The first difference relates to the physical environment. Wireless LANs are designed for the enterprise scenarios with relatively short distances. In such cases the waves bounce of the walls time and again, and it takes a few tens of nanoseconds for the echoes to relax. In the outdoor environment, for which the FBWA systems are designed, the echoes bounce off buildings, and this transforms into echoes spread apart a few hundreds of nanoseconds up to few microseconds. The "delay spread" problem becomes more pronounced as the direct component vanishes in "non line-of-sight" situation.

The original 802.11 standard adopted two physical layers - Frequency Hopping (FH) and Direct Sequence (DS). The DS was extended then into the highly popular 11 Mbits/second 802.11b. In parallel, the 54 Mbits/second 802.11a Orthogonal Frequency Division Multiplex (OFDM) based physical layer was developed for the 5 GHz band, and recently it was adopted for use at 2.4 GHz within 802.11g. The 802.11a products are only now starting to hit the market.

The problem of multipath distortion has long been recognized as a limiting factor in communications.

The multipath robustness of FH stems from its narrow instantaneous bandwidth, and from not staying long on bad frequencies, either in terms of propagation or interference. DS systems typically collect energy from different paths, but there's a residual distortion limiting operation in severe multipath situations. The next step in achieving high spectral efficiency in multipath conditions is OFDM. Here the coded bits are conveyed by multiple densely packed subcarriers. The orthogonality is manifested in the fact that even under multipath the subcarriers do not interfere with each other. In time domain the symbols are composed of the fast Fourier transform interval, from which the data is extracted, and from the guard interval (GI), which serves as a "junkyard" for the echoes from the previous symbol and prevents them from littering the FFT interval. Error correction coding compensates for the nonuniform strength of the subcarriers, due to multipath.

The parameters of the OFDM need to be carefully matched to the environment. In the 802.11a WLAN standard, for example, the FFT interval is 3.2 microseconds, while the guard interval is 0.8 microseconds — commensurate with the longest indoor multipath. For outdoor systems longer guard intervals are needed and the FFT interval is made longer to maintain efficiency.

Going to larger FFT sizes creates a new problem of overhead and payload granularity. For an FFT size of 64 points each OFDM symbol contains 3-27 bytes. For an FFT size of 256 points, this amount quadruples. In many cases the last OFDM symbol is only partially utilized. Add to that the 1-2 training symbols, and it becomes evident that maintaining reasonable efficiency calls for fine-grain allocation.

Orthogonal frequency division multiplexing (OFDM) provides an effective barrier to multipath distortion. Orthogonality is maintained in multiple densely packed subcarriers so that even under multipath conditions the subcarriers do not interfere with each other. In the time domain the symbols are composed of the FFT (fast Fourier transform) interval, from which the data is extracted, and from a Guard Interval (GI), which serves as a 'junkyard' for the echoes from the previous symbol and prevents them from littering the FFT interval. Source:Alvarion

Enter Orthogonal Frequency Division Multiple Access (OFDMA) which allows simultaneous transmission from several users, with only a fraction of the subcarriers assigned to each user. In this way the benefits of large FFT size are combined with the granularity advantage of small FFT size.

An additional advantage of OFDMA is an improved upstream link budget, due to smaller effective bandwidth of each user. Downstream OFDMA provides little benefit. The payloads are concatenated, and granularity overhead is less of an issue. The link budget advantage is not realized since the transmit power is split over the subchannels and no power concentration effect occurs. Supporters of downlink OFDMA claim the capabilities of selective power boosting and of downstream per-user beamforming. IEEE802.16a (draft D2) contains both pure OFDM mode with an FFT size of 256 points, and an OFMDA (up and down) mode with a FFT size of 2048 points.

Alvarion's position within 802.16a is that OFDM downlink with OFDMA uplink is the combination of choice and we are currently working towards applying this methodology to the mode with an FFT size of 256 points.

The second significant issue when comparing FBWA systems to WLANs is the medium access control (MAC) layer of such systems. WLAN systems are mainly used for data services, and the packets on average are long. Fixed wireless access systems support a broader mix of services including voice, which is notorious for its small packet size. In 802.11 WLAN systems each packet, however small and predicted, needs to undergo the process of contention for the medium, incurring a significant overhead. Moreover, the carrier sense multiple access /collision avoidance (CSMA/CA) protocol of 802.11 relies heavily on the assumption that stations hear each other - if one station starts transmitting, other stations hear it and avoid transmitting until the transaction ends.

In FBWA access systems stations seldom hear each other, either because of frequency duplexing or due to directional antennas in the stations. For this reason the FBWA systems typically rely on centralized scheduling of transmissions by the base station. The base station collects information on the traffic demands and allocates time accordingly. By grouping the uplink transmissions together the waste due to turnaround time is minimized as well. This is the baseline mechanism incorporated into 802.16 MAC.

Alvarion entered the FBWA field from the WLAN background. The WLANs, developed initially for the unlicensed bands, incorporate valuable mechanisms which only now begin to be recognized by the FBWA community: built in retransmission mechanisms, and adaptive modulation. Adaptive modulation allows operating most of the network at high data rate, and still having the fallback capability for the few stations with a bad link. The point-to-multipoint BreezeACCESS products use FH, due to its robustness to fades and to interference, and due to availability of multiple frequency channels. DS, having just three frequency channels but a higher data rate is used in point to point products. The MAC of BreezeACCESS started from 802.11 MAC, due to focus on data services. It was subsequently extended to include scheduling oriented extensions for voice support and traffic shaping mechanism for distributing the medium resource according to subscription profiles. As we see, making 802.11 technology work for FBWA involved judicious choices and proprietary extensions to overcome the limitations we outlined earlier.