figure 6. throughput of sora when communicating with a commercial Wifi card. Sora–Commercial presents the transmission throughput when a sora node sends data. Commercial–Sora presents the throughput when a sora node receives data. Commercial–Commercial presents the throughput when a commercial nic communicates with another commercial nic.

25

802.11b

802.11a/g

20

Throughput (Mbps)

15

10

5

0

1M 2M 5.5M 11M 6M 24M 54M
Modulation Mode
Sora–Commercial Commercial–Commercial
Commercial–Sora

generation and transferring can overlap with the demodulation of the data frame. After the entire frame is demodulated and validated, Soft WiFi instructs the RCB to transmit the ACK which has already been stored in the RCB. Thus, the latency for ACK transmission is very small.

Figure 6 shows the transmitting and receiving throughput of a Sora Soft WiFi node when it communicates with a commercial WiFi NIC. In the “Sora–Commercial” configuration, the Sora node acts as a sender and generates 1400-byte UDP frames and unicast transmits them to a laptop equipped with a commercial NIC. In the “Commercial–Sora” configuration, the Sora node acts as a receiver, and the laptop generates the same workload. The “Commercial–Commercial” configuration shows the throughput when both sender and receiver are commercial NICs. In all configurations, the hosts were at the same distance from each other and experienced very little packet loss. Figure 6 shows the throughput achieved for all configurations with the various modulation modes in 11a/b/g. We show only three selective rates in 11a/g for conciseness. The results are averaged over five runs (the variance was very small).

We make a number of observations from these results. First, the Sora SoftWiFi implementation operates seamlessly with commercial devices, showing that Sora Soft WiFi is protocol compatible. Second, Sora SoftWiFi can achieve similar performance as commercial devices. The throughputs for both configurations are essentially equivalent, demonstrating that Soft WiFi ( 1) has the processing capability to demodulate all incoming frames at full modulation rates, and ( 2) it can meet the 802.11 timing constraints for returning ACKs within the delay window required by the standard. We note that the maximal achievable application throughput for 802.11 is less than 80% of the PHY data rate, and the percentage decreases as the PHY data rate increases. This

limit is due to the overhead of headers at different layers as well as the MAC overhead to coordinate channel access (i.e., carrier sense, ACKs, and backoff), and is a well-known property of 802.11 performance.

6. 2. softLte

We have also implemented the 3GPP LTE Physical Uplink Shared Channel (PHUSC) on the Sora platform. 13 LTE is the next generation cellular standard. It is more complex than 802.11 since it uses a higher-order FFT (1024-point) and advanced coding/decoding algorithms (e.g., Turbo coding). Our SoftLTE implementation on Sora provides a peak data rate of 43.8Mbps with a 20-MHz channel, 16QAM modulation, and 3/4 Turbo coding. The most computationally intensive component of an LTE PHY is the Turbo decoder. Our current implementation can achieve 35Mbps throughput using one hardware thread of an Intel Core i7-920 core ( 2. 66 GHz). Since Core i7 supports hyper-threading, though, we can execute the Turbo decoder in parallel on two threads, achieving an aggregated throughput of 54.8Mbps. We can achieve this performance because Turbo decoding is relatively balanced in the number of arithmetic instructions and memory accesses. Therefore, the two threads can overlap these two kinds of operations well and yield a 56% performance gain even though they share the same execution units of a single core. Thus, the whole SoftLTE implementation can run in real time with two Intel Core i7 cores.

7. ReLateD WoRk

Traditionally, device drivers have been the primary software mechanism for changing wireless functionality on general-purpose computing systems. For example, the MadWiFi drivers for cards with Atheros chipsets, 3 HostAP drivers for Prism chipsets, 2 and the rtx200 drivers for RaLink chipsets5 are popular driver suites for experimenting with 802.11. These drivers typically allow software to control a wide range of 802.11 management tasks and non-time-critical aspects of the MAC protocol, and allow software to access some device hardware state and exercise limited control over device operation (e.g., transmission rate or power). However, they do not allow changes to fundamental aspects of 802.11 like the MAC packet format or any aspects of PHY.

SoftMAC goes one step further to provide a platform for implementing customized MAC protocols using inexpensive commodity 802.11 cards. 17 Based on the Mad WiFi drivers and associated open-source hardware abstraction layers, SoftMAC takes advantage of features of the Atheros chipsets to control and disable default low-level MAC behavior. SoftMAC enables greater flexibility in implementing nonstandard MAC features, but does not provide a full platform for SDR. With the separation of functionality between driver software and hardware firmware on commodity devices, time critical tasks and PHY processing remain unchangeable.

GNU Radio is a popular software toolkit for building software radios using general-purpose computing platforms. 1 GNU Radio consists of a software library and a hardware platform. Developers implement software radios