WiMAX and IEEE 802.16. (parts of) Chapter 11. Wireless Local Loop (WLL). Wired technologies are responding to demand for reliable, high-speed access from residential, business, and government subscribers ISDN, xDSL, cable modems…
(parts of) Chapter 11
radius of first Fresnel zone:
If no obstruction within 0.6R along path, then attenuation due to obstructions is negligible
CS = convergence sublayer
Orthogonal Frequency Division Multiplexing (OFDM)
Multicarrier modulation technique
Idea: divide a high bit rate stream into parallel low bit rate streams and modulate each stream on a separate carrier.
Reduces intersymbol interference by reducing the data rate by a factor of N i.e. symbol time is increased by a factor of N
Therefore equalisers may not be necessary
Also: frequency selective fading only affects some channels, not whole signal
OFDM eliminates the need for non-overlapping channels, by selecting subcarriers that are orthogonal to each other over the duration of a bit.
This orthogonality allows us to recover our original signal without distortion from other signals.
The OFDM subcarriers can be packed tightly together because there is little interference between adjacent subcarriers
So far, we’ve presented OFDM as transmitting a single data stream
Split into N reduced bit rate streams
Transmit each of these N streams using a different tone
Clearly, in terms of signal recovery, it does not matter if the N data streams come from the same or different sources
Thus OFDM can be used to provide subchannels…called OFDMA
OFDMA allows users to share both subcarriers and time slots.
This allows resources to be allocated according to both user and system requirements
FDMA + TDMA
In reality, the time slot & frequency allocation is more complex
Slots are defined as one subchannel and a number of OFDM symbols
Users are assigned data regions either as
A contiguous series of slots…
Called Adaptive Modulation and Coding
Good for fixed or low mobility applications
…or using distributed subcarriers to enhance frequency diversity
Better for high mobility situations
Allocations according to Demand, QoS requirements, Channel conditions
In addition, Time Division Duplexing (TDD) means time slots are divided into Uplink and Downlink subframes.
Downlink to uplink ratio varies from 3:1 to 1:1 depending on traffic profiles.
Frequency Division Duplexing (FDD) is also supported.
Various modulation and coding allowed: BPSK, QPSK, 16 QAM, 64 QAM
Modulation and coding can change on burst-by-burst basis
Channel quality indicator used to inform BS about channel quality in downlink direction
BS estimates channel quality in the uplink direction
BS chooses modulation and coding to maximize throughput for available signal-to-noise ratio
Total of 52 combinations of modulation and coding schemes are defined in WiMAX as burst profiles
which MS and BS have reserved resources.
provided at any given time
QoS Parameters include:
maximum sustained traffic rate
maximum burst rate
minimum tolerable rate
scheduling type, ARQ type
service data unit type and size
bandwidth request mechanism
transmission PDU formation rules