “guard intervals” need be included between consecutive transmissions. Two (or more) consecutive uplink transmissions may in fact be “in the air” simultaneously, as far-away stations need to begin transmitting early so their signals will arrive at the base station at the expected time.

The diagram above illustrates this for stations separated by relatively large physical distances (as may be typical for long-range WiMAX). This strategy for uplink scheduling eliminates the full RTT that Wi-Fi polling mode (3.7.7 Wi-Fi Polling Mode) entails.

Scheduled timeslots may be periodic (as is would be appropriate for voice) or may occur at varying intervals. Quality-of-Service requests may also enter into the schedule; LTE focuses on end-to-end QoS while WiMAX focuses on subscriber-to-base QoS.

When a station has data to send, it may include in its next scheduled transmission a request for a longer transmission interval; if the request is granted, the station may send its data (or at least some of its data) in its next scheduled transmission slot. When a station is done transmitting, its timeslot may shrink back to the minimum, and may be scheduled less frequently as well, but it does not disappear. Stations without data to send remain connected to the base station by sending “empty” messages during these slots.

3.8.2 Ranging

The uplink scheduling of the previous section requires that each subscriber station know the distance to the base station. If a subscriber station is to transmit so that its message arrives at the base station at a certain time, it must actually begin transmission early by an amount equal to the one-way station-to-base propagation delay. This distance/delay measurement process is called ranging.

Ranging can be accomplished through any RTT measurement. Any base-station delay in replying, once a subscriber message is received, simply needs to be subtracted from the total RTT. Of course, that base-station delay needs also to be communicated back to the subscriber.

The distance to the base station is used not only for the subscriber station’s transmission timing, but also to determine its power level; signals from each subscriber station, no matter where located, should arrive at the base station with about the same power.

3.8.3 Network Entry

The scheduling process eliminates the potential for collisions between normal data transmissions. But there remains the issue of initial entry to the network. If a handoff is involved, the new base station can be informed by the old base station, and send an appropriate schedule to the moving subscriber station. But if the subscriber station was just powered on, or is arriving from an area without LTE/WiMAX coverage, potential transmission collisions are unavoidable. Fortunately, network entry is infrequent, and so collisions are even less frequent.

A subscriber station begins the network-entry connection process to a base station by listening for the base station’s transmissions; these message streams contain regular management messages containing, among other things, information about available data rates in each direction. Also included in the base station’s message stream is information about when network-entry attempts can be made.

In WiMAX these entry-attempt timeslots are called ranging intervals; the subscriber station waits for one of these intervals and sends a “range-request” message to the base station. These ranging intervals are open to all stations attempting network entry, and if another station transmits at the same time there will be a collision. An Ethernet/Wi-Fi-like exponential-backoff process is used if a collision does occur.

In LTE the entry process is known as RACH, for Random Access CHannel. The base station designates certain 1 ms timeslots for network entry. During one of these slots an entry-seeking subscriber chooses at random one of up to 64 predetermined random access preambles (some preambles may be reserved for a second, contention-free form of RACH), and transmits it. The 1-ms timeslot corresponds to 300 kilometers, much larger than any LTE cell, so the fact that the subscriber does not yet know its distance to the base does not matter.

The preambles are mathematically “orthogonal”, in such a way that as long as no two RACH-participating subscribers choose the same preamble, the base station can decode overlapping preambles and thus receive the set of all preambles transmitted during the RACH timeslot. The base station then sends a reply, listing the preambles received and, in effect, an initial schedule indexed by preamble of when each newly entering subscriber station can transmit actual data. This reply is sent to a special temporary multicast address known as a radio network temporary identifier, or RNTI, as the base station does not yet know the actual identity of any new subscriber. Those identities are learned as the new subscribers transmit to the base station according to this initial schedule.

A collision occurs when two LTE subscriber stations have the misfortune of choosing the same preamble in the same RACH timeslot, in which case the chosen preamble will not appear in the initial schedule transmitted by the base station. As for WiMAX, collisions are rare because network entry is rare. Subscribers experiencing a collision try again during the next RACH timeslot, choosing at random a new preamble.

For both WiMAX and LTE, network entry is the only time when collisions can occur; afterwards, all subscriber-station transmissions are scheduled by the base station.

If there is no collision, each subscriber station is able to use the base station’s initial-response transmission to make its first ranging measurement. Subscribers must have a ranging measurement in hand before they can send any scheduled transmission.

3.8.4 Mobility

There are some significant differences between stationary and mobile subscribers. First, mobile subscribers will likely expect some sort of handoff from one base station to another as the subscriber moves out of range of the first. Second, moving subscribers mean that the base-to-subscriber ranging information may change rapidly; see exercise 7.0. If the subscriber does not update its ranging information often enough, it may transmit too early or too late. If the subscriber is moving fast enough, the Doppler effect may also alter frequencies.